Abstract
Distinctive features of human fetal thymus morphogenesis in early ontogeny in the case of uncomplicated pregnancy have been characterized. A steady increase of thymus dimensions and weight occurred concomitantly to differentiation of morphofunctional zones within the organ. Cell differentiation in the subcapsular and inner cortical zones of the thymus lobes was manifested as changes in parameters of expression of T-lymphocyte antigens CD1, CD2, and CD3 and ultrastructural features of reticuloepithelial cells (REC) type I and II forming a microenvironment for lymphocytes. RECs of the medullar zone formed a glomerular syncytium with desmosomal interepithelial contacts by week 22 of fetal development. Small lymphocytes predominated among thymocytes (66%). Hassall's corpuscles, the structural correlates of morphological and functional maturity, predominated in the fetal thymuses during developmental weeks 25-27.
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