Preantral follicle population and distribution in the horse ovary

Abstract

Characterization of the ovarian preantral follicle population is a necessary step to improve understanding of folliculogenesis and ovarian physiology. Therefore, in the present study, the preantral follicle population in the equine ovary in young and old mares was investigated according to follicular morphology, follicular class, distance from the geometric center using ovarian maps, and follicular density within ovarian portions (lateral vs. intermediary) and regions (dorsal vs. ventral). Ovaries were collected from an abattoir and histologically processed for evaluation, and the follicle population was calculated. Overall, in the current detailed study, a higher preantral follicle population per mare ovary (mean, 82,206 ± 50,022; range, 1,477 to 773,091) than originally reported was identified. Additionally, a mare age effect was observed on the follicle population (young: 152,664 vs. old: 11,750) and the spatial distribution of morphologically normal and abnormal follicles and the density and population of follicular classes. These results demonstrate that, in addition to the preantral follicle population in the mare ovary being comparable to that of other species, the location and spatial distribution of these follicles is dynamic and varies depending on mare age and follicle status (i.e., morphology and developmental stage). The characterization of the distribution and population of preantral follicles in the mare ovary provided by this study can potentially aid in improving reproductive studies and assisted reproductive techniques and may expand the understanding of mechanisms involving ovarian plasticity and follicular migration.

Characterization of preantral follicle clustering and neighborhood patterns in the equine ovary

“…Interestingly, the aforementioned studies reported that follicular clustering increases with age in both species, perhaps due to expected decreases in follicle density and regular follicular activity over time [ 31 ]. However, in the present study, it was found that clustering decreased with age, a finding that is supported by the fact that in old mares, the spatial distribution of follicles exhibits a more dispersive pattern [ 18 , 19 ]. The working hypothesis of ovarian plasticity, in which preantral follicles are suggested to migrate throughout the ovarian cortex, postulates that the rigidity of ovarian tissue is a key component of this migration [ 17 ].…”

“…The working hypothesis of ovarian plasticity, in which preantral follicles are suggested to migrate throughout the ovarian cortex, postulates that the rigidity of ovarian tissue is a key component of this migration [ 17 ]. It has been proposed that primordial follicles may migrate from the more rigid outer regions of the ovarian cortex to the softer, less rigid regions of the cortex, closer to the ovarian geometric center, as they initiate and continue development to the primary stage [ 17 – 19 , 30 ]. However, it has been shown in the ovaries of women that levels of substances that contribute to rigidity and elasticity of the ovarian extracellular matrix (i.e., collagen, elastin, fibrillin) change as aging progresses [ 32 – 34 ].…”

“…Specifically, a decrease in clustering in older mares was observed in the lateral portion and dorsal region of the ovary. Previous work by our laboratory showed that the lateral portion and dorsal region, an outer location of the ovary proposed to be more rigid [ 17 , 30 ], had a high density of preantral follicles in young mares [ 19 ]. It is known that overall collagen levels in the ovary increase and levels of hyaluronic acid, elastin, and fibrillin decrease with age in mice and women [ 33 , 34 ], leading to increased ovarian tissue rigidity; however, it is unknown if this rigidity is homogenous.…”

“…Images of all mounted histological sections were captured and digitized in a photo-editing program (Adobe Photoshop CS4; San Jose, USA). Next, a longitudinal line was drawn through the mathematically determined geometric center [ 19 ] using a marking tool in the editing program (Adobe Photoshop CS4), dividing the ovary into dorsal (above the longitudinal line) and ventral (below the longitudinal line, containing the ovulation fossa) regions. Once follicles were identified via light microscopy (Nikon Eclipse E200; Tokyo, Japan), the 2-dimensional location of each follicle was noted using the x - and y -coordinates (rows and columns) provided by the square grid sheet, with the slide now being referred to as an ovarian map.…”

“…This finding is suggestive of the concept that paracrine signals in the ovary play a large role in follicular activation and the development of primordial follicles [ 20 ]. The potential paracrine activity within follicle clusters and the hypothesis of ovarian plasticity may work together to explain the wide range of follicular heterogeneity observed in the mare, as well as the follicular decrease that occurs during mare aging [ 19 , 21 , 22 ].…”