Ultrastructure of the human egg.

This report on the fine structure of human oocyte is based on 20 years research where over 2000 eggs were examined by TEM in conjunction with our research on various methods of assisted reproduction. The eggs were routinely fixed in glutaraldehyde/osmium tetroxide, flat embedded in araldite, serially sectioned and examined by TEM. The oocytes were usually recovered after gonadotrophin stimulation. The general organisation of the mature human oocyte conforms to that of other mammals but has some unique features. The oocyte has the basic cell organelles such as mitochondria, lysosomes, two types of smooth endoplasmic reticulum (SER)-vesicular and tubular aggregates, multivesicular residual bodies, lipofuschin, microfilaments and microtubules. Golgi, RER and ribosomes are very rare and the egg has no yolk. It's surface has few microvilli, pinocytolic caveolae and 1-3 layers of cortical granules. The zona pellucida is composed of fine fibrils and granules embedded in an amorphous matrix and encloses a perivitelline space containing polar bodies. Remnants of corona cell junctions may be found at the oolemma. The metaphase II spindle is often oriented perpendicular to the surface and is barrel-shaped, anastral and lacks centrioles. Osmiophilic centrosomes are not demonstrable in human eggs since the maternal centrosome is inactive. The sperm centrosome organizes mitotic spindles of the embryo after fertilization, whereas in mice the maternal centrosome is active and dominant during cleavage. The stages of peri-ovulatory maturation and differences in oocyte structure during maturation are also presented. Oocytes ageing in culture show progressive swelling of vesicular SER culminating in vacuolation, denser mitochondria (clouding together or associated with vacuoles) peripheral conglomerations or centripetal migration of cortical granules and increased lysosomal activity. Prolonged culture also causes displacement and disorganisation of metaphase II spindles, loss of microtubules and consequent displacement of chromosomes. Evidently the cytoskeleton becomes disorganized. Some observations on oocyte maturation in vitro and spontaneous activation of oocytes are included.