ONSET OF ELASTOGENESIS AND DOWN-REGULATION OF SMOOTH-MUSCLE ACTIN AS DISTINGUISHING PHENOMENA IN ARTERY DIFFERENTIATION IN THE CHICK-EMBRYO

During development, the arterial system is grossly divided into elasti c and muscular vessel types. Apart from local environmental factors, i t has been suggested that vascular smooth muscle cell origin (mesoderm or neural crest) is involved in this, as yet poorly understood, arter ial differentiation. We describe differentiation of the thoracic arter ial system in the chick embryo, using immunohistochemical techniques s taining for muscle-specific actin, vinculin and desmin and histologica l staining to visualise elastin. The initial developmental stages of t he vessel wall in all arteries appeared to be highly similar, with all arteries showing peri-endothelial actin and vinculin staining. Major alterations did not occur until the start of elastogenesis, which coin cided with complete loss of actin staining from the proximal part of t he great arteries. Later in development, however, actin was re-express ed in a subpopulation of medial cells, which also expressed vinculin a nd desmin. Concomitantly another, nonmuscular, cell type became eviden t in the great arteries. Transient loss of actin expression and segreg ation of very distinct cell populations occurred only in vessels prone to elastic development and known to receive a neural crest contributi on. In contrast, arteries that developed a muscular phenotype never lo st the initially acquired peri-endothelial actin expression. We also s how a significant difference in the organisation of elastic fib bres b etween elastic vessels that contain neural crest derivatives and those that do not. The ductus arteriosus still presents as an enigma in the sense that it is the only part of the pharyngeal arch complex that de velops a muscular phenotype.