Lymphatic vessels in the developing diaphragm of the rat.

Diaphragms of fetal, neonatal and young albino rats have been observed both under light and electron microscopes to examine the presence and distribution of lymphatic vessels and their morphological features. In fetal diaphragms of between 18 and 22 days of gestation, no normal lymphatic vessels can be seen; only after birth, specifically in neonatal and 2-day-old rats, small lymphatic vessels appear; they are in close proximity to the blood vessels in the inner areas of the muscle. As the rats get older, lymphatic vessels are also observed in the subserosa where an abundant connective tissue is present. The fine structure of diaphragmatic lymphatic vessels is different at different ages. In neonatal rats of up to 2 days, the endothelial wall is very thin and often holed. The relationships between contiguous endothelial cells are characterized by simple end-to-end or overlapping structures. The basement membrane is virtually absent. Within the first week of life, the endothelial wall becomes more complex; along the wall, complex interdigitations between two contiguous endothelial cells often touch. A discontinuous basement membrane and collagen and elastic fibers surround the vessels. In the older rats (from 14 to 25 to 140 days), next to the complex interdigitations which characterize the junction between two contiguous endothelial cells, cellular flaps interdigitate forming a channel which opens out either to the exterior or the interior of the vessel. Dense bundles of elastic and collagen fibers are closely apposed to the endothelial wall.