Expression of glucose‐phosphate isomerase in relation to growth of the mouse oocyte in vivo and in vitro

The activity of the enzyme glucose‐phosphate isomerase (GPI‐1) in mouse oocytes is subject to regulation by the cis‐acting gene Gpi‐lta. Electrophoretic analysis of oocytes from 9‐ and 10‐day‐old mice showed that oocyte‐specific regulation of GPI‐1 is not observed in germ cells that have not started to grow (20 μm diameter) but appears as soon as oocyte growth begins (30 μm or larger). Three in vitro culture systems were used to examine the relation of GPI‐1 expression to oocyte growth: culture of intact neonatal ovaries, and co‐culture of dissociated oocytes and somatic cells from neonatal and from 13‐day foetal ovaries. In all three systems modification of GPI‐1 expression always occurred when oocyte growth began, showing that the presence of a normal follicle is not necessary for the expression of the gene Gpi‐lta.