The Anatomy of the Growth Plate
The tibia does not have the same pronounced regional variation within the growth plate as can be seen in the distal femur. Therefore, it became impractical to look for that variation, so this project's focus shifted to examining the general anatomy of the growth plate and its surrounding structures as they are seen through the scanning electron microscope.
Long bones grow longitudinally by means of plates on both ends of the bones. (See figure 1 drawing) The growth plate is composed of cartilaginous cells (chondrocytes) surrounded by an extracellular matrix of collagen fibers and glycosaminoglycans. Figures 2 and 3 show close- up views of the growth plate as seen through a light microscope and a scanning electron microscope, respectively. The light yellow area at the top of Figure 2 is the bone of the secondary ossification center. With the dye used, all cartilage dyed purple. Area A is the reserve zone, where the chondrocytes are randomly oriented throughout the matrix. Figure 4 shows the lacunae left by a chondrocyte. The cell itself was removed in the fixation process, but the collagen matrix surrounding it remails. As the cells grow and divide, they begin to form columns, seen in the proliferative zone (B). This is the only place in the bone where longitudinal growth occurs. In the hypertrophic zone (C), the chondrocytes swell and the matrix begins to calcify. Below this zone chondrocytes undergo programmed cell death (apoptosis) and vascular entry occurs. Bone cells (osteoblasts) are brought in and cell wastes from the expired chondrocytes are transported out.
Figure 1: the structure of bone end
Figure 2: Growth plate, under light microscopy. Photo supplied by Amy Lerner, Department of Mechanical Engineering, University of Rochester
Figure 3: Growth plate, under scanning electron microscopy
Figure 4: A Chondrocyte Lacunae
As seen in figure 2, the growth plate is largely uncalcified cartilage. Closer to the metaphysis the matrix begins to calcify. In figure 3, the calcified cartilage can be seen in the areas that still have transverse septae separating each of the chondrocyte lacunae from one another. Figure 5 shows a transverse septae. As the chondrocytes die, the transverse septae are dissolved. However, the growth plate itself is a largely unsupported structure. As a part of the fixation process, much of the cartilage in the growth plate may have collapsed. Figure 6 shows that directly above the calcified cartilage of the hypertrophic zone, there is an area of densely packed collagen fibers. In vivo, this area would be much more loosely structured, with chondrocytes scattered throughout the matrix. The top of the bone itself, above the secondary center of ossification, contains cartilage similar to that of the growth plate. In figure 7, taken from this area on the top of the bone, the collagen matrix similar to the desired matrix of the growth plate is shown. For a close look at the structure of collagen fibers, refer to figure 8.
Figure 5: A Transverse Septae
Figure 6: The Growth Plate
Figure 7: Collagen Matrix
Figure 8: Collagen Matrix
Figure 9: The Perichondrial Ring
As described above, the soft tissue of the cartilage is surrounded both above and below by bone, a much firmer tissue. However, if there were nothing on the sides of the bone to support the growth plate longitudinally, the cartilage may seep out the sides if pressure is applied. The perichondrial ring, a bony ring that encircles the growth plate, serves the purpose of supporting the growth plate in this fashion. The periosteum (cartilage around the perichondrial ring) provides for latitudinal growth of the bone. This is also an area for entry of blood vessels, which supply nutrients to the bone and carry out waste.
IV. Discussion
Viewing the distinction between cartilage and bone proved to be significantly more difficult in the SEM than using light microscopy. At first we could not discern the transition between cartilage and bone. Under more careful inspection, we realized that the transverse septae indicated the presence of cartilage. Because of its fragile nature, much of the structure could not withstand the fixation in formalin. Perhaps a different fixation process, with gluderaldehyde, would have given us better results in viewing the cartilage of the growth plate.
References
*The Growth Plate and Its Disorders. Mercer Rang. Baltimore: The Williams and Wilkins Company, 1969.
*Lerner, AL; Kuhn, JL. "Variations in bone growth rate in the rabbit distal femur are strongly correlated to the volume of extracellular matrix in the hypertrophic chondrocyte domain." 44th Annual Meeting, Orthopaedic Research Society, 1998.
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