Cracking the code: how piRNA pathway shapes spermatogenesis and combats male infertility.

Male infertility, accounting for approximately 50% of global infertility cases, is a growing concern in reproductive medicine. A fundamental cause lies in disrupted spermatogenesis-a complex, highly regulated process involving mitotic proliferation, meiotic division, and spermiogenic remodeling. Among the key regulatory pathways, PIWI-interacting RNAs (piRNAs) and their associated PIWI proteins have emerged as essential players in maintaining germline genome integrity and ensuring successful sperm development. However, their clinical relevance remain underexplored. This review provides a comprehensive synthesis of the piRNA pathway's multifaceted roles across the full spectrum of spermatogenesis. We describe how piRNAs, together with PIWI proteins, silence transposable elements (TEs), guide chromatin remodeling, regulate mRNA translation, and protect sperm from environmental insults. We detail the stage-specific functions of piRNA machinery during spermatocytogenesis, spermatidogenesis, and spermiogenesis, supported by evidence from gene knockout models and cross-species studies. Particular emphasis is placed on piRNA biogenesis, including the primary processing pathway, the ping-pong amplification cycle, and terminal modifications mediated by enzymes such as PNLDC1 and TDRKH. Genetic disruptions in key piRNA pathway genes-including