Ultrastructural morphometric characterization of alveolar type II cells of perinatal sheep: a comparison with adults.

The quantitative morphologic changes in alveolar type II cells during the perinatal period were characterized morphometrically in the lungs of fetal lambs at 132, 138, and 147 days gestational age (DGA) and in newborns at 2 days postnatal age (2 DPN). Ultrastructural features were compared with those of type II cells of ewes 365 days old. Lamellar body profile number per type II cell profile was highest at term (147 DGA) and 2 DPN. In adults, the number of lamellar body profiles and volume density of lamellar bodies were equal to those of the 132 DGA fetus. Multivesicular bodies were most common at 138 DGA and in adults. The volume density of cytoplasmic glycogen fell dramatically during the latter part of gestation. The volume density of many cellular organelles increased to the level observed in adults by term (147 DGA). Subcellular composition of type II cells of adult sheep differs from that reported for adult rats chiefly by the volume density of lamellar material within the cytoplasm. Plate-like or globe-like inclusions were present only in the type II cells of adults. Cytoplasmic extensions of the type II cell crossing the basal lamina were most abundant in the 132 and 138 DGA fetal sheep. Cytoplasmic extensions were rare in adults. We conclude that morphologic changes of the alveolar type II cell associated with gestational age follow a species-specific time course. In the sheep, this occurs during the later part of gestation and extends into the neonatal period. Morphologic and morphometric changes appear to correspond with cellular interactions between alveolar type II cells and mesenchymal cells of the interstitium.