VAN 211: Veterinary Histology and Embryology (2+2)

INTESTINE

• The intestine is a simple tube beginning in the stomach and ending in the cloaca.The narrow caudal part of the foregut forms the descending duodenum.
• The midgut develops into ascending duodenum, jejunum, ileum, caecum, ascending and transverse colon. The hindgut differentiates into decending colon and rectum.
• Around 20 days, or even earlier, the midgut grows in length farther than it could be accommodated in abdominal cavity.
• The midgut looses connection with the yolksac and at this stage, it is in the form of a loop. This loop has a cranial and a caudal limbs and it undergoes an anti-clockwise rotation. Due to this, the cranial limb goes to the right and behind.
• The caudal limb is taken forwards and to the left. Now the gut begins to elongate rapidly and the loop herniates into the umbilical cord. Then cranial and caudal limbs undergo 180^o rotation.
• The original cranial limb of the intestinal loop forms the ascending duodenum, jejunum and ileum. The caudal limb of the loop forms the ceacum and the first part of the colon.
• The hindgut forms the terminal part of the colon. The caudal end of the hindgut forms the cloaca.
• The subdivision of the cloaca results in the rectum. The endodermal lining obliterates the lumen of the gut in the early stages which later on gets canalized and the lumen gets restored. The lining gets villi throughout initially but later the large intestine loses the villi.

Anomalies

• Stenosis or Atresia of oesophagus - absence of canalization.
• Situs inversus – organs found in exactly opposite situation.
• Umbilical fistula - persistent yolk sac opening to exterior.
• Umbilical hernia – failure of withdrawal of intestine.
• Meckle’s direrticulum of ileum – Persistence of proximal part of yolk stalk.
• Imperforate anus – failure of rupture of the anal membrane.