Formation of intrachondral vessels (cartilage canals) in the proximal femoral epiphysis was studied in 13- to 22-week-old human fetuses using a corrosion casting technique and scanning electron microscopy. Several successive morphological stages of angiogenesis occurring inside the hyaline cartilage were distinguished. The process of cartilage vascularization starts with the formation of hairpin loops sent off from the perichondrial vascular network into the adjacent cartilage. A capillary glomerulus is then formed at the leading end, and the entire vascular unit grows in length, assuming a mushroom-like shape. Its further elongation is accompanied by a backward expansion of the capillary network which surrounds a pair of main vessels (arteriole and venule) like a manchette. The subsequent branching of such primary vascular units proceeds according to the same morphological patterns. The resulting tree-like vascular formations become interconnected via their lateral branches. This study clearly supports the invasion theory of cartilage canal formation.

The internal vascular system of vertebral bodies was investigated in 17-24 wk human fetuses by acrylic dye injection and by corrosion casting\scanning electron microscopy. The regions of intervertebral spaces did not contain blood vessels. The radial metaphyseal vessels were at the stage of centripetal ingrowth into the vertebral body cartilage and their terminal, blindly ending segments had a form of cuff-like capillary plexuses. The anterolateral equatorial arteries communicating with the vessels of the ossification centre were only rarely found. The centre was usually supplied by 2 posterior (nutrient) arteries which branched into an arcade-like array of arterioles equipped with occasional sphincters and giving origin to a dense network of peripherally located capillaries. Numerous blind capillary buds formed the advancing border of the ossification centre. The veins usually accompanied the arteries. In the ossification centre the venous compartment consisted of sinuses drained by larger posterior veins. In the 17 wk fetus, an axial avascular area was observed in the place of notochord localisation, indicating the formation of a ring-shaped ossification centre around the notochord remnants at earlier stages of fetal development.

Vascular architecture of the human pancreas was investigated by corrosion casting combined with scanning electron microscopy in fetuses aged 20 and 25 gestational weeks. The general pattern of the microvascular system was similar to that of the postnatal pancreas, with an evident insulo-acinar portal system and with three types of capillary networks: capillaries of exocrine lobules, islet capillaries and periductal capillaries around large ducts located in the interlobular septa. All these capillary networks were supplied by arteriolar branches of the interlobular arteries. As compared with the postnatal pancreas, capillaries of exocrine lobules formed denser meshworks, had a more sinusoidal character and revealed morphological features indicative of angiogenesis (blind capillary sprouts). The number of efferent (portal) capillaries per islet was lower and the predominant pattern of islet vasculature was top to bottom rather than inner to outer, as observed in adults. These results show that in the second trimester the human pancreatic vascular architecture is almost completely developed and requires only minor remodeling to be fully functional in the postnatal period.

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The organization of the microvascular network of the human pancreas in fetuses aged 18 to 25 gestational weeks is very similar to that of an adult but with more prominent features suggesting active processes of angiogenesis and vascular remodeling.