Ontogeny of endocrine cells in the respiratory system of Syrian golden hamsters. II. Intrapulmonary airways and alveoli.

Results of this and the preceding study reveal 3 patterns of endocrine cell development in hamster airway. The first, a prenatal wave, begins in the larynx and sweeps down the extra- and intrapulmonary conducting airway to the bronchioloalveolar portals. Cells differentiate singly and in groups (presumptive neuroepithelial bodies, pNEBs), colocalize immunoreactivity for serotonin (5-HT) and calcitonin gene-related peptide (CGRP), and persist throughout adulthood. Postnatally a few cells also express calcitonin (CT). Appearance of 5-HT and CGRP staining correlates with the onset of local, NEB-associated mitogenesis in fetal hamster airway epithelium. The second pattern begins after birth and is unique to the larynx and cartilaginous trachea. It involves differentiation of single cells which stain for CGRP but not 5-HT. Later, a proportion also stain for CT. This pattern seemingly accounts for the predominance of single cells in laryngotracheal epithelium of adult animals. In the third pattern, cells immunoreactive for peptide YY (PYY) differentiate, singly at first and later among cells of tiny pNEBs. This begins postnatally in alveoli, spreading centripetally with retrograde differentiation of alveolar epithelium back into the bronchiolar terminations. Restricted distribution and lack of immunoreactivity for 5-HT, CGRP, or CT suggest that the PYY-positive endocrine cells form a regional subset performing special roles in pulmonary homeostasis.