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Expression of fibroblast growth factor‐2 (FGF‐2) in early stages (days 3–11) of the development of the avian lung, Gallus gallus variant domesticus: an immunocytochemical study

In the avian lung, the bronchial system forms from epithelial (endodermal) cells. The intrapulmonary primary bronchus is the focal point of airway development. It originates secondary bronchi (SB) along its proximal–distal extent and parabronchi (tertiary bronchi) arise from and connect the SB. From as early as day 3.5, fibroblast growth factor‐2 (FGF‐2) is diffusely expressed in the epithelial and mesenchymal cells. Up‐regulation of FGF‐2 in discrete areas of the developing lung seem to set the growth rate, trajectories followed, areas appropriated, three‐dimensional symmetry and coupling of the airways. Expressed early in development and persisting into the incubation period, FGF‐2 may be involved in the formation of the avian lung. Morphogenetic differences between the avian and the mammalian lungs may explain the existing structural contrarieties.