Inhibin-Mediated Regulation of Folliculogenesis and Litter Size in Small Ruminants and Non-Ruminants

Litter size is a critical determinant of reproductive efficiency and economic productivity in livestock breeding. Inhibin, a glycoprotein hormone of the transforming growth factor-β (TGF-β) superfamily, plays a pivotal role in regulating follicle-stimulating hormone (FSH) secretion through negative feedback within the hypothalamic–pituitary–gonadal axis. By modulating FSH levels, inhibin influences follicular recruitment, maturation, and ovulation rate—key processes that determine litter size. This review provides a comprehensive analysis of inhibin isoforms (inhibin A and inhibin B), their antagonistic relationship with activin signaling, and their species-specific roles in reproductive regulation. Genetic studies in small ruminants and non-ruminants indicate that polymorphisms in inhibin-related genes, particularly INHA, may contribute to variation in prolificacy-related traits; however, the strength and consistency of these associations differ among species and populations. This review further integrates findings from functional validation studies, particularly in mouse models, demonstrating that partial reduction of inhibin bioactivity can enhance FSH secretion, promote follicular recruitment, and increase ovulation rate and litter size, whereas excessive disruption of inhibin signaling may impair reproductive efficiency. Importantly, the biological effects of inhibin modulation depend on the degree of pathway alteration and the balance between follicular recruitment and prenatal survival. Although inhibin represents an important endocrine regulator of reproductive performance, current genomic evidence suggests that litter-size variation is controlled by multiple interacting pathways, and the contribution of inhibin-related genes requires further functional validation. Future integration of genomic, endocrine, and functional approaches will be essential to clarify the role of inhibin pathways and determine their potential application in sustainable livestock breeding strategies.