Structural aspects of bovine oocyte maturation in vitro

Abstract

Bovine cumulus-oocyte complexes (COCs) were collected from 4-8 mm follicles and graded into four categories on their morphological characteristics. These four categories were matured in vitro and processed for transmission electron microscopy at 24 h after the onset of culture. The morphology of the four groups of oocytes was analysed and compared with that of oocytes that had matured in vivo and were collected 20-23 h after the preovulatory luteinizing hormone peak. After in vivo maturation, oocytes formed a homogeneous group with respect to their morphological characteristics. After in vitro maturation, the oocytes formed a heterogeneous group with respect to their morphology between as well as within the four categories of oocytes. Oocytes from the first three categories showed the same morphology after in vitro maturation. The fourth category showed some specific characteristics: 1) vacuolization, 2) flattening of cumulus cells, and 3) almost complete lack of cortical granules in some category 4 oocytes. These characteristics are interpreted as signs of degeneration. Besides these aspects of degeneration, other deviations from normal development were seen: 1) retraction of cumulus cell process endings from the oocyte without the breaking down of these processes, 2) retardation of some aspects of the cytoplasmic maturation, and 3) incomplete cumulus expansion. It is concluded that oocytes capable of development in vitro show a large morphological variability before the onset of culture. In vitro maturation systems can support normal development, but many oocytes show signs of degeneration and deviant development after in vitro maturation.

In Vitro Fertilization and Cortical Granule Distribution of Bovine Oocytes Having Heterogeneous Ooplasm with Dark Clusters.

“…The present results agreed with previous findings [4,5] that oocytes having heterogeneous ooplasm with dark clusters have potential for nuclear maturation in vitro comparable to that of oocytes with homogeneous ooplasm. However, the maturation rate of oocytes with heterogeneous ooplasm after 24-hr culture was reported to be significantly higher than that of oocytes with homogeneous ooplasm [12].…”

“…It was shown that the difference in the cortical granule distribution was present in the oocytes with various morphologies before in vitro maturation [5], and was no longer observed after in vitro maturation [4]. However, the difference in the cortical granule distribution in each Values are means ± SD.…”

“…One possible reason for this inconsistency might be the difference in the in vitro maturation system. Oocytes were previously co-cultured with granulosa cells in a large well without an oil covering [4]. In spite of the significant difference in the percentage of cleaved zygotes, the proportion of cleaved zygotes that developed to the blastocyst stage did not differ between the two categories.…”

“…The number of oocytes with heterogeneous ooplasm having dark clusters collected from the ovary is more than double (approximately 25%) that of the oocytes with homogeneous ooplasm [8]. Recent studies have suggested that the oocytes with heterogeneous ooplasm with dark clusters have a similar or higher capacity for in vitro maturation [4,5], fertilization [8,12] and development [2,7,8] than the oocytes with homogeneous ooplasm. However, the previous results were inconsistent and fragmental, since in vitro maturation, fertilization, or subsequent development were separately examined by different groups under the different culture conditions.…”

Factors influencing oocyte and embryo quality in cattle

“…Cumulus expansion is usually more extensive following maturation in vivo [75]. In addition, there is a high degree of homogeneity amongst oocytes matured in vivo at the ultrastructural level; this contrasts with the ultrastructural heterogeneity exhibited by in vitro matured oocytes, even when a uniform population of the latter is selected prior to in vitro maturation [17].…”