Chapter 86. Development of The Hindgut - Review of Medical Embryology Book

The hindgut follows the midgut, in the embryo, and extends from the posterior intestinal portal to the cloacal membrane. It gives rise to the left one-third to one-half or distal portion of the transverse colon, the descending colon, the sigmoid or pelvic colon, the rectum, the upper portion of the anal canal, and part of the urogenital system (e.g., the bladder and urethra). It is supplied by the inferior mesenteric artery. The terminal part of the hindgut enters into the cloaca, which is an entoderm-lined cavity that is in direct contact with the surface ectoderm

THE CLOACAL MEMBRANE is composed of entoderm of the cloaca and ectoderm of the proctodeum or anal pit
THE TERMINAL PART OF THE HINDGUT, the cloaca, receives the allantois ventrally and the mesonephric ducts laterally

Fixation of the hindgut: when the mesentary fuses with the peritoneum of the left dorsal abdominal wall and then disappears posteriorly, the descending colon becomes retroperitoneal

DURING DEVELOPMENT, a coronal wedge or ridge of mesenchyme, the urorectal septum, forms in the angle between the allantois and the hindgut

As the septum grows caudal toward the cloacal membrane, it divides the cloaca into an anterior portion, the primitive urogenital sinus, and a posterior part, the anorectal canal
By 7 weeks of age, the urorectal septum reaches the cloacal membrane and fuses with it. Thus, the membrane is divided into a posterior anal membrane and a larger anterior urogenital membrane. The area of fusion of the urorectal septum and the cloacal membrane becomes the primitive perineum or perineal body
In week 9, proliferation of mesenchyme around the anal membrane raises the surrounding ectoderm to form a shallow pit, the anal pit or proctodeum. The surrounding swellings are called the anal folds

Soon after, the anal membrane, at the bottom of the anal pit, ruptures to establish the anal canal, an open pathway from the rectum to the outside (actually, to the amniotic cavity, at this time of development)

The anal canal: its upper two-thirds is derived from hindgut and its lower third from the anal pit

THE JUNCTION of the anal pit ectoderm and the hindgut entoderm is indicated by the anatomic anorectal or pectinate (dentate) line which is at the level of the anal or semilunar valves. This is the former site of the anal membrane and where the epithelium changes from columnar to stratified squamous
AT THE ANUS, the epithelium (anoderm) is keratinized and is continuous with the surface skin of the perineum. The surrounding tissue is derived from splanchnic mesenchyme
THE HINDGUT PORTION of the anal canal is supplied by the inferior mesenteric artery; whereas, the power portion (anal pit) is supplied by the internal pudendal branch of the internal iliac artery
AS ARESULT OF THE DIFFERENT EMBRYOLOGIC ORIGINS of the upper and lower parts of the anal canal, venous (external and internal hemorrhoidal or rectal plexi) and lymphatic drainage (via the iliac and inguinal nodes) and nerve supply (autonomic and peripheral) differ in the various parts of the canal