The ultrastructure of human cumulus-corona cells at the time of fertilization and early embryogenesis. A scanning and transmission electron microscopic study in an in vitro fertilization program.

Abstract

We observed the ultrastructure of the cumulus-corona cells (CC cells) surrounding: 1) human preovulatory oocytes unfertilized after in vitro insemination and 2) in vitro-fertilized polypronuclear ova (PO) at the pronuclear stage (3 pronuclei) and during early cleavage, at the 3-8 cell stage (cleaving PO). All the samples were obtained from women who underwent pharmacological hormonal stimulation during in vitro-fertilization procedures. Both cell groups were composed of irregularly rounded CC cells, showing an oval nucleus with one or more reticular nucleoli. Spermatozoa in close contact with CC cells were also seen. Linear and annular gap junctions between neighbouring cells were present, particularly in Group 1. Lipid droplets were present in both groups, appearing slightly more numerous and electron-dense in Group 2. In Group 1, mitochondria were numerous, polymorphic, and provided with cristae varying from lamellar to tubular. In Group 2, mitochondria also showed polymorphism, with bacilliform organelles with tubular cristae being predominant. In both groups cisternae and associated vacuoles and vesicles belonging to the Golgi complex were scattered in the cytoplasm of CC cells. Similarly, tubular and vesicular profiles of smooth endoplasmic reticulum were abundant and uniformly distributed throughout the cytoplasm of CC cells of both groups. In contrast, the abundant rough endoplasmic reticulum in Group 1 was formed by parallel stacks of flattened cisternae, whereas it was less plentiful and not arranged in stacks in Group 2. The CC-cell surface appeared covered by numerous membrane expansions in both groups. The expansions in Group 1 were mainly composed of blebs of various sizes and a few short microvilli, whereas in Group 2 numerous microvilli covered the cell surface. These observations demonstrate that a gradual establishment and maintainance of a steroidosynthetic capability (luteinization) takes place in CC cells, particularly during and shortly after fertilization, as occurs contemporaneously in the granulosa cells of the postovulatory follicle. Our results may be considered as ultrastructural confirmation of the capability of the CC cells to produce small amounts of steroids (estrogens and mainly progesterone). These hormones, alone or together with other substances (proteins, nutrients, growth factors?), might--around the fertilization time--act positively upon the early embryo itself, as well as on the microenvironment in which the embryo develops, both in vivo and in vitro conditions.