EARLY POSTNATAL GROWTH OF THE INITIAL LYMPHATICS IN THE VENTRAL STRIPE OF SPINOTRAPEZIUS MUSCLE OF THE RAT

The aim of the study was the morphological description of the early postnatal growth of theinitial lymphatics in the ventral stripe of the spinotrapezius muscle of the rat. Electronmicroscopically it was found out that in the muscles of newborn rats no well developedlymphatics were apparent, but the presence of specifically polarized mesenchymal cells in theclose vicinity of central blood vessels was evident. In animals aged one-day through two-weeksold, those modified mesenchymal cells continuously joined with one another, to form simpleintercellular contacts and incomplete lymphatic lumina. Morphologically, they were welldemarcated relative to the surrounding muscular interstitium. In three-week-old rats allintramuscular lymphatics were well developed, including fine intraluminal valves, and theirendothelial cells presented slight pinocytotic activity and a complete absence of a basal lamina.In growing lymphatic endothelial cells no mitoses or signs of sprouting, typical for the growth ofblood capillaries, were found. In conclusion, a possible morphological mechanism enabling theexpansion of the growing lymphatic endothelial cells could be the specific remodeling ofcytoplasmic vacuoles accumulated in the peripheral cellular processes.