Abstract. The nuclei of bovine spermatids and spermatozoa are surrounded by dense cytoplasmic webs sandwiched between the nuclear envelope and the acrosome and plasma membrane, respectively, filling most of the cytoplasmic space of the sperm head. This web contains a complex structure, the perinuclear theca, which is characterized by resistance to extractions in nondenaturing detergents and high salt buffers, and can be divided into two major subcomponents, the subacrosomal layer and the postacrosomal calyx. Using calyces isolated from bull and rat spermatozoa we have identified two kinds of basic proteins as major constituents of the thecal structure and have localized them by specific antibodies at the light and electron microscopic level. These are an Mr 60,000 protein, termed calicin, localized almost exclusively to the calyx, and a group of multiple-band polypeptides (MBP; Mr 56,000-74,000), which occur in both the calyx and the subacrosomal layer. The polypeptides of the MBP group are immunologically related to each other, but unrelated, by antibody reactions and peptide maps, to calicin. We show that these basic cytoskeletal proteins are first detectable in the round spermatid stage. As we have not detected any intermediate filament proteins and proteins related to nuclear lamins of somatic cells in sperm heads, we conclude that the perinuclear theca and its constituents, calicin and MBP proteins, are the predominant cytoskeletal elements of the sperm head. Immunologically cross-reacting polypeptides with similar properties have been identified in the heads of rat and human spermatozoa. We speculate that these insoluble basic proteins contribute, during spermiogenesis, to the formation of the perinuclear theca as an architectural element involved in the shape changes and the intimate association of the nucleus with the acrosome and the plasma membrane.URING the past decade it has become increasingly clear that the overall shape of a cell and its internal architecture is dependent not only on membranous elements but also on a group of structures that resist cell lysis and extraction by nondenaturing detergents and buffers of various pH and ionic strength. Such proteinaceous structures are now usually referred to under the collective terms cytoskeleton, as to the cytoplasm, and karyoskeleton, for nuclear architectural elements. Among the cytoskeletal components some of the most prominent structures are the actin-containing microfilaments and the tubulin polymers, i.e., microtubules. The term cytoskeleton also includes certain filamentous webs such as membranous coats in the cell periphery, many of which contain proteins of the spectrin family, the regular clathrin coats of coated pits and vesicles, and the plaques of adhering junctions. Remarkably, some residual cyto-and karyoskeletal components are preserved even after extraction in high salt concentrations, and maintain typical architectural features. Structures of this category include the intermediate-sized filaments (IFs) ~ of the cytoplasm and the bio...
Fertilization events following coalescence of the gamete plasma membranes and culminating in the formation of the zygote nucleus were investigated by light and electron microscopy in the sea urchin, Arbacia punctulata. Shortly after the spermatozoon passes through the fertilization cone, it rotates approximately 180 ° and comes to rest lateral to its point of entrance. Concomitantly, the nonperforated nuclear envelope of the sperm nucleus undergoes degeneration followed by dispersal of the sperm chromatin and development of the pronuclear envelope. During this reorganization of the sperm nucleus, the sperm aster is formed. The latter is composed of ooplasmic lamellar structures and fasciles of microtubules. The male pronucleus, sperm mitochondrion, and flagellum accompany the sperm aster during its migration. As the pronuclei encounter one another, the surface of the female pronucleus proximal to the advancing male pronucleus becomes highly convoluted. Subsequently, the formation of the zygote nucleus commences with the fusion of the outer and the inner membranes of the pronuclear envelopes, thereby producing a small internuclear bridge and one continuous, perforated zygote nuclear envelope.
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