The intervertebral discs of the dog from the newborn, the 1 year old, and the 5 years old were examined about the cell morphologies and their structural changes according to the advancement of age by histomorphometry and electron microscopy. The three regions of the disc - the nucleus pulposus, the inner and outer annulus fibrosus were considered. Each region was divided into the three zones - the peripheral, intermediate, and central zones. The cell morphology of the three regions were as follows : A. The nucleus pulposus 1. At the newborn, the cells of the nucleus pulposus had long cytoplasmic processes forming large extracellular vacuoles. In the peripheral and intermediate zones, the cells with euchromatic nucleus had fine cytoplasmic granule-like fibers and some of free ribosomes. In the central zone, the cells with heterochromatic nucleus had many cytoplasmic vacuoles filled with free ribosomes. 2. At the 1 years old, the cell-group composed of 4 to 8 cells contacted each other with their cytoplasmic processes. In the peripheral zone, the elliptical chondroblast-like cells with euchromatic nucleus had well-developed rough endoplasmic reticulum and mitochondria. In the intermediate zone, the foam-like large round cells with euchromaic nucleus had multiple cytoplasmic vacuoles filled with granular contents. In the central zone, the cells with heterochromatic nucleus had large cytoplasmic vacuoles, and seemed to be degenerated type of the outer zone-cells. 3. At the 5 years old, the cells in the periphery were rhomboid or polygonal with euchromatic nucleus, and had broad cytoplasmic contact surface with adjacent cells. They had small cytoplasmic vacuoles, well-developed Golgi apparatus and rough endoplasmic reticulum. The cells in the intermediate zone had heterochromatic nucleus and a lot of variable size of cytoplasmic vacuoles, and seemed to be degenerated form of the cells of the periphery. In the central zone, the star-like cells with heterochromatic nucleus had multiple sharp cytoplasmic processes and vacant cytoplasm because of large cytoplasmic vacuoles. B. The cartilaginous part and the inner annulus fibrosus 1. At the newborn, the outer zone of the cartilaginous part of the intervertebral disc showed the isogenous group with 2 to 3 chondrocytes within the cell nest. The cells were distributed mainly around the blood vessels, and had euchromatic nucleus. They had small cytoplasmic vacuoles, well-developed rough endoplasmic reticulum and abundant ribosomes. The cells of the inner zone showed heterochromatic nucleus, well-developed rough endoplasmic reticulum and expanding terminal cisternae. 2. At the 1 year old, the inner annulus fibrosus was divided into the fibrocartilaginous part and the hyaline cartilaginous part. The former, the outer zone showed the cell nest with 2 to 4 of elliptical chondrocytes which had euchromatic nucleus, small cytoplasmic vacuoles and scattered fine granules. The latter, the inner zone showed the chondrocytes within the dissociated cell nest with heterochromatic nucleus. They had disperse rough endoplasmic reticulum and microvilli-like cytoplasmic processes. 3. At the 5 years old, the annulus fibrosus was composed mainly of fibrocartilage. The chondrocyte showed heterochromatic nucleus, long cytoplasmic processes, large cytoplasmic vacuoles and densely packed abundant granules. C. The outer annulus fibrosus 1. At the 1 year old, the outer annulus fibrosus was composed of the palisading collagenous bundles. The chondrocytes with heterochromatic nucleus located in the cartilage cell nest. The cells had well-developed rough endoplasmic reticulum, mitochondria, and Golgi apparatus, and 4 to 5 processes of interterritorial matrix surrounded the collagenous bundles. 2. At the 5 years old, the collagenous bundles were invaded by the processes of interterritorial matrix, and made them incomplete ones. The cell had heterochromatic nucleus and scanty cytoplasm containing small mitochondria and poorly developed organelles. The Summary of The Above Mentioned Findings Are : 1. The cells of the nucleus pulposus were degenerated gradually from the periphery toward the center. 2. The cells of the nucleus pulposus were degenerated gradually according to the aging process. 3. With age, the cells of the annulus fibrosus were degenerated, the hyaline cartilage was replaced gradually by fibrocartilages, and fragmentation of the collagenous bundles appeared. 4. The older the age, the smaller the nucleus of the nucleus pulposus cells, and the larger the nucleus of the annulus fibrosus cells were encountered in the histomorphometric measurement.
Aging ; Animals ; Blood Vessels ; Cartilage* ; Child, Preschool ; Chondrocytes ; Collagen ; Cytoplasm ; Dogs* ; Endoplasmic Reticulum, Rough ; Fibrocartilage ; Golgi Apparatus ; Humans ; Hyalin ; Hyaline Cartilage ; Infant, Newborn ; Intervertebral Disc* ; Microscopy, Electron ; Mitochondria ; Organelles ; Ribosomes ; Vacuoles