Development of connective-tissue components of small arteries in the chick embryo

Development and pathologies of the arterial wall

Arteries consist of an inner single layer of endothelial cells surrounded by layers of smooth muscle and an outer adventitia. The majority of vascular developmental studies focus on the construction of endothelial networks through the process of angiogenesis. Although many devastating vascular diseases involve abnormalities in components of the smooth muscle and adventitia (i.e., the vascular wall), the morphogenesis of these layers has received relatively less attention. Here, we briefly review key elements underlying endothelial layer formation and then focus on vascular wall development, specifically on smooth muscle cell origins and differentiation, patterning of the vascular wall, and the role of extracellular matrix and adventitial progenitor cells. Finally, we discuss select human diseases characterized by marked vascular wall abnormalities. We propose that continuing to apply approaches from developmental biology to the study of vascular disease will stimulate important advancements in elucidating disease mechanism and devising novel therapeutic strategies.

Elastosis in meningioma. A case report and electron microscopic study

A 55-year-old man underwent left frontoparietal craniotomy for total removal of a parasagittal meningioma. The tumor showed a predominant pattern of meningotheliomatous meningioma and a partial pattern of angiomatous, transitional and fibrous meningioma. A striking feature was the appearance of unusually numerous elastinophilic material in the intercellular stroma. We observed a process in which the progress of elastosis changed the appearance of meningotheliomatous meningioma to a hypocellular, and subsequently acellular fibroelastotic node. Three years after the first craniotomy, meningioma recurred at the same site. The recurrent tumor also showed the histological pattern of meningotheliomatous meningioma with stromal elastosis, as previously observed. This is the first report of a case of meningioma with stromal elastosis. The present case represents the first unequivocal demonstration of elastogenesis in meningioma.

Development of arteries in embryonic chick bone marrow with special reference to the appearance of periarterial granulopoietic sheaths

Arterial development was studied in embryonic chick bone marrow starting on day 11 of incubation and continuing until day 18. Arteries first appear on day 11 as vessels with two components to their wall structure, namely a thin, complete endothelium and a nearly complete pericyte layer. With advancing age, three distinct layers--an endothelium, a complete muscularis, and adventitia--appear. Long, narrow endothelial cells form cellular junctions at their basal surfaces. Smooth muscle cells remain relatively undifferentiated and do not consist of more than three layers even in the largest arteries. The adventitia consists of a hypocellular zone containing fibroblasts and a collagenous extracellular matrix. Unmyelinated nerves are located at the peripheral margin of the adventitia. All of the larger arteries are surrounded by a sheath of heterophilic granulopoietic cells. These cells do not intrude into the adventitia, but basophilic or mast cells often do intrude. The stromal cells of the periarterial granulopoietic sheaths remain as either fibroblastic cells or as multilocular preadipocytes. They, unlike those stromal cells beyond the sheath, never become unilocular adipocytes. By day 18, granulopoiesis is restricted to the periarterial sheaths and to the endosteal regions of the marrow. These studies indicate that fibroblastic stromal cells and preadipocytes, but not fully developed adipocytes, support granulopoiesis in the chick marrow. The stromal cells in the periarterial sheaths either represent a subpopulation of cells that do not develop into adipocytes or represent cells whose development into adipocytes is locally inhibited.

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.

Fine mapping of tropoelastin-derived components in the aorta of developing chick embryo

Affinity-purified antitropoelastin antibodies have been used to localize tropoelastin-derived components in aortas from chick embryos of different age by immunoelectron microscopy. Staining in the matrix is first noted at day 3 associated with irregular bundles of filaments resembling microfibrils, in the absence of amorphous elastin deposits. Amorphous material, which rapidly accumulates at later stages, is heavily labelled, while surrounding microfibrils are only poorly labelled. By contrast, a more intense staining of microfibrils persists in regions in which amorphous material is not morphologically evident. These observations indicate that the initial accumulation of elastin requires microfibrils, while the two components are not in close association in the subsequent growth of the amorphous core of the fibre. Intracellular staining is evident in the secretory apparatus of the cell and in peripheral large vesicles. Differentiated cells also show regions of close contact with elastic fibres in which immunological staining for elastin is very close to the cell membrane.

Distribution of hyaluronic acid and sulfated glycosaminoglycans during blood-vessel development in the chick chorioallantoic membrane

This study uses histochemical methods to determine the ultrastructural distribution of specific glycosaminoglycans (GAGs) during the development of blood vessels in the chick chorioallantoic membrane (CAM) and to correlate changes in GAG composition with the significant structural events in the development of these vessels. Tissues were stained with tannic acid, ruthenium red, and high iron diamine and digested in various GAG-degrading enzymes to identify specific GAGs. The results are consistent with a role for hyaluronic acid in the formation, alignment, or migration of the capillary plexus of the CAM and a role for sulfated GAGs (heparan sulfate, chondroitin sulfate, dermatan sulfate) in the differentiation and development of arterial and venous vessels of the chorioallantoic membrane.