The process of lymphangiogenesis was studied in the muscle coat of the rat small intestine by light and scanning and transmission electron microscopy; identification of lymphatic vessels was made by 5’-nucleotidase staining.
Light and scanning electron microscopy demonstrated that the intramuscular lymphatic network formation, which started only postnatally, was attributable to the vascular sprouting of slender lymphatic endothelial projections and to a splitting of the vessels, causing intervascular meshes of various sizes. The growing lymphatics were consistently closed by the endothelial cells, which were characterized by an abundance of cell organelles and prominent cytoplasmic processes. The cells often revealed close contacts with the processes of developing smooth muscle cells in the jejunal muscle coat, suggesting a possible role for the latter cells in the guidance of the lymphatic extension.
The present study is the first to suggest the closed nature of lymphatics persisting throughout their development, even at the initial stage of lymphangiogenesis.