Changes in thickness of collagen fibrils in the endo- and epineurium of the mouse sciatic nerve during development.

Changes in the diameter of collagen fibrils were observed with an electron microscope in the endo- and epineurium of sciatic nerves of mice during development from 12 days of gestation to 5 months after birth. It was noted that endoneurial collagen fibrils appeared in embryonic mice at 15 days of gestation, and at the same time, basal laminae began to appear sporadically on the Schwann cell plasmalemma. No fibroblasts were seen at this developmental stage. Collagen fibrils in the endoneurium remained as thin as they were when they first appeared, being in the narrow range of 250-300 A in diameter, while those in the epineurium became much thicker (400-450 A, 5 months after birth) as is also the case in dermal connective tissues. The present study shows that the endoneurial collagen fibrils were different in their developmental pattern from those of the epineurial or of other connective tissues, lending support to the concept that the endoneurial collagen fibrils are particular in nature, being so-called histological reticular fibers.