A stromal myoid cell line provokes thymic T-cell immigration at the second and third gestational trimesters

Once lymphoid precursors enter the thymus form the blood stream, they come into contact with thymic stromal cells that guide their maturation into functionally competent T cells. Thymic myoid cells are one such cell type. They have been described as a regular constituent of the thymus of embryonic and young vertebrates and express muscle proteins including myosin, desmin, acetylcholine receptor (AChR), C-protein, MyoD, troponin T, rapsyn, and utrophin. It has been emphasized recently that the thymic myoid cells play an important role in the protection of thymocytes from apoptosis, and in the process of T-cell differentiation and maturation.

To provide a quantitative estimation of thymic myoid cells and T-cell population in different stages of development. A probable interaction between these two populations could explain an additional mechanism to the active T-cell migration from the thymus that is a direct contact to a specific myoid cell line.

Paraffin-embedded specimens from the thymus of forty five human embryos at the first, second and third trimester of gestation respectively, were investigated by conventional histology, and immunohistology for the presence in the stroma of the thymic medulla, of myosin in the myoid cells, and UCHL1 (pan T-cell) antigen in the medullary thymocytes.

Our results demonstrated a quantitative difference in the second and third trimester of development concerning the expression of myosin in the stromal myoid cells of the thymic medulla over the equivalent expression of the protein in the first trimester. Similar changes in the above periods were found concerning the population of medullary thymocytes expressing UCHL1 antigen.

Our results indicate that: (1) Thymic myoid cells play an important role in the thymic microenvironment as they are well conserved throughout species evolution. (2) The increased population of myoid cells in the medullary area during mid and late gestational age, in comparison with first trimester, probably reflects the increased demand of the growing fetus for mature T lymphocytes. Contractions of myoid cells mediated by their cytoplasmic structural proteins, including myosin which is well preserved during development, might aid the movement of thymocytes expressing UCHL1 antigen, across or out of the gland, suggesting a potential involvement of myoid cells in the thymic function. Further studies on larger series are needed to corroborate this.