Morphometric and ultrastructural study of postnatal lung growth and development in calves.

The objectives of this study were to characterize basic patterns of postnatal lung growth in 1- to 150-day-old Holstein calves and to identify periods of accelerated lung growth and times of epithelial cell development that might correlate with periods of heightened susceptibility to pulmonary injury by infective agents and dietary toxins. Calves had fully developed alveoli at birth. There was an age-associated increase in total alveolar surface area and alveolar number, which was most marked in calves older than 30 days. Lung volume and body weight increases were also most marked when calves were older than 30 days. Mean bronchiolar cross-sectional area increased significantly (P less than 0.05) with age, and there was a significant (P less than 0.05) decrease in the percentage of terminal bronchioles with diameters less than 3 x 10(4) microns between 1 and 150 days. In newborn calves, nonciliated bronchiolar epithelial cells retained fetal characteristics of differentiation, including abundant cytoplasmic glycogen and sparse agranular endoplasmic reticulum. Glycogen deposits were depleted, and agranular endoplasmic reticulum was developed in nonciliated cells by the time calves were 30 days old. We concluded that bronchiolar epithelium in calves is well differentiated by the time they are 30 days old and that lung growth in calves raised in semi-isolation begins most rapidly when calves are approximately 30 days old.