Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Human Body Structure
Tissues, Organs, Organ Systems, and Apparatuses
Connective Tissue - Skeletal Tissues

Skeletal Tissues include Cartilage and Bone Tissues, which primarily perform mechanical (support and locomotion) and delimiting Functions in the body. Skeletal tissues also take part in Mineral METABOLISM.

Cartilage tissue consists of Cells (chondrocytes, chondroblasts) and a polymerized, dense intercellular matrix. The Extracellular matrix of cartilage exists in a gel-like state and is composed mainly of Glycosaminoglycans and Proteoglycans. Cartilage also contains a large amount of Connective Tissue (Collagen) fibers. The intercellular substance of cartilage exhibits high hydrophilicity. Mature cartilage cells (chondrocytes) are round or oval in shape. These cells are located in specialized cavities (lacunae) and synthesize all Components of the extracellular matrix. Chondroblasts represent immature cartilage cells; they actively synthesize the extracellular matrix of cartilage and are also capable of division. Peripheral (appositional) growth of cartilage occurs through The activity of chondrocytes.

The connective tissue layer covering The surface of cartilage is called the perichondrium. The perichondrium comprises an outer fibrous layer, which consists of dense Fibrous connective tissue and contains Blood Vessels and nerves. The inner layer of the perichondrium is chondrogenic, containing chondroblasts and their precursors (prechondroblasts). The perichondrium drives appositional cartilage growth. Blood Vessels within the perichondrium provide diffuse nourishment to the cartilage tissue and remove Metabolic waste products.

Based on structural features, hyaline, elastic, and fibrocartilage are distinguished.

Hyaline cartilage is characterized by transparency and a bluish-white color. It is found at the junctions of the Ribs with the Sternum, on the articular surfaces of bones, at the epiphyseal-diaphyseal junctions in long bones, within the laryngeal Skeleton, and in the walls of the Trachea and Bronchi.

In addition to collagen fibers, the extracellular matrix of elastic cartilage contains a high concentration of elastic fibers, granting it enhanced flexibility. Elastic cartilage forms the auricle, the cartilage of the external acoustic meatus, the epiglottis, and certain other laryngeal cartilages.

Fibrocartilage contains an Abundance of collagen fibers in its extracellular matrix, imparting high tensile strength. It forms the annulus fibrosus of intervertebral discs, articular discs, and menisci.

Bone tissue is built of bone cells and an extracellular matrix containing a significant amount of various salts and connective tissue fibers. The organic substances of bone are termed "ossein" (from Lat. os — bone). The Inorganic Components consist of salts of calcium, phosphorus, magnesium, and other chemical elements. The combination of organic and inorganic substances provides bone with both strength and elasticity. Children have a higher proportion of organic substances in their bones than adults, which is why bone fractures are rare in children. In elderly individuals, The amount of organic substances decreases, making the bones more brittle and fragile.

The cellular components of bone tissue include osteocytes, osteoblasts, and osteoclasts. Osteocytes are mature, mitotically inactive, process-bearing bone cells ranging from 22 to 55 µm in length. They possess a large, ovoid Nucleus. Osteocytes are spindle-shaped and reside within bone cavities (lacunae). Bone canaliculi containing osteocyte processes radiate from these cavities. A thin layer of tissue (bone) fluid is present between the osteocyte body, its processes, and the walls of the lacuna.

Osteoblasts are young bone cells featuring a round nucleus. They originate from the germinative (deep) layer of the periosteum. As intercellular bone matrix accumulates around the osteoblasts, these cells differentiate into osteocytes.

Osteoclasts are large, multinucleated cells up to 90 µm in diameter. They are involved in the resorption of bone and calcified cartilage.

Two main Types of bone tissue are distinguished: lamellar and woven (coarse-fibered) bone. Lamellar (finely fibered) bone tissue consists of bone lamellae formed by a mineralized extracellular matrix, embedded bone cells, and collagen fibers. The fibers in adjacent lamellae have different orientations. Compact (dense) and spongy substances of the skeletal bones are constructed from lamellar bone tissue (Fig. 10). Compact bone forms the diaphyses (middle shafts) of long bones, the surface plates of their epiphyses (ends), and the outer layer of flat and other bones. Spongy bone forms trabeculae (spicules) located between the plates of compact bone within the epiphyses of long bones and other skeletal elements. These trabeculae are oriented in various directions corresponding to the lines of compression and tension within the bone tissue, which helps maximize bone strength.

Compact bone tissue is formed by concentric lamellae (tubules) numbering from 4 to 20, which encircle the blood vessels running through the bone. A single concentric lamella ranges from 4 to 15 µm in thickness. The tube through which blood vessels up to 100–110 µm in diameter pass is called the osteon canal (Haversian canal). The entire structural complex surrounding this canal is termed an osteon, or Haversian system, which serves as the Structural and functional unit of bone. Differently oriented bone lamellae situated between adjacent osteons are referred to as interstitial lamellae. The inner layer of compact bone, at its border with spongy bone, is formed by inner circumferential lamellae. These lamellae are produced by the endosteum—a delicate connective tissue membrane lining the inner surface of the bone (the walls of the marrow cavity and the spaces of spongy bone) and performing an osteogenic function. The outer layer of compact bone is formed by outer circumferential lamellae, generated by the inner osteogenic layer of the periosteum.

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Fig. 10. Structure of long bones:

1 — periosteum; 2 — compact bone; 3 — layer of outer circumferential lamellae; 4 — osteons; 5 — layer of inner circumferential lamellae; 6 — medullary cavity; 7 — bony trabeculae of spongy bone

The periosteum is a connective tissue membrane covering all bones except for their articular surfaces, which are covered by articular cartilage. It comprises an outer and an inner layer. The outer layer of the periosteum is coarse-fibered and fibrous. This layer is rich in nerve fibers and blood vessels that not only nourish the periosteum but also penetrate the bone through nutrient foramina on the bone surface. The periosteum is firmly anchored to the bone surface by delicate connective tissue fibers (Sharpey's fibers) extending from the periosteum into the bone. The inner layer of the periosteum generates young bone cells, enabling the bone to grow in thickness.