In humans, at 13-14 weeks of fetal development, separate accumulations of epithelial cells appear in the epithelium of the mucous membrane of the trachea and bronchi, which are sources of development of the glands of the mucous membrane and differ in their histochemical characteristics from neighboring epithelial sections. In the cytoplasm of these cells, acidic and neutral mucopolysaccharides are secreted. During intrauterine development, glands and goblet cells are referred to as mucous secretory elements. Given that the goblet cells of the respiratory epithelium secrete only mucins, the submucosal glands are the only sources of the protein component of the surface layer of mucus.