Hair Follicles as Micro-Organs: MicroRNA-Mediated Control of Growth, Cycling, and Fiber Traits

Hair follicles are highly specialized mini-organs within the skin that drive the production of wool and cashmere, traits of major biological and economic importance in sheep and goats. Despite their microscopic size, hair follicles exhibit extraordinary regulatory complexity, integrating genetic programs with seasonal, endocrine, environmental, and epigenetic cues. Although transcriptional networks and signaling pathways underlying follicle morphogenesis and cycling have been extensively investigated, the post-transcriptional mechanisms that fine-tune these processes remain insufficiently understood. MicroRNAs (miRNAs) have emerged as pivotal post-transcriptional regulators that coordinate cell fate determination, lineage commitment, and tissue homeostasis. Growing evidence indicates that miRNAs play essential roles in hair follicle stem cell maintenance, proliferation, differentiation, apoptosis, and organ-level development, functioning through interconnected regulatory networks rather than isolated linear pathways. By modulating the expression of key follicle-determining genes and signaling components, miRNA-mediated regulation shapes follicle formation, cyclic regeneration, and fiber traits. In this review, we synthesize recent advances in miRNA research related to hair follicle biology, with a particular focus on wool- and cashmere-bearing mammals. We integrate findings across species to propose a systems-level framework in which miRNA networks interface with canonical signaling pathways and epigenetic mechanisms to orchestrate follicle development and regeneration. Conserved and species-specific regulatory principles are discussed to bridge fundamental follicle biology with practical applications in fiber production. Overall, this review highlights miRNAs as a critical yet previously underappreciated regulatory layer in hair follicle biology. A deeper understanding of miRNA-mediated control provides new conceptual insights into wool and cashmere development and offers a foundation for future molecular breeding and precision regulation strategies in livestock.