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In vitro model of ewes ovarian follicles: interest and limits

The preantral ovarian follicles of ewe lambs are able to develop to the antral stage within 20 days when grown in vitro. Somatic follicular cells (granulosa and theca) proliferate and differentiate earlier in vitro than in vivo. Oocytes also acquire the ability to finish meiosis earlier. Is this acceleration done at the expense of the quality?

Morphology of follicles developed in vitro. Follicles were classified into one of the five developmental stages according to their diameter and the presence of an antrum © V. Cadoret
By Sylvie André
Updated on 02/22/2018
Published on 01/31/2018

This study compared the morphological, functional and molecular characteristics of granulosa cells and oocytes obtained by a three-dimensional in vitro model of ovine ovarian follicular growth with those of follicles recovered in vivo.
Preantral follicles of 200 μm diameter were recovered and cultured up to 950 μm over a 20-day period.

In vitro development accelerated but less controlled?

Compared with in vivo follicles, the in vitro culture conditions maintained follicle survival, with no difference in the rate of atresia. However, the in vitro conditions induced a slight decrease in oocyte growth rate, delayed antrum formation and increased granulosa cell proliferation rate, accompanied by an increase and decrease in CCND2 and CDKN1A mRNA expression respectively, genes that mark the proliferation acceleration. These changes were associated with advanced granulosa cell differentiation in early antral follicles larger than 400 μm diameter, regardless of the presence or absence of FSH, as indicated by an increase in estradiol secretion, together with decreased AMH secretion and expression, as well as increased expression of GJA1, CYP19A1, ESR1, ESR2, FSHR, INHA, INHBA, INHBB and FST, genes that slow down proliferation. There was a decrease in the expression of oocyte-specific molecular markers GJA4, KIT, ZP3, WEE2 and BMP15,  in vitro compared to that in vivo. Moreover, a higher percentage of the oocytes recovered from cultured follicles 550 to 950 μm in diameter was able to reach the metaphase II meiosis stage.
Overall, this in vitro model of ovarian follicle development is characterized by accelerated follicular maturation, associated with improved developmental competence of the oocyte, compared to follicles recovered in vivo.

References

Cadoret V, Frapsauce C, Jarrier P, Maillard V, Bonnet A, Locatelli Y, Royère D, Monniaux D, Guérif F, Monget P 2017 Molecular evidence that follicle development is accelerated in vitro compared to in vivo. Reproduction. 153:493-508 http://dx.doi.org/10.1530/rep-16-0627