Endocrine secretory granules and crystals in the syncytiotrophoblast

Large membrane-bound crystalline inclusions were seen on electron microscopic examination of the syncytiotrophoblast from the placenta of a HIV infected patient. The syncytium displayed abundant secretory granules (180-240 nm in diameter), with electron-dense centers (i.e., bull's eye granules or targetoid granules) which occasionally showed partial crystallization. Some crystals exhibited a periodicity of 9-10 nm. Electron-dense material was also seen in distended cisternae of the rough endoplasmic reticulum (RER). These granules correspond presumably to human placental lactogen (HPL) secretion, as seen on light microscopic immunoperoxidase stains. The morphological evidence is consistent with the idea that the crystals represent crystalline accumulation of secretory product (HPL), thus forming an analogy with crystal formation in other types of secretory cells. Ribosome-lamella complexes were seen in the many pericytes present in the capillaries of chorionic villi.