Embryology GastrointesInal System

Gastrointes*nal System Embryology Gastrointestinal System: Embryology
1.  The primordial gut at the beginning of the 4th week is closed
a.  At the cranial end by the oropharyngeal membrane
b.  At the caudal end by the cloacal membrane
2.  The embryonic layers of the primordial gut give rise to
a.  Epithelia and glands – endoderm
b.  Stroma – LP splanchnic mesoderm
3.  The epithelium within the stomodeum (cranial to the oropharyngeal
membrane) and epithelium of the proctodeum (caudal to the cloacal
membrane) is derived from surface ectoderm
4.  The primordial gut is divided into three regions:
a.  Foregut – vascularized by celiac trunk
b.  Midgut – vascularized by superior mesenteric artery
c.  Hindgut – vascularized by inferior mesenteric artery
Gastrointestinal System: Embryology
4a. Foregut –
1.  derivatives are:
a.  Primordial pharynx
1.  Oral cavity
2.  Teeth
3.  Pharynx
4.  Tongue
5.  Tonsils
6.  Salivary glands (submaxillary, submandibular, sublingual)
b.  Upper respiratory system (nasal cavities, nasopharynx)
c.  Lower respiratory system (larynx, trachea, bronchi, lungs)
d.  Esophagus
e.  Stomach
f.  Duodenum
g.  Liver
h.  Extrahepatic biliary system (including gallbladder)
i.  Pancreas
All foregut derivatives except pharynx, respiratory system, and most of
esophagus, are supplied by the celiac trunk, the artery of the foregut.
Development of Oral Cavity
Oral Cavity
The pharyngeal apparatus consists of:
1.  Pharyngeal arches
2.  Pharyngeal membranes
3.  Pharyngeal pouches
4.  Pharyngeal grooves
These embryonic structures contribute to
formation of the head and neck.
3
4
Pharyngeal Arches: The pharyngeal arches begin to develop early in the fourth week as neural crest cells (cranial ectomesenchyme) migrate into the future head and neck regions. Pharyngeal Arches: The primordial jaws – first pair of pharyngeal arches – appear as surface elevaAons lateral to the developing pharynx. 1st pair Developing Pharynx Primordial Jaws Primordial Jaws = Maxillary Prominence (green) & Mandibular Prominence (red) Pharyngeal Arches: By the end of the fourth week, four pairs of pharyngeal arches are visible externally. Pharyngeal Arches: The forming pharyngeal arches are separated from each other by fissures – pharyngeal grooves. Pharyngeal grooves Fate of Pharyngeal Arches
Contribute extensively to the formation of the face, nasal cavities, mouth, larynx,
pharynx, and neck.
A typical pharyngeal arch contains:
1.  An aortic arch – an artery that arises from the truncus ateriosus of the
primordial heart.
2.  A cartilaginous rod – that forms the skeleton of the arch
3.  A muscular component – that differentiates into the muscles in the head
and neck
4.  A nerve – that supplies the mucosa (epithelial lining) and muscles derived
from the arch
Pharyngeal Arches Fate of Pharyngeal Arches The pharyngeal arches contribute extensively to the formaAon of the face, nasal caviAes, mouth, larynx, pharynx, and neck. A typical pharyngeal arch contains: 1.  An aor*c arch – an artery that arises from the aorAc sac and connects with the truncus ateriosus of the primordial heart. Pharyngeal Arch: Aor*c Arches 1st AorAc Arch Maxillary artery 2nd AorAc Arch Stapedial artery 3rd AorAc Arch CaroAd arteries: Common, Internal & External 4th leS AorAc Arch Arch of the Aorta 4th right AorAc Arch Right proximal Subclavian artery 5th AorAc Arch Degenerates 6th AorAc Arch, proximal porAon Pulmonary arteries 6th AorAc Arch, distal porAons leS side Ligamentum arteriosum right side Degenerates Pharyngeal Arches Fate of Pharyngeal Arches A typical pharyngeal arch contains: 2. A car*laginous rod – that forms the skeletal structures of the arch Pharyngeal Arch: Car*laginous Rod 1st Arch Meckel’s carAlage (maxillary & mandibular prominences, malleus, and incus 2nd Arch Stapes, Styloid process, Lesser cornu (horn) of hyoid bone, Superior part of body of hyoid bone 3rd Arch Greater cornu (horn) of hyoid, Inferior body of hyoid bone 4th Arch Thyroid carAlage, Cricoid carAlage, Arytenoid carAlages, Corniculate carAlages, Cuneiform carAlages, TriAceal carAlages Pharyngeal Arches Fate of Pharyngeal Arches A typical pharyngeal arch contains: 3. A muscular component – that differenAates into the muscles in the head and neck Pharyngeal Arches: Muscular Component 1st Arch Muscles of masAcaAon: temporalis, masseter, medial and lateral pterygoids; mylohyoid; anterior belly of digastric muscle 2nd Arch Muscles of facial expression: buccinator, auricularis, frontalis, platysma, orbiculais oris, obicularis occuli; stapedius, stylohyoid, posterior belly of digastric 3rd Arch Stylopharyngeus 4th & 6th Cricothyroid, Levator veli palaAni, Constrictors of pharynx (superior, middle, inferior), Intrinsic muscles of larynx (oblique arytenoid, transverse arytenoid, posterior crico-­‐arytenoid, lateral crico-­‐ arytenoid, thyro-­‐arytenoid), Striated muscle of esophagus Pharyngeal Arches Fate of Pharyngeal Arches A typical pharyngeal arch contains: 4. A nerve – that supplies the mucosa (epithelial lining) and muscles derived from the arch Pharyngeal Arches: Nerve Supply 1st Arch Trigeminal nerve (V1, V2, V3) 2nd Arch Facial nerve (VII) 3rd Arch Glossopharyngeal nerve (IX) 4th Arch Vagus nerve (X): Superior laryngeal branch of vagus (CN X) [External laryngeal nerve & Internal laryngeal nerve], Recurrent laryngeal branch of vagus nerve (CN X) First Arch – Mandibular Arch
1.  First aortic arch – maxillary artery.
2.  Skeletal structures –
a.  Maxillary prominence – gives rise to the maxilla (upper jaw), zygomatic
bone, and squamous part of temporal bone
b.  Mandibular prominence – forms the mandible (lower jaw)
c.  Malleus
d.  Incus
e.  Anterior ligament of malleus
f.  Sphenomandibular ligament
3.  Muscles – muscles of mastication: temporalis, masseter, medial and later
pterygoids; mylohyoid; anterior belly of digastric muscle
4.  Nerve – trigeminal nerve (CN V)
The first pair of pharyngeal arches plays a major role in facial development.
Second Arch – Hyoid Arch
1.  Second aortic arch – stapedial artery.
2.  Skeletal structures
a.  Stapes
b.  Styloid process
c.  Lesser cornu (horn) of hyoid bone
d.  Superior part of body of hyoid bone
e.  Stylohyoid ligament
3.  Muscles – muscles of facial expression: buccinator, auricularis, frontalis,
platysma, orbiculais oris, obicularis occuli; stapedius, stylohyoid,
posterior belly of digastric
4.  Nerve – facial nerve (CN VII)
Third Arch
1.  Third aortic arch – common carotid artery.
2.  Skeletal structures –
a.  Greater cornu (horn) of hyoid
b.  Inferior body of hyoid bone
3.  Muscles – stylopharyngeus
4.  Nerve – glossopharyngeal nerve (CN IX)
Fourth & Sixth Arches
1.  A. Fourth Aortic Arch
1.  Left 4th – arch of the aorta
2.  Right 4th – proximal portion of right subclavian artery
1. B. Sixth Aortic Arch
•  Left 6th distal – ductus arteriorsus
•  Left 6th proximal – left pulmonary artery
•  Right 6th distal – degenerates
•  Right 6th proximal – right pulmonary artery
2.  Skeletal structures
a.  Thyroid cartilage
b.  Cricoid cartilage
c.  Triticeal cartilages (paired)
d.  Arytenoid cartilages (paired)
e.  Corniculate cartilages (paired)
f.  Cuneiform cartilages (paired)
Fourth & Sixth Arch
3.  Muscles –
a.  Cricothyroid
b.  Levator veli palatini
c.  Constrictors of pharynx (superior, middle, inferior)
d.  Intrinsic muscles of larynx (oblique arytenoid, transverse arytenoid,
posterior arytenoid, latreal arytenoid, thyroarytenoid)
e.  Striated muscle of esophagus
4.  Nerves
a.  Superior laryngeal branch of vagus (CN X) (a & b, above)
b.  Recurrent laryngeal branch of vagus nerve (CN X) (c - e, above)
Development of the Face
1.  Five facial primordia appear early in the fourth week
a.  Around the large primordial stomodeum
1.  A single frontonasal prominence (1)
2.  Paired maxillary prominences (2)
3.  Paired mandibular prominences (2)
Frontonasal
Prominence
Maxillary
Prominence
Primordial
Stomodeum
Mandibular
Prominence
Development of the Face
1.  Five facial primordia appear early in the fourth week
a.  Five facial primordia are derivatives of the first pharyngeal arch
b.  Form cartilage, bone, and ligaments in facial and oral regions
1.  Are derived from cranial ectomesenchyme:
a.  Maxillary prominence – gives rise to the maxilla (upper jaw),
b.  Zygomatic bone,
c.  Squamous part of temporal bone
d.  Mandibular prominence – forms the mandible (lower jaw)
e.  Malleus
f.  Incus
g.  Anterior ligament of malleus
h.  Sphenomandibular ligament
c.  Form the craniofacial voluntary muscles
1.  Are derived from paraxial and prechordal mesoderm:
a.  temporalis, masseter, medial and later pterygoids; mylohyoid;
anterior belly of digastric muscle
Development of the Face
10. Each lateral nasal prominence is separated from the maxillary prominence by a
cleft, called the nasolacrimal groove.
Lateral
Nasal
Prominence
Nasolacrimal
groove
Maxillary Prominence
Development of the Face
12. By the end of the sixth week, each maxillary prominence has begun to merge
with the lateral prominence along the line of the nasolacrimal groove.
a.  Establishes continuity between the side of the nose (formed by the lateral
nasal prominence) and the cheek region (formed by the maxillary
prominence)
Development of the Face
11. By the end of the 5th week, the primordia of the auricles of the ears have
begun to develop.
a.  Six auricular hillocks (ectomesenchymal swellings) form around:
1.  The 1st pharyngeal groove (three on each side)
a.  The primordia of the auricle
b.  The external auditory meatus (canal).
External
Auditory
Meatus
Auricular hillocks
Development of the Face
14. Between the 7th and 10th weeks the medial nasal prominences merge with each
other. Similarly, the maxillary prominences and lateral nasal prominences merge
with each other.
Development of the Face
15. Merging of medial nasal prominence and maxillary prominence results in
a.  Continuity of the upper jaw
b.  Continuity of the upper lip
c.  Separation of nasal pits from mouth
Development of the Face
16. As medial nasal prominences merge they form an intermaxillary segment
F
Intermaxillary segment
E
Development of the Face
17. Intermaxillary segment forms
a.  Philtrum (middle part) of upper lip
b.  Premaxillary part of the maxilla and associated gingiva (gum)
c.  The primary palate
Primary palate
Philtrum of lip
Premaxillary
part of maxilla
& associated
gingiva
Development of Palate
Palate
Development of the Palate
2. Palatogenesis
a.  Begins at the end of the 5th week
b.  Ends at the 12th week
c.  Critical period for malformations is 6th to 9th week
Primary palate
Developing maxilla
Primordium of
premaxillary part
of maxilla
Development of the Palate – Primary Palate
3. Early in 6th week the primary palate – median palatine process – begins to
develop from deep part of intermaxillary segment of the maxilla
a.  Initially formed by merging of medial nasal prominences
Medial nasal prominences
merging with each other
Development of the Palate – Primary Palate
3. Early in 6th week the primary palate – median palatine process – begins to
develop from deep part of intermaxillary segment of the maxilla
b.  A wedge-shaped mass of ectomesenchyme between internal surfaces of
maxillary prominences of developing maxilla
Developing maxilla
Intermaxillary segment of maxilla
Development of the Palate – Primary Palate
3. Early in 6th week the primary palate – median palatine process – begins to
develop from deep part of intermaxillary segment of the maxilla
c. Primary palate forms premaxillary part of maxilla
1.  Represents small part of adult hard palate
2.  Located anterior to incisive foramen
Developing
maxilla
Primordium of
premaxillary part
of maxilla
c.  Primary palate
forms premaxillary
part of maxilla
1. Represents small
part of adult hard
palate
2. Located anterior
to incisive foramen
Premaxillary part
of maxilla
Incisive foramen
Development of the Palate – Secondary Palate
4.  The secondary palate is the primordium of the hard and soft parts of the
palate
Secondary palate
Hard
palate
Soft
palate
Development of the Palate – Secondary Palate
4.  The secondary palate is the primordium of the hard and soft parts of the
palate
a.  Begin developing early in 6th week
b. Forms from two ectomesenchymal projections – lateral palatine
processes (palatal shelves)
Median palatine process
Median
palatine
process
Lateral palatine processes
Development of the Palate – Secondary Palate
The secondary palate
b. Lateral palatine processes (palatal shelves)
1. Extend from internal aspects of maxillary prominences
2. Initially project inferomedially on each side of tongue
3. During 7th to 8th week, processes elongate and ascend to
horizontal position superior to tongue
Median palatine process
Nasal
septum
Maxillary
prominence
Lateral palatine process
Nasal
septum
Lateral
palatine
process
Development of the Palate – Secondary Palate
4.  The secondary palate
b. Lateral palatine processes (palatal shelves)
4. Gradually processes approach each other and
a.  fuse with posterior part of primary palate
b.  fuse in the median plane
c.  fuse with median nasal septum
Primary palate
Posterior part of
primary palate
Lateral palatine process
Medial nasal septum
Development of the Palate – Secondary Palate
4.  The secondary palate
b. Lateral palatine processes (palatal shelves)
4. Gradually processes approach each other and
a.  fuse with posterior part of primary palate
b.  fuse in the median plane
c.  fuse with median nasal septum
Primary palate
Fuse in the
median plane
Lateral palatine process
Median nasal septum
Development of the Palate – Secondary Palate
4.  The secondary palate
b. Lateral palatine processes (palatal shelves)
4. Gradually processes approach each other and
a.  fuse with posterior part of primary palate
b.  fuse in the median plane
c.  fuse with median nasal septum
Primary palate
Lateral palatine process
Fuse in the
median plane
Median nasal septum
Development of the Palate – Secondary Palate
4.  The secondary palate
b. Lateral palatine processes (palatal shelves)
4. Gradually processes approach each other and
a.  fuse with posterior part of primary palate
b.  fuse in the median plane
c.  fuse with median nasal septum
Nasal septum
Lateral palatine process
Fuse in the median plane with median nasal septum
Development of the Palate – Secondary Palate
5.  The nasal septum develops as a down growth from internal parts of merged
medial nasal prominences
a. Fusion between nasal septum & lateral palatine processes progresses in
an anterior to posterior direction from the 9th week to the 12th week
Nasal septum
Lateral palatine process
Fusion in the median plane with median nasal septum
Development of the Palate – Secondary Palate
5. Bone gradually develops in primary palate
a.  Forms premaxillary part of maxilla
b.  Lodges the incisor teeth
Premaxillary
part of maxilla
Incisor tooth
Development of the Palate – Secondary Palate
6. Concurrently, bone extends from maxilla and palatine bones into lateral
palatine process to form the hard palate
Incisor teeth
Incisive foramen
Suture between
premaxillary part of
maxilla and lateral
palatine process of
maxilla
Lateral palatine
process of
maxilla
Horizontal plate
of palatine bone
Premaxillary part of maxilla
Development of the Palate – Secondary Palate
7. The posterior parts of processes do not ossify
a.  They extend posteriorly beyond the nasal septum
b.  Fuse to form the soft palate, including uvula
c.  The median palatine raphe indicates line of fusion of lateral palatine
processes
Frenulum of lip
Incisive papillae (deep = i. foramen)
Upper lip
Hard palate
Gum
Median palatine raphe
Soft palate
Uvula
Development of the Palate – Secondary Palate
8. A small nasopalatine canal persists in median plane
a. Occurs between premaxillary part of maxilla and palatine processes of
maxilla
b. Represented in adult by incisive foramen (Fig. 10-36)
Incisor teeth
Incisive foramen
Suture between
premaxillary part of
maxilla and lateral
palatine process of
maxilla
Lateral palatine
process of
maxilla
Horizontal plate
of palatine bone
Premaxillary part of maxilla
Development of Teeth
Development of Teeth
Teeth
Teeth: Embryological Development Ectoderm
Ameloblasts
Neural Crest
Odontoblasts
Mesenchyme
Cementocytes
Enamel
• Moves down
• No longer produced
after adulthood
Predentin
• Type I
collagen
Dentin
• Moves outward
• Type I collagen
• Hydroxyapatite
• Fluoroapatite
Cementum
• Covers root
Teeth: Developmental Stages 1.  Bud stage •  Neuroectodermal cells sAmulate overlying ectodermic epithelial cells to proliferate • 
Forms the epithelial tooth bud Ectoderm
2.  Early cap stage NE cells
Mesoderm
•  Cells of the tooth bud proliferate & invaginate into underlying mesoderm = early cap •  BMP-­‐4 & AcAvin βA sAmulate formaAon Teeth: Developmental Stages (cont.) 3.  Late Cap Stage Bud of Permanent Tooth
• Develops from dental lamina
• Remains dormant
Outer Dental Epithelium
• Neural crest
Inner Dental Epithelium
• Neuroectoderm
NC neuroectodermal
cells lining
the epithelial
tooth bud
(“cap”)
Secrete:
• FGF-4
• BMP-2,4,7
Enamel Knot
• Signals tooth development
Regulate tooth shape
Teeth: Developemental Stages (cont.) 4.  Bell Stage •  Mesenchyme adjacent to developing enamel organ proliferates to form cellular mass as the enamel organ becomes cap shaped Inner Dental Epithelium
Layer of the enamel knot
Preameoloblasts
• Ameoloblasts
• Secrete enamel downward
Outermost Cells (apical)
of the dental papilla
Primitive Dental
Papilla
Preodontoblasts
• Odontoblasts
• Secrete non-mineralized predentin which later calcifies
into dentin
Teeth: Developmental Stages (cont.) 5. Tooth Erup*on Enamel
Dentin
Cementoblasts
• Secretes a layer of cementum to cover
root(s) of tooth
Blood
vessel in
dental
pulp
Odontoblasts
Periodontal Ligament
• Holds tooth to alveolus
Dental Sac gives rise to:
• Cementoblasts
• Periodontal ligament
SecAons of Teeth •  Crown: projects from the gingiva •  Root: fixed in tooth socket by periodon6um –  Number of roots varies •  Most of the tooth is composed of den*n –  Covered by enamel over the crown –  Covered by cement over the root •  Set in tooth sockets: alveolar process of maxillae & mandible –  Adjacent sockets are separated by interalveolar septa Sections of Teeth: A) An incisor tooth B) A molar tooth
In living people, the pulp cavity is a hollow space within the crown & neck containing
connective tissue, blood vessels, & nerves. The cavity narrows down to the root canal
in a single-rooted tooth or to one canal per root of a multirooted tooth. The vessels &
nerves enter or leave through the apical foramen.
Development of the Tongue
Development of Tongue
Tongue
Development of the Tongue
1.  Tuberculum Impar (median tongue bud) appears near end of 4th week
2.  It is a median triangular elevation in the floor of the primordial pharynx
3.  It is the first indication of tongue development
4.  Soon, two distal tongue buds (lateral lingual swellings) develop on either
side of the median tongue bud
5.  The three lingual buds develop from proliferation of the prechordal
mesoderm in the first pair of pharyngeal arches
6.  The distal tongue buds rapidly increase in size and fuse (median sulcus of
the tongue) to form the anterior 2/3’s of the tongue
Development of the Tongue
7.  The posterior 1/3 of the tongue (pharyngeal part) develops from two
elevations that develop caudal to the foramen cecum
a.  The copula forms by fusion of the ventromedial parts of the second pair
of pharyngeal arches
b.  The hypopharyngeal eminence develops caudal to the copula from
prechordal mesoderm of the third and fourth pharyngeal arches
8.  As the tongue develops the copula is gradually overgrown by the
hypopharyngeal eminence and disappears
9.  The posterior 1/3 of the tongue develops from the rostral part of the
hypopharyngeal eminence
10. The terminal sulcus is the line of fusion between the anterior 2/3 and
posterior 1/3 of the tongue
Development of the Tongue
11. Cranial ectomesenchyme forms the connective tissue and vasculature of
the tongue
12. Most tongue muscles are derived from myoblasts that migrate from
occipital myotomes
13. The hypoglossal nerve (CN XII) accompanies the myoblasts during their
migration and innervate the tongue muscles as they develop, except for the
palatoglossus muscle which is innervated by the vagus nerve (CN X)
14.  The sensory supply to the anterior 2/3 of the tongue is from the lingual
branch of the mandibular division of the trigeminal nerve (CN V), the
nerve of the first pharyngeal arch
15. The facial nerve supplies the taste buds in the anterior 2/3 of the tongue,
except for the circumvallate papillae
16. The circumvallate papillae are innervated by the glossopharyngeal nerve
(CN IX)
Development of the Tongue
17. Clinical Correlates
a.  Anomalies of the tongue are uncommon, except for
1.  Fissuring of the tongue – infants with Down syndrome
2.  Hypertrophy of the lingual papillae – infants with Down syndrome
b.  Congenital Lingual Cysts and Fistulas
1.  Cysts may be derived from remnants of thyroglossal duct
2.  May emerge to produce symptoms of discomfort and/or dysphagia
(difficulty swallowing)
3.  Fissures are also derived from persistance of lingual parts of
thyroglossal duct
a.  Open through foramen cecum into oral cavity
Mouth: Embryology
Development of the Tongue
17. Clinical Correlates
c.  Ankyloglossia (tongue-tied)
1.  The lingual frenulum normally connects the inferior surface of the
tongue to the floor of the mouth
2.  Sometimes the frenulum is short and entends to the tip of the
tongue
a.  Interferes with free protrusion of the tongue
b.  May make breast feeding difficult
3.  Occurs in about 1:300 live births in North American infants
Mouth: Embryology
Development of the Tongue
17. Clinical Correlates
d.  Macroglossia
1.  An excessively large tongue
2.  Results from generalized hypertrophy of the tongue
3.  Usually results from lymphangioma (a lymph tumor)
4.  May result from muscular hypertrophy
Mouth: Embryology
Development of the Tongue
17. Clinical Correlates
e.  Microglossia
1.  An abnormally small tongue
2.  Usually associated with micrognathia (underdeveloped mandible
and recession of the chin)
3.  May be associated with certain limb defects, Hanhart syndrome
Mouth: Embryology
Development of the Tongue
17. Clinical Correlates
f.  Glossoschisis (bifid or cleft tongue)
1.  Very uncommon anomaly
2.  Results from incomplete fusion of the distal tongue buds
3.  Exhibits as a deep median groove in the tongue
4.  Usually a cleft tongue does not extend to tip of tongue
Salivary Glands
Salivary Glands: Embryology
1.  Salivary glands begin developing during the 6th and 8th weeks
2.  They begin as solid epithelial buds from primordial oral cavity
3.  The club-shaped ends of the buds grow into the underlying cranial
ectomesenchyme
4.  The connective tissue stroma of the glands is derived from cranial
ectomesenchyme (neural crest)
5.  All parenchyma arises by proliferation of oral epithelium
a.  Surface ectoderm – parotid
b.  Endoderm – submandibular (submaxillary) gland and sublingual
gland
Salivary Glands: Embryology
6.  Parotid glands
a.  Are the first to appear – early in the 6th week
b.  They develop from buds of oral ectoderm near the angles of the
stomodeum
c.  The buds grow towards the ears and branch to form solid cords with
rounded ends
d.  Later the cords canalize – develop lumina – and become ducts by 10
weeks
e.  The rounded ends develop into acini
f.  Secretion begins at 18 weeks
g.  The capsule and connective tissue develops from cranial
ectomesenchyme (neural crest)
Salivary Glands: Embryology
7.  Submandibular (Submaxillary) glands
a.  Appear late in the 6th week
b.  Develop from endodermal buds in floor of stomodeum
c.  Solid cellular processes grow posteriorly, lateral to developing tongue
1.  Later they branch and differentiate
d.  Acini begin to form at 12 weeks
e.  Secretory activity begins at 16 weeks
f.  Growth of glands continues after birth with formation of mucus acini
g.  Lateral to tongue, a linear groove forms that soon closes to form
sumandibular duct
Salivary Glands: Embryology
8.  Sublingual glands
a.  Appear in the 8th week, about 2 weeks later than other salivary glands
b.  Develop from multiple endoderm buds in the paralingual sulcus
c.  Buds branch and canalize to form 10 – 12 ducts that open independently
into the floor of the mouth
Development of Larynx
Larynx
Development of Larynx
1.  The epithelium of larynx develops from the endoderm of the cranial end of
the laryngotracheal tube.
2.  The cartilages of the larynx develop from the cartilages in the 4th and 6th
pharyngeal arches.
3.  The cartilages are derived from neural crest cells (cranial ectomesenchyme).
Development of Larynx
4.  The ectomesenchyme are the cranial end of the laryngotracheal tube
proliferates rapidly and produces the paired arytenoid swellings.
a.  Swellings grow towards tongue, converting the slit-like opening – the
primordial glottis – into a T-shaped laryngeal inlet.
b.  Reduces the developing laryngeal lumen to a narrow slit.
Epiglottic swelling
Primitive
glottis
Arytenoid swellings
Development of Larynx
4.  The ectomesenchyme are the cranial end of the laryngotracheal tube
proliferates rapidly and produces the paired arytenoid swellings.
a.  Swellings grow towards tongue, converting the slit-like opening – the
primordial glottis – into a T-shaped laryngeal inlet.
b.  Reduces the developing laryngeal lumen to a narrow slit.
Epiglottis
Laryngeal
inlet
Arytenoid swellings
Cuneiform
tubercle
Development of Larynx
5.  The laryngeal epithelium proliferates rapidly, resulting in a temporary
occlusion of the laryngeal lumen.
6.  Recanalization of the larynx usually occurs by the 10th week.
Endoderm
Esophagus
LP Splanchnic
mesoderm
Laryngotracheal
tube
Epithelium
LP Splanchnic
mesoderm
Developing
cartilages
Development of Larynx
7.  The laryngeal ventricles form during the recanalization process.
8.  The recesses are bounded by folds of mucous membrane that become the
a.  The vestibular folds (cat purring).
b.  The vocal folds (human phonation)
Development of Larynx
9. The epiglottis develops from the caudal part of the hypopharyngeal
eminence.
a.  A prominence produced by the ventral ends of the 3rd and 4th pharyngeal
arches.
b.  The rostral part of the eminence forms the posterior third of the
pharyngeal part of the tongue.
Hypopharyngeal Eminence
Development of Larynx
10. The laryngeal muscles develop from myoblasts of the 4th and 6th
pharyngeal arches and therefore innervated by branches of the vagus (CN
X) nerve.
11. Growth of larynx and epiglottis is rapid during first three years after birth. By
this time the epiglottis has reached its adult form.
Esophagus
Development of Esophagus
Esophagus
4a. Foregut –
1d. Esophagus
1. Development
a. Develops from foregut immediately caudal to pharynx
b. The esophagus is partitioned from the trachea by the
tracheoesophageal septum
c.  Initially, the esophagus is short, but it elongates rapidly
Endoderm
Esophagus
Laryngotracheal
tube
LP Splanchnic
mesoderm
Epithelium
LP Splanchnic
mesoderm
Developing
cartilages
Esophagus: Embryology
4a. Foregut –
1d. Esophagus
1.  Development
d.  It reaches its final length by the 7th week
e.  The epithelium and glands are derived from endoderm
1.  The epithelium proliferates and completely obliterates the
lumen; recanalization of the esophagus occurs by the end of
the 8th week (programmed apoptosis)
f.  The skeletal muscle in the upper 2/3 of the esophagus (overlap in
the middle 1/3 with smooth muscle) is derived from prechordal
mesoderm of the pharyngeal arches
g.  The smooth muscle in the lower 2/3 of the esophagus (overlap in
the middle 1/3 with skeletal muscle) is derived from LP splanchnic
mesoderm
h.  Both types of muscle are innervated by branches of the vagus
nerve (CN X)
Esophagus: Embryology
4a. Foregut –
1d. Esophagus
2.  Clinical correlates
a.  Esophageal atresia
1.  Blockage of the esophagus occurs with an incidence of
1:3000 to 1:4500 live births
2.  About 1/3 of affected infants are born prematurely
3.  Associated with tracheoesophageal fistula in more than 85%
of cases
Esophagus: Embryology
4a. Foregut –
1d. Esophagus
2.  Clinical correlates
b.  Tracheoesophageal Fistula (TEF)
1.  A fistula (abnormal passage) between trachea and
esophagus.
2.  Results from incomplete division of cranial part of foregut
into respiratory and esophageal parts during 4th week.
3.  Incomplete fusion of tracheoesophageal folds results in a
defective tracheoesophageal septum, resulting in a
tracheoesophageal fistula.
4.  Most common anomaly of lower respiratory tract
5.  Occurs in 1/3000 – 4500 births
6.  Males most affected
7.  In more than 85% cases the fistula is associated with
esophageal atresia
Esophagus: Embryology
b. Tracheoesophageal Fistula (TEF)
8.  Four main varieties of tracheoesophageal fistula.
a.  Superior part of esophagus ends blindly (esophageal atresia), the
inferior part of esophagus joins trachea near its bifurcation (gastric
contents may enter trachea and lungs)
b.  Fistula between patent esophagus and trachea (esophageal
contents and gastric contents may enter trachea and lungs)
c.  Fistula between superior part of esophagus and trachea, inferior
part of esophagus ends blindly (esophageal contents enter the
trachea and lungs).
d.  Fistula between superior part of esophagus and trachea, fistula
between inferior part of esophagus and trachea near its
bifurcation (esophageal and gastric contents may enter trachea and
lungs)
Polyhydramnios – is often associated with esophageal atresia and
tracheoesophageal fistula. The excess amniotic fluid cannot pass to the
stomach and intestines for absorption and subsequent transfer through
the placenta to the mother’s blood for disposal.
Varieties of Tracheoesophageal Fistulas
1.  Superior part of esophagus ends blindly (esophageal
atresia)
•  Inferior part of esophagus joins trachea near its
bifurcation (gastric contents may enter trachea and
lungs – aspiration)
Esophagus
Varieties of Tracheoesophageal Fistulas
2. Fistula between superior part of esophagus and
trachea, inferior part of esophagus ends blindly
(esophageal contents enter the trachea and lungs).
Varieties of Tracheoesophageal Fistulas
3. Fistula between patent esophagus and trachea
(esophageal contents and gastric contents may enter
trachea and lungs)
Esophagus
Varieties of Tracheoesophageal Fistulas
4. Fistula between superior part of esophagus and
trachea, fistula between inferior part of esophagus and
trachea near its bifurcation (esophageal and gastric
contents may enter trachea and lungs)
4a. Foregut –
1d. Esophagus
2.  Clinical correlates
c.  Esophageal stenosis
1.  Narrowing of the lumen
2.  Usually occurs in distal 1/3
3.  Occurs as web or long segment with threadlike opening
4.  Results from incomplete canalization during 8th week
5.  May also result from failure of esophageal blood vessels to
develop in affected area – atrophy of segment occurs
4a. Foregut –
1d. Esophagus
2.  Clinical correlates
d.  Short esophagus
1.  Normally esophagus is very short
2.  Esophagus fails to elongate as neck and thorax develop
3.  Part of stomach may be displaced superiorly through
esophageal hiatus into thorax
4.  Congenital hiatal hernia
5.  Most hiatal hernias occur after birth\
6.  Usually in middle-aged people
7.  Result from weakening and widening of esophageal hiatus in
diaphragm
4.  a. Foregut
1d. Esophagus
2. Clinical Correlates
e. Laryngotracheoesophageal Cleft
1.  Uncommon
2.  The larynx and trachea fail to separate completely from the
esophagus
3.  Symptoms similar to tracheoesophageal fistula, EXCEPT
aphonia (absence of voice) is a distinguishing feature
Stomach
Stomach: Embryology
4a. Foregut
1e. Stomach
1.  The distal part of foregut is initially a tubular structure
2.  ~ middle of 4th week, a slight dilation indicates site of stomach
primordium
3.  First appears as fusiform enlargement of caudal part of foregut and
oriented in median plane
4.  Primordium soon enlarges and broadens ventrodorsally
5.  During next two weeks the dorsal border of stomach grows faster than
the ventral border and demarcates
a.  the greater curvature of stomach – dorsal border
b.  The lesser curvature of stomach – ventral border
Stomach: Embryology
4a. Foregut
1e. Stomach
6.  Rotation of the stomach:
a.  as the stomach enlarges it acquires its adult shape,
b.  It slowly rotates 90 degrees in a clockwise direction around its
longitudinal axis
1.  The ventral border (lesser curvature) moves to the right
2.  The dorsal border (greater curvature) moves to the left
3.  The original left side becomes the ventral surface
4.  The original right side becomes the dorsal surface
Stomach: Embryology
4a. Foregut
1e. Stomach
6.  Rotation of the stomach:
a.  as the stomach enlarges it acquires its adult shape,
b.  It slowly rotates 90 degrees in a clockwise direction around its
longitudinal axis
5.  Before rotation, the cranial and caudal ends are in the median
plane
6.  During rotation and growth, its cranial end moves to the left and
slightly inferiorly
7.  Its caudal end moves to the right and superiorly
8.  After rotation, the stomach assumes its final position – its long
axis is almost transverse to the long axis of the body
9.  Explains why left vagus supplies anterior (ventral) wall and right
vagus supplies posterior (dorsal) wall of adult stomach
Stomach: Embryology
4a. Foregut
1e. Stomach
7.  Mesenteries
a.  Stomach is suspended from dorsal wall by a dorsal mesentery =
dorsal mesogastrium
1.  Originally in median plane but carried to left during rotation and
formation of omental bursa (lesser sac of peritoneum)
b.  Stomach is suspended from ventral wall by a ventral mesentery =
ventral mesogastrium
1.  Attaches stomach to liver (hepatogastric ligament) and
duodenum to liver (hepatoduodenual ligament)
2.  Forms anterior wall of lesser sac
3.  Opening into sac is epiploic foramen of Winslow
Stomach: Embryology
4a. Foregut
1e. Stomach
7.  Mesenteries
c.  Omental bursa
1.  Isolated clefts (cavities) develop in the LP splanchnic
mesoderm forming the dorsal mesentery
2.  The clefts soon coalesce to form a single cavity, the omental
bursa or lesser peritoneal sac
3.  As the stomach enlarges, the omental bursa expands and
acquires an inferior recess of the omental bursa between
layers of the dorsal mesentery – the greater omentum
4.  This membranes over hangs the developing intestines.
5.  The inferior recess disappears as the layers of the greater
omentum fuse
6.  The omental bursa communicates with the peritoneal cavity
through the omental (epiploic) foramen of Winslow
Stomach: Embryology
4a. Foregut
1e. Stomach
8.  Clinical correlates
a.  Congenital hypertrophic pyloric stenosis
1.  Anomalies of stomach are uncommon except for this disorder
2.  Affects 1:150 males and 1:750 females
3.  Marked thickening of the pyloric sphincter
4.  The circular, and to a lesser extent, the longitudinal muscle are
markedly hypertrophied – LP splanchnic mesoderm
5.  Results in severe stenosis of the pyloric canal and obstruction
to the passage of food
6.  As a result, the stomach becomes markedly distended
7.  The infant expels the stomach’s contents with considerable
force = projectile vomiting
Liver
Liver: Embryology
1.  The liver, gallbladder, and biliary duct system arise
a.  Early in the 4th week
b.  As a ventral outgrowth from the caudal part of the foregut
1.  Hepatic diverticulum
c.  Develops from embryonic endoderm
d.  Hepatic diverticulum
1.  Extends into septum transversum
a.  A mass of LP splanchnic mesoderm
b.  Between developing heart and midgut
c.  Forms ventral mesentery
Liver: Embryology
1.  The liver, gallbladder, and biliary duct system arise
d.  Hepatic diverticulum
2.  Rapidly divides into two parts as it grows between layers of ventral
mesentery
a.  Cranial portion
1.  Is larger
2.  Forms primordium of liver
3.  Proliferating endodermal cells give rise to
a.  Interlacing cords of hepatic cells
b.  Epithelial lining of intraheptic part of biliary apparatus
4.  Hepatic cords
a.  Anastomose around endothelial-lined spaces – primordia of
hepatic sinusoids
b.  Stroma, hematopoietic tissue, and Kupffer cells are derived
from LP splanchnic mesoderm of septum transversum
Liver: Embryology
1.  The liver, gallbladder, and biliary duct system arise
d.  Hepatic diverticulum
2.a. Cranial portion
4.  Liver grows rapidly
a.  From 5th to 10th weeks fills a large part of the upper abdominal
cavity
b.  The quantity of oxygenated blood from the umbilical vein into the
liver determines the development and functional segmentation
c.  Initially, right and left lobes are about the same
d.  Soon, right lobe becomes larger
e.  Hematopoiesis begins during the 6th week
f.  By 9th week liver accounts for ~ 10% of total weight of fetus
g.  Bile formation by the hepatic cells begins during the 12th week
3.  Bile entering the duodenum through the bile duct after the 13th week gives
meconium (intestinal contents) a dark green color
4.  The left umbilical vein passes in the free border of the falciform ligament
on its way from the umbilical cord to the liver (becomes ligamentum teres
hepatis in the adult)
Liver: Embryology
2.  Clinical Correlates
a.  Anomalies of the liver
1.  Minor variations of liver lobulation are common
2.  Congenital anomalies of the liver are rare
3.  Variations of the hepatic ducts, bile duct, and cystic duct
a.  Are common
b.  Clinically significant
c.  Accessory hepatic ducts may be present
1.  Are narrow channels running from right lobe of liver to
anterior surface of body of gallbladder
2.  Cystic duct may open into accessory hepatic duct rather than
into common bile duct
Development of the Gall Bladder
Gallbladder: Embryology
1.  The liver, gallbladder, and biliary duct system arise
d.  Hepatic diverticulum
2.  Rapidly divides into two parts as it grows between layers of ventral
mesentery
b. Caudal portion
1.  Becomes the gallbladder
2.  The stalk of the hepatic diverticulum becomes the cystic duct
3.  The extrahepatic biliary apparatus
a.  Initially, is occluded with epithelial cells
b.  Is later canalized due to programmed apoptosis
4.  The stalk connecting the hepatic and cystic ducts to the
duodenum becomes the bile duct
3.  Bile entering the duodenum through the bile duct after the 13th week gives
meconium (intestinal contents) a dark green color
Gallbladder: Embryology
2.  Clinical Correlates
a.  Extrahepatic biliary atresia
1.  Most serious anomaly of extrahepatic biliary system
2.  Occurs in 1:10,000 to 1:15,000 live births
3.  Most common form
a.  ~ 85% cases
b.  Obstruction of ducts at or superior to porta hepatis (gateway
to the liver)
1.  Deep transverse fissure on the visceral surface of the liver
c.  Caused by failure of ducts to canalize
d.  Could also result from liver infection during late fetal period
4.  Jaundice occurs soon after birth
a.  If not corrected surgically, child may die if a liver transplant is not
performed
Exocrine
Pancreas
Pancreas: Embryology
1.  Pancreas develops between layers of the mesentery from
a.  dorsal pancreatic bud of endodermal cells
b.  ventral pancreatic bud of endodermal cells
c.  Arises from caudal part of foregut
b.  Most of pancreas is derived from dorsal pancreatic bud
2.  The larger dorsal pancreatic bud appears first and develops a slight
distance cranial to the ventral bud
a.  It grows rapidly between layers of the dorsal mesentery
3.  The ventral pancreatic bud develops near the entry of the bile duct into the
duodenum
a.  It grows between layers of the ventral mesentery
b.  As the duodenum rotates to the right and becomes “C”-shaped the
ventral pancreatic bud is carried dorsally with the bile duct.
1.  It soon lies posterior to the dorsal pancreatic bud and fuses with it
c.  Forms the uncinate process and part of the head of the pancreas
Pancreas: Embryology
4.  As the stomach, duodenum and ventral mesentery rotate, the pancreas
comes to lie on the dorsal abdominal wall.
5.  As the pancreatic buds fuse, their ducts anastomose.
a.  The main pancreatic duct forms from
1.  The duct of the ventral bud
2.  The distal part of the duct of the dorsal bud
b.  The accessory pancreatic duct, if present,
1.  Forms from the proximal part of the duct of the dorsal bud
2.  Opens into the minor duodenal papilla
a.  Located about 2 cm cranial to the main duct
c.  The main pancreatic duct and the accessory pancreatic duct often
communicated with each other
d.  In about 10% of the people the pancreatic ducts fail to fuse
Pancreas: Embryology
6.  Histogenesis:
a.  The parenchyma of the pancreas is derived from endoderm of the
pancreatic buds
1.  Form a network of tubules
2.  Early in the fetal period (9 weeks), acini begin to develop from cell
clusters around the ends of these tubules (primordial ducts)
b.  The stroma of the pancreas develops from LP splanchnic mesoderm
Development of the Duodenum
Duodenum: Embryology
1.  Early in 4th week the duodenum begins to develop from the caudal part of the
foregut and the cranial part of the midgut
2.  The parenchyma develops from endoderm and the stroma develops from LP
splanchnic mesoderm
3.  The junction of the two parts of the duodenum is just distal to the origin of the
bile duct
4.  The developing duodenum grows rapidly, forming a C-shaped loop that
projects ventrally
5.  As the stomach rotates, the duodenal loop rotates to the right and comes to lie
retroperitoneally
6.  Because of its dual origin (foregut and midgut), it is vascularized by both the
celiac trunk (foregut) and superior mesenteric artery (midgut)
7.  During 5th and 6th weeks the lumen is obliterated due to epithelial proliferation
8.  It recanalizes (programmed apoptosis) by the end of the 8th week
Duodenum: Embryology
9.  Clinical correlates
a.  Duodenal stenosis
1.  Partial occlusion of the lumen results from incomplete recanalization
2.  Due to defective programmed apoptosis
3.  Most stenoses involve the horizontal (third) and/or ascending (fourth)
parts of the duodenum
4.  Because of the occlusion, the stomach’s contents (usually containing
bile and therefore green in color) are often vomited
Duodenum: Embryology
9.  Clinical correlates
b.  Duodenal atresia
1.  Complete occlusion of the lumen
2.  Not common
3.  20-30% of infants have Down syndrome
4.  An additional 20% are premature
5.  In 20% of cases, the bile duct enters the duodenum just distal to the
opening of the hepatopancreatic ampulla
6.  The lumen is completely obliterated with epithelial cells
7.  Recanalization fails to occur
8.  Most atresias involve descending (2nd) and horizontal (3rd) parts
9.  Located distal to opening of bile duct
10. Vomiting usually begins a few hours after birth
11. Vomitus always contains bile
12. Often there is extension of the epigastrium – upper central area of
abdomen
a.  Resulting from an overfilled stomach and superior part of
duodenum
Duodenum: Embryology
9.  Clinical correlates
c.  Polyhydramnios
1.  Occurs because of duodenal atresia
2.  Atresia prevents normal absorption of amniotic fluid by the intestines
3.  Diagnosis of duodenal atresia is suggested by the “double bubble
sign” on plain radiographs or ultrasound scans
a.  The “double bubble sign” is caused by a distended, gas-filled
stomach and proximal duodenum
Midgut: Embryology
1.  The derivatives are:
a.  Distal ½ of duodenum (segments 3 & 4)
b.  Uncinate process of Pancreas (ventral Pancreatic Bud)
c.  Jejunum
d.  Ileum
e.  Cecum
f.  Appendix
g.  Ascending colon
h.  Proximal ½ - 2/3 of transverse colon
2.  All derivatives are supplied by the superior mesenteric artery
3.  The midgut loops are suspended from the dorsal abdominal wall by an
elongated mesentery
4.  As the midgut elongates it forms a ventral, U-shaped loop of gut – the midgut
loop – projects into the proximal part of the umbilical cord
5.  Designated as the physiological umbilical herniation
a.  Occurs because there is not enough room in the abdomen for the rapidly
growing midgut
Midgut: Embryology
6.  Occurs at beginning of 6th week
7.  The midgut communicates with the yolk sac through the narrow yolk stalk
(vitelline duct) until the 10th week
8.  The midgut loop has a cranial limb and a caudal limb
9.  The yolk stalk is attached to the apex of the midgut loop where the two limbs
join
10. The cranial limb grows rapidly and forms the small intestines
11. The caudal loop grows more slowly and forms the cecal diverticulum, the
primordia of the cecum and appendix
Midgut: Embryology
12. Rotation of the midgut
a.  While in the umbilical cord, the midgut loop rotates 90 degrees
counterclockwise around the axis of the superior mesenteric artery
b.  This brings the cranial limb of the midgut loop to the right and the caudal
limb to the left
c.  During rotation, the midgut elongates and forms the intestinal loops, e.g.,
jejunum and ileum
Midgut: Embryology
13. Return of Midgut to Abdomen
a.  During the 10th week the intestines return to the abdomen
1.  Unknown cause
2.  Proposed decrease in size of liver and kidneys
3.  Process called reduction of physiological midgut hernia
4.  The small intestine returns first
a.  Passes posterior and superior to superior mesenteric artery
b.  Occupies central part of abdomen
5.  As large intestine returns, it undergoes a further 180-degree
counterclockwise rotation
a.  Later it comes to lie on the right side of the abdomen
b.  Ascending colon becomes recognizable as the posterior
abdominal wall progressively elongates
Midgut: Embryology
14. Fixation of intestines
a.  Rotation of stomach and duodenum causes the duodenum and pancreas
to fall to the right
1.  They are pressed against the posterior abdominal wall by the colon
2.  Adjacent layers of peritoneum fuse and subsequently disappear
3.  Consequently most of the duodenum (2nd and 3rd segments) and the
pancreas become retroperitoneal
Midgut: Embryology
14. Fixation of intestines
b.  The attachment of the dorsal mesentery to the posterior wall of the
abdominal wall is greatly modified after the intestines return to the
abdominal cavity
1.  At first, the dorsal mesentery is in the median plane
2.  As the intestines enlarge, lengthen, and assume their final positions,
their mesenteries are pressed against the posterior abdominal wall.
3.  The mesentery of the ascending colon fuses with the parietal
peritoneum on the wall and disappears
4.  Consequently, ascending colon becomes retroperitoneal
Midgut: Embryology
14. Fixation of intestines
c.  The enlarged colon presses the duodenum against the posterior
abdominal wall
1.  As a result, most of the duodenal mesentery is absorbed
2.  Consequently, the duodenum, except for about the first 2.5 cm
(derived from foregut), has no mesentery and lies retroperitoneally
d.  Other derivatives of the midgut loop (e.g., jejunum and ileum) retain
their mesenteries
1.  The mesentery is at first attached to the median plane of the posterior
abdominal wall
2.  After the mesentery of the ascending colon disappears, the fanshaped mesentery of the jejunum and ileum acquires a new line of
attachment
a.  Passes from the duodenal-jejunal junction to the ileocecal
junction
Midgut: Embryology
15. Cecum and Appendix
a.  Cecal diverticulum
1.  Primordium of cecum and vermiform (wormlike) appendix
2.  Appears in 6th week
3.  A swelling on the antimesenteric border of the caudal limb of the
midgut loop
4.  The apex does not grow as rapidly as the rest of the diverticulum
a.  Thus the appendix is initially a small diverticulum of the cecum
b.  The appendix increases rapidly in length
c.  At birth it is a relatively long tube arising from the distal end of the
cecum
d.  After birth, the wall of the cecum grows unequally
1.  Results in the appendix comes to enter its medial side
Midgut: Embryology
15. Cecum and Appendix
a.  Cecal diverticulum
4.  The apex does not grow as rapidly as the rest of the diverticulum
e.  The appendix is subject to considerable variation in position
1.  As ascending colon elongates, the appendix may pass
posterior to cecum – retrocecal appendix (~64% of people)
2.  The appendix may pass posterior to the colon – retrocolic
appendix
3.  The appendix may descend over the brim of the pelvis –
pelvic appendix
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut – are common, most are anomalies of gut rotation
1.  Malrotation of the gut
a.  Result from incomplete rotation and/or fixation of the intestines
2.  Congenital Omphalocoele
a.  Persistence of the herniation of abdominal contents into the
proximal part of the umbilical cord
b.  Results from failure of the intestines to return to the abdominal
cavity during the 10th week
c.  Herniation of intestines into cord occurs in 1:5,000 births
d.  Herniation of liver and intestines occur in 1:10,000 births
e.  Size of hernia depends on contents
f.  The abdominal cavity is proportionately small when an
omphalocoele is present, due to impetus to grow being absent
g.  Immediate surgical repair is required
h.  Covering of hernia sac is epithelium of umbilical cord – a
derivative of the amnion
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
3.  Umbilical hernia
a.  When the intestines enter the abdominal cavity during the 10th
week and then herniate through an imperfectly closed umbilicus
b.  Different from an omphalocoele
c.  In umbilical hernia the protruding mass (usually greater omentum
and some small intestines) are covered by subcutaneous tissue
and skin
d.  Hernia does not reach its maximum size until a month after birth
e.  Size ranges from 1 to 5 cm
f.  Defect through which the hernia occurs is the linea alba
g.  Hernia protrudes during increased abdominal pressure, e.g.,
crying, straining, coughing
h.  Can be easily reduced through fibrous ring at umbilicus
i.  Surgery is not usually performed unless hernia persists to the age
of 3 to 5 years
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
4.  Gastroschisis
a.  Relatively uncommon
b.  Gastroschisis is a misnomer, actually the anterior abdominal wall
is split open, not the stomach
c.  Results from a defect lateral to the median plane of the anterior
abdominal wall
d.  The linear defect permits extrusion of the abdominal viscera
without involving the umbilical cord
e.  The viscera protrude into the amniotic cavity and are bathed by
amniotic fluid
f.  Usually occurs on the right side lateral to the umbilicus
g.  More common in males than females
h.  Results from incomplete closure of the lateral folds during the 4th
week
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
5.  Nonrotation of Midgut
a.  Relatively common condition
b.  Sometimes called left-sided colon
c.  Is generally asymptomatic, but volvulus (twisting) of the intestines
may occur
d.  Occurs when the midgut loop fails to rotate as it reenters the
abdomen
e.  As a result,
1.  The caudal limb of the loop returns to the abdomen first
2.  The small intestines lie of the right side of the abdomen
3.  The entire large intestine lies on the left side of the abdomen
f.  When volvulus occurs, the superior mesenteric artery may be
obstructed
1.  Results in infarction and gangrene of the intestines supplied
by the artery
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
6.  Mixed Rotation and Volvulus
a.  The cecum lies just inferior to the pyloric region of the stomach
and is fixed to the posterior abdominal wall by peritoneal bands
that pass over the duodenum
b.  Duodenal obstruction, caused by
1.  The peritoneal bands
2.  Intestinal volvulus
c.  This type of malrotation results from failure of the midgut loop to
complete the final 90 degree rotation
d.  Consequently, the terminal part of the ileum returns to the
abdomen first
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
7.  Reversed Rotation
a.  Very unusual
b.  Midgut loop rotates in a clockwise rather then counterclockwise
direction
c.  As a result,
1.  The duodenum lies anterior to the superior mesenteric artery
rather than posterior to it
2.  The transverse colon lies posterior to the superior mesenteric
artery instead of anterior to it
d.  In more unusual cases
1.  The small intestine lies on the left side of the abdomen
2.  The large intestine lies of the right side of the abdomen
3.  The cecum is in the center
4.  Results from malrotation of the midgut followed by failure of
fixation of the intestines
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
8.  Subhepatic Cecum and Appendix
a.  Occurs in about 6% of all fetuses
b.  More common in males
c.  Results if the cecum adheres to the inferior surface of the liver
when it returns to the abdomen
1.  It will be drawn superiorly as the liver diminishes in size
2.  The cecum remains in its fetal position
d.  Subhepatic cecum is not common in adults
1.  When it occurs, it may create a problem in diagnosis of
appendicitis and during appendectomy (surgical removal of
the appendix)
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
9.  Mobile cecum
a.  Occurs in ~ 10% of people
b.  Cecum has an abnormal amount of freedom
c.  In very unusual cases it may herniate into right inguinal canal
d.  Results from incomplete fixation of the ascending colon
e.  Clinically important
1.  Possible variations in position of appendix
2.  Because volvulus (twisting) of the cecum may occur
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
10. Internal Hernia
a.  Very uncommon condition
b.  The small intestine passes into the mesentery of the midgut loop
during the return of the intestines to the abdomen
c.  As a result, a hernia-like sac forms
d.  Usually does not produce symptoms
e.  Often detected at autopsy or during anatomical dissection
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
11. Midgut Volvulus
a.  The small intestine fails to enter the abdominal cavity normally
b.  The mesenteries fail to undergo normal fixation
c.  As a result, volvulus (twisting) of the intestines occurs
d.  Only two parts of the intestines are attached to the posterior
abdominal wall, the duodenum and the proximal colon
e.  The small intestine hangs by a narrow stalk that contains the
superior mesenteric artery and vein
1.  Vessels are usually twisted in the stalk and become
obstructed at or near the duodenal-jejunal junction
2.  The circulation to the twisted intestine is often restricted
3.  If vessels are completely obstructed, gangrene develops
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
12. Stenosis and Atresia of the Intestine
a.  Partial occlusion (stenosis) and complete occlusion (atresia) of
the intestinal lumen account for about 1/3 of the cases of
intestinal obstruction
1.  Occurs most often in ileum – 50% and duodenum – 25%
2.  Length of area affected varies
b.  May result from failure of an adequate number of vacuoles to
form during recanalization of intestine
1.  In some cases, a transverse diaphragm forms, forming a
diaphragmatic atresia
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
12. Stenosis and Atresia of the Intestine
c.  May occur by interruption of blood supply to a loop of fetal
intestine resulting from a fetal vascular accident
1.  An excessively mobile loop of intestine may twist, interrupting
its blood supply
2.  Leads to necrosis of section of bowel involved
3.  The necrotic segment later becomes a fibrous cord
connecting patent ends of the intestine
4.  Most atresias of ileum are caused by infarction of fetal bowel
resulting from impairment of its blood supply due to volvulus
5.  Impairment most likely occurs during 10th week as intestines
return to abdomen
6.  Malfixation of the gut predisposes it to volvulus,
strangulation, and impairment of its blood supply
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
13. Ileal (Meckel’s) Diverticulum (“rule of 2’s”)
a.  This out pouching is one of the most common anomalies of the
digestive tract
b.  Congenital ileal diverticulum (Meckel’s diverticulum) occurs in
2% to 4% of all people
c.  Is 3 to 5 times more likely in males than females
d.  Clinically significant
1.  Can become inflamed
2.  Mimics symptoms of appendicitis
e.  Wall of diverticulum contains all layers of ileum
1.  May also contain small patches of gastric and pancreatic
tissues
2.  Gastric mucosa often secretes acid, producing ulceration
and bleeding
f.  Is the remnant of the proximal portion of the yolk stalk
g.  Typically appears as a finger-like pouch about 3 to 6 cm (2”) long
h.  Arises from antimesenteric border of ileum ~ 40-50 cm (2’) from
ileocecal junction
i.  May be connected to umbilicus by fibrous cord, omphaloenteric
fistula
Midgut: Embryology
16. Clinical correlates
a.  Anomalies of the Midgut
14. Duplication of the Intestine
a.  Most intestinal duplications are cystic duplications or tubular
duplications
b.  Cystic duplications are more common
c.  Tubular duplications usually communicate with intestinal lumen
d.  Almost all duplications result from failure of normal recanalization
of the lumen
e.  As a result, two lumen form
f.  The duplicated segment of the bowel lies on the mesenteric side
of the intestine
Development of the Hindgut
Hindgut: Embryology
5.  Cloaca
a.  The terminal part of the hindgut is an endodermal-lined chamber that is in
contact with surface ectoderm at the cloacal membrane
b.  This membrane is composed of endoderm of the cloaca and ectoderm of
the protodeum, or anal pit
c.  The cloaca, the expanded part of the hindgut, receives the allantois (a
fingerlike diverticulum) ventrally
Hindgut: Embryology
5.  Cloaca
d.  Partitioning of Cloaca
1.  The cloaca is divided into dorsal and ventral parts by a wedge of
mesoderm – the urorectal septum
2.  Develops in the angle between the allantois and the hindgut
3.  As the septum grows towards the cloacal membrane, it develops
forklike extensions that produce infoldings of the lateral walls of the
cloaca
4.  These folds grow towards each other and fuse, forming a partition that
divides the cloaca into two parts
a.  Dorsally - the rectum and cranial part of the anal canal
b.  Ventrally – the urogenital sinus
Hindgut: Embryology
5.  Cloaca
d.  Partitioning of Cloaca
5.  By the 7th week, the urorectal septum has fused with the cloacal
membrane, dividing it into
a.  A dorsal anal membrane
b.  A larger ventral urogenital membrane
6.  Perineal body in the adult represents the area of fusion of the urorectal
septum with the cloacal membrane
a.  The tendinous center of the perineum
b.  This fibromuscular node is the landmark of the perineum
c.  Location for convergence and attachment of several muscles
Hindgut: Embryology
5.  Cloaca
d.  Partitioning of Cloaca
7.  The urorectal septum also divides the cloacal sphincter into
a.  Anterior part
1.  Develops into superficial perineal, bulbospongiosus, and
ischiocavernosus muscles
2.  Reason why pudendal nerve innervates these muscles
b.  Posterior part
1.  Becomes external anal sphincter
8.  Mesodermal proliferations produce elevations of the surface ectoderm
around the anal membrane
a.  As a result, this membrane is soon located at the bottom of the
ectodermal depression – the proctodeum (anal pit)
b.  The anal membrane usually ruptures at the end of the 8th week
1.  Brings distal part of digestive tract (anal canal) into
communication with amniotic cavity
Hindgut: Embryology
6.  Anal Canal
a.  The superior 2/3s (about 25 mm) of the adult anal canal are derived
from the hindgut
b.  Inferior 1/3 develops from the proctodeum
c.  Pectinate line indicates the junction of the epithelium from the endoderm of
the hindgut and the surface ectoderm of the protodeum
1.  Located at inferior limits of anal valves
2.  Former site of the anal membrane
d.  About 2 cm superior to the anus is the anocutaneous line = white line
1.  Position where epithelium changes from stratified columnar epithelium
(recturm) to non-keratinized stratified squamous epithelium (anal canal)
2.  At the anus the skin is keratinized and continuous with the skin around
the (natal cleft) anus
Hindgut: Embryology
6.  Anal Canal
e.  The other layers of the wall of the anal canal is derived from LP splanchnic
mesoderm
f.  Superior 2/3 of anal canal
1.  Supplied by the superior rectal artery, a branch of the inferior
mesenteric artery
2.  Venous drainage via the superior rectal vein, a tributary of the
inferior mesenteric vein
3.  Lymphatic drainage eventually flows into the inferior mesenteric
lymph nodes
4.  Innervation from autonomic nervous system
Hindgut: Embryology
6.  Anal Canal
e.  The other layers of the wall of the anal canal is derived from LP splanchnic
mesoderm
g.  Inferior 1/3 of anal canal
1.  Supplied by the inferior rectal arteries, branches of the internal
pudendal artery
2.  Venous drainage via the inferior rectal vein, a tributary of the
internal pudendal veins
3.  Lymphatic drainage eventually flows into the superficial inguinal
lymph nodes
4.  Innervation from inferior rectal nerve
a.  Sensitive to pain, temperature, touch, and pressure
Hindgut: Embryology
6.  Anal Canal
e.  The other layers of the wall of the anal canal is derived from LP splanchnic
mesoderm
h.  Clinically important
1.  Differences of blood supply, nerve supply, and venous and lymphatic
drainage of the anal canal are important, especially when considering
the metastasis (spread) of cancer cells
a.  Tumors in the superior part are painless
b.  Tumors arise from simple columnar epithelium
c.  Tumors in the inferior part are painful
d.  Tumors arise from stratified squamous epithelium
Hindgut: Embryology
7.  Clinical correlates
a.  Anomalies of hindgut
1.  Most anomalies are located in the anorectal region and result from
abnormal development of the urorectal system
2.  Clinically, they are divided into high and low anomalies depending on
whether the rectum terminates superior or inferior to the puborectal
sling, formed by puborectalis muscle, a part of the levator ani
musculature
Hindgut: Embryology
7.  Clinical correlates
b.  Congenital Megacolon – Hirschsprung Disease (neural crest)
1.  Most common cause of neonatal obstructions
2.  Males affected more than females (4:1)
3.  Results from failure of neural crest cells to migrate into colon
during 5th to 7th weeks
a.  Failure of parasympathetic ganglion cells to develop into
Auerbach’s and Meissner’s plexuses
4.  The dilation results from failure of peristalsis in the aganglionic segment
a.  Prevents movement of the intestinal contents
5.  A part of the colon is dilated because of absence of autonomic ganglion
cells in the myenteric plexus distal to the dilated colon segment
6.  The enlarged colon – megacolon – has the normal number of ganglion
cells
7.  In most cases only the rectum and sigmoid colon are involved
Hindgut: Embryology
7.  Clinical correlates
c.  Imperforate Anus and Anorectal Anomalies
1.  Occurs in 1:5,000 newborns
2.  More common in males
3.  Abnormal development of urorectal segment, resulting in incomplete
separation of the cloaca into urogenital and anorectal protions
4.  Lesions are classified as low or high depending on whether the rectum
ends superior or inferior to the puborectalis muscle
Hindgut: Embryology
7.  Clinical correlates
c.  Imperforate Anus and Anorectal Anomalies
5.  Low Anorectal anomalies
a.  Anal agenesis with or without fistula
1.  The anal canal may end blindly, may be an ectopic anus, or an
anoperineal fistula that opens into the perineum
2.  May open into the vagina in females or the urethra in males
3.  More than 90% of low anorectal anomalies are associated with
an external fistula
4.  Anal agenesis with a fistula results from incomplete separation
of the cloaca by the urorectal septum
Hindgut: Embryology
7.  Clinical correlates
c.  Imperforate Anus and Anorectal Anomalies
5.  Low Anorectal anomalies
b.  Anal Stenosis
1.  The anus is in the normal position, but the anus and the anal
canal are narrow
2.  Is probably caused by a slight dorsal deviation of the urorectal
septum as it grows caudally to fuse with the cloacal membrane
3.  As a result, the anal canal and the anal membrane are small
4.  Sometimes only a small probe can be inserted into the anal
canal
Hindgut: Embryology
7.  Clinical correlates
c.  Imperforate Anus and Anorectal Anomalies
5.  Low Anorectal anomalies
c.  Membranous Atresia of Anus
1.  The anus is in the normal position, but a thin layer of tissue
separates the anal canal from the exterior
2.  The anal membrane is thin enough to bulge during straining
3.  The anal membrane appears blue due to the presence of
meconium superior to it
4.  Results from failure of the anal membrane to perforate during
the 8th week
Hindgut: Embryology
7.  Clinical correlates
c.  Imperforate Anus and Anorectal Anomalies
6.  High Anorectal anomalies
a.  Anal agenesis with or without fistula
1.  Anorectal agenesis with a fistula is the result of incomplete
separation of the cloaca by the urorectal septum
2.  The rectum ends superior to the puborectalis muscle when
there is anorectal agenesis
3.  Most common type of anorectal anomaly, accounting for 2/3 of
anorectal defects
4.  Although rectum ends blindly, there is a fistula to the bladder
(rectovesical fistula) or urethra (rectourethral fistula) or to the
vestibule of the vagina (rectovestibular fistula) in females
Hindgut: Embryology
7.  Clinical correlates
c.  Imperforate Anus and Anorectal Anomalies
6.  High Anorectal anomalies
a.  Anal agenesis with or without fistula
5.  Passage of meconium or flatus (gas) in the urine is diagnostic
of a rectourinary fistula
6.  In newborn males, meconium (feces) may be observed in the
urine
7.  In newborn females, meconium may be observed in the
vestibule of the vagina
Hindgut: Embryology
7.  Clinical correlates
c.  Imperforate Anus and Anorectal Anomalies
6.  High Anorectal anomalies
b.  Rectal Atresia
1.  The anal canal and rectum are present, but separated by a
fibrous cord
2.  Cause may be abnormal recanalization of the rectum
3.  Cause may be defective blood supply