I. Differentiation of the Lutein Cell from the Granulosa Follicle Cell during the Preovulatory Stage and under the Influence of Exogenous Gonadotrophins

The granulosa follicle cell of the Graafian follicle of the rabbit ovary differentiates into a lutein cell involved in steroid synthesis. Cytological events which occur within the granulosa cell of the normally stimulated follicle prior to ovulation have been duplicated by the intrafollicular injection of exogenous gonadotrophin. The luteinization of the granulosa cells involves the accumulation of 250to 300-A, electron-opaque, spherical granules, dispersed within the cytoplasmic matrix, which have been identified as glycogen with the PASstaining procedure. Further development of the granulosa cell following ovulation involves an increase in cell size, a decrease in the number of RNP particles, and an accumulation of an abundant system of intracellular membranes (agranular endoplasmic reticulum). Glycogen granules first appear in the granulosa cells as the separate, monoparticulate form. After follicle rupture and the formation of agranular endoplasmic reticulum, glycogen particles are present in a rosette arrangement within membrane-bounded vacuoles. The rosette arrangement of glycogen particles is also found dispersed within the cytoplasmic matrix of the lutein cell during the later stages of the cell life-span. Injection of luteinizing hormone or human chorionic gonadotrophin into a mature follicle also produces a marked accumulation of monoparticulate glycogen in the majority of granulosa ceils, within 30 rain. Cytoplasmic extensions which contain the glycogen masses are noticeably free of RNP particles. I N T R O D U C T I O N The ovary is a heterogeneous tissue containing endocrinologically active structural subunits including the Graafian follicle and corpus luteum which provide an ideal system to study the differentiation, maturation, and degeneration of cells involved in steroid synthesis and secretion. A timed series of stages during cell differentiation prior to follicle rupture may be obtained from the rabbit, since it is a reflex ovulator. Recent investigations, both in vivo and in vitro, have provided information concerning the role of the vesicular follicle in steroid metabolism (12, 16). Although preovulatory progestin release after mating (10) or gonadotrophic stimulation (16, 17) has been de501 on O cber 9, 2017 jcb.rress.org D ow nladed fom termined in the rabbit, the cellular site of origin of this secretion is not known. Both the theca follicle cells and granulosa cells have been implicated in the formation of steroid metabolites present in the follicular fluid (9, 12). The synthetic activity of the granulosa cells has been established in the formation of mucopolysaccharides using S 3~ as a tracer (24) and in relation to protein turnover in the follicular fluid (23). Variat ion in function is to be expected between the parietal granulosa cells and the cells surrounding the oocyte that comprise the corona radiata, which is released with the secondary oocyte at follicle rupture. The parietal granulosa cells remain, hypertrophy, and reorganize into a compact lutein body (6, 32). I t is these cells that have been examined in the present study. Although gonadotrophins such as luteinizing hormone (LH) and human chorionic gonadotrophin (HCG) have been shown to stimulate steroid synthesis in vitro and in vivo (11, 16) and also follicle rupture in vivo (14), their effect on cellular differentiation has not previously been elucidated. I t is the purpose of this study to report the fine structural changes which occur in the granulosa cell compar tment of the preand postovulatory follicles in the rabbit : ( 1 ) during the differentiation of the granulosa cell into a lutein cell, and (2) after a local injection of gonadotrophins into the follicle. Subsequent reports will deal with structural changes in the cells of other ovarian compar tments during various physiological conditions