Follicle environment and quality of in vitro matured oocytes

Abstract

In mammalian reproduction, the oocyte depends on the ovarian follicle for most of its growth. They form a bipolar partnership and the status of one will impact the functioning of the other. When oocytes are removed from their follicle by ovulation, they have normally completed all the steps required to begin their journey into the oviduct and drive the early embryonic development. When oocytes are removed from their follicle before natural ovulation, the process by which they acquire all the important components for their journey might not be completed and their ability to mature, fertilize or develop into embryos or to term might be compromised. Animal models have been useful to define the important steps required for the oocyte's growth phase, and in the mouse, when the oocyte has reached its full size, the program is ready. This is not the case in larger mammals where the completion of growth does not ensure that the oocyte is fully capable of undergoing all the steps to the embryo and to term. The final steps of oocyte preparation also involve a progressive condensation of the chromatin that may facilitate normal maturation but may also indirectly reduce the lifespan of the oocyte. In such a scenario, the oocyte would have an expiration date when fully competent. In humans, a number of indications may justify the aspiration of oocytes from unstimulated patients and the development of an in vitro maturation (IVM) process that would allow fertilization and subsequent development. This objective could be realized by a better understanding of the essential follicular contribution required before removing the oocyte. Therefore, this review will focus on the large animal models where IVM has been used and studied for more than 25 years. The status of the follicle at the time of oocyte recovery and the status of the oocyte's chromatin will be described in detail as they have a significant impact on the outcome.

Role of gap junction-mediated communications in regulating large-scale chromatin configuration remodeling and embryonic developmental competence acquisition in fully grown bovine oocyte

“…In particular, in human ART this must be taken into account when considering the origin of the oocyte after repetitive FSH treatment [11]. In this case, success in attempting to improve the oocyte developmental capability in vitro may be strongly dependent on the large-scale chromatin configuration and junctional coupling status once the oocytes have been recovered from the follicle.…”

“…Thus, if coupling is prematurely interrupted -i.e., when oocytes have not yet acquired full competence-sudden chromatin condensation can be triggered, thus preventing proper and gradual differentiation of large-scale chromatin configuration and function [11]. In view of all given considerations, knowledge of the molecular mechanism(s) leading the oocyte to remodel its chromatin configuration under physiological conditions will be beneficial for ARTs.…”

“…In assisted reproductive technologies (ART), the physical removal of the cumulus-oocyte complex (COC) from middle size antral follicles initiate oocyte's meiotic resumption before the journey of the oocyte development has been completed and causes asynchrony between cytoplasmic and nuclear maturation, thus decreasing oocyte developmental competence (Reviewed in [9][10][11]). Spontaneous resumption of meiosis is mainly due to a decrease in the concentration of intraoocyte cyclic adenosine monophosphate (cAMP), which has a major role in controlling mammalian oocyte maturation [12][13][14].…”

Intracellular glutathione content, developmental competence and expression of apoptosis-related genes associated with G6PDH-activity in goat oocyte

“…It is widely accepted that oocyte quality is a key factor for optimizing the efficiency of reproductive techniques in animal and human assisted reproductive technologies (ARTs) [1][2][3]. Visual assessment of the compactness of the cumulus investment as well as the homogeneity of the ooplasm is routinely used for selecting the more competent oocytes in ART programs [3].…”

Obtaining farm animal embryos <i>in vitro</i>

“…Mature oocytes (Metaphase II) can be isolated, although in the majority of cases, immature ones are harvested (Miyano, 2003). The frequency of oocyte isolation is often a function of ovarian follicle development and maturation, which is species specific (Hendriksen, 2000;Hirao, 2011;Sirard, 2011).…”