Cytochemical demonstration of modification of carbohydrates in the mouse zona pellucida during folliculogenesis.

In the present study, lectin-gold cytochemistry and antibodies against ZP2 and ZP3 glycoproteins were used to investigate the oligosaccharide content of mouse ovarian zona pellucida (ZP) during follicular development. The entire thickness of the ZP and several organelles of the oocyte (cortical granules, Golgi apparatus, and vesicular aggregates) were reactive to RCA-I, DSA, AAA, WGA, MAA, and LFA throughout follicular development. HPA labeling was not detected at the earliest stages of follicular folliculogenesis. HPA reactivity was first observed in the ZP, Golgi apparatus, and the vesicles of oocytes at the trilaminar primary follicle stage. HPA labeling in the ZP was always restricted to the inner region of the zona matrix. After neuraminidase treatment, HPA reacted with the entire ZP in ovarian follicles at different stages of development. Immunolabeling with specific antibodies showed that, although ZP2 and ZP3 glycoproteins were uniformly distributed in the zona matrix of ovarian oocytes, there was a progressive increase in thickness of the ZP in parallel with the proliferation of follicular cells. ZP3 glycoprotein was also localized to the Golgi apparatus and vesicular aggregate. The present results suggest: (1) a difference in composition of carbohydrate content between the inner and outer region of the fully developed ZP generated probably by a modification in the biosynthetic pathway of oligosaccharides in the oocyte during folliculogenesis, (2) that newly synthesized ZP glycoproteins displace previously synthesized ZP components in a direction toward the follicular cells and, therefore, no redistribution of the ZP matrix occurs during folliculogenesis, and (3) that the vesicular aggregates in the ooplasm constitute an intermediate step in the secretory pathway of ZP glycoproteins.