lec15The Digestive System

The Digestive System

Dr. Haythem Ali Alsayigh
Department of Human Anatomy And Histology

DIVISIONS OF THE GUT TUBE

Formation :
Folded endoderm layer
Ventral (floor) of gut from portion of yolk sac
As a result of cephalocaudal and lateral folding of the embryo, a portion of the endoderm-lined yolk sac cavity is incorporated into the embryo to form the primitive gut. Two other portions of the endoderm-lined cavity, the yolk sac and the allantois, remain outside the embryo

DIVISIONS OF THE GUT TUBE

In the cephalic and caudal parts of the embryo, the primitive gut forms a blind-ending tube, the foregut and hindgut, respectively.
The middle part, the midgut, remains temporally connected to the yolk sac by means of the vitelline duct, or yolk stalk

DIVISIONS OF THE GUT TUBE

Parts of gut :
• Foregut
• Midgut
• Hindgut

Mesenteries:

Definition
Intraperitoneal organ
Retroperitoneal organ
Peritoneal ligaments
Initially foregut, midgut, and hindgut are in broad contact with posterior abdominal wall

Development

Development of the primitive gut and its derivatives is usually discussed in four sections:
(a) The pharyngeal gut, or pharynx, extends from the oropharyngeal membrane to the respiratory diverticulum and is part of the foregut; this section is particularly important for development of the head and neck and is
(b) The remainder of the foregut lies caudal to the pharyngeal tube and extends as far caudally as the liver outgrowth.
(c) The midgut begins caudal to the liver bud and extends to the junction of the right two-thirds and left third of the transverse colon in the adult.
(d) The hindgut extends from the left third of the transverse colon to the cloacal membrane


Development
Endoderm forms the epithelial lining of the digestive tract and gives rise to the specific cells (the parenchyma) of glands,
such as hepatocytes and the exocrine and endocrine cells of the pancreas.
The stroma (connective tissue) for the glands is derived from visceral mesoderm.
Muscle, connective tissue, and peritoneal components of the wall of the gut also are derived from visceral mesoderm.

MOLECULAR REGULATION OF GUT TUBE DEVELOPMENT

Regional specification of the gut tube into different components occurs during the time that the lateral body folds are bringing the two sides of the tube together
Specification is initiated by transcription factors expressed in the different regions of the gut tube.
Thus, SOX2 “specifies” the esophagus and stomach;
PDX1, the duodenum;
CDXC, the small intestine; and
CDXA, the large intestine and rectum.

MOLECULAR REGULATION OF GUT TUBE DEVELOPMENT

This initial patterning is stabilized by reciprocal interactions between the endoderm and visceral mesoderm adjacent to the gut tube).
This epithelial-mesenchymal interaction is initiated by Sonic Hedge Hog (SHH) expression throughout the gut tube.
SHH expression upregulates factors in the mesoderm that then determine the type of structure that forms from the gut tube, such as
the stomach, duodenum, small intestine, etc. For example, in the region of the caudal limit of the midgut and all of the hindgut, SHH expression establishes anested expression of the HOX genes in the mesoderm.
Once the mesoderm is specified by this code, then it instructs the endoderm to form the various components of the mid and hindgut regions, including part of the small intestine, cecum, colon, and cloaca


MESENTERIES
Portions of the gut tube and its derivatives are suspended from the dorsal and ventral body wall by mesenteries, double layers of peritoneum that enclose an organ and connect it to the body wall. Such organs are called intraperitoneal
whereas organs that lie against the posterior body wall and are covered by peritoneum on their anterior surface only (e.g., the kidneys) are considered retroperitoneal

MESENTERIES

Peritoneal ligaments are double layers of peritoneum (mesenteries) that pass from one organ to another or from an organ to the body wall. Mesenteries and ligaments provide pathways for vessels, nerves, and lymphatics to and from abdominal viscera

Mesenteries:

5th week … Portions of the gut tube and its derivatives are suspended from the dorsal and ventral body wall by mesenteries, double layers of peritoneum that enclose an organ and connect it to the body wall Extension of dorsal mesentery
Mesogastrium
Mesodoudenum
Mesocolon
Mesentery proper

Ventral mesentery, which exists only in the region of the terminal part of the esophagus, the stomach, and the upper part of the duodenum is derived from the septum transversum.
Growth of the liver into the mesenchyme of the septum transversum divides the ventral mesentery into
(a) the lesser omentum, extending from the lower portion of the esophagus, the stomach, and the upper portion of the duodenum to the liver and
(b) the falciform ligament, extending from the liver to the ventral body wall

Mesenteries:

Ventral mesentery:
Extension
Divided by liver into:
• Lesser omentum:
• Roof of epiploic foramen contain portal triad
• Falciform ligament
• Its free margin contain umblical vein forming round lig (lig teres hepatis)

Parts of the gut tube

Foregut: consists of 2 parts
• Pharyngeal gut ( pharynx)
• Foregut caudal to the pharyngeal tube until the liver outgrowth.

Parts of the gut tube

• Midgut: begins caudal to liver bud and end at the junction between Rt 2/3rd with Lt1/3rd of transverse colon
• Hindgut : begins from there to the cloacal membrane

Derivatives of the Foregut:

• The Esophagus;
• tracheoesophageal septum
• (4th w) divide foregut :
ventral part .tracheobronchial diverticulum dorsal part -..esophagus, extends to stomach.
At first, the esophagus is short, but with descent of the heart and lungs, it lengthens rapidly
muscle coat derived from surrounding splanchnic mesoderm;
upper 2/3rd striated (vagus supply)
lower 1/3rd smooth (splanchnic supply)

1-When the embryo is approximately 4 weeks old, the respiratory diverticulum (lung bud) appears at the ventral wall of the foregut at the border with the pharyngeal gut

Clinical Derivatives of the Foregut:

Esophagus anomalies:
• Atresia with or without fistula
• Stenosis
• Shortenning with hiatal hernia
tracheoesophageal fistula in order of their frequency of appearance: A, 90%; B, 4%; C, 4%; D, 1%; and E, 1%.

Esophageal atresia and/or tracheoesophageal fistula

results either from spontaneous posterior deviation of the tracheoesophageal septum or from some mechanical factor pushing the dorsal wall of the foregut anteriorly.
In its most common form, the proximal part of the esophagus ends as a blind sac, and the distal part is connected to the trachea by a narrow canal just above the bifurcation.
Other types of defects in this region occur much less frequently
Atresia of the esophagus prevents normal passage of amniotic fluid into the intestinal tract, resulting in accumulation of excess fluid in the amniotic sac (polyhydramnios).

Esophageal atresia and/or tracheoesophageal fistula

In addition to atresias, the lumen of the esophagus may narrow, producing esophageal stenosis, usually in the lower third.
Stenosis may be caused by incomplete recanalization, vascular abnormalities, or accidents that compromise blood flow.
Occasionally, the esophagus fails to lengthen sufficiently, and the stomach is pulled up into the esophageal hiatus through the diaphragm.
The result is a congenital hiatal hernia.

Derivatives of the Foregut:

• Stomach :
• 1-At 4th week stomach appaere as a fusiform dilatation of the foregut During the following weeks, its appearance and position change greatly as a result of the different rates of growth in various regions of its wall and the changes in position of surrounding organs. Positional changes of the stomach are most easily explained by assuming that it rotates around a longitudinal and an anteroposterior axis
2-Rotates 90⁰ clockwise about longitudinal axis
Dorsal border grow faster than ventral……2 curvatures rotates 90⁰ clockwise around its AP axis

Stomach :

causing its left side to face anteriorly and its right side to face posteriorly
Hence, the left vagus nerve, initially innervating the left side of the stomach, now innervates the anterior wall; similarly, the right nerve innervates the posterior wall.
During this rotation, the original posterior wall of the stomach grows faster than the anterior portion, forming the greater and lesser curvatures

Stomach :

The cephalic and caudal ends of the stomach originally lie in the midline, but during further growth, the stomach rotates around an anteroposterior axis, such that the caudal or pyloric part moves to the right and upward, and the cephalic or cardiac portion moves to the left
and slightly downward The stomach thus assumes its final position, its axis running from above left to below right.

Since the stomach is attached to the dorsal body wall by the dorsal mesogastrium and to the ventral body wall by the ventral mesogastrium , its rotation and disproportionate growth alter the position of these mesenteries.
Rotation about the longitudinal axis pulls the dorsal mesogastrium to the left, creating a space behind the stomach called the omental bursa (lesser peritoneal sac)
This rotation also pulls the ventral mesogastrium to the right. As this process continues in the fifth week of development,


Derivatives of the Foregut:
• Stomach :
Attach to body wall by dorsal and ventral mesogasterium
Rotation about long axia will pull dorsal mesogasterium to left and formation of lesser sac
Spleen (5th w) between the 2 leaves of dorsal mesogasterium





the spleen primordium appears as a mesodermal proliferation between the two leaves of the dorsal mesogastrium
With continued rotation of the stomach, the dorsal mesogastrium lengthens, and the portion between the spleen and dorsal midline swings to the left and fuses with the peritoneum of the posterior abdominal wall
The posterior leaf of the dorsal mesogastrium and the peritoneum along this line of fusion degenerate.
The spleen, which remains intraperitoneal, is then connected to the body wall in the region of the left kidney by the lienorenal ligament and to the stomach by the gastrolienal ligament Lengthening and fusion of the dorsal mesogastrium to the posterior body wall also determine the final position of the pancreas

Derivatives of the Foregut:

• Stomach :
Lengthening, fusion and degeneration of posterior leaf of dorsal mesogastrium and peritoneum will place the tail of pancreas in a retroperitoneal position (secondarily retroperitoneal).


Lengthening and fusion of the dorsal mesogastrium to the posterior body wall also determine the final position of the pancreas.
Initially, the organ grows into the dorsal mesoduodenum, but eventually its tail extends into the dorsal mesogastrium
Since this portion of the dorsal mesogastrium fuses with the dorsal body wall, the tail of the pancreas lies against this region

Once the posterior leaf of the dorsal mesogastrium and the peritoneum of the posterior body wall degenerate along the line of fusion, the tail of the pancreas is covered by peritoneum on its anterior surface only and therefore lies in a retroperitoneal position. (Organs, such as the pancreas, that are originally covered by peritoneum, but later fuse with the posterior body wall to become retroperitoneal, are said to be secondarily retroperitoneal.)

As a result of rotation of the stomach about its anteroposterior axis, the dorsal mesogastrium bulges down
It continues to grow down and forms a double-layered sac extending over the transverse colon and small intestinal loops like an apron
This double-leafed apron is the greater omentum; later, its layers fuse to form a single sheet hanging from the greater curvature of the stomach .
The posterior layer of the greater omentum also fuses with the mesentery of the transverse colon

Derivatives of the Foregut:

• Stomach :
Stomach Rotation about anteroposterior axis will make double layered greater omentum;
layers fuse to form a single sheet
The posterior layer of the greater omentum fuses with the mesentery of the transverse colon.














The lesser omentum and falciform ligament form from the ventral mesogastrium, which itself is derived from mesoderm of the septum transversum.
When liver cords grow into the septum, it thins to form
• the peritoneum of the liver;
the falciform ligament, extending from the liver to the ventral body wall; and
the lesser omentum, extending from the stomach and upper duodenum to the liver

. The free margin of the falciform ligament contains the umbilical vein, which is obliterated after birth to form the round ligament of the liver (ligamentum teres hepatis).
The free margin of the lesser omentum connecting the duodenum and liver (hepatoduodenal ligament) contains the 1-bile duct, 2-portal vein, and 3-hepatic artery (portal triad).
This free margin also forms the roof of the epiploic foramen of Winslow, which is the opening connecting the omental bursa (lesser sac) with the rest of the peritoneal cavity (greater sac) .


Derivatives of the Foregut:
• Stomach :
Stomach anomalies:
• Pyloric stenosis: develope during fetal life and it results from hypertrophy of the circular musculature of the pyloric region of the stomach. The fetus complains of sever projectile vomiting.
• Duplication of the stomach.
• Prepyloric septation

Derivatives of the Foregut:

• Doudenum :
• Caudal part of foregut down to major doudenal papilla (hepatopancreatic duct)…… upper1/3rd
• cephalic part of midgut; after the papilla…..lower 2/3rd

Derivatives of the Foregut:

Duodenum

The terminal part of the foregut and the cephalic part of the midgut form the duodenum.
The junction of the two parts is directly distal to the origin of the liver bud
As the stomach rotates, the duodenum takes on the form of a C-shaped loop and rotates to the right.
This rotation, together with rapid growth of the head of the pancreas, swings the duodenum from its initial midline position to the right side of the abdominal cavity .


Derivatives of the Foregut:
• Liver and GB:
Hepatic diverticulum ( liver bud) appears (middle 3rd week) as endodermal outgrowth at the distal end of the foregut
This outgrowth, the hepatic diverticulum, or liver bud, consists of rapidly proliferating cells that penetrate the septum transversum,
(that is, the mesodermal plate between the pericardial cavity and the stalk of the yolk sac)

Liver and GB

While hepatic cells continue to penetrate the septum, the connection between the hepatic diverticulum and the foregut (duodenum) narrows, forming the bile duct.
A small ventral outgrowth is formed by the bile duct, and this outgrowth gives rise to the gallbladder and the cystic duct

Liver and GB

During further development,
epithelial liver cords intermingleيختلط with the vitelline and umbilical veins, which form hepatic sinusoids.
Liver cords differentiate into the parenchyma (liver cells) and form the lining of the biliary ducts.
Hematopoietic cells, Kupffer cells, and connective tissue cells are derived from mesoderm of the septum transversum.

Derivatives of the Foregut:

• Liver and GB:
hepatic sinusoids; liver cords fuse with vitelline and umbilical veins
parenchyma (liver cells);from epith of liver cords
Hematopoietic cells, Kupffer cells, and connective tissue cells derived from mesoderm of the septum transversum.
The 90⁰ rotation will shift bile duct opening from anterior to posterior (or posteromedial).

When liver cells have invaded the entire septum transversum,

so that the organ bulges caudally into the abdominal cavity,
mesoderm of the septum transversum lying between the liver and the foregut and the liver and the ventral abdominal wall becomes membranous, forming the lesser omentum and falciform ligament, respectively.

Together, having formed the peritoneal connection between the foregut and the ventral abdominal wall, they are known as the ventral mesentery
Mesoderm on the surface of the liver differentiates into visceral peritoneum except on its cranial surface
In this region, the liver remains in contact with the rest of the original septum transversum. This portion of the septum, which consists of densely packed mesoderm, will form the central tendon of the diaphragm.
The surface of the liver that is in contact with the future diaphragm is never covered by peritoneum; it is the bare area of the liver


In the 10th week of development, the weight of the liver is approximately 10% of the total body weight. Although this may be attributed partly to the large numbers of sinusoids, another important factor is its hematopoietic function.
Large nests of proliferating cells, which produce red and white blood cells, lie between hepatic cells and walls of the vessels.
This activity gradually subsides during the last 2 months of intrauterine life, and only small hematopoietic islands remain at birth.
The weight of the liver is then only 5% of the total body weight.

Another important function of the liver begins at approximately the 12th week, when bile is formed by hepatic cells.
Meanwhile, since the gallbladder and cystic duct have developed and the cystic duct has joined the hepatic duct to form the bile duct
bile can enter the gastrointestinal tract. As a result, its contents take on a dark green color.
Because of positional changes of the duodenum, the entrance of the bile duct gradually shifts from its initial anterior position to a posterior one, and consequently, the bile duct passes behind the duodenum .

Derivatives of the Foregut:

• Liver and GB:
• Accessory hepatic duct
• Double GB
• Biliary duct atresia

Derivatives of the Foregut:

• Pancreas :
• Ventral bud:
• from ventral wall close to the bile duct
• Form lower half of the head and the uncinate process.
• Dorsal bud:
• From dorsal wall
• Form the upper part of head, neck, body and tail

Derivatives of the Foregut:

• Pancreas :
Because of 90⁰ rotation, ventral bud rotates dorsally and finally lie below and behind dorsal bud and then fuse together.
major pancreatic duct ; distal part of dorsal pancreatic duct and entire ventral pancreatic duct.
accessory pancreatic duct, if present, will be formed by proximal part of dorsal pancreatic duct.

Derivatives of the Foregut:

• Pancreas :
• annular pancreas; may causes duodenal obstruction.
• Accessory pancreatic tissues; may be found in the stomach or in meckels diverticulum.

Development and Derivatives of the Midgut:

Midgut attached to posterior body wall by dorsal mesentery
it gives rise to the following structures:
• The lower 2/3 of the duodenum below the opening of the bile duct.
• The jejunum and the ileum.
• The cecum, appendix, ascending colon, and right 2/3 of the transverse colon.

Development and Derivatives of the Midgut:

4 stages of midgut developments:
• the preherniation stage:
• The herniation stage:
• Reduction stage:
• Fixation stage:

Development and Derivatives of the Midgut:

• Preherniation stage:
• rapid elongation of gut and its mesentery, with formation of U- shaped loop with a cranial and caudal limbs.
• The tip of the loop is connected to the yolk through the vitelline duct.

Development and Derivatives of the Midgut:

• Herniation stage:
• rapid growth and elongation of intestinal loop, parts of midgut will enter the umbilical cord during the 6th week of development (physiological umbilical herniation)
• During herniation midgut loops rotate 90⁰ counterclockwise direction around the axis formed by superior mesenteric artery.

Development and Derivatives of the Midgut:

• Herniation stage:
• This 90⁰ rotation will bring the cranial limb to the right and the caudal limb to the left side
• right (cranial) limb will elongate faster and becomes coiled and it will develops into distal part of duodenum, the jejunum, and part of ileum.
• left (caudal) limb becomes lower portion of ileum, cecum, appendix, ascending colon, and proximal 2/3rd of transverse colon

Development and Derivatives of the Midgut:

• Reduction stage:
• In 10th week of development
• During the process of reduction, the intestinal loops rotate 180⁰counterclockwise (so the ultimate rotation of the midgut is 270⁰ counterclockwise)

Development and Derivatives of the Midgut:

• Reduction stage:
• the180⁰ rotation make right limb on the left and left limb on right
• proximal jejunum is first part to reenter abdominal cavity, and be on left side.
• cecum, ascending and transverse colon deviates upward and to the right

Development and Derivatives of the Midgut:

• Fixation stage:
• Most of dorsal mesentery persists but that of ascending and descending colon will disappear making them retroperitoneal.
• The appendix, lower end of the cecum, and sigmoid colon retain their free mesenteries.
•

Anomalies of Midgut

• abnormalities of the mesenteries:
volvolus of cecum or colon; due to persistance of the mesocolon
retrocolic hernia; entrapment of portions of the small intestine behind the ascending mesocolon.
• Body wall defects:
Omphalocele
Gastroschisis

CLINICAL CORRELATES

• herniation of abdominal viscera through an enlarged umbilical ring.
• The viscera, are covered by amnion.
• Due to a failure of the bowel to return to the body cavity from its physiological herniation during the 6th to 10th weeks.

Omphalocele

CLINICAL CORRELATES
Gastroschisis
herniation of abdominal contents through the body wall directly into the amniotic cavity.
It occurs lateral to the umbilicus, usually on the right, through a region weakened by regression of the right umbilical vein, which normally disappears.
Viscera are not covered by peritoneum or amnion, and the bowel may be damaged by exposure to amniotic fluid.
Both omphalocele and gastroschisis result in elevated levels of α-fetoprotein in the amniotic fluid, which can be detected prenatally.

Gastroschisis mainly occurs in young women with cocaine use

Unlike omphalocele, gastroschisis is not associated
with chromosome abnormalities or other severe
defects.
Therefore the survival rate is excellent, although
volvulus (rotation o f the bowel) resulting in a
compromised blood supply may kill large regions of the
intestine and lead to fetal death.

Anomalies of Midgut

• Vitelline duct abnormalities:
Meckels diverticulum (ileal diverticulum); out pocketing of the antimesenteric border of the ileum through a persistent vitelline duct.
Enterocystoma (vitelline cyst); that may be twisted causing strangulation and gangrene.
Patent vitelline duct forming umbilical fistula with fecal discharge from the intestine out of the umbilicus.

Anomalies of Midgut

• Gut rotation defect
• Gut atresia and stenosis; occur as a result of vascular accidents
•

Derivatives of the Hindgut:

1. The left 1/3 of the transverse colon.
2. Descending colon.
3. Sigmoid colon.
4. The rectum.
5. Upper part of the anal canal
-The endoderm of the hindgut also forms the epithelium of the bladder and urethra-

Derivatives of the Hindgut:

Hindgut enters posterior region of the cloaca (future anorectal canal),
Allantois enters anterior region (future urogenital sinus).
Boundary between endoderm and ectoderm forms cloacal membrane.
Breakdown of cloacal membrane provides communication to the exterior for the anus and urogenital sinus.

Derivatives of the Hindgut:

The urorectal septum, separates the allantois and hindgut. This septum comes to lie close to the cloacal membrane and its tip will form the perineal body.

Derivatives of the Hindgut:

Lower 1/3rd of anal canal derived from ectoderm around the proctodeum.
Ectoderm proliferates and invaginates to create the anal pit.
Later, degeneration of the cloacal membrane (now called the anal membrane) establishes continuity between the upper and lower parts of the anal canal, which is delineated by the pectinate line,

Derivatives of the Hindgut:

Anomalies :
1. Imperforated anus; occurring due to failure of cloacal membrane to rupture.
2. Rectoanal atresia.
3. rectovaginal or rectourethral fistula.
4. Congenital megacolon is due to an absence of parasympathetic ganglia in the bowel wall ( aganglionic megacolon or Hirschsprung disease)

THE END

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