Summary. Ovaries of neonatal rats were transplanted on the day after birth into ovariectomized (high gonadotrophin levels) or ovariectomized-hypophysectomized adult female rats (extremely low gonadotrophin levels). Although transplantation caused a reduction of the number of growing follicles in all transplants, it did not seem to induce a clearly abnormal course of follicle development. After 15 days of development, the hormone-rich transplants contained a greater number of follicles larger than type 4 and a smaller number of oocytes with diameters < 21 μm, than did the hormone-poor transplants. In the transplants with high hormone levels, the smallest follicles (types 2–3a) showed advanced transition of flattened into cuboidal follicle cells and oocyte growth. It is concluded that in these experimental conditions gonadotrophins, presumably especially FSH, seem to have a stimulatory effect on early follicle cell development and early oocyte growth.
E. Arendsen de Wolff-Exalto
E Wolff, M M Suplicki and R Behr
Primordial germ cells (PGCs) are the embryonic precursors of spermatozoa and eggs. In mammals, PGCs arise early in embryonic development and migrate from their tissue of specification over a significant distance to reach their destinations, the genital ridges. However, the exact mechanism of translocation is still debated. A study on human embryos demonstrated a very close spatial association between migrating PGCs and developing peripheral nerves. Thus, it was proposed that peripheral nerves act as guiding structures for migrating PGCs. The goal of the present study is to test whether the association between nerves and PGCs may be a human-specific finding or whether this represents a general strategy to guide PGCs in mammals. Therefore, we investigated embryos of different developmental stages from the mouse and a non-human primate, the marmoset monkey (Callithrix jacchus), covering the phase from PGC emergence to their arrival in the gonadal ridge. Embryo sections were immunohistochemically co-stained for tubulin beta-3 chain (TUBB3) to visualise neurons and Octamer-binding protein 4 (OCT4 (POU5F1)) as marker for PGCs. The distance between PGCs and the nearest detectable neuron was measured. We discovered that in all embryos analysed of both species, the majority of PGCs (>94%) was found at a minimum distance of 50 µm to the closest neuron and, more importantly, that the PGCs had reached the gonads before any TUBB3 signal could be detected in the vicinity of the gonads. In conclusion, our data indicate that PGC migration along peripheral nerves is not a general mechanism in mammals.
E. Arendsen de Wolff-Exalto and A. C. Groen-Klevant
Summary. Growth rates of oocytes in the ovary cannot be measured directly. However, oocyte growth can be determined by relating oocyte diameter to the time the enclosing follicle has been growing, calculated from kinetic studies of granulosa cell doubling time. For the ovaries of immature rats aged 16 and 28 days, oocyte diameter plotted against time gave straight and parallel lines, showing that the diameter is positively and linearly related to time and that oocytes grow at the same rate at both ages. However, during earliest developmental stages (type 3a follicles with 10–20 granulosa cells) the growth rates differed, being faster at 16 days and slower at 28 days than at later stages.