The ultrastructure of lung tissue from newborn and embryo mice

Lung tissue from embryo, newborn and adult mice was studied by means of the electron microscope. Discontinuities were not found in either the endothelial lining of the capillaries or the epithelial cell lining of the alveoli. Capillary endothelial cells of newborn and embryo mice were similar in appearance to those of adult mice. Surface configurations of certain of the epithelial and endothelial cells were suggestive of pinocytosis. Numerous microvilli projected from some of the epithelial cells into the alveoli. The outer nuclear membrane of alveolar epithelial cells was found to be in direct continuity with membranes of the ergastoplasm. Certain epithelial cells contained characteristic inclusion bodies. Some alveolar cells may have been undergoing degeneration, as indicated by the general appearance of the cytoplasm as well as the presence of the inclusion bodies. Structures similar to the cristae of normal mitochondria could be seen in many of these bodies in newborn, adult lungs and occasionally in lungs of 18-day embryos. Inclusion bodies were not found in embryos younger than 18 days. There might be a relationship between mitochondria and these inclusion bodies. In many areas of the lungs of newborn mice a homogeneous basement membrane was apparently just being formed. Collagen and elastic fibers were frequently found in newborn and late embryo as well as in adult lung tissue. Newborn mouse lung tissue was found to contain areas of striated muscle. There was evidence that in newborn mice many alveoli were just developing. Microvilli from adjacent epithelial cells frequently projected into such areas. Evidence for and against the possibility of the existence of two cell types in alveolar epithelium has been presented and discussed.