A stromal myoid cell line provokes thymic erythropoiesis between 16th to 20th weeks of intrauterine life.

BACKGROUND The thymus provides an optimal cellular and humoral microenvironment for cell line committed differentiation of haematopoietic stem cells. The immigration process requires the secretion of at least one peptide called thymotaxine by cells of the reticulo-epithelial (RE) network of the thymic stromal cellular microenvironment. The thymic RE cells are functionally specialised based on their intrathymic location and this differentiation is modulated by various interaction signals of differentiating thymocytes and other non lymphatic haematopoietic stem cells. OBJECTIVES To study the role of another cell line in fetal thymic haematopoietic proliferation and differentiation in different stages of development: the stromal myoid cells. DESIGN Fifteen cases of fetal thymic specimens (4th to 8th weeks: five cases 16th to 20th weeks: five cases and 28th to 32nd weeks: five cases respectively) were studied. Tissue paraffin samples were stained immunohistochemically using (i) a monoclonal antibody recognising alpha-smooth muscle actin, a contractile microfilament expressed exclusively by smooth muscle cells, myofibroblasts and related cells, (ii) a monoclonal antibody glycophorin C recognising the erythropoietic cells. SETTING Histology-Embryology Department of Democritus University of Thrace (Alexandroupolis) over ten year period (1991-2001). RESULTS The number of alpha-smooth muscle actin-positive cells significantly increased during the late second and third trimester of gestation. In the above period a relevant increase in the number of glycophorin C positive cells were observed. CONCLUSION Our data suggest that a myoid cell line is involved in the formation of an appropriate microenvironment for homing and proliferation of erythropoietic cells.