Fine structure of spermatogonia, spermatocytes, and spermatids of the sea cucumbers Cucumaria lubrica and Leptosynapta clarki (Echinodermata: Holothuroidea)

Abstract

Spermatogonia, which usually lie in close contact with the basal lamina of the testicular wall, and are joined together by desmosomes, contain a roughly circular nucleus with one or two nucleoli. Small proacrosomal granules are first evident in late spermatogonia. A tubular body (0.2 × 1.5 microns (μ)) consisting of a parallel array of tubules is observed in spermatogonia of Cucumaria. Spermatocytes were identified by the presence of flagellar formation, large proacrosomal granules, and centriolar satellite projections. Striated rootlets of the two species differ morphologically. A dense chromatoid body is present in spermatocytes of Leptosynapta. Spermatids of both species are joined by cytoplasmic bridges. By late spermiogenesis, the nucleus of Leptosynapta is roughly circular, whereas that of Cucumaria is tapered at both ends. No microtubules were observed during the elongation of the Cucumaria nucleus. Fibrous projections extend from the proximal centriole of both species into the centriolar fossa. It is suggested that only one mitochondrion is present throughout spermatogenesis. This implies that the mitochondrion transforms from a highly branched tubular structure to a simple compact organelle. Morphogenesis of the acrosome is described.