Amniotic membrane from human term placenta: a morphological study

Human term placental tissues arouse great interest as a source of stem/progenitor cells useful in regenerative medicine (1). Human term placenta, a discarded organ after parturition, appears ethically accepted as a new source of stem/progenitor cells and easy to obtain with a non-invasive method. Moreover, placental stem cells show interesting immunomodulatory properties as well as phenotypic plasticity. Due to the complexity of human term placenta, it is necessary to clearly define its histological structure, to better understand morphology, function and appropriate use of its cells for cell therapy. Our study was focused on the amniotic membrane, the innermost layer of human term placenta consisting of a thin epithelium placed on an avascular stromal matrix on foetal and amniotic fluid side. Human term placentas were obtained from donors after informed consent according to the guidelines of the Ethical Committee of the Catholic Hospitals (CEIOC). The amnion was manually separated from the chorion as previously described (2). Three tissue samples were taken from the area in close proximity to the umbilical cord toward the periphery and processed for light and transmission electron microscopy. Light microscopy observations showed changes in the epithelium organization but not in the structure of the underlying loose avascular connective tissue, containing several star-shaped mesenchymal stem cells. In particular, from the centre toward the periphery, the surface epithelium changed from squamous stratified, with a number of exfoliating cells, to a monolayer of cubic and cylindrical cells with a brush border and, at the periphery, to a monolayer of squamous epithelial cells covering a thinner connective tissue. Immunocytochemical analysis demonstrated the expression of cKit and OCT3/4, known markers of pluripotency. Transmission electron microscopy observations showed a mixed cell population mainly of epithelial nature. Both cell types displayed euchromatic nuclei and prominent nucleoli and aspects of secretory activity. At cytoplasmic level numerous mitochondria and different types of vesicles and granules, both in size and in chemical nature (protein and/or lipid), were detected. The surface of the epithelial cells had numerous microvilli of different length, with functions of resorption/secretion of the amniotic fluid. All the observed morphological features are closely related to the characteristics of plasticity and immunomodulation of these cells.