STRUCTURAL AND ENDOCRINE ASPECTS OF EQUINE OOCYTE MATURATION IN-VIVO

The objectives were to describe the ultrastructure of equine oocytes a spirated from small and preovulatory follicles, and to relate the ultr astructural features to follicle size and follicular fluid steroid con centrations. Mares were examined every second day by transrectal ultra sonography, and follicles measuring >30 mm were aspirated (in vivo) us ing a 20-cm-long 12-gauge needle through the flank. Following slaughte r, both large and small follicles were aspirated (in vitro) from six m ares. The oocytes were isolated under a stereomicroscope and processed for transmission electron microscopy, and the follicular fluid was as sayed for progesterone (P4) amd estradiol-17 beta (E2). A total of 29 oocytes (32% recovery rate) were aspirated in vivo, and 15 oocytes wer e recovered in vitro. According to the stage of nuclear maturation, th e oocytes could be divided into the following six categories: 1) the c entral oocyte nucleus (CON) stage, 2) the peripheral spherical oocyte nucleus (PON-I) stage, 3) the peripheral flattened oocyte nucleus (PON -II) stage, 4) the oocyte nucleus breakdown (ONBD) stage, 5) the metap hase I (M-l) stage, and 6) the metaphase II (M-II) stage. The maturati on of the preovulatory follicle was reflected by alterations in the fo llicular fluid concentrations of steroid hormones. E2 was high in all preovulatory follicles, whereas P4 concentration exhibited a 10-fold i ncrease during follicle maturation, particularly associated with the p rogression from M-I- to M-II-stage oocytes. The nuclear oocyte maturat ion included flattening of the spherical oocyte nucleus, followed by i ncreasing undulation of the nuclear envelope, formation of the metapha se plate of the first meiotic division, and, finally, the extrusion of the first polar body and the subsequent formation of the metaphase pl ate of the second meiotic division. The cytoplasmic oocyte maturation changes comprised breakdown of the intermediate junctions between the cumulus cell projections and the oolemma, enlargement of the perivitel line space, the formation and arrangement of a large number of cortica l granules immediately beneath the oolemma, the rearrangement of mitoc hondria from a predominantly peripheral distribution to a more central or semilunar domain, and the rearrangement of membrane-bound vesicles and lipid droplets from an even distribution to an often semilunar do main, giving the ooplasm a polarized appearance. It is concluded that the final equine oocyte maturation includes a series of well-defined n uclear and cytoplasmic changes that are paralleled by an increase in P 4 concentration in the follicular fluid, whereas E2 concentration rema ins constantly high. (C) 1995 Wiley-Liss, Inc.