Showing papers in "Current Topics in Developmental Biology in 1978"

Mechanisms of activation of sperm and egg during fertilization of sea urchin gametes.

Abstract: Publisher Summary This chapter discusses the fertilization of the sea urchin egg. The metabolic and morphological changes of sea urchin gametes during fertilization are the best characterized of all embryos. In organization, this chapter explains the activation of both gametes, first considering the activation of the sperm upon its contact with egg jelly and the resultant cascade of the events leading to sperm–egg attachment and sperm–egg fusion. It then examines the consequences of sperm fusion with the egg that leads: (1) to the responses, excluding other sperm from fusing with the egg and (2) to the responses that result in the activation of embryonic development. The chapter describes the important roles played by intracellular calcium and cytoplasmic pH in turning on the cell metabolism. In sperm plasma membrane, the receptors interact with some component of the egg jelly or egg surface. This leads to increased Ca2+ content, the induction of membrane fusion, and the acrosomal exocytosis. At the same time, there is an acid efflux, polymerization of internal actin, and exposure of sperm lysins and sperm bindins that are attached to the newly formed membrane of the acrosomal process.

Biochemistry of male germ cell differentiation in mammals: RNA synthesis in meiotic and postmeiotic cells.

Abstract: Publisher Summary This chapter discusses a few aspects of ribonucleic acid (RNA) and protein synthesis at different stages of germ cell differentiation in mammals that are relevant to the problem of regulation of spermatogenesis. Spermatogenesis is a highly orderly process that begins with the stem cell and terminates with the release of the mature spermatid into the lumen of the seminiferous tubule. In the mouse, the total duration of spermatogenesis, from the stem cell to the mature spermatid, is about 34.5 days and is subdivided into three phases: (1) the period of multiplication and maturation of spermatogonia or mitotic phase of spermatogenesis that lasts about eight days, (2) meiosis that lasts 13 days, and (3) spermiogenesis, from the early spermatid to the release of the spermatozoon into the lumen, which is about 13.5 days long. Biochemistry of spermatogenesis is one of the most promising areas of research in developmental biology and mammalian reproduction. The development of germ cell is a field, where different experimental approaches, cytology, ultrastructure, cytogenetics, genetics, biochemistry, immunology, endocrinology, can interact in a coordinated view of cell differentiation. The results described provide clear evidence for gene expression during the haploid phase of spermatogenesis in the mouse; the transcriptional activity during early spermiogenesis involves both polyadenylated RNA and ribosomal RNA.

Sperm-egg association in animals.

Abstract: Publisher Summary This chapter discusses the sperm–egg fusion in mammals. Eggs fusing with spermatozoa can be obtained, by flushing the oviducts of mated females shortly after ovulation, but the chance of obtaining eggs in the initial stage of the fusion is very poor because of asynchronous sperm penetration and the rapidity of the fusion process. The fusion of a spermatozoon with an egg seems to be facilitated by the presence of numerous microvilli on the egg surface. The process of sperm–egg fusion in mammals can be readily studied by inseminating zona pellucida, free eggs, with acrosome reacted spermatozoa. The sperm membrane that initially fuses with the egg plasma membrane is the plasma membrane in the posterior region of the sperm head, not the inner acrosomal membrane. The inability of the acrosomal membrane to fuse with the egg membrane could be due to its lack of “fluidity.” The acrosome reaction appears to be essential for successful fusion of the spermatozoon with the egg. In some mammals (e.g., the hamster), the egg plasma membrane remains capable of fusing with the spermatozoa even long after the penetration of the fertilizing spermatozoon into the egg.

Transformations of sperm nuclei upon insemination

Abstract: Publisher Summary This chapter describes the transformations of the sperm nuclei upon insemination. Normal interactions between the nucleus and cytoplasm are of fundamental importance at all stages of embryonic development. However, during fertilization, a unique situation exists that demonstrates rather dramatically the presence of the conditions within the eggs that bring about major changes in nuclear morphology and function. The relationship and possible functional equivalence of the transformations, the maternally and paternally derived chromatin, undergoes at fertilization and the changes in size and behavior of the transplanted nuclei has been discussed in the chapter. At fertilization, the sperm nucleus is reorganized to form the male pronucleus. Transformation of the sperm nucleus into a male pronucleus involves morphological changes similar to those that have been described for the transplanted nuclei; they include: chromatin decondensation and nuclear enlargement. The activity of the genes has been shown to depend on the association of regulatory proteins with particular regions of chromosomes. The transformations of the sperm nucleus at fertilization into a male pronucleus may be grouped into three processes that are described ultrastructurally in a number of animals and include: (1) breakdown of the sperm nuclear envelope, (2) chromatin dispersion, and (3) formation of a “new” nuclear envelope.

Cell communication, cell union, and initiation of meiosis in ciliate conjugation.

Abstract: Publisher Summary This chapter discusses cell communication, cell union, and the initiation of meiosis in ciliate conjugation. For ciliate cells, conjugation is an exceptionally social occasion. During this time, cells of complementary mating types first communicate by mating signals, gain the capacity to unite, and temporarily form bicellular conjugant pairs. In the united cells, a series of nuclear changes, including meiosis, exchange, and fusion of gametic nuclei, is set in motion. Then, the cells separate, returning to their unicellular phase, but developmental changes continue. The fertilization nucleus repeatedly divides mitotically and the division products differentiate into somatic and germ nuclei that eventually replace the pre-existing nuclear system. Certain cortical structures are also reconstructed. Thus, ciliate conjugation encompasses two important features of fertilization— namely, karyogamy and the initiation of development. These processes all commonly occur in fertilization and in related phenomena in many organisms, but some of the characteristics of ciliate conjugation appear to provide unique investigative opportunities. The physical association of cells in conjugation may begin with a subtle contact of cilia, but the association soon becomes more intimate, eventually turning into a tight cell union with cytoplasmic bridges.

Chapter 5 Sperm and Egg Receptors Involved in Fertilization

Abstract: Publisher Summary This chapter discusses the sperm and egg receptors involved in fertilization. Specific cell-cell binding and specific cell-surface interaction with the substances in solution (for example, toxins, drugs, hormones, and agglutinins) implies interaction of specific receptors on the apposed or exposed surfaces of the cells involved. By analogy, with specific immunological reactions, specific cell attachment and adhesion are commonly explained, by assuming attachment and/or binding by the receptor substances that interact in “lock-and-key” fashion. Several specific sperm–egg interactions can be attributed to the cell-surface receptors comparable to those involved in other specific cell-cell interactions. Certain events are characterized by species specificity and are evidently mediated, by the interaction of specific receptors of complementary gametes, comparable to the cell-specific receptor systems of the other cell types. Certain receptors are released into the solution from gametes, notably eggs, which can act on sperm at a distance from the source. Others are evidently attached to or built into cell coats and the cell membrane. The former include: chemotactic agents for sperm and acrosome initiating agents that presumably interact with complementary sperm-bound receptors. At least, two additional complementary sperm- and egg-bound receptor systems bind sperm to the egg envelopes and the egg plasma membrane.

Chapter 1 Patterns in Metazoan Fertilization

Abstract: Publisher Summary This chapter discusses the two distinctive areas in the field of metazoan fertilization— namely, sperm penetration and the block to polyspermy. Basically, gamete union is a simple event, involving the fusion of egg and sperm plasma membranes and the formation of a single cell. In sponges and coelenterates, this event remains relatively uncomplicated, but in most metazoans the spermatozoon must penetrate one or more egg investments before starting upon the true process of fertilization. The surface block to polyspermy is found to depend on the changes of a related nature in a wide range of animals, from marine invertebrates to mammals. Species, for which the details are available, include several sea urchins, such as Clypeaster ; the starfish, Asterias; and the polychaete worm, Nereis . A homology between the sperm acrosome and the egg cortical vesicle surely exists in their general structure, in their genesis with the Golgi complex, in their disposition in close proximity to the plasma membrane and the faculty for fusion with it, and in their assorted content of the enzymes and mucopolysaccharides. In the spermatozoon and the egg, the same organelle has, thus, been exploited for different ends, but involving identical mechanisms.