Rat oocytes in early tertiary follicles can attain appropriate maturation in combination culture system.

We have developed a "combination culture system", composed of primary differentiated granulosa cells, proestrus anterior pituitary lobe and delta-4-androstenedione. This cultured system effectively induce the maturation of oocytes from early tertiary follicles of diethylstilbestrol (DES)-treated rats. After DES stimulation, an average of 189 oocytes were obtained from early tertiary follicles of unilateral ovaries; 78% of these oocytes, when cultured in this culture system, extruded the first polar body, a criterion of oocyte maturation. Of the mature oocytes, the rate of normal fertilization and egg cleavage were 88% and 93%, respectively. After 96 h of culturing in vitro, 59% of the zygotes developed into morulae or blastocysts. Embryos implanted at the two-cell stage were able to develop into healthy individuals. Thus the combination culture system provides an experimental model in which the molecular mechanisms of egg maturation, fertilization, and embryonic development can be studied.