Electron Microscope Observations on the Endoplasmic Reticulum in the Human Fetal Adrenal

The human fetal adrenal is proportionally 10 to 20 times larger than the adult gland. Its large size is attributable to a histologically distinct central reticular zone known as the fetal zone. This zone is prominent throughout fetal life, but undergoes rapid involution in the first postnatal week (1). In the course of a study of the human fetal adrenal, electron micrographs were obtained of cells in the fetal zone. These cells are distinguished by having a very extensive, densely packed system of tubules which occupies most of the cytoplasmic volume and are considered to be morphologically homologous to the endoplasmic reticulum found in other cells. In addition to its tubular structure, and dense packing, the endoplasmic reticulum of the fetal zone ceils differs from that of other cell types described by Porter (2) and Palade (3) in being predominantly free of attached particles. I t is felt that the unique character of the endoplasmic reticulum in the fetal zone justifies a brief description of our findings. The adrenal specimens were obtained in Stockholm, from human fetuses between 6 and 17 weeks of gestation. The fetuses were delivered either by Caesarian section or vaginal hysterotomy. One adrenal gland was dissected free of the surrounding tissue and placed in a pool of cold OsO4. In the case of the Caesarian sections the time lapse between cessation of blood flow from placenta to fetus until the tissue was placed in OsO4 was approximately 3 to 4 minutes. Fetuses delivered by vaginal hysterotomy necessitated a longer time lapse due to the procedure involved. However the degree of postmortal change between the two pro-