Development
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Ectoderm gives rise to the CNS and PNS as well as the epidermis and subcutaneous
glands, including mammary glands.
Mesoderm gives rise to: dermotome which forms the dermis and subcutaneous tissue
of the skin; myotome which forms the deep back muscles; and sclerotome which
surrounds the notochord and spinal cord and forms the vertebral column; kidneys,
ureters, gonads, lateral and ventral body walls, serous linings of the body cavities or
pleura, pericardium and peritoneum, blood cells and blood vessels, including the heart
tube.
Endoderm: gastrointestinal tract, epithelial lining of the trachea, bronchi, lungs,
parenchyma of the liver and pancreas, thyroid and parathyroid glands, and epithelial
lining of the bladder and urethra.
Vitelline veins return blood from the yolk sac; the right and left hepatic veins and the
portal vein of the developing liver are derived from the right vitelline vein.
The umbilical veins bring oxygenated blood from the placenta; the left umbilical vein
persists and carries blood from the liver and connects the umbilical vein to the
inferior vena cava, after birth, the umbilical vain and ductus venosus become the
ligamentum teres and ligamentum venosum of the adult.
The common cardinal veins return blood from the body of the embryo; the anterior
cardinal vein on the left side gives rise to the brachiocephalic vein; a small portion of
the proximal common cardinal vein on the left persists as the coronary sinus; the
anterior and common cardinal veins on the right side persist as the superior vena
cava; as the result of the series of changes in the posterior cardinal veins and their
tributaries, the azygos, renal gonadal veins and the inferior vena cava develop.
Foramen ovale is between the right and left atria. It allows most of the blood entering
the right atrium from the inferior vena cava to pass into the left atrium. After birth,
the interatrial septum is completed when the septum premium fuses with the septum
secundum, closing the foramen ovale and forming the fossa ovalis.
As development proceeds, part of the sinus venosus forms the coronary sinus, and the
rest becomes incorporated into the wall of the right atrium. Because it is derived
from the sinus venosus, the smooth part of the right atrial wall is called the sinus
venarum and it is separated from the rough part by the crista terminalis. The wall of
the left atrium is smooth and is formed by incorporation of the primitive pulmonary
veins.
Ductus arteriosus is a shunt of blood that has gone to the right ventricle from the
lungs. Therefore it goes from the pulmonary trunk to aorta. The ligamentum
arteriosum in the adult is the remnant of ductus arteriosus. The left recurrent
laryngeal nerve (branch of the vagus nerve) runs under this structure.
Derivatives of the embryonic foregut in adults are the:
1. lower respiratory system
2. pharynx
3. esophagus
4. stomach and its mesenteries
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superior or first part of the duodenum
liver and its mesenteries
gall bladder and biliary ducts
pancreas
spleen
As the stomach acquires its adult shape, it rotates slowly 90 degrees in a clockwise
direction around its longitudinal axis. This rotation explains LARP (the Left vagus
serves the Anterior part of the stomach and the Right vagus serves the Posterior part
of the stomach.)
Dorsal mesogastrium in the embryo is the greater omentum in the adult.
Ventral mesogastrium in the embryo is the lesser omentum in the adult.
The liver develops in the ventral mesentery between ventral body wall and stomach.
Thus, the lesser omentum in the adult attaches between the liver and stomach and first
part of the duodenum and can be divided into hepatogastric and hepatoduodenal parts
of ligament.
Omental bursa or lesser sac communicates with the rest of the pritoneal cavity
through the epiploic foramen, which can be found behind the free edge of the lesser
omentum.
Ventral mesentery gives rise to the lesser omentum (from the liver to the lesser
curvature of the stomach (hepatogastric ligament) and from the liver to the first part
of the duodenum (hepatoduodenal ligament)), falciform ligament (extends from the
liver to the anterior abdominal wall), coronary ligament (from the liver to the
diaphragm and surrounds the bare area on the posterior surface of the liver), and the
visceral peritoneum surrounding the liver. The umbilical vein passes in the free
margin of the falciform ligament from the umbilical cord to the liver. In the adult, the
obliterated umbilical vein is called the ligamentum teres hepatis or round ligament of
the liver.
Spleen is not part of the digestive system, but it gets its blood supply from the artery
of the foregut. It is not retroperitoneal and has tow mesenteric attachments called the
gastrosplenic and splenorenal ligaments.
Derivatives of the midgut are
1. Small intestine (except the first part and the portion of the second part proximal
to the duodenal papilla of the duodenum)
2. Dorsal mesentery of the small intestine
3. Cecum and vermiform appendix
4. Ascending or right colon
5. Proximal one-half to two-thirds of the transverse colon and its transverse
mesocolon.
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Rotation of the midgut loop refers to a process involving a physiological umbilical
hernation. During the tenth week, the intestine return to the abdominal cavity. This
process is called reduction of the midgut hernia.
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The duodenum and pancreas, the ascending or right colon and the descending or left
colon come to lie against the posterior body wall and their dorsal mesenteries become
resorbed. Thus they become retorperitoneal as a consequence of development.
The hindgut derivatives are:
1. Distal one-half to one-third of the transverse colon and its transverse mesocolon
2. Descending or left colon
3. Sigmoid colon
4. Rectum
5. Upper portion of the anal canal
Three different sets of excretory organs develop in the human from the intermediate
mesoderm: the pronephros, the mesonephros, and the metanephors.
Pronephros degenerates but the pronephric duct is retained and used by the
mesonephric kidney.
Mesonephric kidney appears caudal to the rudimentary pronephros. There is a
transient formation of glomerulus-like structures that grow laterally and contact the
mesonephric duct.
Metanephric diverticulum or ureteric bud gives ruse to the ureter, renal pelvis, major
and minor calyces, and collecting tubules. This begins as a dorsal outgrowth from the
mesonephric duct near the cloaca.
Metanephrogenic mass gives rise to glomerular capsule, proximal convoluted tubule,
nephric loop, and distal convoluted tubule.
Urorectal septum divides the cloaca into a dorsal rectum and a ventral urogenital
sinus. The urogenital sinus is divided into a cranial vesical part, a middle pelvic part,
and a caudal phallic part.
Bladder is continuous with the allnatois, but the lumen of this structure is soon
obliterated and it is then called the urachus. The remnants of this can be seen on the
posterior aspect of the anterior abdominal wall and is referred to as the median
umbilical ligament. As the bladder enlarges, the caudal portions of the mesoneghric
duct are incorporated into its dorsal wall and contribute to the formation of the vesical
trigone. As this occurs, the metanephric diverticulum (ureters) open separately into
the bladder.
In the male, the epithelium of the vesical portion of the urogenital sinus also gives
rise to the part of the epithelium of the prostatic urethra. The remaining epithelium of
the prostatic urethra and that of the membranous urethra are derived from the
endoderm of the pelvic part of the urogenital sinus. The penile or spongy urethra
develops from the phallic portion of the urogenital sinus. The prostate gland and
bulbourethral glands arise as outgrowths of the prostatic urethra and the membranous
urethra, respectively.
In the female, the urethra is derived entirely from the endode5rm of the vesical part of
the urogenital sinus. Also, outgrowths from the urogenital sinus will form the greater
vestibular glands.
Primary sex cords grow into the mesenchyme and the indifferent gonads have a
cortex and a medulla. Large, spherical primordial germ cells are thought to originate
in the yolk sac, and during folding of the embryo they migrate along the dorsal
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mesentery of the hindgut to the gonadal ridges. These cells migrate into the
mesenchyme of the gonad and become incorporated into the primary sex cords.
Under the influence of the Y chromosome, the medulla of the gonad develops into a
testis and the cortex regresses. A thick albuginea develops on the surface of the
gonad and primary sex cods ultimately develop into the seminiferous tubules, the
tubuli recti, and the rete testis.
The cortex differentiates into an ovary and the medulla regresses in the female
embryo. There is proliferation of the germ cells during fetal life, and the primordial
germ cells become incorporated into the sex cords.
The mesonephric ducts are important in the male while in the female the
paramesonephric ducts are important.
Mesonephric ducts gives rise to vas deferens and an outgrowth from their caudal end
will give rise to the seminal vesicles.
The cranial unfused portion of the paramesonephric ducts develop into the uterine
tubes and the caudal fused portions form the uterovaginal primordium which gives
rise to the uterus and vagina. Fusion of the caudal portions of the paramesonephric
ducts brings together two peritoneal folds, which become known as the broad
ligament, and forms the rectouterine and vesicouterine pouches.
A ligament called the gubernaculum descends on each side of the abdomen from the
inferior pole of the gonads. It passes through the developing inguinal region and
attaches on the anterior abdominal wall to a labioscortal swelling. Later, an
evagination of peritoneum, known as the processus vaginalis, develops ventral to the
gubernaculum and herniates through the lower abdominal wall. The opening in the
transversalis fascia produced by the processus vaginalis is the deep inguinal ring, and
the opening in the external oblique aponeurosis is the superficial inguinal ring.
The cranial part of the gubernaculum becomes the ovarian ligament and the caudal
part, which blends with the connective tissue of the labia majora after passing through
the inguinal canal, is the round ligament.