Three-dimensional structures of uterine elastic fibers: scanning electron microscopic studies.

We report findings that demonstrate for the first time that the structure of the elastic fibers of the uterus and cervix are characteristic to these tissues. Elastic fibers of the uterine corpus and cervix were studied by scanning electron microscopy (SEM). Elastic tissues were prepared from non-pregnant human uteri and pregnant rat uteri by three different methods: extraction from the tissue homogenates, in situ digestion by autoclaving of sliced tissue, and in situ formic acid digestion of sliced tissue. In addition, in situ formic acid digestion of the glutaraldehyde-fixed uterine wall of pregnant rats was done. Under SEM, the uterine elastic fibers revealed two distinct structures--fibrils and thin sheets of elastic membranes. Isolated fibers and membranes ranged in thickness from 0.1 to 0.4 micron which is thinner than aortic elastic lamellae (1-2.5 microns in thickness). These thin sheets of elastic membranes and elastic fibrils probably allow the uterus to maintain its elasticity without exerting excess pressure on the growing fetus. Formic acid digestion of fixed uterine walls of pregnant rats preserved the structural organization of elastic tissues near in vivo conditions. In these tissues, the fibers were arranged in a honeycomb structure made up largely of membranes, although sparse fibrils were present. These elastic, membranous sheets were arranged parallel to the plane of the uterine surface and interconnected with the threads of the membranes or fibrils. In the rat uterine wall, at least 12 parallel layers of elastic sheets were present. In contrast, at low magnification, the elastic tissues in the non-pregnant human uterus had no specific architectural arrangement and exhibited a sponge-like structure. Elastic fibers of the cervix were also made up of membranes and fibrils, and these fibers were organized into fishnet-like structures. These cervical membranes had fenestrations and pits with a diameter of 3-5 microns. In these studies, the concentrations of insoluble elastin in human uteri were found to be 1.38 and 1.32-1.41% of dry-defatted tissues for uterine body and cervix, respectively. The concentrations of total collagen were 38.8 and 64.3-72.4% of dry-defatted tissues for uterine body and cervix, respectively.