New features in the development of the submucosal gland of the respiratory tract

The development of submucosal glands in the respiratory tract was studied by light and scanning electron microscopy in the rat, fetal dog and fetal sheep. From the results obtained the present concepts about the formation of these glands in man were questioned and an alternative hypothesis proposed. With scanning electron microscopy the development of the submucosal gland was seen to begin with an aggregation of low electron-responsive cells. Within such an aggregate, a pit, several microns in diameter, was formed. This pit was usually surrounded by medium electron-responsive cells possessing primary cilia in the rat, and by low electron-responsive cells in the fetal dog. In the rat medium electron-responsive cells appeared in other areas of the aggregate, preceded by apical elevations on the low electron-responsive cells. Further development in the rat led to a disappearance of the low electron-responsive cells, differentiation of ciliated and brush cells, and enlargement of the gland orifice. With light microscopy it was observed that the initial gland buds in both the rat and fetal sheep contained lumina several microns in size. These have not been reported by previous investigators. The bud extended into the underlying tissue and developed many simple tubules. The lumina of these tubules were consistently larger than the channel close to the epithelial surface. The cells of these tubules were also the first to differentiate into mucous and serous cells. The development of glands in the rat, in contrast to the sheep, began after birth. In the sheep, unlike the rat, the lumina of the developing glands were often filled with acidic mucosubstances, even though the cells of these glands did not stain for such material. Hence it is suggested that this material is derived from the mucin-containing cells of the surface epithelium and is carried into the interior of the developing gland by the fluid present in the respiratory tract during intrauterine life.