Formation of cartilage in cardiac semilunar valves of chick and quail

The development of cartilage in the aortic and pulmonary valves of chick an d quail was studied using histological, histochemical and immunohistochemic al techniques. In both species, the earliest evidence of chondrogenesis is the formation of smooth muscle a-actin-negative prechondrogenic (type II co llagen-negative:) cellular condensations in the tunica media of the proxima l aorta and pulmonary trunk, in front of or slightly distal to the valvular commissures. Such condensations are present as early as stage 37 of Hambur ger-Hamilton in the aortic and pulmonary valves of the chick. In quail embr yos, they form somewhat later, namely, at stage 38 in the aortic valves and stage 39 in the pulmonary valves. In the chick, synthesis of type II colla gen starts in the central core of the aortic cellular condensations at stag e 38. In the pulmonary valves of chick and aortic and pulmonary valves of q uail, production of type II collagen does not begin until stage 40. This pr oduction then gradually increases toward the periphery of the condensations , which remain devoid of perichondrium prior to hatching. After birth, the condensations become transformed into hyaline cartilaginous foci. In the ao rtic valves of some chickens and quails, more or less extensive deposits of hyaline cartilage or fibrocartilage form along the attachments of the leaf lets to their supporting sinuses. They develop later than the commissural c artilages. The present findings, together with previous data from the liter ature, suggest that the aortic and pulmonary valve cartilages differentiate from neural crest-derived nonmuscular cells.