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Following the preovulatory surge of gonadotropins, the compact layer of cumulus cells in the antral follicle secretes a hyaluronic acid-enriched extracellular matrix and undergoes a morphological change referred to as cumulus expansion. It has been previously shown that a soluble factor(s) produced by the oocyte is required, in combination with FSH, to promote this process. Since such matrix is sensitive to proteases we have now studied the effect of the oocyte on another gonadotropin-controlled follicle cell function, i.e., the synthesis of plasminogen activator (PA). Our data indicate that isolated cumulus cells secrete uPA in the medium and that FSH or dbcAMP increases this production. The presence of the oocyte or the oocyte-conditioned medium greatly reduces uPA synthesis induced by FSH and dbcAMP in cumulus cells by modulating the abundance of its mRNA. The ability of the mouse oocyte to produce such a factor(s) is dependent upon its stage of development, with fully grown oocytes but not growing oocytes or two-cell embryos being able to inhibit uPA synthesis. A preliminary characterization of this factor suggests that it is a heat-unstable protein with an apparent molecular weight above 100 kDa. Thus, the mouse oocytes appear to promote preovulatory matrix accumulation that occurs just prior ovulation by modulating the gonadotropin action on both the synthesis and the degradation of specific matrix component.

A soluble factor(s) produced by fully grown oocytes is essential, together with follicle stimulating hormone (FSH), to stimulate in vitro hyaluronic acid (HA) synthesis by mouse cumulus cells (CCs). The stability of the response to this stimulus by CCs in culture was investigated. The data showed that preculture for 8 hr in basal medium reduced to approximately 30% the ability of CCs to synthesize HA in response to FSH or dibutyryl cyclic AMP (Bt2cAMP) and soluble oocyte factor(s). However, if CCs were precultured for the same period of time as intact cumulus cell-oocyte complexes, or in the presence of fully grown oocytes, or in medium conditioned by fully grown oocytes, their ability to synthesize HA was 75-95% preserved. In vitro stimulation of dermatan sulfate (DS) synthesis by CCs does not require oocyte factors and is induced by FSH or Bt2cAMP treatment alone. However, the preservation of such activity, like that of HA synthesis, depended on the presence of a soluble oocyte factor(s) during preculture. The presence of isolated oocytes or of oocyte-conditioned medium also prevented the spreading of CCs in culture. However, inhibiting CC spreading by culture on agar-coated plates or in serum-free medium did not preserve their HA or DS synthetic activity, thus suggesting that the two oocyte actions on CCs are independent. Growing oocytes were unable both to induce HA synthesis in freshly isolated CCs stimulated with FSH and to preserve the ability to synthesize HA and DS in 8-hr precultured CCs. The results suggest that the stability of the differentiated state of mouse CCs in vitro depends upon continued exposure to a soluble factor(s) produced by fully grown oocytes.

gamma-Aminobutyric acid (GABA)-sensitivity was studied in mouse and human oocytes using electrophysiological techniques. Isolated mouse oocytes at the germinal vesicle (GV) or metaphase II stage, and human oocytes at the GV stage or following resumption of meiosis in culture, were sensitive to GABA. The transmitter usually hyperpolarized the membrane, with a concomitant decrease followed by an increase in membrane conductance, at threshold concentrations as low as 10(-10) M. GABA response was not evoked in Cl-free medium. Bicuculline (10(-5)-10(-6) M) reversibly blocked GABA (10(-9)-10(-4) M) responses. In contrast mouse fertilized eggs were insensitive to GABA at concentrations of 10(-5) M or lower. A possible biological role of the neurotransmitter GABA is discussed.