Early estrogen-induced nuclear changes in rat hypothalamic ventromedial neurons: an ultrastructural and morphometric analysis.

Alterations in the fine structure of nuclei of ventromedial hypothalamic neurons in ovariectomized (OVX) rats after either a 2-hour exposure to estradiol (E2) or a discontinuous exposure (2 hours E2/7 hours off/2 hours E2), previously shown sufficient for female rat sexual behavior (Parsons et al., '82a), were examined with the electron microscope. Morphometric measurements of nucleolar, nuclear, and somal areas, and nuclear shape and perimeter were accomplished at the light microscope level. After 2 hours of E2, the appearance of the nucleoplasm was altered, with a decrease in the small, scattered clumps of heterochromatin. Nuclear shape appeared dramatically altered from nonspherical, invaginated profiles toward spherical, smooth profiles. Nucleolar, nuclear, and somal hypertrophy were evident. In addition, stacked rough endoplasmic reticulum was present more frequently in E2-treated than control OVX neurons. After the discontinuous (2 hours/7 hours/2 hours) E2 treatment, progressive loss of small clumps of heterochromatin along the nuclear envelope as well as in the nucleoplasm had occurred. Decrease in a large heterochromatin clump along the nuclear envelope was correlated with an increase in nucleolus-associated chromatin. As determined by a distribution analysis, these estrogen-induced nuclear changes co-occurred more frequently than predicted from mutual independence. These findings, the marked co-occurrence of E2-induced changes in 30% or more of the cells, and the differences between the 2-hour E2 and the 2-hour/7-hour/2-hour group are consistent with a cascade of cell nuclear changes in the first few hours after estrogen onset.