Histological characteristics of ovarian follicle atresia in dairy cows with different milk production.

Follicle atresia in mammals is a universal phenomenon characteristic by degenerative morphological changes in granulosa and theca cells. The unfavourable effect of milk production in relation to fertility has been studied starting from the 70s of the last century; however, there is no unambiguous and persuasive data on association of ovarian atresia with milk yield of dairy cows. The aim of this study was to define histological signs of ovarian follicle atresia in dairy cows in relation to their milk production. The ovaries were recovered from slaughtered Holstein dairy cows assigned into two groups according to average level of annual milk production: Group 1 (n = 25)-low (≤8,000 kg/year) and Group 2 (n = 23)-high (≥8,000 kg/year). Atresia of antral follicles was evaluated on the basis of histopathological image (staining with basic fuchsine and toluidine blue) of nonovulated follicles, classified into five categories: an initial atresia, cystic atresia, obliterated atresia, atresia with luteinization of the granulosa and follicle structures of the fibrous body-corpus fibrosum. We found that the histopathological image of follicle atresia in groups of low-milk- or high-milk-producing cows is essentially similar. Prevalent form of atresia in follicles of all experimental cows was the formation of fibrous bodies and obliterated atresia. The occurrence of fibrous bodies was significantly higher (55.44%) in low-milk-producing cows compared with high-milk-producing cows (34.61%). In the same way, the higher incidence of obliterated atresia was recorded in ovarian follicles from cows with the lower milk production (36.96%) compared to the cows with the higher milk production (25.48%). In contrast, ovaries from lower milk-producing cows showed lower (p < 0.05) incidence of initial (p < 0.001) and cystic (p < 0.05) follicle atresia than ovaries from the higher milk-producing cows. Our results show that cows in the higher lactation group showed more initial and cystic atresia, what may adversely affect the fertility of dairy cows.