Gl. Xia et al., CUMULUS CELLS SECRETE A MEIOSIS-INDUCING SUBSTANCE STIMULATION WITH FORSKOLIN AND DIBUTYRIC CYCLIC ADENOSINE-MONOPHOSPHATE, Molecular reproduction and development, 39(1), 1994, pp. 17-24
The role of the cumulus cells in initiating the resumption of meiosis
after exposure to forskolin and dbcAMP was studied in the mouse. The r
esumption of meiosis was monitored by the percentage of germinal vesic
le breakdown (GVBD) and polar body formation (PB). The cumulus-enclose
d oocytes (CEO) and denuded oocytes (DO) were cultured with and withou
t hypoxanthine (HX) in the culture medium. Three types of experiments
were performed: (1) Effect of forskolin on spontaneous resumption of m
eiosis, i.e. cultures without HX, and two experiments in which HX is p
resent throughout the culture: (2) Effect of transient exposure to for
skolin or dibutyric-cyclic adenosinemonophosphate (dbcAMP) on GVBD pri
or to continued culture without forskolin or dbcAMP (oocyte priming).
(3) Priming of CEO with forskolin for 2 hr, separation of cumulus cell
s and oocytes, followed by coculture of rejoined cumulus cells and ooc
ytes, or coculture of the cumulus cells and new, unprimed DO. (1) Fors
kolin inhibited a spontaneous resumption of meiosis in a dose-dependen
t manner during the first 5 hr of culturing. After 22 hr all controls
and CEO resumed meiosis, whereas only half of the DO did. (2) At least
1 hr of priming the CEO with forskolin is needed to induce GVBD and P
B formation, but forskolin inhibited the resumption of meiosis when pr
esent for 24 hr. Similar results were obtained with a high concentrati
on of dbcAMP. (3) A separation and rejoining of oocytes and cumulus ce
lls after priming induced the resumption of meiosis in a significantly
greater number of oocytes than in the control oocytes which were not
primed. The GVBD of unstimulated DO also increased significantly when
cocultured with cumulus cells from primed CEO. The percentage of GVBD
in unprimed DO and in DO isolated from primed CEO was the same. We sug
gest that within 1-2 hr, forskolin and cAMP stimulate cumulus cells to
produce a diffusible meiosis-inducing substance which overcomes HX-in
hibition and induces oocyte maturation, including both GVBD and PB for
mation. The CEO must be primed for more than 2 hr before the resumptio
n of meiosis in DO isolated from such CEO is induced. Oocyte-cumulus c
onnections are crucial as far as initiating the production of a meiosi
s-inducing substance is concerned. Oocyte-cumulus connections are not
needed for transferring this substance to the oocyte. (C) 1994 Wiley-L
iss, Inc.