Ovarian Follicle Ultrastructure and Changes in Levels of Ovarian Steroids during Oogenesis in Chalcalburnus tarichi Pallas, 1811


Abstract: The ultrastructure of the ovarian follicle and the levels of the ovarian 11-dehydrocorticosterone (11-DHC), estradiol-17\beta (E_{2}), 17\alpha-hydroxyprogesterone (17\alpha-OHP), and progesterone (P) were studied during cortical alveoli, vitellogenesis and oocyte maturation in Chalcalburnus tarichi. The microvilli began to form on the oocyte surface during cortical alveoli phase and during vitellogenesis, came into contact in the pore canals of the zona radiata with microvilli formed from granulosa cells. While the special thecal cells posses specific organelles, which are characteristic of steroid-producing cells, namely, mitochondria with tubular cristae, smooth endoplasmic reticulum and lipid droplets, the granulosa cells contain organelles typical for protein-secreting cells. These findings suggest that during vitellogenesis the special thecal cells are the sites of steroid synthesis in the C. tarichi ovary. The levels of 11-DHC and progesterone were measured at lower concentration than other hormones. While there was a decline in 11-DHC level during cortical alveoli phase (from 52.60 ± 6.54 to 20.33 ± 6.74 ng/ml), during vitellogenesis it increased gradually and reached a peak at the end of vitellogenesis (68.90 ± 9.99 ng/ml). At the beginning of maturation phase, it decreased to baseline level (22.90 ± 1.87 ng/ml) and again increased at the end of the maturation (54 ± 0.75 ng/ml). E_2 levels started to increase at cortical alveoli phase and this increase continued during vitellogenesis too. In February, a small decline was noted. However, at the end of vitellogenesis the level of E_2 reached its maximum value (756.10 ± 26.5 ng/ml). At the maturation phase (in April) it decreased to 213.50 ± 25.5 ng/ml and again a slight increase was perceived (in May). 17\alpha-OHP level was 100.10 ± 14.4 ng/ml at the beginning of the cortical alveoli phase and it decreased to 13.40 ± 1.3 ng/ml at the end of the phase. It started to increase during vitellogenesis and reached its maximum value at the end of vitellogenesis (213.90 ± 8.14 ng/ml). At the beginning of the maturation phase (in April) an evident decline was noted (68.00 ± 8.58 ng/ml). It increased again to 142.20 ± 4.45 ng/ml at the end of maturation (in May). P level was measured to 50.90 ± 2.37 ng/ml at the beginning of the cortical alveoli phase (in August), and it decreased to 22.80 ± 0.50 ng/ml at the end of the phase. This low value was kept at the beginning of vitellogenesis, but during vitellogenesis a small increase was noted and at the end of the phase it was measured as 50.80 ± 4.03 ng/ml. This low level is kept during maturation phase too. These results suggest that in C. tarichi none of these hormones (11-DHC, E_{2}, 17\alpha- OHP and P) is effective during the cortical alveoli phase, E_{2}, 11-DHC and 17\alpha- OHP are effective during vitellogenesis while P has no effect and 11-DHC and 17\alpha- OHP are effective at the end of maturation while E_2 and P have no role.

Keywords: Chalcalburnus tarichi, ovarian follicles, ovarian steroids, ultrastructure

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