This week’s embryology lecture attempted to link the processes that form the gastrointestinal (GI) tract with the anatomy that you’re seeing in the lab. We started by recapping the end of the last lecture (gastrulation) and showing how the flat sheet of the early embryo can be rolled up to form a tube. We looked at the role of the yolk sac in this, and how the ectoderm, mesoderm and endoderm roll up and contribute to the different parts of the embryo.
We divided the simple gut tube into foregut, midgut and hindgut regions based on the attachment of the gut tube to the yolk sac. These regions each have their own artery branching from the aorta (coeliac trunk, superior mesenteric artery and inferior mesenteric artery respectively). This simple tube will dilate, lengthen and twist to form what we recognise as the adult GI tract.
The foregut forms the oesophagus, stomach, part of the duodenum, liver, gallbladder and pancreas. We talked about how each of these forms, and how a 90 degree left to right rotation moves these structures from a midline starting position to their final locations on the left or right side. Most of the tube will become closed by epithelial proliferation during development, and then cleared. Importantly we also looked at the mesentery, which starts as a very simple sheet of connective tissue running in the midline from the dorsal to ventral walls of the embryo’s developing abdomen. With the twisting of the gut tube this simple sheet will form all the more complex connecting bits of peritoneum in the adult that tie these organs together, such as the lesser omentum, greater omentum, falciform ligament, gastrosplenic ligament, etc.
The midgut will form the rest of the small intestine and the large intestine as far as the transverse colon. It starts as a simple loop that becomes longer and rotates 270 degrees, again from left to right. As it lengthens it pushes out into the umbilical cord and then is pulled back into the abdomen. The simple mesentery initially holding the single loop of midgut in place has become much larger as the midgut lengthened, and now carries many branches of the superior mesenteric artery to all parts of the tube.
The hindgut will form the remainder of the GI tract and part of the urogenital system. The cloaca is an endoderm lined cavity at the anus end of the gut tube, and becomes split in two by the urogential sinus. The dorsal part will become the rectum and the superior parts of the anal canal, and the ventral part will help form the bladder and the urethra. The cloacal membrane that had closed off the tube up to this point ruptures, and two openings are formed (rectum and urethra). The last part of the anal canal is formed from the external ectoderm, explaining the two separate routes of blood supply to the anal canal in the adult (inferior mesenteric artery and internal pudendal artery).
With these normal developmental processes in mind, what could go wrong and what congenital defects would be observed?
Links:
Pyloric stenosis – kidshealth.org, emedicine.com
Gastroschisis & omphalocoele.
emedicine.com: imperforate anus
Simbryo
