Electron Microscope Studies on Sperm Differentiation in Marine Annelid Worms. Ii. Sperm Formation in Arenicola Brasiliensis

Abstract

The course of spermiogenesis in Arenicola brasiliensis was observed with the electron microscope. The spermatogonia floating in the body cavity seem to proliferate and differentiate to mature spermatozoa in the coelomic fluid. More than a hundred spermatids are connected t o one large central mass of cytoplasm and spermiogenesis proceeds synchronously in one cluster, which changes into a sperm-disc during maturation. The pre-acrosomal vesicle originates from the Golgi-body and gradually changes into the acrosomal vesicle of peculiar structure like a cup upside down. In the process of differentiation of the acrosome, a part of the material in the acrosomal vesicle is transferred into the space between the vesicle and the nucleus. The posterior one-third of the cylindrical nucleus is surrounded by four middle-piece mitochondria. The flagellar axoneme originates from one of the centrioles, which is located near a posterior pit in the nucleus.Arenicola brasiliensis (formerly A . cristata) is a very common species distributed widely in the world. The processes of gametogenesis and early development of this annelid have been described in detail by OKADA (1 941 ), using the light microscope. The early spermatogonia are liberated from gonads into the coelomic fluids as clusters, which he termed "spermatophores", and they develop in the form of sperm-discs until these are disintegrated to free the spermatozoa in the process of discharge from the nephridia.In the present paper, I will report observations on the ultrastructure of the spermatozoon and the process of its formation, obtained with the electron. microscope. Since the structural changes take place during spermiogenesis, my interest is mainly directed to the stages following sperm-disc formation. MATERIALS AND METHODSArenicola brasiliensis were collected from the seashore near Matsuyama City in June when the worms shed egg clusters very frequently. Some worms had many fully formed sperm-discs and others had many immature spermatids in the coelomic fluids. The sperm-discs and spermatids 89

An ultrastructural study of spermatogenesis and sperm morula breakdown inArenicola marina (L.) (Annelida: Polychaeta)

“…Therefore. a more thorough evaluation of the sperm biology of this species can now be made in addition to a comparison between the sperm structure of A. marina and the closely related Arenicola brasiliensis (Sawada, 1975}. Sperm ultrastructure can often be related to aspects of the reproductive mode or fertilisation biology of a given species and the sperm ultrastructure of A. marina, for example, is typical of species which utilize external fertilisation following broadcast spawning: an oval shaped head, a simple acrosorne, a single flagellum and four to six unmodified mitochondria (see Franz4n, 1956). In many respects the sperm structure of A. marina is very similar to the sperm morphology of the closely related A. brasiliensis.…”

Electron Microscope Study on Sperm Differentiation in Perinereis brevicirris (Polychaeta)

“…The synchronous specialization in the later stages is limited to the four spermatids derived from the one primary spermatocyte. Observations suggest that spermatids having a poorly developed cytoplasmic central mass and detached from the mass at late spermatogenesis maintain a certain quantity of cytoplasm that is shed off at the terminal stage of maturation, whereas spermatids connected to a well-developed central mass or anucleate nutrient mass lose the cytoplasm that surrounds the nucleus (13,15,16).…”

“…In Lumbricus (1), Ikedosoma (15), Arenicola (16) and Travisia (13), the anucleate mass or cytoplasmic central mass may have the function of reabsorbing and remetabolizing the excess cytoplasm sloughed off during spermiogenesis, but the central mass of Perinereis does not appear to have such a function, as the mass is poorly developed and the cells are separated from the cluster at the late stage of spermatogenesis and each spermatid has sufficient cytoplasm until the final stage of differentiation. The synchronous specialization in the later stages is limited to the four spermatids derived from the one primary spermatocyte.…”

“…In annelid worms, a number of undifferentiated male germ cells form large cell clusters floating in the coelomic cavity. In some species, cells in clusters are connected with the cytoplasmic central mass until final sperm differentiation (13,15,16) and in others the clusters are disintegrated into small cell groups in the late stage of spermatogenesis (3,9,14).In the present paper, the intercellular communication in the Perinereis brevicirris male germ cells at differentiation and the ultrastructure of mature spermatozoa are demonstrated with the ATPase cytochemical reaction. These intercellular bridges connecting cells are responsible for the synchronization of cell differentiation.…”

“…In annelid worms, a number of undifferentiated male germ cells form large cell clusters floating in the coelomic cavity. In some species, cells in clusters are connected with the cytoplasmic central mass until final sperm differentiation (13,15,16) and in others the clusters are disintegrated into small cell groups in the late stage of spermatogenesis (3,9,14).…”