Microanatomy of spermatophore formation and male genital ducts in the holocephalan, Callorhynchus milii

Abstract

The present study is the first to consider male genital tract histology and spermatophore development among the holocephalans. Spermatogenesis in Callorhynchus milii occurs in a spermatocyst containing sperm laterally embedded in the apex of Sertoli cells. Individual sperm occur in the proximal tubule of the epididymis where they commence to associate into laterally aligned bundles. The Leydig gland, a modified anterior mesonephros, lies laterally to the genital ducts and supplies a seminal matrix to the epididymis and ductus deferens. The epididymis consists of a thin-walled large lumen with a smooth epithelium of simple ciliated columnar cells and secretory pyriform cells. The lumen contains a flocculent granular seminal matrix with a peripheral area of hyalin material. Sperm are present in the ductus deferens as bundles embedded in an ovoid-to- elongate spermatophore matrix with a distinct cortex and medulla. The epithelium of the ductus deferens consists of simple ciliated columnar cells and secretory pyriform cells. Fully formed spermatophores occur in the seminal vesicle where laterally aligned sperm are embedded in a matrix. The development of spermatophores as a mode of sperm storage may reflect a breeding strategy that is both opportunisitic and competitive.