The vascularisation of the central-peripheral transitional zone of rat lumbar ventral rootlets: a morphological and morphometric study.

The blood vessels supplying the central-peripheral transitional zone of rat fifth lumbar ventral spinal nerve rootlets were examined during development and at maturity. At all stages all vessels were either capillaries or postcapillary venules. They lay in the spaces between the rootlets, being entirely absent from the endoneurial spaces. A proportion of these vessels communicated with those supplying the adjacent spinal cord. In this respect they differed from those supplying the dorsal rootlet transitional zone, at least in the cat, where no such communication occurs. During the first week after birth, at least one capillary was directly related to each rootlet, generally over about half the length of the transitional zone. Subsequently vascularity increased considerably. At three weeks postnatum, and subsequently, capillaries outnumbered rootlets by up to 50% and almost the entire length of the transitional zone was related to one capillary or more. This change was related to the maturation of the transitional nodes of gamma axons, which is likely to be related to increased alpha and gamma motoneuron activity. These changes were somewhat offset due to the fact that rootlet diameter increased with age. As a result, the distance between the capillary wall and the centre of the rootlet almost doubled between 20 and 300 days postnatum. The diameter of the capillaries did not change with age but that of the postcapillary venules increased.