Ultrastructural changes during meiotic maturation in mammalian oocytes: Unique aspects of the human oocyte

Abstract

This paper reviews the process of peri-ovulatory oocyte maturation and the ultrastructural organization of the human egg and compares it with that of the mouse. The main thrust of the paper is on the human, since there are several reviews on the mouse. Both preovulatory and postovulatory events at fertilization, as well as some of the aberrant features of maturation are covered. Some changes induced by oocyte culture and cooling in the human are also included. The report attempts to focus on unique features of the human oocyte and shows a variety of ultrastructural differences between human and murine oocytes, which may well reflect differences in their physiology and biochemistry. Based on these differences and further observations on the process of fertilization of both species, particularly with respect to the inheritance of paternal centrioles, it is concluded that the mouse may not be a suitable model for the development and refinement of current procedures in human assisted reproductive technology.

Ultrastructural analysis of egg membrane abnormalities in post-ovulatory aged eggs

“…The pits in aged fertilized mouse eggs seem similar to the "membrane-delimited craters…termed cortical crypts" observed in zebrafish eggs, structures which are described as remnants of exocytosed cortical granules (Becker and Hart, 1999). The vesicles observed here on aged eggs appear to be membrane blebs, in agreement with reports of membrane blebbing, abnormally shaped microvilli, and release of cytoplasmic fragments observed in aged rabbit, hamster and human eggs (Longo, 1974a, Longo, 1974b, Sathananthan, 1994, and similar to membrane blebs observed in mouse eggs treated with the microfilament disruptor cytochalasin B (Longo and Chen, 1985). Interestingly, cytoplasmic and DNA fragmentation has been observed in aged eggs activated with an extract of soluble sperm (Gordo et al, 2002, Gordo et al, 2000, suggesting that sperm-induced Ca 2+ signaling can trigger apoptosis-like responses in aged eggs; the membrane abnormalities observed here in fertilized aged eggs may be linked with the way an aged eggs responds to increased cytosolic Ca 2+ , clearly differing from the responses in a young, healthy egg.…”

“…This polarity in the mature egg develops during oocyte maturation. The surface of pre-ovulatory prophase I oocytes in rodents and in humans is characterized by uniformly distributed microvilli (Sathananthan, 1994). As the oocyte progresses from prophase I arrest into meiosis I, the meiotic spindle forms in the central position (where the germinal vesicle was localized), and then migrates to a more peripheral location, such that the spindle is now positioned for the asymmetric cytokinesis that occurs with completion of meiosis I.…”

“…Studies of mouse eggs have reported no significant morphological surface changes developing with post-ovulatory age (Longo, 1981, Szollosi, 1971, while aged eggs in other species (rabbit, hamster, human) have noteworthy abnormalities on the egg surface, including membrane blebbing, abnormally shaped bulbous microvilli, and release of fragments containing organelles such as cortical granules and endoplasmic reticulum (Longo, 1974a, Longo, 1974b, Sathananthan, 1994. It should be noted that spontaneous activation often accompanied post-ovulatory aging in hamster eggs (Longo, 1974a), and thus it is unknown if the changes that were observed due to agingassociated changes, activation-associated changes, or a combination.…”

Comparative Studies on Lipid Analysis and Ultrastructure in Porcine and Southern Minke Whale (Balaenoptera Bonaerensis) Oocytes

“…Lipid droplets interact with the elements of the cytoskeleton in the oocytes. These interactions are destroyed during cryopreservation [16,17], which causes irreversible damage to the oocyte. In the present study, the lipid contents were expressed both per oocyte and per volume as shown in Table 1.…”

ART failure: oocyte contributions to unsuccessful fertilization

“…One such event involves specialized organelles, known as cortical granules. In response to Ca 2þ rise, cortical granules migrate toward the oolemma to release their enzyme contents into the perivitelline space in a process known as the cortical reaction (Sathananthan et al, 1985;Sathananthan, 1994). Cortical granule release results in alteration of zona pellucidae, cleaving ZP2 to ZP2f, and modifying ZP3, thus preventing further sperm binding and polyspermic penetration (Ducibella et al, 1990;Schroeder et al, 1990).…”