
Embryology: Gastrointestinal
o Formation of Gut Tube
 Incorporation of yolk sac (endoderm) during body folding
 Gut tube extends form oropharyngeal membrane to cloacal membrane
 Divisions
 Foregut
 Midgut
 Hindgut
o Histology of GI Tube (adult)
 Sources of GI Tube
 Endoderm
o Epithelium
o Glands
 Splanchnic mesoderm
o Smooth muscle
 Circular
 Longitudinal
o Vessels
o Adventitia (Connective Tissue)
o Visceral layer of peritoneum
 Neural Crest
o Autonomic innervation
 Sympathetic
 Postganglionic fibers
o Contained in pre-vertebral ganglia (ex.
Celiac)
 Parasympathetic
 Preganglionic fibers
o Mostly from Vagus (CN X)
 Ganglia
o Postganglionic neurons
o Organization of GI Tract
Foregut
Midgut
Hindgut
Arteries
Celiac Trunk
Superior Mesenteric A. Inferior Mesenteric A.
Veins
Splenic V. +
Superior Mesenteric V. Inferior Mesenteric V.
Lymphatics
Parasympathetics
Sympathetics
Mesenteries


Celiac Nodes
Superior Mesenteric
Nodes
Inferior Mesenteric
Nodes
Vagus N.
Vagus N.
Pelvic Splanchnics nn.
Celiac Ganglion
Superior Mesenteric
Ganglion
Inferior Mesenteric
Ganglion
Ventral (falciform lig.,
lesser omentum) &
Dorsal (mesentery,
Dorsal (greater
transverse mesocolon)
omentum, gastrolienal,
lienorenal)
Dorsal (sigmoid
mesocolon)
Mesenteries
o GI tube (caudal foregut thru hindgut) is initially suspended by a dorsal mesentery
(double layer of peritoneum)
 Connects the splanchnic and parietal layers of the lateral plate mesoderm
o Mesenteries conduct vessels and nerves to and from tract
o Dorsal Mesentery
 Divisions
 Dorsal mesogastrium (greater omentum)
 Dorsal mesoduodenum
 Mesentery proper
 Dorsal mesocolon
o Abdominal portion of foregut has a ventral mesentery
 Formed by septum transversum
 Growth of liver separates it into
 Falciform ligament
 Lesser omentum
Foregut
o Stomach
 Stomach appears as a dilation in the foregut just caudal to the diaphragm
 Dorsal side expands more than ventral side
 Dorsal side = greater curvature
 Ventral side = lesser curvature
 Stomach rotates 90 degrees clockwise along longitudinal axis
 Left side of stomach faces anteriorly
o CN X follows this rotation
 Left vagus becomes anterior vagal trunk

o
Stomach also rotates along its anteroposterior axis so that the cardiac
region lies on the left and the pyloric region on right

Dorsal mesogastrium
 With longitudinal foregut rotation, dorsal mesogastrium shifts to
the left and creates a space
o Omental bursa (lesser peritoneal sac)
 Spleen
o Forms from mesoderm and is populated mostly by
lymphocytes
 Separates the dorsal mesogastrium into
 Gastrolienal ligament
 Lienorenal ligament
o Mostly fuses to body wall
 With Anteroposterior foregut rotation the dorsal mesogastrium
bulges caudally creating the greater omentum
o Redundant fold develops (like an apron)
o Fuses with transverse mesocolon

o
o
o
o
Ventral Mesogastrium
 Liver grows in septum transversum forming
 Falciform ligament
o Between the anterior body wall and liver
 Free edge contains the umbilical vein which in the
adult turns into the ligamentum teres hepatis
 Lesser omentum
o Between the liver and stomach
o Hepatogastric ligament
 Liver to stomach
o Hepatoduodenal ligament
 Liver to duodenum
 Free edge contains portal triad
 Forms roof of epiploic (omental) foramen
 Entrance into omental bursa (lesser
peritoneal sac)
Clinical Correlation
 Pyloric Stenosis
 Pyloric muscular hypertrophy
o Can completely obstruct gastric outlet
o Projectile vomiting without bile
Duodenum
 Formed from caudal end of the foregut and the cephalad end of the midgut
 Point of junction is just distal to liver bud
 With foregut rotations, forms C-shaped loop
 Loses its mesentery to become retroperitoneal
 During 2nd month lumen is obliterated by proliferating epithelium and then
recanalized

Liver and Gallbladder

Ventral diverticulum of the duodenum endoderm forms liver bud which
forms:
 Hepatocytes
 Bile duct
 Gallbladder

Liver bud penetrates septum transversum (mesoderm), forms:
 Hepatic sinusoids – intermingle with umbilical and vitelline veins
 Hematopoietic cells
 Connective tissue
 Through foregut rotation, bile duct comes to the left side of duodenum
 Functions of fetal liver
 Produce albumin
 Store glycogen
 Hematopoiesis (formation of blood cellular components)
 Produce bile
Pancreas
 Ventral pancreatic bud off bile duct
 Forms uncinate process
 Dorsal pancreatic bud off duodenum
 Forms head, body, & tail
 Two buds unite during foregut rotation
 Ventral duct and distal dorsal duct from main pancreatic duct
 Dorsal duct may persist as accessory pancreatic duct

o

Pancreas becomes retroperitoneal with duodenum
Both exocrine and endocrine pancreas (islets of langerhans) derive from
endoderm
Clinical Correlation
 GI Atresia/Stenosis
 Most often in esophagus & duodenum


o


 Many others due to vascular accidents
Annular pancreas
 Ventral pancreatic bud may split and surround duodenum
 May result in duodenal stenosis
Midgut
o Yolk sac remains connected to midgut loop by vitelline duct
o Midgut elongates rapidly, forming the primary intestinal loop
 Cephalic limb will form the duodenum, jejunum and proximal 2/5 of ileum
 Caudal limb will form distal 3/5 of the ileum, cecum, ascending colon, and
proximal 2/3 of the transverse colon
 Vitelline duct remains connected at apex of the loop
o Loop herniates into umbilical cord during the 6th week
o
o
o
o
Midgut Rotation
 270 degree counterclockwise
 Around superior mesenteric artery
 First 90 degrees when loop herniated into umbilical cord
 Cephalic limb on right
 Caudal limb on left
 Remaining 180 degrees when the loop returns to the abdomen
 Cephalic limb enters first and goes to the left
 Caudal limb enters last and goes to the right
 Cecum descends into right iliac fossa
 Appendix develops during this descent
Mesenteries
 Mesentery proper is twisted in midgut rotation
 Ascending and descending colon are pushed against posterior abdominal
wall and lose their mesenteries
 Transverse mesocolon fuses with greater omentum
Clinical Correlation
 Omphalocele
 Failure of the midgut loop to return from umbilical cord herniation
o Associated with other defects (heart, neural tube)




o
Gut Rotation Defects
 Partial or reversed midgut rotations
 May cause obstruction volvulus
Meckle’s Diverticulum
 Persistent vitelline duct

Hindgut
o Develops into the distal 1/3 of transverse colon, descending colon, sigmoid colon,
rectum and upper part of the anal canal
o Terminal hindgut enters the cloaca
o Urorectal septum separates cloaca into anorectal canal and the urogenital sinus
o Cloacal membrane (proctodeum) breaks down
o Upper 2/3 of anal canal from hindgut, lower 1/3 from ectoderm – line of union is
the pectinate line
o
Embryology: Urogenital system
o Kidneys
 Develop from intermediate mesoderm
 Amniotic fluid comes from embryonic kidneys

o
o
3 phases of development (cranial to caudal)
 Pronephros
o Never functional
 Mesonephros
o Briefly functional
 Metanephros
o Definitive kidney
Mesonephric Kidney
 Upper thoracic through upper lumbar region
 Structure
 Excretory tubule (~35 pair)
o Associates with dorsal aorta
 Renal corpuscle
o Presumptive urine (plasma)
o Glomerulus
 Tuft of capillaries
o Bowman’s capsule
 Mesonephric duct
o Collecting duct, leads to cloaca
 Functional between 6-10 weeks
 With developing gonad, forms the urogenital ridge
 Paramesonephric duct develops
 Parallels mesonephric duct
o Important in genital duct development

Metanephric Kidney
 Becomes adult kidney
 Develops in a similar manner to the mesonephric kidney





Collecting system
 Ureteric bud
o Outgrowth of the mesonephric duct near cloaca
o Penetrates metanephric tissue
o Forms
 Renal pelvis
 Major/minor calyces
 Collecting tubules
o Ureteric bud will become the ureter
o
Each collecting tubule covered by metanephric cap, forms renal vesicles
Nephron
 Renal vesicle
o Bowman’s capsule
o Proximal convoluted tubule
o Loop of Henle
o Distal convoluted tubule
 Glomerulus
~1 million nephrons form until birth, none after

o
Kidneys ascend to their final position and receive arteries from the aorta at
successively higher levels
 Functional at ~12th week
 Urine contributes to amniotic fluid
 Clinical correlation
 Renal agenesis
o “Lack of origin”
o Unilateral
 Birth with only one kidney
o Bilateral
 Birth without kidneys
 Fatal within minutes to hours w/o treatment
 Pelvic kidney
o Failure of kidney to ascend to normal position
o Increased incidence of ureteric obstruction
 Horseshoe kidney
o Kidneys fuse at inferior pole
o Failure to ascend
Bladder and Urethra
 Come from endoderm around cloaca
 Cloaca divided into urogenital sinus and anorectal canal by the urorectal
septum



Urogenital sinus can be divided into
 Urinary bladder
o Continuous with the allantois (urachus, median umbilical
ligament)
 Pelvic part
o Prostatic & membranous urethra
 Phallic part
o Penile urethra
Caudal part of mesonephric ducts (intermediate mesoderm) absorbed into
the urogenital sinus resulting in separate openings for the ureter and genital
duct


o
o
Ducts switch positions
Trigone of bladder forms from absorbed mesonephric duct tissue,
later overgrown by endodermally – derived bladder epithelium

Gonads (intermediate mesoderm)
 Genital ridges form by proliferation of mesodermal epithelium near the
mesonephric kidneys
 Germ cells (1000-2000)arise from endoderm in yolk sac, migrate to the
genital ridges forming primitive sex cords
 Germ cells surrounded by support cells from genital ridge epithelium
(intermediate mesoderm)
 “Intermediate mesoderm creates a home for the germ cells to live”

Testis
 XY genotype leads to production of testis-determining factor (SRY [Sex –
region of Y chromosome] gene product) from support cells
 Primary sex cords proliferate, form testis cords
 Composed of germ calls (pre-spermatogonia) + support cells (preSertoli cells)
 These remain solid until puberty where they then transform into
seminiferous tubules, the site of sperm production
o Connects to mesonephric tubule and these connections
become efferent ductules
 Testis cords in the hilum form tubules celled rete testis
 Dense connective tissue covering (tunica albuginea) forms around testis
 Leydig cells form from mesenchyme of the genital ridge, start to produce
androgens in 8th week which will affect the development of the ducts and
genitalia. The fetal Leydig cells involute in weeks 17-18. Reappear at
puberty.
o
o

Ovary
 XX sex chromosome – absence of testis-determining factor
 Primary sex cords degenerate in the medulla of the ovary
 Surface epithelium (support cells) of the ovary proliferates forming follicular
cells
 Follicular cells surround germ cells (oogonia) to form primordial
follicles
 Follicles remain close to the surface of the ovary
 Oogonia proliferate by mitosis (up to the 4th month) then enter prophase of
the 1st meiotic division where they stay until recruited for ovulation after
puberty (now called primary oocytes)
 ~6-7 million at peak, ~1-2 million at birth, ~400,000 at puberty, ~400
ovulate
 Thecal cells (produce estrogens) form from mesenchyme of genital ridge

Male genital ductus
 Rete testis establishes contact with remaining mesonephric tubules which
form the efferent ductules
 Mesonephric duct will form
 Epididymis
 Ductus (vas) deferens
 Seminal vesicles
 Ejaculatory ducts
 Under the influence of Mullerian Inhibiting Substance (MIS) from support
cells, the paramesonephric ducts degenerate, except for
o
o
o
 Appendix testis
 Prostatic utricle
 Prostate gland develops from urogenital sinus (endoderm)
Female genital ducts
 Cranial opening of the paramesonephric ducts comes to lie close to the
ovary – form the uterine tube
 Caudally, the two paramesonephric move medially forming the broad
ligament then fuse to form the uterine canal
 Mesonephric ducts degenerate except for
 Epoophoron
 Paroophoron
 Gartner’s cysts

Vagina
 Sinovaginal bulb grows out of urogenital sinus and fuses with the fused
paramesonephric ducts (uterine canal)
 Later canalizes to form most of the vagina
 Upper part of the vagina and the vaginal fornices form from the
paramesonephric ducts
 The lumen of the vagina closed by a thin membrane, the hymen

External genitalia
 Indifferent stage
 Mesenchyme around cloacal membrane forms cloacal folds
o Urethral folds


o
o Anal folds
Urethral folds unite cranially to form genital tubercle
Second pair of elevations form genital swellings

 Male – stimulate by androgens
 Genital tubercle elongates to form phallus
o Urethral folds fuse to form the penile urethra
 Genital swellings fuse to form the scrotum
 Female – stimulate by estrogens
 Genital tubercle form clitoris
 Urethral folds form labia minora
 Genital swellings form labia majora
 Clinical correlation
 Hypospadias (males)
o Failure of urethral folds to fuse fully
Summary of Homologous Structures
Male
Female
Testis
Ovary
Spermatogonia
Oogonia
Support Cells
Sertoli cells
Follicular Cells
Mesenchymal Cells
Leydig Cells
Thecal Cells
Semiferous Tubules
__
__
Primordial Follicles
Gonads
Germ Cells
Medulla
Cortex
Genital Ducts
Mesonephric Ducts
(& tubules)
Efferent Ductules,
Epididymis, Ductus
Degenerate (ex.,
Deferens, Ejaculatory Duct, Epoophoron, Paraoophoron)
Seminal Vesicles
Degenerate (ex., Appendix
Testis, Prostatic Utricle)
Uterine Tube, Uterus, Upper
Vagina
Prostate, Membranous &
Prostatic Urethra
Urethra, Lower Vagina
Penile Urethra
Labia Minora
Genital Tubercle
Penis
Clitoris
Genital Swellings
Scrotum
Labia Majora
Paramesonephric
Ducts
External Genitalia
Urogenital Sinus
Urethral Folds
o
Descent of Testes
 Gubernaculum testis develops from mesenchyme and extends from testes
to scrotal swellings
 Through differential body growth testes are “pulled” into the scrotal sacs
and carry layers from the abdominal wall with them
 Peritoneum evaginates along the gubernaculum forming the processus
vaginalis
 Proximal part degenerates
 Distal part forms the tunica vaginalis surrounding the testes in the
scrotum


Clinical Correlation
 Congenital Hernia
o Patent processus vaginalis allows intestinal loops to
herniate
o Cysts from processus vaginalis are called hydroceles
o
o
Cryptorchidism
o Undescended testis
Clinical Correlation
 Defects in Sex Differentiation
 Klinefelter Syndrome – 47 chromosomes, XXY
o Male phenotype
o Infertility, gynecomastia, impaired sexual maturation
 Pseudohermaphroditism – 46 chromosomes, XX or XY
o Male or female
o Genotype masked by external phenotype due to
disturbance of normal sex steroid production
 Testicular feminization – 46 chromosomes, XY
o Defect in androgen receptors
o Female external phenotype
o Uterus and upper vagina absent (due to MIS secretion)

o