Histological and Colour Doppler‐Based Vascularity Analysis of Ovarian Follicles at Different Developmental Stages in Cattle

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Abstract
Follicular dynamics represent the fundamental aspect of bovine reproduction, which relies on complex physiological and hormonal processes. To investigate the haemodynamics and characteristics of ovarian follicles at different stages of development in Atlas Brown cattle, 31 cycling non‐lactating cows were submitted to the Ovsynch oestrous synchronisation protocol to standardise follicular development. Follicular growth and vascularisation were monitored using 2D and Doppler ultrasonography. Moreover, follicles were classified into dominant, largest subordinate and third‐largest follicle. Also, 28 ovary samples were collected for histological examination focusing on the theca interna capillary density. The follicular deviation was identified between Days 1 and 3 after GnRH injection. Statistical analysis revealed that before the deviation, no differences were found between the two parameters in all types of follicles studied (p > 0.05); furthermore, after follicular deviation, the dominant follicles maintained capillary density (p < 0.05) and maintained detectable blood flow. However, the largest subordinate and third‐largest follicles marked a reduction of both parameters (p < 0.05). Histological findings confirmed ultrasonographic data, demonstrating maintained angiogenesis in dominant follicles and reduced vascularisation in atretic follicles. These results demonstrate the critical role of vascularisation for follicular dominance and selection, providing helpful information for optimising reproductive management practices in cattle. Also, this study advances knowledge of bovine reproductive physiology and suggests potential improvements in synchronisation and ovulation stimulation protocols. As a result, increasing the outcome of artificial insemination and embryo transfer.

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