Axonal degeneration and regeneration in rat uterus during the estrous cycle

Uterine innervation of the adult virgin rat changes throughout the estrous cycle. Nerves immunoreactive for the pan-neuronal marker protein gene produ ct 9.5 and the sympathetic marker dopamine beta -hydroxylase are maximal at diestrus and minimal at estrus, whereas presumptive sensory and parasympat hetic axons are unchanged. In the present study, we used quantitative elect ron microscopy to determine if depletion of immunoreactive nerves from the myometrium is due to loss of structurally intact axons, and whether this oc curs through degeneration or retraction. Numbers of intact myometrial axons per unit sectional area were greatest at diestrus and least at: estrus, wh ile myometrial area was smallest at diestrus and greatest at estrus. Howeve r, depletion of intact axons at estrus was evident even after correcting fo r changes in uterine size. Varicosities adjacent to smooth muscle cells did not vary significantly with respect to their ultrastructural features or d istance to the nearest smooth muscle target cell. Because retracting axons show increases in organelle content and distances to target cells, retracti on probably does not play a major role in reducing uterine innervation. In contrast, axons with ultrastructural features consistent with degeneration (organelle and axolemmal disintegration, abnormal electron opacity, dense i nclusion bodies) were significantly increased at proestrus and estrus. Grow th cones were observed only at metestrus and diestrus. We conclude that cyc lical degeneration and regeneration of myometrial innervation is a normal f eature of the virgin adult rat. (C) 2000 Elsevier Science BN. All rights re served.