Cumulus expansion, chromatin configuration and meiotic competence in horse oocytes: a new hypothesis.
Abstract: When recovered from the follicle, horse oocytes may be categorised as having either a compact or an expanded cumulus. Cumulus expansion is strongly associated with follicle atresia. Oocytes with expanded and compact cumuli have similar proportions in the germinal vesicle stage when recovered from the follicle. However, during in vitro culture, a higher proportion of oocytes with expanded cumuli mature, and they do so more quickly, than do oocytes with compact cumuli. Using Hoechst 33258 to label chromatin, in the germinal-vesicle stage horse oocytes can be divided into those in which the nucleus fluoresces diffusely (FN), and those in which the chromatin is condensed as a mass within the germinal vesicle (condensed chromatin; CC). The CC configuration is more common in oocytes with expanded rather than compact cumuli. The occurrence of the condensed chromatin configuration in oocytes with compact cumuli rises with increasing follicle size to the point that, in follicles >20 mm diameter, almost 80% of oocytes with compact cumuli have condensed chromatin. This suggests that the chromatin of fully grown oocytes condenses prior to the gonadotrophin stimulation that induces ovulation. The number of oocytes maturing in vitro correlates strongly with the presence of the condensed chromatin configuration at the beginning of culture for both cumulus types. This suggests that the condensed chromatin configuration represents the meiotically competent oocyte. In contrast, FN oocytes appear to be meiotically incompetent, although 61% of oocytes from small, apparently viable follicles, have the FN configuration. This leads to the hypothesis that the FN configuration represents viable germinal vesicle stage oocytes that have yet to acquire meiotic competence. As oocytes achieve meiotic competence, the chromatin appears to condense to the CC configuration. This change occurs during growth of the preovulatory follicle but it can also occur during follicular atresia. Information about the inherent meiotic competence of different populations of oocytes will assist in the selection of oocytes for experiments involving in vitro maturation and fertilisation in equids.
Publication Date: 1998-05-21 PubMed ID: 9593526DOI: 10.1111/j.2042-3306.1997.tb05098.xGoogle Scholar: Lookup
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- Journal Article
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- U.S. Gov't
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Summary
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The research involved the study of horse oocytes and their varying chromatin levels, to understand the factors associated with their ability to mature and be fertilized. The data showed that oocytes with expanded cumuli mature more quickly and that chromatin condensation within the oocyte is an indicator of its readiness for meiotic, i.e., reproductive, activity.
Understanding Horse Oocytes
- The study focused on horse oocytes (egg cells) and examined them with respect to two variables: the condition of their cumulus, a protective layer of cells, and their chromatin configuration.
- Oocytes can either have a rigid (compact) or a more relaxed (expanded) cumulus; it was noted that an expanded cumulus is more common in instances where the follicle, which houses the oocyte, has undergone atresia or deterioration.
- However, it was observed that oocytes with both types of cumuli were found in the germinal vesicle stage (early stage of an egg cell’s life) when they were recovered from the follicle. Despite this similarity, in vitro or artificially stimulated culture revealed that oocytes with an expanded cumulus tend to mature more rapidly and in greater numbers than their counterparts with a compact cumulus.
The Role of Chromatin
- The researchers used a dye called Hoechst 33258 to label chromatin, which is the substance within a cell that contains DNA and proteins.
- Oocytes could be classified into two types based on their chromatin structure: FN (fluorescent nucleus), where the nucleus of the cell is diffused and illuminates under the dye, and CC (condensed chromatin), where the chromatin is gathered and appears as a mass under the dye.
- More oocytes with expanded cumulus were observed to have condensed (CC) chromatin. Furthermore, it was noted an increase in appearance of CC chromatin as follicle size grew, suggesting that full-grown oocytes ready for ovulation tend to have condensed chromatin.
Chromatin Configuration and Meiotic Competence
- The study found a clear association between the occurrence of condensed chromatin and an oocyte’s ability to mature or achieve meiotic competence, suggesting that CC oocytes are more suitable for the reproductive process.
- In contrast, FN oocytes, despite being predominant in healthy, small follicles, were generally incompetent for meiosis, leading to the hypothesis that such oocytes are still in the early stages of their development and not yet ready for reproduction.
- Even compact cumuli can transform into the CC configuration, either through the growth of the preovulatory follicle or through follicular atresia. This implies that an oocyte’s meiotic competence can perhaps be enhanced or established via controlled interventions.
- This understanding of oocyte chromatin configuration and meiotic competence could be beneficial in selecting the right oocytes for in vitro fertilization experiments in horses.
Cite This Article
APA
Hinrichs K.
(1998).
Cumulus expansion, chromatin configuration and meiotic competence in horse oocytes: a new hypothesis.
Equine Vet J Suppl(25), 43-46.
https://doi.org/10.1111/j.2042-3306.1997.tb05098.x Publication
Researcher Affiliations
- Tufts University School of Veterinary Medicine, North Grafton, Massachusetts 01536, USA.
MeSH Terms
- Animals
- Bisbenzimidazole / chemistry
- Chromatin / chemistry
- Chromatin / ultrastructure
- Female
- Fertilization in Vitro / methods
- Fertilization in Vitro / veterinary
- Fluorescent Dyes / chemistry
- Horses / physiology
- Meiosis / genetics
- Meiosis / physiology
- Oocytes / physiology
- Oocytes / ultrastructure
- Ovarian Follicle / physiology
References
This article includes 31 references
Citations
This article has been cited 4 times.- Yao MH, Cheng WJ, Liu LW, Zheng H, Gu WY, Miao F, Zhang JF, Wang L, Su YP, Liu YL, Sui HS. Relationship between chromatin configuration and in vitro maturation ability in guinea pig oocytes. Vet Med Sci 2021 Nov;7(6):2410-2417.
- Bogolyubova I, Bogolyubov D. Heterochromatin Morphodynamics in Late Oogenesis and Early Embryogenesis of Mammals. Cells 2020 Jun 19;9(6).
- Abdoon AS, Abdel-Rahman HA, Shawki SM, Kandil OM, Fathalla SI. Influence of follicle size, methods of retrieval on oocytes yield and morphology in Egyptian Jennies ovaries with special reference to maturation rate in vitro. Vet Res Commun 2014 Dec;38(4):287-95.
- Luciano AM, Lodde V, Franciosi F, Tessaro I, Corbani D, Modina S. Large-scale chromatin morpho-functional changes during mammalian oocyte growth and differentiation. Eur J Histochem 2012 Aug 10;56(3):e37.
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