BIOLOGY Volume 1 - A Guide to General Biology - 2004

6. HISTOLOGY

6.4. Animal Connective Tissue

6.4.2. Skeletal Tissues

Cartilage

Cartilage is a Connective Tissue consisting of Cells embedded in a ground substance (matrix) composed of chondrin. Chondrin is secreted by cells called chondroblasts and contains numerous fine fibers made primarily of Collagen. Eventually, the chondroblasts become enclosed within cavities known as lacunae. In this state, they are referred to as chondrocytes. The outer surface of cartilage is covered by the perichondrium, a dense sheath composed of cells and fibers. New chondroblasts are formed here and are continuously added to the inner matrix of the cartilage.

Cartilage is a firm yet flexible tissue, exceptionally well adapted to resist all forms of deformation. The cartilage matrix possesses elasticity and Shock-absorbing properties, which are crucial for buffering the impacts frequently occurring between the articular surfaces of bones at their points of contact. The collagen fibers are capable of withstanding significant tensile stresses to which this tissue is often subjected.

There are Three types of cartilage. Each is characterized by a distinct composition of organic components within its matrix.

HYALINE CARTILAGE (FIG. 6.25). "Hyaline" means glassy or translucent. The matrix of this cartilage is a translucent substance traversed by fine collagen fibers. Peripheral chondrocytes are flattened, whereas those located closer to the center are angular in shape. Each chondrocyte is surrounded by a clear space called a lacuna, which may contain one, two, four, or eight chondrocytes.

Class="center">

Fig. 6.25. Hyaline cartilage.

Unlike osteocytes, chondrocytes lack cytoplasmic processes extending from the lacunae into the ground substance, nor are there any Blood Vessels present. Exchange of substances between chondrocytes and the matrix occurs via diffusion.

Hyaline cartilage is an elastic tissue located at the ends of bones and in the Nose. C-shaped rings of hyaline cartilage support the walls of the respiratory Airways (Trachea, Bronchi, and larger bronchioles), preventing them from collapsing. It also forms the Skeleton of cartilaginous fish (such as sharks) and the embryonic skeleton of vertebrates that later develop bony skeletons.

YELLOW ELASTIC CARTILAGE. The matrix of yellow elastic cartilage is translucent and contains an interwoven network of yellow elastic fibers. These fibers make the cartilage more flexible than hyaline cartilage and give it The ability to rapidly spring back to its original shape after deformation. Elastic cartilage forms the pinna of the ear, the Eustachian tube, and the epiglottis.

WHITE FIBROCARTILAGE. This cartilage is formed from numerous bundles of densely packed white collagen fibers embedded in a matrix. It possesses greater tensile strength than hyaline cartilage, though less flexibility. White fibrocartilage forms the intervertebral discs, where it acts as a shock absorber. It is also found in the Pubic Symphysis (the region between the two pelvic pubic bones) and within joint capsules.

Bone

Bone is the primary structural material of the skeleton in most vertebrates, fulfilling supportive, protective, and certain metabolic Functions. Bone cells are embedded in a dense, calcified matrix composed of approximately 30% Organic compounds—mainly collagen fibers and Glycoproteins—and 70% inorganic salts. The principal inorganic component consists of needle-like crystals of hydroxyapatite, Са₁₀ (РО₄)₆ (ОН)₂, a form of calcium phosphate. It also contains varying amounts of sodium, magnesium, potassium, chlorine, fluorine, hydrogen carbonate, and citrate ions.

Bone cells, termed osteoblasts, reside in lacunae distributed throughout the matrix. These cells deposit the Inorganic Components of the bone. The lacunae are interconnected by tiny canaliculi containing Cytoplasm, through which blood vessels run to facilitate the exchange of various substances among the osteoblasts.

The Structure of bones is adapted to withstand compressive forces and resist tensile stresses. Through processes of resorption and remodeling, the architecture of each individual bone continuously adapts to Changes in the mechanical loads experienced by the animal during its development. The release of calcium and phosphate into the bloodstream as needed is regulated by two Hormones: parathyroid hormone and Calcitonin (Chap. 17).

COMPACT (DENSE) BONE (FIG. 6.26). Compact bone forms the long diaphysis and epiphyses of tubular (limb) bones. A cross-section of compact bone reveals that it consists of numerous cylinders, through the center of which runs a Haversian canal. Each such cylinder, along with its canal, constitutes a Haversian system, or osteon. Each cylinder is itself composed of several concentric layers called lamellae, which are also cylindrical in shape; this structural arrangement enhances bone strength.

Between the lamellae lie numerous lacunae (spaces) containing living bone cells known as osteoblasts. Each such Cell is capable of depositing bone. As they mature, osteoblasts become less active and exhibit a reduced number of cellular Organelles. At this stage, they are referred to as osteocytes. If structural Modification of the bone is required, osteocytes can be reactivated and rapidly transform back into osteoblasts.

Radiating from each lacuna are numerous fine canaliculi containing cytoplasmic extensions (processes) of the osteoblasts; these canaliculi may connect with the central Haversian canal, with other lacunae, or extend from one lamella to another.

Each Haversian canal contains one artery and one vein, which branch into capillaries extending via the canaliculi to the lacunae of that Haversian system. They supply the cells with nutrients and O₂ while removing Metabolic waste products and СО₂. Each Haversian canal also houses a lymphatic vessel and nerve fibers. Transverse Volkmann's (Haversian) canals communicate with the Bone Marrow cavity as well as with the longitudinal Haversian canals. The latter contain larger blood vessels and are not surrounded by concentric lamellae.

The matrix of compact bone consists of collagen produced by osteoblasts and hydroxyapatite, along with magnesium, sodium, carbonates, and nitrates. This combination of organic and inorganic Materials creates an exceptionally robust structure.

The bone lamellae are arranged in a specific orientation to enable the bone to withstand the forces and loads imposed upon it.

Externally, bone is covered with a layer of Cytology/practical/45.html">Dense connective tissue known as the periosteum. Bundles of collagen fibers extending from the periosteum penetrate into the bone, firmly anchoring it and providing a strong foundation for tendon attachment. The inner layer of the periosteum contains a rich network of blood vessels and gives rise to a population of cells capable of differentiating into osteoblasts.

Fig. 6.26. A. Section of a transverse cut through a long tubular bone. B. Schematic representation of the Haversian system in cross-section. This system forms a cylindrical unit. The presence of numerous lamellae within each cylinder imparts high mechanical strength to the bone despite its light weight.

CANCELLOUS BONE. Cancellous bone consists of a lattice of thin, anastomosing bony struts known as trabeculae. Its Extracellular matrix contains less inorganic material (60-65%) than that of compact bone. The organic component is composed primarily of collagen fibers. The spaces between the trabeculae are filled with soft bone marrow.

The trabeculae are aligned along the lines of mechanical stress experienced by the bone. This structural Organization provides maximum resistance to tension and compression while keeping bone mass to a minimum.

Cancellous bone is present in embryos and growing organisms, as well as in the epiphyses of long bones in adults.