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In recent electron microscope studies of the corpus callosum i n 60-80 gm male rats, some of the cells were identified as rnicroglia by staining with thc weak silver carbonate method of del Rio-Hortega, and others as astrocytes by staining with the gold chloride sublimate method of Kamcin y Cajal. In the present work, the cells which did not belong i n one of these two groups were examined i n the light microscope using semithin sections stained with toluidine blue and in the electron microscope using thin sections stained with uranyl-lead.These cells make up a largc, somewhat heterogeneous group, but they have a few common features: regular nucleus, cytoplasm rich in ribosomes and microtubules, and a variahle number of narrow, non-branching fine processes of uniform diameter. Because of these features, the cells are all taken to be oligodendrocytes. Three classes may be described within this group of cells: ( a ) light oligodendrocytes, which are large cclls with pale nucleus, a large nucleolus, a cytoplasm containing rather small organelles as well as many free ribosomes, and giving off nunierous fine processes; they make up about 6% of the glial cells in the corpus callosum; and they undergo mitosis at a rapid ratc; ( b ) mediumahade oligodendrocytes, which are somewhat smaller cells with moderately dense nucleus and cytoplasm, containing well developed organelles, and giving ofl' a fair number of fine processes; they make up about a quarter of the glial cells; and they undergo mitosis at a moderate rate; ( c ) dark oligodendrocytes, which are even smaller cells with very dense nucleus and cytoplasm, containing a prominent Golgi and structures referred to as lamellar bodies, and giving OR' very few fine processes; these cells make up about 40% of the glial cells; they do not undergo mitosis and probably arise from divisions of medium oligodendrocytes.

The elaboration of dentin collagen precursors by the odontoblasts in the incisor teeth of 30-40-g rats was investigated by electron microscopy, histochemistry, and radioautography after intravenous injection of tritium-labeled proline .At 2 min after injection, when the labeling of blood proline was high, radioactivity was restricted to the rough endoplasmic reticulum, indicating that it is the site of synthesis of the polypeptide precursors of collagen, the pro-alpha chains .At 10 min, when the labeling of blood proline had already declined, radioactivity was observed in spherical portions of Golgi saccules containing entangled threads, and, at 20 min, radioactivity appeared in cylindrical portions containing aggregates of parallel threads . The parallel threads measured 280-350 nm in length and stained with the low pHphosphotungstic acid technique for carbohydrate and with the silver methenamine technique for aldehydes (as did extracellular collagen fibrils) . The passage of label from spherical to cylindrical Golgi portions is associated with the reorganization of entangled into parallel threads, which is interpreted as the packing of procollagen molecules .Between 20 and 30 min, prosecretory and secretory granules respectively became labeled . These results indicate that the cylindrical portions of Golgi saccules transform into prosecretory and subsequently into secretory granules . Within these granules, the parallel threads, believed to be procollagen molecules, are transported to the odontoblast process .At 90 min and 4 h after injection, label was present in predentin, indicating that the labeled content of secretory granules had been released into predentin . This occurred by exocytosis as evidenced by the presence of secretory granules in fusion with the plasmalemma of the odontoblast process .It is proposed that pro-alpha chains give rise to procollagen molecules which assemble into parallel aggregates in the Golgi apparatus . Procollagen molecules are then transported within secretory granules to the odontoblast process and released by exocytosis . In predentin procollagen molecules would give rise to tropocollagen molecules, which would then polymerize into collagen fibrils .

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The role played by cell addition, cell enlargement, and matrix deposition in the endochondral growth of the condyle was assessed in weanling rats by four approaches making use of the light microscope: morphometry, 3H-thymidine radioautography, 3H-proline radioautography, and immunostaining for the cartilage-specific type II collagen. From the articular surface down, the condyle may be divided into five layers made up of cells embedded in a matrix: 1) the articular layer composed of static cells in a matrix rich in fibers presumed to be of type I collagen, 2) the polymorphic cell layer including the progenitor cells from which arise the cells undergoing endochondral changes, 3) the flattened cell layer in which cells produce a precartilagenous matrix devoid of type II collagen while undergoing differentiation in two stages: a "chondroblast" stage and a short "flattened chondrocyte" stage when intracellular type II collagen elaboration begins, 4) the upper hypertrophic cell layer, in which cells are "typical chondrocytes" that enlarge at a rapid rate, actively produce type II collagen, and deposit it into a cartilagenous matrix, and 5) the lower hypertrophic cell layer, composed of chondrocytes at a stage of terminal enlargement while the cartilagenous matrix is adapting for mineralization. 3H-thymidine radioautographic results indicate that the turnover time of progenitor cells in the polymorphic cell layer is about 2.9 days. The time spent by cells at each stage of development is estimated to be 1.4 days as chondroblasts, 0.5 days as flattened chondrocytes, 2.3 days as the chondrocytes of the upper hypertrophic cell layer, and 1.1 days as those of the lower hypertrophic cell layer. Calculations referring to a 1 x 1-mm square-sided column extending from the articular surface to the zone of vascular invasion provide the daily rate of cell addition (0.0077 mm3), extracellular matrix deposition (0.0127 mm3), and cell enlargement (0.0302 mm3). Hence the respective contribution of the three factors to condyle growth is in a ratio of about 1:1.6:4. This result emphasizes the role played by cell enlargement in the overall growth of the condyle.

A peptide that is rich in organically bound phosphorus and contains abundant serine residues has been identified in the dentin of man (1), fetal bovine (2, 3), and rat (4). This phosphoprotein may play a role in mineralization (5-9). Butler et al. (4) reported that the phosphoprotein of rat incisor dentin constituted 10.8% of the proteinaceous material recovered from decalcified incisor teeth while collagen comprised 84%. Since the phosphorus content of the phosphoprotein was estimated at 3.29% and that of collagen at 0.19% (4), much of the phosphorus taken up in organic form by the incisor would likely be present as phosphoprotein. With this in mind, it was decided to inject [~3P]phosphate into rats and examine the demineralized incisor teeth by radioautography in the hope of tracing phosphoprotein formation.The organic phosphorus of dentin phosphoprotein is believed to be attached to serine residues (6). In the rat incisor dentin, this amino acid comprises 35 residues per cent of the phosphoprotein and only four residues per cent of the cyanogen bromide peptides of collagen (4). Hence serine also appeared to be a suitable amino acid precursor to trace phosphoprotein formation by radioautography.Finally, the radioautographic pattern of the deposition of labeled phosphorus and serine was compared to that of [3H]proline. Proline may be used as a precursor to trace collagen, since it makes up 22.0 residues per cent of dentin collagen and only 2.4 residues per cent of the phosphoprotein (4). The results indicated that the pattern of phosphoprotein deposition into the dentin matrix is strikingly different from that of collagen. The fixative consisted of 2.5% glutaraldehyde in 0.05 M S6rensen's phosphate buffer with the addition of 0.1% sucrose and 0.5% dextrose. In the experiments using L-[2,~-gH]proline, the fixative employed was 3% formaldehyde (TAAB Laboratories, Emmer Green, Reading, England) in 0.1 M S6ren-sen's phosphate buffer with 0.1% sucrose added. The final pH of either fixative was 7.2-7.3. After perfusion for 15 min at room temperature the maxillary incisor teeth were immersed in fresh fixative for 2-3 h and demineralized in EDTA for 2 wk at 4°C (10). Although a 2-wk period is sufficient for exhaustive demineralization, some teeth were denfineralized for 3 or 4 wk in the experiments conducted with [3~P]phosphate. Specimens were washed overnight in 0.15 M S6rensen's buffer, sliced transversely with razor blades into 1-mm thick sections, postfixed for 1-2 h in 1% OsO4 in 0.1 IV[ S6rensen's buffer, dehy-838 MATERIALS AND METHODS Sherman

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