The site of iodination of thyroglobulin in the thyroid gland revealed through morphological studies was critically reviewed in relationship to biochemical points of view. The phylogenetic and evolutional aspects of this problem were also reviewed and discussed. To conclude, the present author wishes to emphasize the following:
1. In adult vertebrates whose thyroid follicular structure has been well completed, the main site of the iodination of thyroglobulin is generally the follicular lumen and the apical plasma membrane region.
2. The iodination may also take place partly in the cytoplasm. Autoradiographic data from the chick embryo and from the dissociated thyroid cell of the rat and sheep, and the electron microscopic histochemical data for peroxidase reaction indicate the possibility that the iodination could occur also in the cytoplasm, such as in the rough endoplasmic reticulum, Golgi apparatus and subapical vesicles.
3. In usual adult animals, however, thyroglobulin in the follicular lumen is far larger in quantity than that in the cell cytoplasm and numerous molecules of thyroglobulin in the follicular lumen might have not yet completely iodinated and therefore injected iodine is considered to be combined with luminal colloid preferentially. This seems to be the reason why the iodination takes place almost entirely in the follicular lumen (and apical plasma membrane region) in adult animals having usual follicular structures.
4. Based on the data of larval lampreys and ascidians, iodine metabolism in the thyroidhomologous cells in cyclostomes and protochordates seems to show the similar pattern to that of higher vertebrates. The main site of iodination of protein in these animals is the apical plasma membrane region of certain types of endostylar cells, and the possibility of iodination taking place also in the endostylar lumen is not ruled out. Some of iodinated protein must be reabsorbed into the certain types of endostylar cells in these animals.

View full abstract

The cells, fibers and ground substance of the connective tissue of the snake Xenodon merremii were studied by morphological and histochemical methods. The main results obtained were:
1. Xenodon has macrophages, plasma cells, lymphocytes and fibroblasts comparable to these cells in mammals.
2. No mast cells were found in this species while a characteristic granular acidophilic cell was described.
3. In the mesentery, the collagen appears less polymerized than in mammals. Thus, no collagen fibers were observed while fibrils and microfibrils were abundant.
4. Smooth muscle cells appear frequently in the mesentery.
5. In this same structure patches of ciliated cells and cells analogous to the lung septal cells appear substituting the mesothelial covering.

View full abstract

The parathyroid gland of the laying hen, Gallus domesticus, was studied in the electron microscope. The parathyroid gland was comprised of a single cell type, the chief cell. Most of the chief cells contained a small amount of glycogen granules, numerous aggregated ribosomes, abundant mitochondria, cisterns of granular endoplasmic reticulum occurring close to the mitochondria, well developed Golgi apparatus and numerous secretory granules. Prosecretory granules (50-100mμ in diameter) enclosed in a loosely fitting membrane occurred numerously in and near the Golgi field.
Secretory granules (200-400mμ in diameter) were round and homogeneously dense bodies were bounded by a limiting membrane. Some granules were seen in the vicinity of, or even in contact with the plasma membrane. The granule content composed of finely granular, electron dense materials was sometimes continuous to the intercellular space through an opening in the fused limiting and plasma membranes. Irregularly shaped dense masses presumably corresponding to the granule content were occasionally seen in widened portions of intercellular spaces. It was thus considered that the secretory granules may be released by eruptocrine secretion.
Round bodies (200-400mμ in diameter) containing heterogeneously dense structures seem to be derived from secretory granules and to be lysosomal in nature. These bodies may be involved in the control mechanism of secretion.

View full abstract

The pars tuberalis of the hypophysis and the external layer of the median eminence were studied by electron microscopy using hypophysectomized rats. Twenty day-old and adult rats were hypophysectomized and examined at various intervals (from 10 to 150 days) afterwards. Several hypophysectomized adult rats were adrenalectomized and examined 3 days later.
Ten days after hypophysectomy, the external layer of the median eminence indicated several neurosecretory axons containing large vesicles (1, 500-3, 000Å) with osmiophilic interior and aminergic axons with small cored vesicles (800-1, 200Å) and clear vesicles (300-500Å). Thereafter these axons steadily increased in number, especially the neurosecretory axons, and extended deeply into the pars tuberalis making contact with the surface of the glandular cells.
In spite of the alterations mentioned above, the fine structures of the tuberalis cells showed no remarkable changes, while the anterior hypophysial cells in the remnant of the sectioned stalk changed remarkably in their cytoplasmic appearance.
In the adrenalectomized adult rats, whose hypophysis had been removed 30 days earlier, the anterior hypophysial tissue in the remnant of the stalk sectioned indicated many active ACTH cells (KUROSUMI and KOBAYASHI, 1966), while the tuberalis cells did not show any alteration in their cytoplasmic details.
These observations may suggest that the tuberalis cells, unlike the anterior hypophysis, are not under the neurohumoral control of the hypothalamus in spite of their topographical and embryological association.

View full abstract

The mesonephros from human embryos ranging 7-20mm in total length was investigated with light and electron microscopes. The fine structure of the mesonephric glomerulus is characterized by a thick endothelium, wide subendothelial space and poorly differentiated podocytes. These features which are prominent in the 7-12mm embryos are obscure in the glomerulus of larger embryos which are rather similar in fine structure to that of the metanephros.
The proximal tubule of the mesonephros consists of columnar cells bearing long microvilli and apical invaginations and vesicles suggesting pinocytotic activity. The distal tubule consists of tall columnar cells with smooth apical surfaces doming into the lumen and lacks the basal infoldings.
In the subendothelial space of the mesonephric glomerulus there are found fine filaments approximately 100Å in diameter extending from the glomerular basement membrane toward the endothelium. After treatment with ruthenium red, their diameter increases to 200Å. The glomerular basement membrane strongly reacts to ruthenium red. There are many granules approximately 100-200mμ in diameter in the basal part of the podocytes. These granules and fine filaments are supposed to participate in the formation of the glomerular basement membrane.
Mesangial cells are found in the intercapillary space surrounded by a basement membrane and at the vascular pole accompanied by capillaries. A juxta-glomerular apparatus cannot be found.

View full abstract

Spherical objects appear as circular profiles on sections studied with the electron microscope. The three-dimensional size distribution of such objects has been derived from the distribution of profile sizes on sections. For this purpose, the profile diameters have generally been measured. However, it is proved to be more convenient and easy to obtain the size distribution of spherical objects from measurements of the profile areas rather than the diameters on sections. In this article, a new stereological method is proposed and its principle and procedure are described. In addition, the method was applied for determination of the sizes of small lymphocytes of the mouse thymus to reexamine the results reported in our previous paper (ABE and ITO, 1970).

View full abstract