Skeletal Tissues Ms. Hazel Anne L. Tabo

Skeletal Tissues Skeletal System is made up of various tissues which have originated from mesenchyme. 1. Bone – provides rigid protection and supporting framework. Ca2+ salts within collagen and matrix  rigidity 2. Cartilage – provides smooth articular surface at epiphyses of long bones and structural support in special areas (eg. Trachea, Pinna or ear auricle). Hyaline form is important in bone formation. 3. Joints – structures that connect bones of skeleton. Its different forms may permit degrees of movement. 4. Ligaments – support joint stability made up of collagenous tissue (dense regular CT). 5. Tendons – connect muscles into bones.

CARTILAGE A specialized CT in which the firm ECM allows the tissue to bear mechanical stresses without permanent distortion. Supports soft tissues since it is smooth surfaced and resilient, cartilage is a shock-absorbing and sliding area for joints and facilitates bone movements. Essential for the development and growth of long bones both before and after birth (ossification).

Cartilage: Ground substance ECM - enriched with GAGs and proteoglycans, macromolecules that interact with collagen and elastic fibers. Variations in the composition of these matrix components produce three types of cartilage. Hyaluronic acid (non-sulfated GAG) forms central backbone and predominant sulfated forms are chondroitin sulfate and keratan sulfate. Chondronectin – mediates adherence of chondrocytes to ECM. It binds GAGs and collagen II. 3 types: Hyaline, Elastic and Fibrocartilage.

Molecular organization in cartilage matrix Molecular organization in cartilage matrix. Link proteins noncovalently bind the protein core of proteoglycans to the linear hyaluronic acid molecules. The chondroitin sulfate side chains of the proteoglycan electrostatically bind to the collagen fibrils, forming a cross-linked matrix. The oval outlines the area shown larger in the lower part of the figure.

Cartilage development Mesenchymal cells differentiated into chondroblasts which became its precursorial cells. Chondroblast mitosis  isogenous groups grow and begin synthesis of ground substance and fibrous extracellular (EC) materials. Secretion of EC materials trap each chondroblasts in the matrix thereby separating the cells (interstitial growth). Chondrocytes develop and maintain matrix integrity. Perichondrium – made up of collagen fibers and fibroblasts. It is capable of transforming into chondroblasts (appositional growth).

Histogenesis of hyaline cartilage Histogenesis of hyaline cartilage. A: The mesenchyme is the precursor tissue of all types of cartilage. B: Mitotic proliferation of mesenchymal cells gives rise to a highly cellular tissue. C: Chondroblasts are separated from one another by the formation of a great amount of matrix. D: Multiplication of cartilage cells gives rise to isogenous groups, each surrounded by a condensation of territorial (capsular) matrix.

Mitotic chondroblast (arrow) Cc-chondrocyte Territorial matrix (green arrow) Violet - Interterritorial matrix (ECM) Cc Perichondrium

Perichondrium Perichondrium harbors the vascular supply for the avascular cartilage and also contains nerves and lymphatic vessels. Articular cartilage, which covers the surfaces of the bones of movable joints, is devoid of perichondrium and is sustained by the diffusion of oxygen and nutrients from the synovial fluid.

Electron micrograph from young animal: Chondrocytes in their lacunae Electron micrograph from young animal: Chondrocytes in their lacunae. Note the abundance of rough ER. Chondrocytes synthesize the cartilage matrix.

Cartilage Nutrition: Chondrocytes respire under low oxygen tension since it is devoid of capillaries. Hyaline cartilage cells metabolize glucose mainly by anaerobic glycolysis to produce lactic acid. Nutrients from the blood cross the perichondrium to reach more deeply placed cartilage cells by diffusion and transport of water and solute promoted by the pumping action of intermittent cartilage compression and decompression. Because of this, the maximum width of the cartilage is limited. Chondrocyte function is dependent on hormonal balance: Synthesis of sulfated GAGs - Growth hormone, thyroxin, and testosterone. Cortisone, hydrocortisone, and estradiol – inhibits sulfated GAGs. Somatotropin (pituitary gland) - Cartilage growth depends mainly on this hypophyseal growth hormone by acting on the liver cells promoting synthesis of somatomedin C. Somatomedin C acts directly on cartilage cells, promoting their growth.

Hyaline Cartilage The most common and best studied of the three forms. Fresh hyaline cartilage - bluish-white and translucent. In embryo: A temporary skeleton until it is gradually replaced by bone. In adult mammals: This cartilage is located in the articular surfaces of the movable joints, in the walls of larger respiratory passages (nose, larynx, trachea, bronchi), in the ventral ends of ribs, where they articulate with the sternum, and in the epiphyseal plate, where it is responsible for the longitudinal growth of bone. Perichondrium is well-defined.

Hyaline Cartilage

Elastic Cartilage Fresh form – yellowish due to elastin in the fibers. Found in the auricle of the ear, the walls of the external auditory canals, the auditory (eustachian) tubes, the epiglottis, and the cuneiform cartilage in the larynx. Identical to hyaline cartilage except that it contains an abundant network of fine elastic fibers in addition to collagen type II fibrils. Perichondrium is defined.

Photomicrograph of elastic cartilage, stained for elastic fibers Photomicrograph of elastic cartilage, stained for elastic fibers. Cells are not stained. This flexible cartilage is present, for example, in the auricle of the ear and in the epiglottis. Resorcin stain. Medium magnification.

Fibrocartilage A tissue intermediate between dense connective tissue and hyaline cartilage. It is found in intervertebral disks, in attachments of certain ligaments to the cartilaginous surface of bones, and in the symphysis pubis. It is always associated with dense CT, and the border areas between these two tissues are not clear-cut, showing a gradual transition. Chondrocytes are either singly or in isogenous groups, arranged in long rows separated by coarse collagen type I fibers . Because it is rich in collagen type I, the fibrocartilage matrix is acidophilic. Perichondrium is poorly defined.

Intervertebral Disks (IVD) Situated between two vertebrae and is held to them by means of ligaments. The disks have two components: the fibrous annulus fibrosus and the nucleus pulposus. IVD acts as a lubricated cushion that prevents adjacent vertebrae from being eroded by abrasive forces during movement of the spinal column. The nucleus pulposus serves as a shock absorber to cushion the impact between vertebrae. The annulus fibrosus has an external layer of dense connective tissue, but it is mainly composed of overlapping laminae of fibrocartilage in which collagen bundles are orthogonally arranged in adjacent layers. The multiple lamellae, with the 90° registration of type I collagen fibers in adjacent layers, provide the disk with unusual resilience that enables it to withstand the pressures generated by impinging vertebrae. The nucleus pulposus is derived from the embryonic notochord and consists of a few rounded cells embedded in a viscous matrix rich in hyaluronic acid and type II collagen fibrils. In children, the nucleus pulposus is large, but it gradually becomes smaller with age and is partially replaced by fibrocartilage.