Abstract
We have addressed the question whether the epithelial stroma in the thymus is derived from a common stem cell or whether cortical and medullary epithelial cells are derived from different embryonic stem cells emerging, for example, from endoderm and ectoderm. By the use of rapidly expanding cultures of thymic epithelial cells (TEC) from 14 to 16 day-old murine fetuses and by specific antibodies against cortical and medullary epithelium, respectively, we were able to demonstrate a small subpopulation of double-labeled TEC in the cultures. These cells were not present in TEC cultures initiated from thymuses of neonatal mice. Double-labeled TEC were also found in tissue sections from fetal thymuses. These findings may indicate that TEC populations of the cortex and the medulla are derived from a common stem cell, with potential for differentiation toward both cortical and medullary TEC.
Highlights
It is generally accepted that the thymus develops from the endoderm of th third pharyngeal pouches and the ectoderm of the corresponding branchial cleft
The present results confirm previous results obtained from culture of thymic epithelial cells (TEC) from newborn mice and from human thymuses, showing essentially all cultured cells to be of epithelial nature when grown in the present serum-free medium with added, defined growth factors (R6pke et al, 1990; R6pke and Elbroend, 1992)
We cannot explain why the frequency of TR5 + cells, as determined by FACScan analysis, is lower than the frequency estimated by fluorescence microscopy, but the proportion of TR5 + to TR4 + cells is clearly reflected by these FACScan results
Summary
It is generally accepted that the thymus develops from the endoderm of th third pharyngeal pouches and the ectoderm of the corresponding branchial cleft. The notion that thymic epithelial cells (TEC) are derived from both ectoderm and endoderm is based on morphological evidence showing that pouches from the two derms approach each other and stay in close contact for some time (Cordier and Heremans, 1975), and that a normal thymus fails to develop if they do not meet (Cordier and Haumont, 1980). This notion is supported by the use of antibodies specific for subpopulations of TEC and immunohistochemistry.
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