Morphogenesis and histogenesis of the cartilaginous tracheal rings in the domestic fowl (gallus domesticus).

The morphogenesis, histogenesis and growth pattern of the tracheal rings were studied in chick embryos and in chickens up to 4 months of age. The blastemas representing the earliest ring primordia are seen to arise in the embryo in the cranial portion of the tracheal tube on the 10th day of incubation, and to extend rapidly caudalwards. They form first in the anterior wall of the tracheal tube and expand successively laterally and posteriorly. The mesenchymal blastemas that have acquired the shape of complete rings differentiate into precartilage and then into typical hyaline cartilage. During growth, the tracheal rings undergo striking changes in both shape and position. In the embryo, an active growth rate in a craniocaudal direction prevails, and after hatching each ring outstretches cranially and caudally into two long expansions (winglike projections). Moreover, these rings, which in early embryos were regularly aligned in a longitudinal row, are seen to become alternately located with their higher halves on an inner plane and their lower halves on a more superficial plane. Such a peculiar ring displacement on two different planes may reasonably be assumed to obey spatial requirements, since ring growth rate according to a craniocaudal direction is far more vigorous than growth, in the same direction, of the tracheal wall housing them.