Human trophoblast differentiates by the fusion of cytotrophoblasts to form syncytiotrophoblast. To determine factors controlling this process, the effects of epidermal growth factor (EGF) on trophoblast differentiation were studied using long term serum-free culture of isolated trophoblast. Only trophoblast was present in the cultures, as demonstrated by positive immunoperoxidase staining with beta hCG, cytokeratin, and trophoblast-specific H315 monoclonal antisera and by the absence of contaminating endothelial cells, fibroblasts, and macrophages, as shown by negative staining with vimentin and OKM1 monoclonal antisera. EGF induced large sustained increases in hCG and human placental lactogen (hPL) secretion in a dose-dependent manner. The minimum effective dose was 0.1 ng/mL, and the maximum effective dose was 1 ng/mL. Light and electron microscopic studies showed EGF-induced differentiation of cytotrophoblast to form syncytiotrophoblast. DNA content and cell number did not change during the process. The formation of syncytia thus probably accounted for the increase in hCG and hPL secretion. We conclude that EGF causes morphological differentiation, but not cell proliferation, of trophoblasts, and the differentiation results in increased hCG and hPL secretion from the syncytia.

The structure and morphogenesis of respiratory syncytial (RS) virus particles in a green monkey kidney cell line (Vero) were examined. Infected cells contained dense intracytoplasmic inclusions composed of filamentous structures. In places where inclusion material was associated with membranes, structural modifications were induced. There was a thickening of the membrane and an addition of projections 12 to 15 nm in length. The same changes were most frequently observed after association of isolated filamentous structures with the cytoplasmic membrane. The buddingoff process was clearly visualized. The diameter of mature virus particles varied between 90 and 130 nm and that of the internal component varied between 11 and 15 nm. The similarities between ultrastructural features of cells infected with RS virus and pneumonia virus of mice are pointed out. It is proposed that these two viruses should be classified together in a third subgroup of myxoviruses.

Electron microscopy of purified preparations of equine arteritis virus (EAV) revealed enveloped, spherical particles with an average diameter of 55 m~. The envelope was found to carry tiny projections on the surface, 3 to 5 m~ in length. No detailed structure of an internal component could be seen.In sections of EAV infected BHK cells 24 hours after inoculation, viral particles were shown to bud from the cytoplasmic matrix into cisternae of the endoplasmic reticulum, the process starting in the vicinity of the Golgi apparatus. The mature particles had an average diameter of 50 m~, with an inner core measuring 25 m~.

No abstract