Spermatogenesis in the hologonic testis of the trichuroid nematode, Capillaria hepatica (Bancroft, 1893)

Abstract

Development of spermatozoa in the hologonic testis of Capillaria hepatica (Bancroft, 1893) has been examined by light and electron microscopy. The testis contains a region of developing germ cells that may encircle the testis, or be restricted to a circumferential zone. This germinal region extends the entire length of the testis. Spermatogonia occur closest to the periphery. Spindle remnants are characteristic of division stages of these cells. Primary spermatocytes are identified by the breakdown of the nuclear envelope and expansion of the endoplasmic reticulum. Nuclear material becomes ringed by mitochondria. Karyokinesis is followed by incomplete cytokinesis. The second meiotic division follows rapidly, giving four spermatids that remain connected via cytoplasmic bridges. The cell membrane of both primary and secondary spermatocytes is elaborately folded. During the second meiotic division, superfluous cytoplasm, containing Golgi apparatus and endoplasmic reticulum, is extruded as a residual body. After separation of spermatids, spermateleosis begins. Large diameter tubules occur in the cytoplasm of early spermatids. These probably collapse to form individual membrane loops which coalesce to form a system of interconnected membrane specializations of the developing sperm. Nuclear material progressively condenses to form the dense, nonmembrane-bound nucleus. Spermateleosis continues in the luminal portion of the testis. Sperm develop a head region including the trilobed nucleus, some mitochondria and β-glycogen, and a tail containing closely packed peripheral membrane specializations and centrally disposed mitochondria. Pseudopodia occur on sperm in the vas deferens and probably in the testis. Throughout reduction division stages, germ cells are enclosed by processes of sustentacular cells. These cells may have three functions: nutritive, regulatory and supportive. The first may be evidenced by phagocytic activity and the elaborations of adjacent spermatocyte membranes. Regulation of sperm development is not a striking feature but may be a function of sustentacular cells along with cytoplasmic bridges between spermatocytes. Sustentacular cells probably hold early developmental stages in the testis against the flow of mature sperm through the lumen.