Icon module 20

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.
Fig. 19 - Development of the cloaca
Stage 13, roughly 32 days

Urorectal septum
Cloacal membrane

Fig. 20 - Migration of the kidneys, Stage 23, roughly 56 days

Urogenital orifice
Anal orifice
Urogenital sinus
Future bladder

Fig. 19

The white arrow indicates the direction of growth of the urorectal septum. This separates the cloaca into a urogenital sinus (ventrally) and the rectum (dorsally).

Fig. 20

The kidneys are found in their definitive position at the level of the upper lumbar region. The urorectal septum (white arrow) has divided the cloaca. The urogenital sinus, bounded on the outside by the urogenital orifice, lies ventrally. The rectum, which opens to the outside through the anal orifice, is found behind it. The cloacal membrane has ruptured in stage 19.

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.

Fig. 21 - Development of
the urorectal septum

  1. Peritoneal cavity
  2. Upper fold (Tourneux) (pink arrow)
    Lower fold (Rathke) (blue arrow)

Fig. 21

The urorectal septum consists of two different mesoderm structures.
Over the course of the 4th week an upper fold (Tourneux) divides the cloaca in the cranio-caudal direction. The two lower, lateral folds (Rathke) are responsible for the division in the lower section.

Fig. 22 - Separation of the cloaca and the formation of the perineum

  1. Peritoneal cavity
  2. Upper fold (Tourneux; pink arrow)
    Lower folds (Rathke; blue arrows)
  3. Primary urogenital sinus
  4. Anal canal

Fig. 23 - Separation of the cloaca and the formation of the perineum

Urogenital orifice
Anal orifice

Fig. 22

When the two lower folds (Rathke) are fused in the median level, cranially meet at the upper fold (Tourneux), and merge with it, the separation of the cloaca is completed. (see the transverse section in Fig. 23 O).

Fig. 23

Transverse section from Figs. 21 and 22 at the N and O levels.