Cytokinesis in the stratum intermedium of the rat molar enamel organ

Clinical, histological and ultrastructural features of a possibly virus-induced oral leukoplakia

Two male patients with oral leukoplakias exhibiting a peculiar type of apparent dysplastic changes have been followed for 4 and 6 years, and a series of biopsies have been examined by light and electron microscopy. The apparent lack of normal epithelial stratification below the keratinized cell layers was mainly caused by the frequent appearance of large ballooned cells and multinuclear giant cells. The centre of the large ballooned cells contained aggregations of chromatin and evenly-dispersed microtubulus surrounded by a thick rim of tonofibril bundles. In the peripheral cytoplasm large numbers of smooth-surfaced endoplasmic reticulum were found, but no Golgi apparatus was observed. In addition, several autophagocytic bodies were recorded. Along the cell membrane only a few desmosomes were present, whereas aggregations of digested desmosomes were found in the cytoplasm. On the basis of the ultrastructural findings, it is suggested that the large ballooned cells represented epithelial cells arrested in early stages of mitotic division. The epithelial cells in interphase exhibited a normal ultrastructure except for large nucleoli with varying degrees of condensation of nucleolonema and vacuolization. Further, atypical dense granular aggregations and strands of fine fibrillar material were recorded in the nuclei. It is suggested that this new type of oral leukoplakia has a viral etiology.

The development of the elastic cartilage of the mouse pinna

The development of the elastic cartilage in the mouse pinna was studied with the electron microscope. For descriptive purposes, the developmental process was divided into five stages. These begin with the onset of cartilage matrix production by chondroblasts. The second stage is the major mitotic stage although the cells now resemble chondrocytes and cartilage matrix production evidently continues. This is followed by a period during which cartilage matrix production appears to be the major activity of the chondrocytes. In Stage IV, the cells continue to produce cartilage matrix but at the same time they also produce lipid which comes to occupy most of the cell as a large lipid droplet. In the adult, Stage V, matrix production appears to be at an end and each cell contains a lipid droplet which is so large that the cells bear some resemblance to white fat cells and might be designated as lipochondrocytes. The adult cartilage matrix contains elastic plates in a three-dimensional honeycomb pattern. Between the elastic plates and the cells, the matrix contains elements similar to those seen in hyaline cartilage but it does not contain matrix vesicles such as those which have been associated with the calcification of cartilage. Two types of filaments differing in appearance, size, and location are seen within the chondrocytes. They have been designated cytofilaments and cortical filaments. Cytofilaments are dispersed throughout the cell and also contact the substance of the lipid droplet; cortical filaments (sub-plasmalemmal microfilaments) are located directly under the plasma membrane. They are thought to be involved in movement of the plasma membrane as a concomitant to extracellular fibrogenesis.