Formation of primary microvessels in the early stages of prenatal human morphogenesis

: By means of light and electron microscopy, the most general mechanisms of formation and development of primary blood microvessels in functionally different organs (adenohypophysis, thyroid, thymus, liver, spleen, small and large intestine) have been studied in human embryos 4-8 weeks of age. Ultrastructure of cells in the extra- and intraorganic mesenchyme is described; to the latter belongs the leading role in organization the pathways of the prevascular microcirculation. The primary microvessels are formed as a result of canalization of the intercellular clefts, lining with mesenchymal cells, that gradually transfer into primordial endotheliocytes. Basing on ultrastructural analysis, certain stages of differentiation of protocapillary endotheliocytes have been defined and described in different organs. The change of the prevascular microcirculation into the intraorganic protocapillary bed (the primary blood bed) is an essential and necessary stage of the organogenesis.