Number of Primordial Follicles in Juvenile Ringed Seals (Pusa hispida) from the Gulf of Bothnia and West Greenland

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.

Keywords: Baltic Sea; Greenland; primordial follicles; ringed seal; stereology.

Figure 1

Examined Area. The collection of the samples originates from the Gulf of Bothnia in the Baltic Sea and in West Greenland. The samples were predominantly collected in the locations marked with a circle in the Gulf of Bothnia and with a triangle at the west coast of Greenland.

Figure 2

Setup for the volume determination of each ovary (A). After complete immersion of the ovary in water, the displaced volume of water corresponds to the volume of the ovary and can be measured as the weight change. The histological preparation for each ovary is shown in (B). Cross sections of the ovary were made.

Figure 3

A part of the stained ovary slice with the test grid of 64 points. Points hitting the primordial follicles as well as the four green-colored points hitting the reference space (ovary) were counted and afterwards multiplied with 16 (coarse grid).

Figure 4

Volume determination of different primordial follicles. All central sectioned primordial follicles, which cross the green line or are inside the marked area are taken into account for volume measurements.

Figure 5

Total primordial follicle number of Baltic and Greenland ringed seals. It is shown that the quantity of primordial follicles in seals from the Baltic Sea varies more.

Figure 6

Correlation between number-weighted mean volume of follicles and ovary volume in juvenile ringed seals. The number-weighted mean volume shows a significant positive relationship with the ovary volume (LM, p = 0.04).