Subdividing the cloaca
We have seen that the upper urinary system - consisting of the collecting ducts, the calices, the renal pelvis, and the ureters - arises from the ureter anlage.
The lower urinary system - composed of the bladder and the urethra - is formed from the endoderm of the posterior intestine.
Separating the cloaca
In stage 13, the cloaca is the common end of the rectal tube and the urogenital tract. Towards the outside it is closed by the cloacal membrane.
Between the 4th and 6th weeks the urorectal septum separates the cloaca into a primary urogenital sinus (ventrally) and the rectum (dorsally).
- The bladder and the pelvic limb of the urethra arise from the primary urogenital sinus and the caudal portion of the urethra comes from the definitive urogenital sinus (see Fig. 26.
- The urorectal septum divides the cloacal membrane into two membranes: the urogenital membrane (ventrally) and the anal membrane (dorsally).
These two membranes disolve (stage 19), like the bucco-pharyngeal membrane (stage 11), in order to form the intestinal and urogenital openings.
The perineum and the urorectal septum
Today, the urorectal septum is no longer regarded as an isolated cellular layer of mesoderm cells that slowly grow towards the cloacal membrane. It consists of two mesodermal structures that are fused together.
An upper fold (Tourneux), located frontally, grows caudally. Near the cloacal membrane two lateral folds (Rathke) form that fuse at the median level.
They subdivide the cloaca and also grow in the direction of the upper fold (Tourneux) that is located frontally. A disorder in the formation of these two structures leads to recto-urethral or rectovesical fistulas. Connective tissue and the perineal musculature, which keep the pelvic organs in place, arise from the mesoderm, which surrounds the rectal tube. The central fibrous part of the perineum corresponds anatomically to the region between the anal and urogenital orifices.