A novel source of mast cells: the human placenta.

The presence of moderate amounts of histamine in the human placenta was confirmed (0.72 +/- 0.10 microgram/g wet weight), and the hitherto unknown storage site of this biogenic amine was elucidated. Mast cells were identified by their characteristic morphology, staining reactions and secretory activity measured in terms of histamine release. Human placental tissue contains 7.6 x 10(5) mast cells/g wet weight, identified by staining with toluidine blue or alcian blue, and these cells were positive for chloro-acetate-esterase. Light microscope studies of placental tissue stained with HRP-conjugated anti-human IgE demonstrated cells with a typical 'halo' effect indicating cell-bound IgE, and electron microscopy revealed cells containing membrane-bound electron dense granules. A single mast cell was calculated to contain approximately 1 pg of histamine. Enzymatic digestion of placental tissue with collagenase (1.5 mg/ml) yielded viable cell suspension. containing mast cells in a purity of 0.6% which exhibited a low spontaneous output of histamine (12%). Placental mast cells released histamine in a concentration dependent manner upon challenge with anti-human IgE and the calcium ionophore A23187. Also, unlike other human mast cells so far studied. with the exception of skin, those dispersed from human placenta were responsive to the polybasic secretagogue compound 48/80. These findings represent a novel source of human mast cells and, since placentas are readily available in quantity, such tissue is proposed as an ideal source of mast cells for biochemical and pharmacological use.