Germ cell dynamics during nest breakdown and formation of primordial follicles in the domestic turkey (Meleagris gallopavo)

Authors: HALL, GEORGE - University Of Guelph; Long, Julie; WOOD, BENJAMIN - Hybrid Turkeys; BEDECARRATS, GREGOY, - University Of Guelph

Submitted to: Research Journal of Poultry Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2019
Publication Date: 3/20/2020
Citation: Hall, G.B., Long, J.A., Wood, B.A., Bedecarrats, G.Y. 2020. Germ cell dynamics during nest breakdown and formation of primordial follicles in the domestic turkey (Meleagris gallopavo). Research Journal of Poultry Sciences. 99(5):2746-2756. https://doi.org/10.1016/j.psj.2019.12.050.
DOI: https://doi.org/10.1016/j.psj.2019.12.050

Interpretive Summary: Similar to mammals, bird ovaries contain a finite number of oocytes at sexual maturation. The final number of oocytes is determined during the first few weeks after the female bird hatches through a process where groups of cells (germ cell nests) break down and the remaining groups of cells form mature structures known as primordial follicles that will ultimately be ovulated. This period of cellular depletion and reorganization has not been studied in the turkey. The objective of this study was to define the time line of germ cell nest break down and the subsequent formation of primordial follicles. Ovaries from turkey poults between 1 and 21 days post-hatch were evaluated for the size, density, percent volume, and number of pre-follicular germ cells and primordial follicles. Based on our results, germ cell nest breakdown begins between day 5 and 7 post-hatch, where a decrease in pre-follicular germ cell density occurred on day 5 followed by the subsequent appearance of primordial follicles on day 7. On day 9, both pre-follicular germ cells and primordial follicles increased in size. This growth leveled off by day 13 post-hatch, with no further change. The majority of germ cell nest breakdowns concluded by day 15 post-hatch, seen by a lack of change in both pre-follicular germ cell and primordial follicle densities. Further, there was an 84% decrease in the total germ cell population between day 5 and 15. This 84% drop in germ cell numbers during nest breakdown in the turkey is comparable to the domestic chicken, but is greater than the average two-thirds which are lost in mammalian species.

Technical Abstract: This study aimed to assess the different subpopulations of germ cells and stereological changes within the cortex of the left ovary during germ cell nest breakdown in the domestic turkey (Meleagris gallopavo). This was accomplished by measuring the size, density, percent volume, and count of pre-follicular germ cells and primordial follicles, in white breasted turkey poults, 1 - 21 days post-hatch. Based on our results, germ cell nest breakdown initiates between day 5 and 7 post-hatch, characterized by a decrease (P < 0.001) in pre-follicular germ cell density and the subsequent appearance of primordial follicles on the latter day. Nest breakdown is temporally followed on day 9 by the first increase (P < 0.05) in size of both pre-follicular germ cells and primordial follicles. This growth levels off by day 13 post-hatch, with no further change (P > 0.05). The average diameter of primordial follicles after day 11 was 52.45 +/- 53.96 m with minimum and maximum values of 30.04 +/- 1.34 m and 86.73 +/- 7.73 m, respectively. The majority (>90%) of germ cell nest breakdowns conclude by day 15 post-hatch, seen by a lack of change in both pre-follicular germ cell and primordial follicle densities, thereafter. Before and after nest breakdown, the total germ cell counts were 1,057,402 +/- 194,627 and 169,578 +/- 31,594 on day 5 and 15 post-hatch, respectively, which marks an 84% decrease in the total germ cell population. With pre-follicular germ cells still being present at day 21 post-hatch, a further decrease in the overall germ cell pool is a reasonable assumption. This 84% drop in germ cell numbers during nest breakdown in the turkey is comparable to the domestic chicken, but is greater than the average two-thirds which are lost in mammalian species.