Light microscopic, electron microscopic and histochemical studies were carried out on the segmental mesometrial arteries of non-pregnant guinea pigs and on pregnant ones at each of the nine weeks of gestation. Pregnant animals show drastic changes in arterial structure and dimensions. Hypertrophy and structural dilatation of the arterial wall are obvious. In midpregnancy, the elastic membranes begin to disappear; only small fragments remain. From the fifth week on, mononuclear cells appear in the media; they form aggregates and occasionally giant cells with signs of phagocytosis in the seventh week of gestation. In the eighth week further degenerative changes can be observed, resulting in widespread destruction of the arterial wall. Deposition of necrotic cell debris is obvious in the ninth week. By this time there appear in the endothelial layer conspicuous single cells, cell aggregates and giant cells with heavily folded nuclei, prominent nucleoli, abundant vesicles, free ribosomes, intracellular lacunae and the histochemical properties of placental trophoblast. These cells in the endothelium are distinctly different from the medial giant cells of mononuclear origin. According to these observations, the segmental mesometrial arteries of pregnant guinea pigs show cytological and structural changes similar to those described for the mesometrial arteries in the hamster and the spiral arteries in man. The results show that, beside structural dilatation, degenerative changes and apparent trophoblastic giant cell invasion occur in the arteries studied. Trophoblastic invasion occurs later than structural dilatation and obviously does not trigger or control the structural dilatation of the segmental mesometrial arteries.

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Growth of mesometrial arteries in guinea pigs during pregnancy.

Massive growth was observed in mesometrial arteries supplying the placenta of the guinea pig in the first 60 days of pregnancy. The wet weight of the vessel wall increased 50 times, with an almost constant protein concentration. The DNA content rose 30 times. The length of the arteries increased 3.5 times and there was a 13-fold increase in the cross-sectional area of the wall. The changes in cross-sectional area were most pronounced in the peripheral part of the vessel. The observed changes followed an exponential time course. Histological sections showed proliferation, particularly of connective tissue with collagen fibres, while the synthesis of elastic material was suppressed. After parturition, involution with reduction of internal diameter occurred with a half-time of around five days.

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