The structure of the chromosomes in human primordial oocytes

Primordial oocytes (oocytes in primordial follicles) from human ovaries aged 51/2 months post conception to 11 3/4 years post partum were examined in: (a) squash preparations of fresh and fixed tissue; (b) histological preparations; and (c) thin sections by electron microscopy, in order to study the structure of the chromosomes. — The light microscope shows that the chromosome consists of a thread bearing numerous fine lateral appendages. Cytochemical tests indicate that the thread contains DNA, and is surrounded by material containing RNA and protein. — The electron microscope shows that there are three main structural components in the chromosome: (i) an axis or core containing at least two longitudinal strands about 200 Å thick; (ii) a surrounding sheath composed of coiled fibrils which form symmetrically arranged columns and loops, and (iii) clusters of large granules which are associated with the outer parts of the sheath. Small nucleoli and other granular bodies are also present. — These observations indicate the presence of lampbrush chromosomes in the human oocyte. The significance of this type of chromosome in mammals is discussed in relation to the differential radiosensitivity of the oocytes, and to the form of chromosomes at the dictyate stage in rodents.