Ultrastructural features of the development of the venular segment of the hemomicrocirculatory bed in the first half of human prenatal ontogenesis

Annotation. The roots of the venous system are postcapillary venules and collecting venules, which form the venular segment of the hemomicrocirculatory bed. The venular vessels of the hemomicrocirculatory bed perform transport, resistive, depot, and drainage functions of the cardiovascular system. In the prenatal period of ontogenesis, the hemomicrocirculatory bed ensures the normal processes of organogenesis and histogenesis. The aim of the study is to investigate the ultrastructural features of the formation of venular microvessels in the hemomicrocirculatory bed during the prenatal period of human ontogenesis. Using transmission electron microscopy, the structural features of the development of venular segment vessels of the hemomicrocirculatory bed were examined from 4–5 to 20 weeks of prenatal ontogenesis. During the differentiation of the efferent vessels of the precapillary bed into the venular vessels of the hemomicrocirculatory bed, the ultrastructural organization of endothelial cells, which belong to the somatic-type endothelial cells of venular microvessels, is established. The cells of the paravascular connective tissue differentiate into pericytes, the number of which increases with fetal growth and the enlargement of the microvessel diameter. Around the vessel, an adventitial layer is formed, composed of fibroblasts and bundles of collagen fibers. The study results on the normal development of the venular segment of the hemomicrocirculatory bed allow for the development of pharmacological protection strategies against adverse factors affecting the maternal body during pregnancy.