A morphologic and morphometric analysis of fetal lung development in the sheep.

In the sheep, fetal lung development proceeds to a later stage of maturity than in smaller laboratory animals. Of the four stages in pulmonary development recognizable in this species - embryologic, pseudoglandular, canalicular, and aveolar - the latter three are described in the present study using histologic, morphometric, and ultrastructural techniques. During the pseudoglandular stage, the major airways developed centrifugally. Cartilaginous, glandular, muscular, vascular, and neural elements were present in major airway walls from an early age. During the canalicular stage, volume expansion of the lung was accomplished by rapid growth of large terminal spaces. In the final stage, alveoli were formed following subdivision of the large terminal spaces by alveolar crests. The alveolar lining epithelium differentiated during the latter two stages producing a large increase in alveolar surface area, particularly during the alveolar stage; a large increase in pulmonary capillary surface area also accompanied alveolar development. Thus, just prior to birth, the fetal sheep lung has a well-developed air-way system and alveolar network, in preparation for postnatal gas exchange.