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

Musculoskeletal System
Skeleton
Classification and Structure of Bones

Skeletal bones vary in shape and Structure. They are classified into tubular (long), spongy (short), flat, irregular, and pneumatic bones (Fig. 15). Tubular bones are further subdivided into long bones (humerus, Femur, BONES OF THE forearm and leg) and short bones (bones of the tarsus and carpus, Phalanges of the fingers). Except for their articular surfaces, bones are covered by a Connective Tissue membrane known as the periosteum, which performs osteogenic (bone-forming) and protective Functions. The periosteum is firmly fused to the bone via connective tissue fibers that penetrate deep into the bone matrix. Its outer layer is coarse-fibrous, consisting of intricately interwoven connective tissue fibers and Cells. This layer is richly supplied with Blood Vessels and nerve fibers that sustain the bone's vitality. The inner layer of the periosteum is thin and contains osteogenic cells that differentiate into osteoblasts—young bone-forming cells. Through the osteogenic function of the periosteum, bones grow in thickness and heal after fractures.

Inside the bones lie the marrow cavity (in tubular bones) and the spaces of the spongy bone, which house the Bone Marrow. In newborns and children, the marrow cavities are filled with Cytology/practical/86.html">Red bone marrow, which performs hematopoietic (blood-forming) and protective functions. Stem cells in the red bone marrow give rise to Blood Cells (erythrocytes, leukocytes) as well as cells of The Immune System (lymphocytes). In adults, red bone marrow persists only within the spaces of the spongy bone. Other bone cavities contain fatty (yellow) bone marrow, which has lost its active functions.

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Fig. 15. Types of bones:

1 — long (tubular) bone; 2 — flat bone; 3 — spongy (short) bones; 4 — irregular bone

Facing the marrow cavity and the spaces of the spongy bone, the bone is lined with a delicate connective tissue layer—the endosteum—which also produces Bone tissue and contains osteogenic cells.

The bones of the Skeleton, and the Skeletal System as a whole, perform supportive, locomotor, and protective functions. Additionally, skeletal bones serve as a reservoir for mineral salts—phosphorus, calcium, iron, copper, and other Trace Elements.

Bone strength is ensured by the physicochemical unity of organic and Inorganic Components, as well as the specialized architecture of bone tissue. In terms of hardness and elasticity, bones can be compared to bronze and cast iron. Compact and spongy Bone Tissues form the structural basis of the bones. Compact (dense) bone tissue forms the outer layer of every bone. Spongy tissue, composed of interconnected bony trabeculae (spicules), lies beneath the compact layer. In tubular bones, the compact bone layer is thick (up to 1 cm) along the shaft (diaphysis). At the ends of tubular bones, as well as in flat and other bones, this layer is thin. The compact substance is penetrated by a system of bone canals housing blood vessels and nerve fibers. Each bone canal (osteonal canal) is surrounded by concentric lamellae arranged as 4–20 thin tubes nested inside one another. This system of tubes together with the canal is termed an osteon, or Haversian system (Fig. 16, see color insert). The spaces between osteons are occupied by interstitial lamellae, which serve as material for The formation of new osteons during bone remodeling driven by changing physical loads. The Superficial layer of compact bone consists of outer circumferential lamellae, which are produced by the osteogenic activity of the periosteum. The inner layer of the bone, bordering the marrow cavity and spongy spaces, is formed by inner circumferential lamellae and covered with Fibrous connective tissue—the endosteum.

Spongy bone tissue, located beneath the compact layer, is found in the ends of tubular bones (epiphyses), in the bodies of spongy and irregular bones, and within flat and pneumatic bones. Spongy bone consists of bony trabeculae oriented in various directions and interconnected with one another. The distribution of these trabeculae corresponds to the main lines of compression (pressure) and tension acting on the bone. This angular arrangement of the trabeculae ensures the uniform distribution of pressure and Muscle forces across the skeletal bones.

Bone exhibits remarkable plasticity. Depending on the magnitude of mechanical load, the number of osteons increases or decreases, and their arrangement within the compact bone adapts accordingly. Under regular muscular load, athletic training, and physical labor, the number and size of osteons increase, the compact bone layer in tubular and other bones thickens, and the marrow cavities narrow. The trabeculae of the spongy bone also thicken and acquire a more complex, branched architecture. As a result, bones become thicker and stronger. Conversely, with reduced physical load, a sedentary lifestyle, or prolonged bed rest during illness, bones become thinner and weaker.

Organic and inorganic substances are equally vital for bone strength. Organic compounds impart flexibility and elasticity to bones, whereas inorganic substances (calcium phosphate, calcium carbonate, and other salts) provide hardness. In a dry, living bone, organic matter accounts for about 60% of its mass, with the remainder consisting of Inorganic Compounds.

The respective roles of organic and inorganic components in bone strength can be demonstrated experimentally. When organic matter is burned away by calcining a bone over fire, the bone becomes brittle. Conversely, removing inorganic salts by soaking a bone in acid leaves it soft and pliable. The combination of inorganic hardness and organic elasticity guarantees optimal bone strength.

REVIEW AND SELF-Control Questions:

1. Which Organ Systems form the apparatus of support and movement?

2. Name the Regions of the skeleton and the bones that comprise them.

3. What is contained within the marrow cavities and spongy bone spaces in children and adults?

4. Describe the characteristics of compact and spongy bone tissue.

5. What is an osteon, and what is its structure?

6. What properties do organic and inorganic substances impart to bones?