Scanning electron microscopy analysis of the human zona pellucida: influence of maturity and fertilization on morphology and sperm binding pattern

Human oocytes from the same as well as from different patients have an extr emely heterogeneous morphology of the zona pellucida surface as shown by sc anning electron microscopy, For years it has been believed that this hetero geneous morphology plays an important part in the sperm-oocyte interaction. It was the aim of this investigation to analyse the morphology and the spe rm binding patterns of the human zona pellucida. Oocytes were divided into four categories: mature, immature, fertilized and unfertilized. Four differ ent types of zona morphology were detectable. They ranged from a porous, ne t-like structure to a nearly smooth and compact surface. No correlation cou ld be established between zona type and oocyte maturity or zona type and ac hieved fertilization. However, fertilized (polyploid) oocytes had a more co mpact and smooth zona surface than unfertilized ones. The analysis of the n umber and distribution patterns of bound spermatozoa on the zona pellucida revealed extremely variable patterns regardless of the zona morphology, Sig nificant differences between mature and immature oocytes did not appear. In both groups there were oocytes with either no or numerous bound spermatozo a on the zona pellucida, Oocytes overloaded with spermatozoa could only be found in the mature group. Unfertilized oocytes had fewer bound spermatozoa on average than polyploid zygotes.