Endoderm differentiates into a transient epidermis in the mouse perineum

In eutherian mammals, the embryonic cloaca is partitioned into genitourinary and anorectal canals by the urorectal septum. In the mouse embryo, the urorectal septum contributes to the perineum, which separates the anus from the external genitalia. During the growth of the urorectal septum, endodermal epithelium of the cloaca is displaced to the surface of the perineum, where endodermal cells are integrated into the developing skin. However, it is unknown whether the endodermal lineage of the perineum acquires true epidermal identity, an enigmatic fate for endodermal cells.

We find that endodermal cells that reach the surface of the perineum express markers of basal, spinous, and granular epidermis. During postnatal development, the endodermal lineage of the perineum epidermis undergoes terminal differentiation and desquamation and is replaced by adjacent ectoderm. Live imaging and single-cell tracking show that ectodermal cells move at a faster velocity in a lateral-to-medial direction, indicating convergence toward the narrow band of endoderm that lies between the anus and external genitalia.

Cloacal endoderm differentiates into a non-renewing, transient epidermis at the midline of the perineum. Differences in directionality and velocity of cell movement patterns between endodermal and ectodermal cells suggest that the perineum epidermis develops by convergent extension.