Specialized Connective Tissue: Cartilage and Bone
Hyaline cartilage (lavender matrix), with perichondrium (pink) outside it. The latter is a dense regular collagenous c.t.. There are collagenous and elastic fibers lying in the cartilage matrix but they are invisible because their refractive index is the same as that of the matrix. Cartilage cells = chondrocytes, and they are lying in the lacunae.
Two chondrocytes completely filling their lacunae. If the cells were to drop out, you would see spaces in the matrix. The matrix appears very smooth, clear, and glassy (or "hyaline").
Electron micrograph of a chondrocyte in its lacuna and almost entirely filling the lacunar space. Notice that the cell has many fine cytoplasmic projections when viewed by electron microscopy. There are surrounded by heavily condensed ground substance which appears less dense on the other side of the cell.
Appositional growth of cartilage by conversion of long, thin perichondrial cells (at the right) into the round, large chondrocytes. Notice how they change shape as they lay matrix down around themselves. The cells of the outer perichondrium are fibroblasts; the inner perichondrial cells include some primitive connective tissue cells which differentiate into chondroblasts and then into chondrocytes as they lay down matrix and become embedded in it.
Hyaline cartilage with quite basophilic matrix immediately surrounding the lacunae. Cells are often grouped in "nests" (or isogenous groups) as a result of earlier mitoses and nowhere for cells to move apart. (This is called interstitial growth). (The "ripple lines" in the matrix here are due to uneven cutting of the section.)
Elastic cartilage, with chondrocytes and matrix as before, but elastic fibers predominate and take a specific stain. They always look very distinct and dark and show many branchings.
More elastic cartilage. The matrix immediately surrounding each cell is typically not traversed by fibers.
Fibrocartilage, with wispy, broad collagenic fibers predominating in the matrix. They look "cotton-y", unlike the sharply defined elastic fibers seen before. Notice that the cells are lying in lacunae.
Fibrocartilage, at the point of junction between hyaline cartilage (lavender) above and dense collagenous tissue (pink) below. The combination of chondrocytes, matrix, and visible wispy collagenic strands or fibers identifies this as fibrocartilage.
Section of compact ground bone - dry and unstained - showing cross-cuts of Haversian systems. In the center of each system is an Haversian canal which carries blood vessels. With so many such systems per unit volume of bone, we can say that bone is a well vascularized tissue. (By contrast, cartilage is avascular.)
Higher power of ground compact bone. You can see on the left that a central vascular channel (Haversian canal) is surrounded by concentric lamellae (layers) of bone. These lamellae are made up of collagenous fibers and inorganic salt matrix. The lamellae in the center of the picture are interstitial lamellae, left over from earlier Haversian systems that have been partially resorbed as new systems were laid down during the constant remodelling of the bone as it formed. Black spaces air-filled lacunae in which osteocytes once lived.
Detail of Haversian system, showing the tiny, spidery canaliculi extending from one lacuna to the next. In life these canaliculi held the processes of osteocytes thus permitting diffusion of nutrients from the central blood vessels to the outer lamellae of the Haversian system.
Detail of lacuna, showing radiating canaliculi. Tissue fluid from the capillaries and connective tissue of the Haversian canal can seep through these spaces and channels, bringing nutrients to the stellate osteocytes residing there.
EM of osteocyte in lacuna. The cytoplasm of the cell contains rough endoplasmic reticulum for the production of protein collagen, some of which can be seen lying immediately around the cell. The collagen becomes masked by black apatite (CaPO4) crystals as the matrix becomes mineralized.
High power EM of contact between two neighboring osteocytes whose processes have met in a canaliculus. Close examination of the contact shows fused outer leaflets of cell membrane (note three dark lines), indicating that this is a tight junction. Osteocytes are also known to make contact by means of gap junctions.
Low power view of a cross-cut shaft of decalcified long bone. The bone itself is pink and lies in the center of the field. The pinkness is due to the staining of collagen fibers in the lamellae. To the left is bone marrow; to the right is attaching skeletal muscle.
Early compact bone, decalcified so it can be stained. This has been cut so that the Haversian systems are cut in cross section. Vascular channels cut longitudinally are parts of Volkmann's canals.
Vascular elements from bone marrow (on the left) are continuous with vascular spaces within the bone. The endosteum lining the marrow cavity is therefore continuous with the endosteal linning of Haversian canals.
Detail of bone-forming osteoblasts lined up along the inner (endosteal) edge of bone next to the marrow cavity. In young bones growth continues in width, constantly laying down bone and resorbing it and laying down more. Real width, of course, increases by the laying down of periosteal bone on the outside of the bone, but activity continues on the endosteal surface also. Notice osteocytes inside the bony substance, lying in lacunae.
Detail of osteocytes in lacunae. The collagenous fibers of the decalcified matrix are quite acidophilic, as always. Osteocytes like these are present in both compact and spongy bone; their arrangement, however, is in concentric lamellae in compact bone and in randomly arranged lamellae in spongy bone. Remember, too, that osteocytes have processes which extend out into canaliculi in both kinds of bone.