Within the male reproductive system, peritubular myoid (PTM) cells are situated just outside the seminiferous tubules of the testes. Their primary roles involve providing structural integrity to the tubules and facilitating the transport of sperm. The organization and number of PTM cell layers can differ between species. These cells are active throughout the reproductive lifespan, continuing to divide even after sexual maturity, which necessitates ongoing regulation of their growth.
Anatomy of Peritubular Myoid Cells
Peritubular myoid cells have smooth muscle-like characteristics and a flattened, spindle-shaped appearance. They are arranged in multiple, thin layers that encircle the seminiferous tubules, creating a cellular boundary that provides structural support. In humans, this arrangement consists of approximately five to seven distinct layers of these cells, helping the seminiferous tubules maintain their shape.
These cells produce and organize the extracellular matrix, which forms the basal lamina, or basement membrane, of the tubule wall. This matrix is a scaffold composed of proteins like fibronectin and various types of collagen. The arrangement of the cell layers and the matrix they secrete contribute to the architecture of the seminiferous tubules.
The cytoskeletal components within PTM cells contain abundant actin filaments, which are proteins that enable contraction. The organization of these filaments varies by species, but they can form a lattice-work pattern running both along and around the tubule. This network of filaments gives the cells their contractile ability.
Function in Sperm Motility
Peritubular myoid cells transport spermatozoa through the seminiferous tubules. Developing sperm are not yet motile and must be moved from their creation site toward the rete testis and epididymis for maturation. PTM cells accomplish this through coordinated, rhythmic contractions that generate peristaltic-like waves, squeezing the tubules to propel immature sperm and testicular fluid forward.
This contractile ability allows the cells to shorten and squeeze the tubule in a controlled manner. The force generated by this collective action ensures the steady flow of sperm out of the tubular network. This mechanical transport is a necessary step for sperm to reach the epididymis for maturation.
Regulation of these contractions involves various signaling molecules. The hormone oxytocin, produced by adjacent Leydig cells, is a factor in stimulating these contractions. It may act on PTM cells indirectly by activating vasopressin receptors, though the complete pathways are still under investigation. This biochemical control ensures the peristaltic waves are timed to move the tubule contents.
Role in the Blood-Testis Barrier
Peritubular myoid cells are a component of the blood-testis barrier, a physical separation that isolates developing sperm cells from the bloodstream. This barrier prevents the male’s immune system from recognizing mature sperm as foreign and attacking them. An attack would lead to an autoimmune response and infertility, so the barrier’s integrity is required for spermatogenesis.
PTM cells form the outermost cellular layer of this barrier, working with Sertoli cells that form the inner boundary. Together, these two cell types create a sealed environment that regulates which substances can pass from the blood to the developing sperm. The extracellular matrix secreted by the PTM cells also reinforces this partition.
The flattened shape and layering of PTM cells create a physical wall that complements the junctions formed by Sertoli cells. This multi-layered system ensures the environment within the seminiferous tubules remains stable and immunologically privileged. This protection allows sperm development to proceed without interference from the body’s immune system.
Hormonal Influence and Cell Communication
The function of peritubular myoid cells depends on hormonal signals and communication with neighboring cells. Androgens, such as testosterone produced by Leydig cells, influence the development and contractile activity of PTM cells. PTM cells possess androgen receptors, indicating they are a direct target for testosterone.
Communication occurs through paracrine signaling, where cells release chemical factors that influence adjacent cells. PTM cells communicate with Sertoli cells, exchanging signaling molecules that maintain the tubule’s structural integrity and support sperm production. For example, factors secreted by PTM cells can influence Sertoli cell function, and this interaction is reciprocal.
The interplay between Leydig cells, PTM cells, and Sertoli cells creates a highly regulated microenvironment. This environment is controlled to support the stages of spermatogenesis, from stem cell division to the transport of mature sperm. The signaling pathways involved include various growth factors and other secreted proteins.