Three-dimensional structure of the smooth muscle in the term-pregnant human uterus

To identify the structural elements and describe the three-dimensional microanatomy of the term-pregnant human uterus.

Biopsy specimens were taken from the funduses and the lower uterine segments of two term-pregnant uteri. Tissue was sectioned serially and stained with Masson trichrome. Microscope images were captured with a digital camera, aligned, and computer-processed. Computer-assisted three-dimensional reconstruction of structures was performed.

Five structures were identified: 1) fasciculus-the macroscopic organization of myocytes and connective tissue, 2) cylindric bundle-300+/-100-microm diameter cylinder of densely packed myocytes, 3) sheet-like bundle-densely packed sheet of myocytes, 4) fiber bundle-a loosely packed bundle of small numbers of myocytes, and 5) communicating bridge-a large cylindric bundle that bridges adjacent fasciculi. The cylindric bundles merged, dichotomized, and intertwined with each other and with sheet-like bundles. Essentially all myocytes except those in fiber bundles were contiguous.

The microanatomy of the term-pregnant human uterus contains clearly defined structural elements. These structures merge with each other and potentially form an interlacing network, connecting the vast majority of the myocytes of the uterus in a contiguous pathway.