Sperm of Galeorhinus galeus (Elasmobranchii, Triakidae) Traverse an Excurrent Duct System Characterized by Pronounced Regionalization: A Scanning Electron and Light Microscopy Study.

Abstract

The transport and subsequent maturation of spermatozoa in the vertebrate excurrent duct require the creation of a series of biochemically defined luminal milieus along the length of the duct. Such specialization is accomplished, among others, by changes in the epididymal histoarchitecture. Here we show that the intratesticular and extratesticular genital ducts of mating Galeorhinus galeus exhibit pronounced regionalization both in terms of epithelial histology and lumen diameter size. Findings also reveal distinct differences in the manner in which the spermatozoa were found in each segment of the duct. Novel scanning electron microscopy evidence is presented showing that the wide lumen ductuli epididymides, which ultimately convey the spermatozoa to the proximal epididymis, show functional specialization as well. The wall of the former consisted of cuboidal ciliated and nonciliated cells whose spatial arrangement in the duct wall resulted in a luminal surface showing lengthy rows of cilia-free areas, with each row bordered on both sides by a single row of cilia. The proximal epididymis comprised several subregions whose epithelial histology varied widely. The distal epididymis and ampulla of the epididymis possessed many fingerlike projections and transverse septa, respectively. As the main storage site for spermatozoa, the ampulla completed the bundling of spermatozoa into spermatozeugmata. These were circular sperm masses in which the heads of the spermatozoa were aligned side by side and embedded in a seminal matrix, while their tails extended outward. These findings of pronounced regionalization differ greatly from the rather uniform epididymal histology seen in some rays.