Cells of the follicular epithelium and interstitial cells of the rabbit ovary, spinal ganglion cells of dogs, and oocytes of a sea-urchin (
Heliocidaris crassispina) and the rabbit were examined with the electron microscope. The minute structures of nucleoli of these cell types were compared with each other and the following results were obtained.
1. All of the nucleoli studied consist of a great number of extremely minute granules, whose diameter in mammalian nucleoli measures about 150-200Å, while the granules in the nucleolus of sea-urchin oocyte are larger than 300Å. No limiting membrane is observed between the karyoplasm and the nucleolus.
2. The mode of the aggregation of the granules in nucleoli is widely variable and may be classified into the following three types.
i. Nucleolonema (according to ESTABLE and SOTELO 1951). An aggregate of the granules seen as a thread or filament about 0.2μ thick,
ii. Pars amorpha (according to ESTABLE and SOTELO 1951). An irregular mass of the aggregated granules,
iii.
Pars sphaeroidea (designated by the present authors). A regular spheroidal body of the granular aggregate having a variable size.
3. It was observed that more than two types of the above classification could be mingled within a single nucleolus in some cases, and that a nucleolus was made up of one type of the granular aggregation exclusively in others.
Cells of the follicular epithelium and interstitial cells of rabbit ovaries consist of only the nucleolonemata or of a mixture of nucleolonema and pars amorpha. The nucleolus in spinal ganglion cells of the dog is a round glomerular body which consists of closely entangled nucleolonemata. In the nucleolus of rabbit oocytes, nucleolonemata compose a reticulum at the center of the nucleolus and several round bodies (
partes sphaeroideae) are scattered around the reticulum. Direct continuation between the nucleolonema and the spheroidal part was seen. In contrast to the nuleolar structures in a variety of cell types metioned above, none of the corrresponding features to the nucleolonema has observed in a nucleolus of the the sea-urchin oocyte, but the nucleolus, in this case, is composed of only one spheroidal body (
pars sphaeroidea) of enormously large dimension.
4. As described above, the morphology of the nucleolus is by no means uniform, but it varies widely according to the cell type and to the animal species. There exist certain evidences suggesting that such a variability in form of the nucleolus may depend upon a varying functional state of the nucleolus.
An appearance of complicated filaments, the nucleolonemata, results in a vast increase of the surface area of the nucleolus and may activate the interaction between the nucleolus and the karyoplasm or the cytoplasm, whereas a condition with pars amorpha or
pars sphaeroidea may represent a rather inert stage of the nucleolar function.
5. In ovarial interstitial cells of rabbit, the nucleolus frequently situates near the periphery of the nucleus. A part of this nucleolonema extends itself toward the nuclear membrane and attatches to the inner surface of the membrane. Such a figure is sufficient to suggest an active interaction between the nucleolus and the cytoplasm. We have not encountered, however, any feature to show an outflow of a substance of similar electron density to that of the nucleolus through the nuclear membrane. Thus, the authors can only suggest a possibility that a substance of the nucleolus may flow out to the cytoplasm, but cannot yet prove this phenomenon.
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