The development of the intestine and its congenital abnormalities Fig 69
The primitive endodermal tube of the gut is divided into:
1 the fore-gut (supplied by the coeliac axis) extending as far as the entry of the bile duct into the duodenum;
2 the mid-gut (supplied by the superior mesenteric artery) continuing as far as the distal transverse colon;
3 the hind-gut (supplied by the inferior mesenteric artery) extending thence to the ectodermal part of the anal canal.
Fig. 69 Stages in rotation of the bowel. (a) The prolapsed mid-gut loop, seen in lateral view. (b) The mid-gut returns to the abdomen. (c) The caecum descends to its definitive position. Note the completion of stomach-rotation with the formation of the lesser sac (omental bursa).
Fig. 69 Stages in rotation of the bowel. (a) The prolapsed mid-gut loop, seen in lateral view. (b) The mid-gut returns to the abdomen. (c) The caecum descends to its definitive position. Note the completion of stomach-rotation with the formation of the lesser sac (omental bursa).
At an early stage rapid proliferation of the gut wall obliterates its lumen and this is followed by subsequent recanalization.
The fore-gut becomes rotated with the development of the lesser sac so that the original right wall of the stomach comes to form its posterior surface and the left wall its anterior surface. The vagi rotate with the stomach and therefore lie anteriorly and posteriorly to it at the oesophageal hiatus.
This rotation swings the duodenum to the right and the mesentery of this organ then blends with the peritoneum of the posterior abdominal wall —this blending process is termed zygosis (see p. 98).
The mid-gut enlarges rapidly in the 5-week fetus, becomes too large to be contained within the abdomen and herniates into the umbilical cord. The apex of this herniated bowel is continuous with the vitello-intestinal duct and the yolk sac, but this connection, even at this early stage of fetal life, is already reduced to a fibrous strand.
The axis of this herniated loop of gut is formed by the superior mesen-teric artery, which demarcates a cephalic and a caudal limb. The cephalic element develops into the proximal small intestine; the caudal segment differentiates into the terminal 2 feet (62 cm) of ileum, the caecum and the colon as far as the junction of the middle and left thirds of the transverse colon.
A bud which develops on the caudal segment indicates the site of subsequent formation of the caecum; it may well be that this bud delays the return of the caudal limb in favour of the cephalic gut during the subsequent reduction of the herniated bowel.
At 10 weeks this return of the bowel into the abdominal cavity commences. The mid-gut loop first rotates anti-clockwise through 90° so that the cephalic limb now lies to the right and the caudal limb to the left.
The cephalic limb returns first, passing upwards and to the left into the space left available by the bulky liver. In doing so, this mid-gut passes behind the superior mesenteric artery (which thus comes to cross the third part of the duodenum) and also pushes the hind-gut—the definitive distal colon—over to the left.
When the caudal limb returns, it lies in the only space remaining to it, superficial to, and above, the small intestine with the caecum lying immediately below the liver.
The caecum then descends into its definitive position in the right iliac fossa, dragging the colon with it. The transverse colon thus comes to lie in front of the superior mesenteric vessels and the small intestine.
Finally, the mesenteries of the ascending and descending parts of the colon blend with the posterior abdominal wall peritoneum by zygosis. This embryological fusion of peritoneal surfaces is of major surgical importance. Thus, in mobilising the right or left colon, an incision is made along this avascular line of zygosis lateral to the bowel, allowing it to be mobilised with its mesocolon and blood supply. In a similar fashion, the duodenum, head of pancreas and termination of the common bile duct can be mobilised bloodlessly by incising the peritoneum along the right border of the duode-num—Kocher's manoeuvre (see page 77).
Numerous anomalies may occur in the highly complex developmental process.
1 Atresia or stenosis of the bowel may result from failure of recanalization of the lumen. Another cause of this may be damage to the blood supply to the bowel within the fetal umbilical hernia with consequent ischaemic changes. Imperforate anus—see page 83.
2 Meckel's diverticulum represents the remains of the embryonic vitello-intestinal duct (communication between the primitive mid-gut and yolk sac) and is, therefore, always on the anti-mesenteric border of the bowel. As an approximation to the truth it can be said to occur in 2% of subjects, twice as often in males as females, to be situated at 2 feet (62 cm) from the ileocae-cal junction and to be 2 in (5 cm) long. In fact, it may occur anywhere from 6 in (15 cm) to 12 feet (3.5 m) from the terminal ileum and vary from a tiny stump to a 6 in (15 cm) long sac. Occasionally the diverticulum ends in a whip-like solid strand.
As well as a diverticulum—the commonest form—this duct may persist as a fistula or band connecting the intestine to the umbilicus, as a cyst hanging from the anti-mesenteric border of the ileum or as a 'raspberry tumour' at the umbilicus, formed by the red mucosa of a persistent umbilical extremity of the diverticulum pouting at the navel (Fig. 70).
The mucosa lining the diverticulum may contain islands of peptic epithelium with oxyntic (acid-secreting) cells. Peptic ulceration of adjacent intestinal epithelium may then occur with haemorrhage or perforation.
3 The caecum may fail to descend; the peritoneal fold which normally seals it in the right iliac fossa passes, instead, across the duodenum and causes a neonatal intestinal obstruction. The mesentery of the small intes
Fig. 70 Abnormalities associated with persistence of the vitello-intestinal tract. (a) Meckel's diverticulum. (b) Patent vitello-intestinal duct. (c) Cyst within a fibrous cord passing from the anti-mesenteric border of the intestine to the umbilicus. (d) Meckel's diverticulum with terminal filament passing to umbilicus.
Fig. 70 Abnormalities associated with persistence of the vitello-intestinal tract. (a) Meckel's diverticulum. (b) Patent vitello-intestinal duct. (c) Cyst within a fibrous cord passing from the anti-mesenteric border of the intestine to the umbilicus. (d) Meckel's diverticulum with terminal filament passing to umbilicus.
tine in such a case is left as a narrow pedicle, which allows volvulus of the whole small intestine to occur (volvulus neonatorum).
4 Occasionally, reversed rotation occurs, in which the transverse colon comes to lie behind the superior mesenteric vessels with the duodenum in front of them; this may again be accompanied by extrinsic duodenal obstruction due to a peritoneal fold.
5 Exomphalos is persistence of the mid-gut herniation at the umbilicus after birth.
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