An egg-that took weeks to months to make in the adult-can be extraordinarily transformed within minutes during its fertilization. This review will focus on the molecular biology of the specialized secretory vesicles of fertilization, the cortical granules. We will discuss their role in the fertilization process, their contents, how they are made, and the molecular mechanisms that regulate their secretion at fertilization. This population of secretory vesicles has inherent interest for our understanding of the fertilization process. In addition, they have import because they enhance our understanding of the basic processes of secretory vesicle construction and regulation, since oocytes across species utilize this vesicle type. Here, we examine diverse animals in a comparative approach to help us understand how these vesicles function throughout phylogeny and to establish conserved themes of function.

The biology of cortical granules

Here we characterize the germinal epithelia of both sexes of Strongylocentrotus droebachiensis, the green sea urchin, throughout its annual gametogenic cycle, using light and electron microscopy and cytochemistry. In both sexes, germinal epithelia include two interacting cellular populations: nutritive phagocytes (NPs) and germ cells. After spring spawning, NPs accumulate nutrients; amitotic oogonia and often mitotic spermatogonia occur in clusters beneath NPs; and subsequent gametogenic stages are residual or absent. During the summer, NP nutrients are mobilized for use in vitellogenesis by residual primary oocytes or to support limited spermatogenesis. In addition, some residual primary oocytes may degenerate and be phagocytized by NPs. Significant nutrient mobilization from NPs and substantial gonial cell mitoses (indicative of new gametogenesis) occur in the fall. In both sexes, all of these changes are facilitated by NPs that form basal incubation chambers near the gonadal wall and within which germ ...

Nutritive phagocyte incubation chambers provide a structural and nutritive microenvironment for germ cells of Strongylocentrotus droebachiensis, the green sea urchin.

Publisher Summary This chapter discusses the gametogenesis and reproduction of sea urchins. Gametogenesis and intra-gonadal nutrient storage and utilization are linked processes in sea urchin reproduction. Reproduction is divided into distinct stages based on the prevailing interactions between gametogenic cells and nutritive phagocytes (NPs): (1) inter-gametogenesis and NP phagocytosis, (2) pre-gametogenesis and NP renewal, (3) gametogenesis and NP utilization, and (4) the end of gametogenesis, NP exhaustion and spawning. Dynamically interacting germinal and somatic cellular populations make up the germinal epithelium of the sea urchin gonad. The chapter describes structural changes in the germinal epithelium of the sea urchin gonad using the staging systems based on germinal cells and NPs. Sea urchin gonads grow in size because gametogenesis increases the size and/or the numbers of germinal cells present, and somatic cells within the germinal epithelium—the nutritive phagocytes—store extensive nutrient reserves before gametogenesis begins. The rough handling of sea urchins during harvesting results in the disintegration of the gonads, as fully mature ova or spermatozoa are released. Gametogenesis has a negative influence on the quality of sea urchin gonads.

Chapter 2 Gametogenesis and reproduction of sea urchins

Sixty-seven specimens of the common North Atlantic asteroid, Asterias vulgaris, were collected at seasonal intervals over a 2-year period and their testes observed with both light and electron microscopy. In the germinal epithelium, a predictable series of interactions between versatile somatic cells and germinal cells is repeated annually in relation to sequential events in spermatogenesis. For example, massive proliferation and differentiation of spermatogenic cells depend on the elaboration of thousands of spermatogenic columns, which are distinct cellular subdivisions of the germinal epithelium. Each fully developed column is composed of at least one somatic cell surrounded by ≈ 400 germinal cells. Such columns form only after intensive spermatogonial mitosis begins in the germinal epithelium. Single annual periods of spermatogenic proliferation and differentiation are initiated from 1 to 3 months out of phase in different individuals and overlap incompletely. Therefore, it is possible to observe testes that are entirely in the proliferative phase, entirely in the differentiative phase, or in both phases simultaneously. Detailed ultrastructural observations and preliminary autoradiographic data demonstrate that columns maintain their height for a variable period of time as germinal cells are generated near their bases, pass along their lengths, and differentiate near their tips; therefore, simultaneous proliferation and differentiation of more than one generation of germinal cells occur in the same column. Finally, formation of primary spermatocytes ceases basally, (terminating proliferation), and remaining columns degrade completely as germinal cells composing them differentiate or are phagocytized (terminating differentiation and spermatogenesis); resulting spermatozoa ultimately accumulate in the expandable lumen. It is proposed that spermatogenic columns provide the structural basis for organization of the microenvironment of small groups of spermatogenic cells (≈ 400 at a time) during proliferation and differentiation. Preliminary evidence from A. vulgaris and other species also suggests that somatic cells are temporally pluripotent and are variously involved in the formation, structure, and activities of columns, in extensive phagocytosis, and probably in contributing intrinsic (e.g., 1-methyl adenine and steroids) and mediating extrinsic (e.g., gamete shedding substance and nutrients) microenvironmental factors influential during spermatogenesis in asteroids. The prodigious spermatogenic capabilities of asteroids apparently depend on the generation of spermatogenic columns, on the progressive interaction of germinal and somatic cells before, during, and after columns form, and on the predictable effects of microenvironmental factors received and interpreted at the structural level of the spermatogenic column.

Spermatogenic columns, somatic cells, and the microenvironment of germinal cells in the testes of asteroids

This chapter discusses about Gametogenesis and intra-gonadal nutrient storage and utilization that are linked processes in sea urchins. These processes involve the two cellular populations that make up the germinal epithelium of the sea urchin gonad. Sea urchin gonads grow in size because not only gametogenesis increases the size and/or number of germinal cells present, but also because somatic cells within the germinal epithelium and the nutritive phagocytes store extensive nutrient reserves before gametogenesis begins. Knowledge of these phenomena has lead to successful manipulation of sea urchin reproduction. A thorough understanding of sea urchin reproduction will provide increased opportunities for aquaculture. A diagrammatic representation of the sea urchin reproductive system in aboral side view and gonadal wall is also shown in the chapter.

Reproduction of sea urchins

1. Specimens of the sea urchins, Anthocidaris crassispina and Hemicentrotus pulcherrimus, were collected each month, their ovaries were fixed and embedded in Epon 812, and thick sections were observed with the light microscope.2. The sea urchin ovary has a clearly defined state in which nutrient globules are produced and stored in accessory cells. Especially in Hemicentrotus pulcherrimus, this state continued from May to November. The number of globules in the accessory cells fluctuated with the course of the reproductive cycle.3. Toward the end of the breeding season, strong acid phosphatase activity was detected in the unshed (relict) ova as they degenerated, and numerous large mosaic globules containing cortical granules appeared in the accessory cells. It is proposed that the cytoplasm of relict ova is disrupted by the activity of their lysosomes.

Reiko Masuda

Papers

Studies on the annual reproductive cycle of the sea urchin and the acid phosphatase activity of relict ova