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1. • With the establishment of head and tail folds, part of the endoderm lined cavity of the
secondary yolk sac is enclosed within the embryo to form the primitive gut .
• The primitive gut is in free communication with the rest of yolk sac that is now called
the definitive yolk sac.
• The part of the gut cranial to this communication is the foregut; the part caudal to the
communication is the hindgut, while the intervening part is the midgut.
• The communication between foregut and midgut is called anterior intestinal portal
• The communication between the midgut and hindgut is called posterior intestinal
portal.
INTRODUCTION

2. • The foregut is in the head fold of the embryo. Cranially, the foregut is separated from
the stomodeum by the buccopharyngeal membrane.
• The hindgut is in the tail fold of the embryo. Caudally, the hindgut is separated from the
proctodeum or ectodermal cloaca by the cloacal membrane.
• At a later stage of development, the buccopharyngeal and cloacal membranes
disappear, and the foregut and hindgut are in communication with stomodeum and
proctodeum respectively
INTRODUCTION

4. • While the gut is being formed, the circulatory system of the embryo undergoes
considerable development.
• A midline artery, the dorsal aorta, is established and comes to lie just dorsal to the gut.
It gives off a series of branches to the gut.
• Those in the region of the midgut, initially, run right up to the yolk sac and are,
therefore, called vitelline arteries.
INTRODUCTION

5. • Subsequently, most of these ventral branches
of the dorsal aorta disappear and only three
of them remain; one for the foregut, one for
the midgut and one for the hindgut
• The artery of the abdominal part of the
foregut is celiac, that of midgut is superior
mesenteric and that of hindgut is inferior
mesenteric
INTRODUCTION

6. • The wide communication between the yolk sac and the midgut is gradually narrowed
down with the result the midgut becomes tubular.
• Thereafter, the midgut assumes the form of a loop. The superior mesenteric artery now
runs in the mesentery of this loop to its apex.
• The loop can, therefore, be said to have a proximal or prearterial segment and a distal
or postarterial segment.
• A bud (cecal bud) soon arises from the postarterial segment very near the apex of the
loop
INTRODUCTION

8.  Prelaryngeal part:
–– Part of the floor of the mouth, including the tongue
–– Pharynx
–– Salivary glands
–– Various derivatives of the pharyngeal pouches, and the thyroid
–– Respiratory system
 Postlaryngeal part:
–– Esophagus
–– Stomach
–– Duodenum: Whole of the superior (first) part and upper half of the descending (second)
part (up to the major duodenal papilla)
–– Liver and extrahepatic biliary system
–– Pancreas
DERIVATIVES OF FOREGUT

9.  Prearterial segment:
–– Duodenum: Descending (second) part distal to the major papilla; horizontal
(third) and ascending (fourth) parts
–– Jejunum
–– Ileum except terminal part
 Postarterial segment:
–– Terminal ileum
–– Cecum and appendix
–– Ascending colon
–– Right two-thirds of transverse colon
DERIVATIVES OF MIDGUT

10. ARTERIES OF THE GUT
◆The celiac artery is the artery of the foregut. It supplies the gut from the lower part of
the esophagus to the middle of the second part of duodenum.
◆The superior mesenteric artery is the artery of the midgut. It supplies the gut from the
middle of second part of
duodenum to the junction of right two-thirds with the left one-third of transverse colon.
◆The inferior mesenteric artery is the artery of the hindgut. It supplies the gut from the
junction of right two-thirds with the left one-third of transverse colon to the upper part of
anal canal.

11. DERIVATION OF GUT AND ITS LAYERS

12.  Esophagus
• Develops from the part of the foregut between the pharynx and the stomach.
• At first short but elongates with the
– formation of neck
– descent of diaphragm
– enlargement of pleural cavities.
• Musculature of esophagus - derived from mesenchyme surrounding foregut. Around
the upper two-thirds of the esophagus, the mesenchyme forms striated muscle. Around
the lower one-third, the muscle formed is smooth (as over the rest of the gut).
DERIVATION OF INDIVIDUAL PARTS OF ALIMENTARY TRACT
FOREGUT DEVELOPMENT

13.  Stomach
• It is first seen as a fusiform dilatation of the foregut just distal to the esophagus.
• A line connecting its cranial (cardiac) and caudal (pyloric) ends marks the long axis of
stomach.
• Its dorsal border is attached to the posterior abdominal wall by a fold of peritoneum
called the dorsal mesogastrium.
• Its ventral border is attached to the septum transversum by another fold of peritoneum
called the ventral mesogastrium
DERIVATION OF INDIVIDUAL PARTS OF ALIMENTARY TRACT
FOREGUT DEVELOPMENT

17. • Rotation of the stomach can be explained as follows:
–– First rotation is 90° clockwise along longitudinal axis. This changes orientation of
its surfaces and change in the position of vagus nerves.
–– Second rotation: It is in transverse / anteroposterior axis. This brings about
changes in position of fundus and duodenum and in the position of ends of
stomach.
DERIVATION OF INDIVIDUAL PARTS OF ALIMENTARY TRACT
FOREGUT DEVELOPMENT

18. • One surface of stomach grows faster than the other resulting in formation of greater
and lesser curvatures.
• The rotation and differential growth of surfaces explains the change in relationship of
right and left vagus nerves to posteroinferior and anterosuperior surfaces respectively.
• During rotation the cranial end tilts to the left and the caudal end to the right to assume
the adult position.
DERIVATION OF INDIVIDUAL PARTS OF ALIMENTARY TRACT
FOREGUT DEVELOPMENT

19. • Rotation and disproportionate growth of the stomach alters the position of dorsal and
ventral mesogastria.
• Rotation along longitudinal axis pulls dorsal mesogastrium to the left thus forming the
lesser sac/omental bursa behind the stomach.
• During this rotation, the ventral mesogastrium is pulled to the right.
• With the growth of stomach, the dorsal mesogastrium lengthens and the spleen
develops between the layers of dorsal mesogastrium splitting it into gastrosplenic and
lienorenal ligaments.
DERIVATION OF INDIVIDUAL PARTS OF ALIMENTARY TRACT
FOREGUT DEVELOPMENT

20. • Due to rotation along anteroposterior axis, the dorsal mesogastrium bulges downward
and continues to grow to form a double-layered greater omentum.
• The lesser omentum and falciform ligament are formed from the ventral mesogastrium
(a derivative of septum transversum).
DERIVATION OF INDIVIDUAL PARTS OF ALIMENTARY TRACT
FOREGUT DEVELOPMENT

21.  Duodenum
• The superior (or first) part and the upper half of the descending (or second) part of the
duodenum are derived from the foregut.
• The rest of the duodenum develops from the most proximal part of the midgut.
• The part of the gut that gives rise to the duodenum forms a loop attached to the
posterior abdominal wall by a mesentery called mesoduodenum . Later, this loop falls
to the right. The mesoduodenum then fuses with the peritoneum of the posterior
abdominal wall. With the result most of the duodenum becomes retroperitoneal.
DERIVATION OF INDIVIDUAL PARTS OF ALIMENTARY TRACT
FOREGUT DEVELOPMENT

23. • The midgut elongates to form U-shaped intestinal loop that is suspended from the
posterior abdominal wall by a short mesentery.
• Anteriorly, it communicates with the yolk sac by the narrow vitellointestinal duct.
• The superior mesenteric artery runs in the middle of midgut loop and divides it into a
prearterial (cranial) and a postarterial (caudal) segment
DERIVATION OF INDIVIDUAL PARTS OF ALIMENTARY TRACT
MIDGUT DEVELOPMENT

24.  Jejunum and Ileum
• The jejunum and most of the ileum are derived from the prearterial segment of the
midgut loop.
• The terminal portion of the ileum is derived from the postarterial segment proximal to
the cecal bud
DERIVATION OF INDIVIDUAL PARTS OF
ALIMENTARY TRACT (MIDGUT DEVELOPMENT)

26. • During 3rd week of development, the prearterial segment of midgut loop elongates
rapidly. There is rapid growth of liver during this period. Because of rapid elongation of
midgut loop and rapid growth of liver and the developing mesonephric kidney, the
abdominal cavity becomes too small to accommodate all the intestinal loops.
• The midgut loop enters the extraembryonic coelomic cavity in the umbilical cord during
6th week of development. This herniation of intestinal loop is called physiological
umbilical hernia.
PHYSIOLOGICAL UMBILICAL HERNIA

27. • During 10th week of development the herniated midgut loop begins to return to the
abdominal cavity.
• The contributing factors for the return of midgut loop are reduction in size of
developing liver, regression of the mesonephric kidney with associated expansion of
abdominal cavity
REDUCTION OF PHYSIOLOGICAL UMBILICAL HERNIA

28. • When we view the midgut loop from the ventral aspect, it makes a rotation of 270° in
counter clock-wise direction around the axis of superior mesenteric artery.
• During rotation elongation of intestinal loop and coiling of jejunum and ileum also takes
place. Large intestine also shows elongation but without coiling.
• The total rotation of midgut can be divided into three stages of each 90°. First 90°
rotation occurs during the herniation and the remaining 180° during the return of
intestinal loop into the abdominal cavity.
ROTATION OF GUT

29. • Initially, the loop lies in the sagittal plane outside the umbilical ring. Its proximal
segment is cranial and ventral to the distal segment. The midgut loop now undergoes
rotation.
• This rotation plays a very important part in establishing the definitive relationships of
the various parts of the intestine.
• Viewed from the ventral side the loop undergoes an anticlockwise rotation by 90°, with
the result that it now lies in the horizontal plane. The prearterial segment comes to lie
on the right side and the postarterial segment on the left of the superior mesenteric
artery which forms the axis.
FIRST STAGE ROTATION

30. • The prearterial segment now undergoes great increase in length to form the coils of the
jejunum and ileum. These loops still lie outside the abdominal cavity, to the right side of
the distal limb
• During 10th week, the herniated intestinal loops return to the abdominal cavity due to
the regression of mesonephric kidney, reduced growth of liver and expansion of
abdominal cavity.
• The coils of jejunum and ileum (prearterial segment) now return to the abdominal
cavity. As they do so, the midgut loop undergoes a further anticlockwise rotation of 90°.
As a result, the coils of jejunum and ileum pass behind the superior mesenteric artery
into the left half of the abdominal cavity.
SECOND STAGE ROTATION

31. • The postarterial segment of the midgut loop returns to the abdominal cavity.
• As it does so, it also rotates in an anticlockwise direction of 90°.
• With the result, the transverse colon lies anterior to the superior mesenteric artery,
and the cecum comes to lie on the right side.
• At this stage, the cecum lies just below the liver, and an ascending colon cannot be
demarcated.
• Gradually, the cecum descends to the iliac fossa, and the ascending, transverse and
descending parts of the colon become distinct.
THIRD STAGE ROTATION / RETRACTION OF HERNIATED LOOP

32. ROTATION OF GUT

34. • After the completion of rotation of gut, the duodenum, the ascending colon, the
descending colon and the rectum become retroperitoneal by fusion of their
mesenteries with the posterior abdominal wall.
• The original mesentery persists as mesentery of the small intestine, the transverse
mesocolon, and the pelvic mesocolon.
FIXATION OF GUT

35. CLINICAL CORRELATION

40. CLINICAL CASE SCENARIOS

41. RECAP

43. SELF ASSESSMENT