Meeting Abstract
Four ventrolateral muscular layers consistently support the thorax and abdomen of most tetrapods, while only two muscular layers support the perineum. However, many, if not all, placental mammals have four ventrolateral muscular layers supporting the perineum, that function to allow precise muscular control of the structures that develop as part of full septation of the cloaca into urogenital and anorectal portions, including the rectum, anal canal, urethra, vagina, vulva, and paired vascular erectile tissues. The organization and function of muscular layers of the thorax and abdomen are conserved across taxa, with the muscular layers of the perineum serving a diverse set of functions and are more varied in structure. From superficial to deep, the perineal subcutaneous layer usually regulates orifice closure; the external layer usually supplements erectile and micturition function; the internal layer usually provides primary micturition and defecation regulation, and the transversus layer provides structural support for pelvic organs. However, we observe that some well-known muscles that are present across many mammals may derive from alternative muscle layers. In carnivorans, the retractor penis/clitoris muscle is derived from the external layer, but in ungulates, retractor penis is derived from the transversus muscle. In dogs, constrictor vulvae and constrictor vestibulae are both derived from the external layer, whereas in horses it is derives from the subcutaneous layer and constrictor vestibulae is derived from the external layer. We identify perineal muscular homologies and analogies in a variety of placental mammals, and suggest variations in perineal muscle layering taxa likely represent independent evolutionary adaptations that serve specific reproductive and excretory functions.
