Sex difference in target seeking behavior of developing cremaster muscles and the resulting first visible sign of somatic sexual differentiation in marsupial mammals

Cremaster muscles are present in both male and female developing and adult marsupial mammals. They are complex structures and composed of several dist inct bundles of striated muscle fibers provided with: (1) a distinct and ex tensive innervation; (2) a distinct blood vascular supply; (3) a distinct t endineous origin on the anterosuperior iliac spine; and (4) distinct target structures. The muscles thus seem to be separate anatomical entities and n ot a part of one or more of the layers of the ventral abdominal wall muscul ature. Cremaster muscles in males are elongated, are larger than in females, and f or the most part are a component of the funiculus spermaticus. They insert on the distal part of the tunica vaginalis. The distal Darts of the muscles in females are flattened ("fan shaped") and insert over a broad area on th e dorsal borders of the mammary glands. Muscles in males have no relation w hatsoever to the male mammary glandular rudiments. Muscles in females are a ttached at the base of the uterine round ligament. The remarkable sex difference in target structures of marsupial cremaster m uscles becomes noticeable during perinatal life when outgrowing muscles tak e a different path in males and females. The initial appearance of this sex ually dimorphic trait precedes the sexual differentiation of the genital du cts and external genitalia. In fetal males, the cremaster muscles grow in the direction of the site whe re scrotal bulges initially appear in the subcutaneous layers and later on the inguinal skin surface. They also take the gubernacular core of the vent ral abdominal wall and the attached peritoneal epithelium with them during this outgrowth process. Consequently, this results in the development of a slitlike evagination of the abdominal lumen as the primary step to developm ent of the processus vaginalis, while the testis and adjacent mesonephros a nd its duct are still attached to the posterior abdominal wall. In fetal females, the outgrowing cremaster muscles pass along the gubernacu lar core and, subsequently, this structure develops further as the tip (att ached to the tubo-uterine junction) of the intra-abdominally protruding and further developing uterine round ligament. The female cremaster muscles gr ow further into caudal direction to shape a dorsal border of the developing mammary glands. The early onset, of this sexually dimorphic outgrowth of cremaster muscles indicates that the "classical hormones" of sexual differentiation (anti-Mul lerian hormone [AMH] and steroidal androgens) are not involved in this proc ess. It could thus depend on primary genetic control with male development associated with the male-limited activity of genes on the Y-chromosomes and female development as the default process. Alternatively, the process in m ales could be under the control of an as yet unidentified third fetal testi cular hormone involved in sexual differentiation processes which must then show an unexpectely early (i.e., perinatal) onset of its secretion. Anat Re c 255:130-141, 1999. (C) 1999 Wiley-Liss, Inc.