Cumuli oophori in tissue culture: hormone production, ultrastructure, and morphometry of early luteinization.

The steroidogenic activity of cumulus oophorus granulosa cells obtained from unstimulated rabbits in estrus has been investigated at closely timed intervals in short-term cultures. The presence of viable, structurally well preserved oocytes and the addition of large follicle follicular fluid had no apparent inhibitory effect on progesterone or estrogen synthesis and secretion. Progesterone secretion was especially stimulated by the addition of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) to the culture medium. A similar increment was also seen, although inconsistently, for estrogen secretion. Ultrastructural evidence of luteinization was demonstrated by quantitative morphometric techniques in control cultures as early as 6 hours after explantation and was shown to relate well to biochemical luteinization. It is suggested that the main modifications in cytoplasmic organelles concomitant with luteinization may be represented only by volume and surface changes rather than by alterations in morphology. Therefore, such changes need to be assessed quantitatively. Oocytes exhibited isolated maturative changes almost exclusively in FSH-LH cultures, such as cortical granule synthesis, resumption of meiosis, polar body extrusion, and two-cell parthenogenetic cleavage. It is suggested that cumulus granulosa cells may play a significant role in the resumption of "spontaneous" maturation in oocytes after their release in vitro from their follicles. Characteristic, thin-walled vesicles, either isolated or in rosette-like formation with cumulus granulosa cells, were described. A possible common nature was suggested for these vesicles and the so-called "Call-Exner bodies."