Fine structure of the micropylar cell and its change during oocyte maturation in the chum salmon, Oncorhynchus keta

Abstract

In the ovarian follicle, the micropylar cell (MPC) is distinguished from neighboring granulosa cells by its larger cell size and its thick cytoplasmic process. The micropylar cell body fits into a shallow depression (micropylar vestibule) on the outer surface of the egg envelope; its process extends through the micropylar canal, which extends from the bottom of the vestibule through the full thickness of the zona pellucida interna. At its distal end, the cell process expands into a bulb which fits into an indentation of the ooplasmic surface immediately beneath the inner opening of the micropylar canal. Intermediate and desmosomelike junctions establish an intimate association between MPC process and oocyte. Various kinds of organelles and inclusions in the MPC show a characteristic pattern of cytoplasmic distribution; rough endoplasmic reticulum with markedly dilated cisternae is found exclusively in the main cell body, while microtubules and thin filaments are observed in the cytoplasmic process. Immediately before or during the breakdown of the germinal vesicle in the intrafollicular oocyte, the cytoplasmic process of the MPC gradually decreases in length and begins to withdraw from the micropylar canal. At the same time, the ooplasmic surface protrudes outward to form a papilla in the canal. The intimate MPC-oocyte association disappears during formation of the ooplasmic papilla. Hydration of the oocyte apparently occurs at the final stage of maturation and probably participates in papilla formation. Although the MPC undergoes degenerative changes as ovulation draws near, it remains attached to the inner surface of the granulosa cell layer even after its association with the oocyte has completely disappeared. We speculate that the micropyle develops during fish oogenesis through the combined activity of the MPC and neighboring granulosa cells. It appears that the cell body of the micropylar cell and nearby granulosa cells exert mechanical pressure on the external surface of the growing oocyte and thus participate in formation of the micropylar vestibule. The cytoplasmic process of the MPC evidently forms a passive barrier to deposition of material for the egg envelope in the animal pole, thereby resulting in formation of the micropylar canal.

Communications of Oocyte‐Granulosa Cells in the Chum Salmon Ovary Detected by Transmission Electron Microscopy

“…A similar association was observed in the oocyte-micropylar cell relationship in chum salmon follicles (21). Since desmosomes are known to be important in cell-to-cell adhesion in general (29), they may serve to fasten the granulosa cell or the micropylar cell to the surface of the intrafollicular oocyte.…”

Structural analysis of fertilization in the fish Brycon orbignyanus

“…The larger granules located in the cytoplasm opposite to the animal pole resulted from the coalescence of yolk granules. Kobayashi & Yamamoto (1985) observed that yolk coalescence in Oncorhynchus keta began at the vegetative pole, while the nuclear membrane was disintegrated.…”

Factors Controlling Sperm Entry into the Micropyles of Salmonid and Herring Eggs

“…If the micropyle area of the egg is covered by another egg or foreign object, no spermatozoa would gain access to the micropyle. It is tempting to speculate that a large micropylar cell, which is believed to play an important role in the formation of micropyle during oogenesis (13,21), produces the SGF although there has been no evidence to support or deny this view.…”