Myoid cells are a unique class of contractile cells distributed throughout various non-muscular tissues. Although they share characteristics with muscle cells, they are not classified as true muscle tissue. Instead, they are a specialized cell type adapted to perform muscle-like functions in specific organs. Their presence across a wide range of vertebrates indicates a conserved role, and these cells are integral to the function of the reproductive and immune systems.
Structural and Functional Hallmarks
The name “myoid,” derived from the Greek for “muscle-like,” points to their defining feature. Myoid cells are filled with filaments of actin and myosin, the same proteins that allow skeletal and smooth muscles to contract. This protein machinery enables them to generate force and movement. These cytoskeletal proteins are often arranged in a lattice-work pattern, allowing for coordinated contraction.
However, myoid cells are distinct from true smooth muscle cells. They often originate from different precursor cells, such as epithelial or fibroblastic lineages. Structurally, their contractile filaments are less organized than those in smooth muscle cells. This structural difference reflects their specialized roles, which require more subtle, localized movements rather than the powerful, sustained contractions of tissues like the intestinal wall.
Myoid Cells in the Reproductive System
In the male reproductive system, peritubular myoid cells (PMCs) support testicular function. These flattened, spindle-shaped cells form several layers that encircle the seminiferous tubules, where sperm production occurs. In humans, these layers can be five to seven cells deep, though this organization varies between species; for instance, many rodents have only a single layer of PMCs.
The primary function of these cells is propulsion. Through slow, rhythmic contractions, PMCs generate peristaltic waves that gently push immature, non-motile sperm and testicular fluid out of the seminiferous tubules. This transport moves sperm toward the epididymis for maturation. Beyond this mechanical role, PMCs are secretory cells that produce components of the extracellular matrix. These secretions help form the basement membrane, contributing to the blood-testis barrier, which isolates developing sperm from the bloodstream.
Myoid Cells in the Immune System
Within the immune system, myoid cells are found in the thymus, a gland behind the sternum that is responsible for T-cell maturation. These thymic myoid cells are located in the medulla, the central region of the thymus. Research suggests a role in establishing immune tolerance, a process that teaches developing T-cells to distinguish the body’s own proteins (“self”) from foreign invaders.
Thymic myoid cells express muscle-specific proteins, including the acetylcholine receptor (AChR), on their surface. By presenting these self-antigens to immature T-cells, they help eliminate or inactivate any T-cells that might otherwise recognize muscle tissue as foreign and launch an autoimmune attack. This function is a part of preventing autoimmunity. Myoid cells are also present in other immune tissues like the spleen and lymph nodes, where their contractility may assist in moving lymph and immune cells.
Clinical Relevance and Research
Myoid cell dysfunction is linked to several health conditions. In the reproductive system, impaired contractility of peritubular myoid cells can disrupt the transport of sperm from the testes. This contributes to certain forms of male infertility, like non-obstructive azoospermia, where no sperm is present in the ejaculate despite production. Alterations in these cells can impair the peristaltic movement needed to propel tubule contents.
In immunology, thymic myoid cells are associated with the autoimmune disease Myasthenia Gravis (MG). In this condition, the immune system produces antibodies that attack acetylcholine receptors on muscle cells, causing weakness. The process may be initiated in the thymus, where abnormal immune responses to the AChR expressed on myoid cells trigger the production of these harmful autoantibodies. Research into these cells continues to provide insight into disease mechanisms and potential therapeutic targets.