Electron microscopic study of the prenatal development of the thoracic aorta in the rat.

Prenatal development of the thoracic aorta of the rat during the period ranging from gestational days 12 to 21 was examined by transmission electron microscopic and morphometric studies. The process of wall formation occurred in four major phases. At phase I (gestational day 12), the dorsal aorta consists of an endothelium and loosely surrounding mesenchymal cells. Collagen fibrils and fine filamentous materials are sparsely present in the intercellular space. At phase II (days 13 to 16), the mesenchymal cells begin to differentiate to myoblasts, which have small clusters of myofilaments with dense bodies, rough endoplasmic reticulum, and a discontinuous basal lamina. The differentiating cells form a few compact cell layers around the endothelium. Elastic fibers first occur sparsely in juxtacellular spaces at days 13-14. The thickness of the aorta increases rapidly from 1-3 layers of cells at day 13 to 5-8 layers at day 17, leading to a maximum of 5-9 cell layers at day 20. The differentiation of myoblasts and elastogenesis are initiated in the inner layers, and later progress toward the outer layer of the aortic wall. At phase III (days 17 to 19), the myoblasts continue to develop into typical smooth muscle cells, and elastic fibers rapidly increase in both size and number. At phase IV (day 20 and later), smooth muscle cells have well-developed myofilaments in the cell periphery, and rough endoplasmic reticulum and other organelles tend to accumulate in the apical portion of the cytoplasm. Elastic laminae appear in a few inner layers of the aortic wall.