Characterization of cellular and vascular changes in equine follicles during hCG-induced ovulation

contrast to other species, the histology of the equine follicle during ovul ation has not been described. Preovulatory follicles were isolated during o estrus at 0, 12, 24, 30, 33, 36 and 39 h (n = 5-6 follicles per time point) after an ovulatory dose of hCG to characterize the cellular and vascular c hanges associated with ovulation in mares. Pieces of follicle wall were for malin-fixed and processed for Light microscopy to evaluate the general foll icular morphology and quantify selected parameters. Marked changes were obs erved in the histology of equine follicles in the hours before ovulation. T he thickness of the granulosa cell layer doubled between 0 and 39 h after h CG (77.8 +/- 4.8 versus 158.8 +/- 4.8 mu m, respectively; P < 0.01). This e xpansion was caused primarily by a pronounced accumulation of acid mucosubs tances between granulosa cells, which was first detected at 12 h after hCG and peaked at 36-39 h. In contrast, a significant thinning of the theca int erna was observed after hCG treatment. Fewer cell layers were present; thec a interna cells appeared smaller than before hCG; and the presence of occas ional pyknotic cells was noted at 36 and 39 h after hCG. In addition, the t heca layers were invaded by numerous eosinophils. No eosinophils were obser ved in preovulatory follicles isolated between 0 and 24 h after hCG, but th e number increased to 14.0 +/- 0.8 and 5.6 +/- 0.3 eosinophils per field (x 400) in theca interna and theca externa, respectively, 39 h after hCG trea tment (P < 0.01). Severe oedema, hyperaemia and haemorrhages, and significa nt increases in the number of blood vessels in theca interna and externa we re observed at 33, 36 and 39 h after hCG. This study provides the first in- depth characterization of the sequential cellular and vascular changes that occur in equine follicles before ovulation.