Changes in ultrastructure of cytoplasm and nucleus during spermiogenesis in Chara vulgaris

Abstract

In the present study much attention was paid to the first and middle stages of spermiogenesis in Chara vulgaris. The spermiogenesis was divided into 8 phases (A-H). In telophase leading to spermatid formation (A phase) there are no structural changes, indicating that cells pass into a new functional stage-spermiogenesis.B phase: After the formation of a posttelophase nucleus, the difference is striking in chromatin of newly formed spermatids and chromatin nuclei in cycling cells. The nuclei of spermatids form big clusters of dense chromatin and nucleoli remain as in telophase. However, in cycling cells condensed chromatin forms fine reticulum while nucleoli contain abundant granular component. C phase: The movement of the nucleus to the side wall, characteristic for early spermiogenesis, is not preceded by changes in the structure of the nucleus. D phase: In D phase and later free spaces between plasmalemma and the cell wall appear. In these spaces two flagella have a typical structure (9 + 2). They are surrounded by plasmalemma with rhombus-shaped scales on its surface. The structure of nucleus is characterised by a thick, dense layer of chromatin and gradual disappearance of nucleoli by extrusion to the cytoplasm. In electron-transparent ground cytoplasm, numerous ribosomes forming clusters, spirals and chains are present. Numerous rough ER cisternae with light content also appear. Bodies similar to secondary lysosomes and intensified activity of Golgi structures are observed. E phase: Large amounts of proteins seem to be synthesized and accumulated in rough ER cisternae which are filled with an electron dense substance. The same kind of substance can be seen within the perinuclear space. One may assume that proteins migrate through the nuclear envelope to the cell nucleus. Probably at that moment the main proteins taking part in the reorganisation of chromatin structure and in the exchange of somatic proteins into generative ones are formed. Simultaneously synthesis of other types of proteins continues. Active Golgi structures producing numerous light and coated vesicles complete the picture of intensive metabolism. Parallelly reduction of cytoplasm takes place. F phase: The nuclear chromatin becomes netlike after DNP staining, without a thick layer of condensed chromatin close to the nuclear envelope. Simultaneously, all types of structures with positive contrast after EDTA migrate polarly from nucleus to the cytoplasm. Further reduction of cytoplasm takes place and protein synthesis continues as suggested by the presence of numerous free polysomes. G phase: The structure of chromatin fibrils undergoes a complete transformation. They become much thicker, parallel to each other, changing the orientation along with the formation of nucleus coils. The nucleus is devoid of most nucleoplasm structures. It has significantly elongated shape and shows the presence of manchette microtubules on the outside of nuclear envelope and lamina attached to the inner membrane of nuclear envelope. Condensed mitochondria are located near flagella and nucleus while plastids with starch grains lined up on the opposite side of the spermatozoid are also surrounded by microtubules. H phase: Nuclear fibrils form lamellar interconnections and the nucleus has a form of dense reticular structure surrounded by the nuclear envelope without pores.