Follicle structure influences the availability of oxygen to the oocyte in antral follicles

Abstract

The ability of an oocyte to successfully mature is highly dependent on intrafollicular conditions, including the size and structure of the follicle. Here we present a mathematical model of oxygen transport in the antral follicle. We relate mean oxygen concentration in follicular fluid of bovine follicles to the concentration in the immediate vicinity of the cumulus-oocyte complex (COC). The model predicts that the oxygen levels within the antral follicle are dependent on the size and structure of the follicle and that the mean level of dissolved oxygen in follicular fluid does not necessarily correspond to that reaching the COC.

Figure 1

The antral follicle with the follicle wall, the antrum, and the COC labelled. In some follicles (particularly smaller follicles), the COC sits partially within the follicle wall; however, in many follicles it rests within the antrum, either next to the follicle wall or attached to it by a “stem” of granulosa cells as shown here.

Figure 2

The model geometry used to represent the antral follicle. This geometry is described using polar coordinates (ρ, θ) with axial symmetry. The origin of the geometry is taken to be the centre of the COC, and thus the origin varies with respect to the antrum and the follicle wall. The geometry is shown for (a) the COC entirely within the antrum and (b) the COC partially enclosed within the antrum and partially within the follicle wall, intersecting the inner boundary of this layer at θ = θ*.

Figure 3

The O₂ concentrations (normalized by c _P) in a follicle of 1 mm radius when (a) the COC rests on the inner boundary of the follicle wall and (b) the COC is a distance of one radius (r _C) away from the follicle wall. Explicit inclusion of the COC in the model shows a decrease in O₂ concentration within the antrum in the vicinity of the COC.

Figure 4

Mean follicular fluid (FF) O₂ concentration versus follicle radius for (A) the COC incorporated into the follicle wall, (B) the COC resting on the inner boundary of the follicle wall, (C) the COC a distance of one radius (r _C) away from the follicle wall, and (D) the COC at the centre of the follicle wall.

Figure 5

(a) Mean O₂ concentration at the surface of the COC versus follicle radius for (A) the COC incorporated into the follicle wall, (B) the COC resting on the inner boundary of the follicle wall, (C) the COC a distance of one radius (r_C) away from the follicle wall, and (D) the COC at the centre of the follicle wall. (b) The ratio of mean O₂ concentration at the surface of the COC to mean follicular fluid (FF) O₂ concentration plotted against follicle radius for the cases when the COC is explicitly included in modelling (cases B–D).