Alterations in nuclear envelope invaginations in axotomized fetal and early postnatal hamster facial motoneurons

Abstract

In this study, changes in the amount of nuclear envelope invaginations (NEI) were morphometrically assessed after axotomy during late fetal and early postnatal developmental stages in hamster facial motoneurons. These changes were expressed as boundary density or B A (length of nuclear envelope per unit area of nucleus). Axotomy-induced changes in nuclear area and perimeter were also quantitatively determined. At 17 h after axotomy in the fetal operative series, no changes in any of the parameters were seen. At 1 day postoperative (dpo) in newborn, 2 and 4 postnatal day animals, the boundary densities of the total and invaginated portion of the nuclear envelope increased significantly. No corresponding qualitative changes were observed. At 2 dpo in 4 and 7 postnatal day animals, there were significant increases in the boundary densities of both invaginated and total nuclear envelope and a decrease in nuclear area. These changes were not seen at 2 dpo in the 9-day operative series. At 4 dpo in 7 and 9 postnatal day animals, scalloping of the normally smooth nuclear profile, as well as a flattening and elongation in nuclear shape, occurred. These qualitative changes in the 7 and 9 day operated groups were also accompanied by significant changes in all the measured parameters. The boundary density of the invaginated, non-invaginated and total nuclear envelope increased; whereas, nuclear area and perimeter decreased. These results argue against the generally held hypothesis that an increase in nuclear envelope invaginations is indicative of an allied increase in cellular metabolism. Rather, it appears that the incidence and extent of nuclear envelope invaginations depend on factors that operate differentially under conditions imposed by cell type, age and stress, such that the circumstances leading to nuclear membrane invaginations are more complex than previously supposed.