Abstract
Primordial follicles are important for the reproduction cycle and, therefore, also for the survival of the whole population of a species. Mammals have a large pool of primordial follicles, and it is thought that this pool represents the total number of oocytes. The aim of the present study was to determine the total primordial follicle number of juvenile ringed seals (Pusa hispida) from the Gulf of Bothnia and Greenland. Overall, 52 ovaries from two ringed seal populations (West Greenland (N = 6), Gulf of Bothnia, region in the Baltic Sea (N = 46)) were examined. All ovaries were cut into 2 mm thick slices and every slice was embedded in paraffin. Out of each tissue block, a 5 µm thick section was cut and stained with haematoxylin-eosin. The mean volume of the follicles and the total volume of primordial follicles per ovary were estimated by stereology and used to calculate the total estimated number of primordial follicles. The median of the total estimated number of primordial follicles seemed to be higher in Baltic individuals than in Greenland individuals (Gulf of Bothnia = 565,657; Greenland Sea = 122,475). This widens the total range of primordial follicles in ringed seals overall and might bear some potential for discussions regarding the influence of endocrine disruptors and environmental influences depending on different regions/populations and their exposure to various factors. Thus, this study aims to provide basic reference data of the number and mean volume of ringed seal primordial follicles.
Highlights
Mammalian ovaries have a large pool of primordial follicles [1,2]
The 46 ovaries from the Gulf of Bothnia, including 13 ovaries from Swedish individuals and 33 Finnish individuals, were provided from the Swedish Museum of Natural History in Stockholm (SMNH) and from the Natural Resources Institute in Finland (LUKE), the six ovaries from Greenland were provided from the Aarhus University in Denmark
The total estimated number of primordial follicles was analyzed in 46 ovaries from juvenile ringed seals from the Baltic Sea
Summary
Mammalian ovaries have a large pool of primordial follicles [1,2]. Different species develop this pool at different stages of development. A primordial follicle consists of an oocyte arrested in prophase 1 of meiosis [1]. Each primordial follicle will go through folliculogenesis, where it first develops into a primary and secondary follicle with support from granulosa and precursor theca cells [2,6]. Granulosa cells form the microenvironment for the oocytes and together with the theca cells, they synthesize a number of hormones, which control follicular development [2]. The tertiary, or antral, follicle contains follicular fluid where the oocyte undergoes maturation. Tertiary follicles can respond to specific signals and ovulate, for the ringed seals (Pusa hispida) one in every normal oestrus cycle [7]
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