Ultrastructural Studies on Oocyte Development and Vitellogenesis associated with Follicle Cells in Female Scapharca subcrenata (Pelecypoda: Arcidae) in Western Korea

Abstract

Ultrastructural studies on oocyte development and vitellogenesis in oocytes, and the functions of follicle cells during oogenesis and oocyte degeneration were investigated to clarifyb the reproductive mechanism on vitellogenesis of Scapharca subcrenata using electron microscope observations. In this study, vitellogenesis during oogenesis in the oocytes occured by way of autosynthesis and heterosynthesis. Of two processes of vitellogenesis during oogenesis, the process of endogenous autosynthesis involved the combined activity of the Golgi complex, mitochondria and rough endoplasmic reticulum. However, the process of exogenous heterosynthesis involved endocytotic incorporation of extraovarian precursors at the basal region of the oolema of the early vitellogenic oocytes before the formation of the vitelline coat. In this study, follicle cells, which attached to the previtellogenic and vitellogenic oocytes, were easily found. In particular, the follicle cells were involved in the development of previtellogenic oocytes by the supply of nutrients, and vitellogenesis in the early and late vitellogenic oocytes by endocytosis of yolk precursors. Based on observations of follicle cells attached to degenerating oocytes after spawning, follicles of this species are involved in lysosomal induction of oocyte degeneration for the resorption phagosomes (phagolysosomes) in the cytoplasm for nutrient storage, as seen in other bivalves. In this study, the functions of follicle cells can accumulate reserves of lipid granules and glycogen particles for vitellogenesis from degenerating oocytes after spawning.

Keywords: Follicle cells; Oogenesis; Phacosoma japonicus; Vitellogenesis.

Fig. 1. Map showing the sampling area.

Fig. 2. Electron micrographs showing oogenesis of Scapharca subcrenata (A-H).

A, The primary and secondary oogonia in the oogonial phase, with large nucleas, several mitochondria in the cytoplasm; B, A photomicrograph of oogenic follicle, with oogonia, previtellogenic oocytes, early vitellogenic oocytes, follicle cells in the ovary. C, A Previtellogenic oocyte with a nucleolus in the nucleus and several mitochondria in the cytoplasm several follicle cells attaching to a previtellogenic oocyte, D, an early vitellogenic oocyte and the attached follicle cells, with an nucleolus in the nucleus, mitochondria, lipid droplets rough endoplasmic reticulum in the cytoplasm of the oocyte; E, the same stage oocyte, with the Golgi complex containing the Golgi product; F, An eariy vitellogenic oocytes, with several vacuoles and several lipid droplets in the vacuoles; G, An early vitellogenic oocytes, with the coated vesicles formed through the coated pits by endocytosis at the basal region of the oolemma; H, An coated pits by endocytosis at the basal region of the oolemma; H, An early vitellogenic oocytes, with the detached follicle cell (containing a number of vacuoles and myelin-like bodies). Abbreviations: AM, amorphous material, CP, coated pit, CV, coated vesicle; EVO, early vitellogenic oocyte, FC, follicle cell, GPR, golgi product; LD, lipid droplet; M, mitochondria, N, nucleus; NU, nucleolus, OG, oogonium; OL, oolemma, POG, primary oogonium; PVO, previtellogenic oocyte; RER, rough endoplasmic reticulum; SOG, secondary oogonium, V, vacuole.

Fig. 3. Electron micrographs showing vitellogenesis and degeneration in the oocytes of Scapharca subcrenata (A-F).

A, An early vitellogenic oocytes, with a number of lipid droplets near the mitochondria, endoplasmic reticulum in the cytoplasm; B, lipid droplets and glycogen particles in the germinal epithelium in the late vitellogenic oocytes, with modified mitochondria; C, The late vitellogenic oocytes, detached follicle cells with lots of cortical granules in the cortical region; D, a late vitellogenic oocytes, with proteinaceous yolk granules, immature yolk granules and a number of mature yolk granules (the mature yolk granules were composed of three components; (1) a crystalline core, (2) an electronlucent cortex, and (3) a limiting membrane; E, F, degenerated oocytes, with a number of vacuoles, degenerating yolk granules, distended endoplasmic reticulum, a few phagosomes (lysosomes) and lipid droplets appeared in the cytoplasm. Abbreviations: CC, crystalline core; DER, distended endoplasmic reticulum, DO, degenerated oocyte, EVO, early vitellogenic oocyte; GP, glycogen particle; LD, lipid droplet, LVO, late vitellogenic oocyte; M, mitochodrion; MM, modified mitochodrion; MV, microvillus; MVB, multivesicular body; MYG, mature yolk granule; PYG, primary yolk granule; PHA, phagosome; RER, rough endoplasmic reticulum; YP, yolk precursor.