Ovarian antral follicular dynamics and their associations with peripheral concentrations of gonadotropins and ovarian steroids in anoestrous Finnish Landrace ewes

Daily transrectal ultrasonography of ovaries was done in seven Finn ewes du ring three 17-day periods from May to July. Blood samples were collected ea ch day for estimation of the serum follicle-stimulating hormone (FSH), oest radiol and progesterone concentrations, and also every 15 min for 6 h, half way through each period of ultrasonographic examination, to determine the p atterns of gonadotropic hormone secretion. Four ewes ceased cycling from Ma rch to mid-April (ewes entering anoestrus early) and three in May (ewes ent ering anoestrus late). In all ewes cyclicity resumed during the period from mid-August to mid-September. The growth of ovarian antral follicles to per iovulatory sizes of greater than or equal to 5 mm in diameter was seen at a ll stages of anoestrus. An average of four waves of follicular development (follicles growing from 3 to greater than or equal to 5 mm in diameter befo re regression) with a periodicity of 4 days were recorded during each of th e three scanning periods. There was a close temporal relationship between d ays of follicular wave emergence and peaks of successive FSH fluctuations. Ewes entering anoestrus late exceeded ewes that became anoestrus early in n umbers of large(greater than or equal to 5 mm in diameter) ovarian antral f ollicles and maximum follicle diameter. Peak concentrations of transient FS H increases were higher (P < 0.05) in ewes entering anoestrus late than in ewes entering anoestrus early. The secretion of luteinising hormone, (LH; m ean and basal level, and LH pulse frequency, but not amplitude) was lowest during the month of June in all ewes. Oestradiol production was markedly su ppressed throughout anoestrus. Peaks of progesterone secretion appeared to occur at regular intervals and were associated with the end of the growth p hase of the largest follicles of sequential waves. In conclusion, the growt h of ovarian follicles to ostensibly ovulatory diameters is maintained thro ughout anoestrus in Finn ewes and periodic emergence of follicular waves is correlated with an endogenous rhythm of FSH secretion. The present study a lso provides evidence for the inverse relationship between the time of the onset of seasonal anoestrus and the number and size of antral follicles dev eloping throughout anoestrus in Finn ewes, and indicates that differences e xist in both the secretion of and ovarian responsiveness to gonadotropic ho rmones among early and late anoestrous ewes. (C) 2000 Elsevier Science B.V. All rights reserved.