Human Anatomy and Physiology - N. I. Fedyukovich 2003

Bones and Joints
Bone Structure

One of the primary human Functions is movement through space. Locomotion is the fundamental adaptive response of the Organism to its environment. In mammals (including humans), this function is performed by The Musculoskeletal System. Movement is carried out with the participation of bones acting as levers, and skeletal Muscles, which together with Bones and Their Joints form the musculoskeletal system. The latter consists of two parts: passive and active. The passive part includes interconnected bones, while the active part consists of muscles, whose contraction changes the body's position in space. The Biological Significance of the Skeletal System is also associated with its involvement in Mineral METABOLISM (serving as a reservoir for phosphorus, calcium, iron, etc.). The BONES OF THE Skull form a well-protected vault for the Brain; the bones of the spine and lower limbs perform a supportive function; the bones of the rib cage protect The Heart and Lungs from external impacts; and the pelvic bones protect the Urinary Bladder and rectum, as well as the Uterus and its appendages in women.

A human bone (os) is a complex organ: it occupies a specific position, has a characteristic shape and Structure, and performs functions unique to it.

Blood Vessels and nerves penetrating the bone facilitate its interaction with the body, its participation in general metabolism, the execution of its functions, and the necessary remodeling during growth, development, and changing environmental conditions. In a living organism, bone contains about 50 % Water, 28 % organic matter, including 16 % fat, and 22 % inorganic matter. The organic component of bone is represented by Proteins, while the inorganic component consists of hydroxyapatite. In addition, bones also contain varying amounts of sodium, magnesium, potassium, chlorine, fluorine, carbonates, and nitrates.

The predominance of organic substances in bones (in children) provides them with flexibility and elasticity. A shift in this ratio toward inorganic substances leads to bone fragility (in the elderly) and more frequent fractures.

Bone is formed by Bone tissue, which is a type of Connective Tissue. It consists of Cells and a dense Extracellular matrix rich in Collagen and mineral components.

Bone tissue contains Two Types of cells: osteoblasts and osteoclasts. Osteoblasts are young, polygonal bone cells rich in rough Endoplasmic reticulum, Ribosomes, and a well-developed Golgi apparatus. They contain large amounts of ribonucleic acid and alkaline phosphatase. Osteoblasts gradually differentiate into osteocytes, which is accompanied by a decrease in the number of Organelles. The extracellular matrix produced by osteoblasts surrounds the osteocytes on all sides and becomes impregnated with calcium salts.

Osteocytes are mature, branching cells residing in bone lacunae; they produce the extracellular matrix and are typically embedded within it. The number of cellular organelles in osteocytes is reduced, and they often store Glycogen. If structural remodeling of the bone is required, osteoblasts become active, rapidly differentiate, and transform into osteocytes. The system of bone canaliculi ensures Metabolic exchange between osteocytes and interstitial fluid.

In addition to the aforementioned cells, bone tissue also contains osteoclasts—large, multinucleated cells poor in Chromatin. The Cytoplasm of these cells has numerous projections covered by a Cell/33.html">Plasma Membrane. The cells contain Mitochondria, Lysosomes, vacuoles, hydrolytic Enzymes, and prominent Golgi complexes. In this region, The Plasma Membrane forms numerous folds and is called the ruffled border.

Osteoclasts are capable of resorbing calcified Cartilage and the extracellular matrix of bone tissue during Bone Development and remodeling. According to modern data, osteoclasts are of monocytic origin and belong to the macrophage system.

Externally, the bone is covered by a layer of Cytology/practical/45.html">Dense connective tissue called the periosteum (periosteum). It is a thin, dense connective tissue membrane rich in Blood Vessels, Lymphatic vessels, and nerves. The periosteum consists of outer and inner layers.

The outer layer of the periosteum is fibrous, while the inner layer is osteogenic (bone-forming). The inner layer is directly adjacent to the bone tissue and forms young cells (osteoblasts) located on the bone surface. Thus, due to the bone-forming Properties of the periosteum, the bone grows in thickness. The periosteum is firmly attached to the bone by penetrating fibers that extend deep into the bone.

The outer layer of bone is represented by a layer of compact bone, which is thicker in the diaphyses of long bones than in the epiphyses. Within the compact bone, bone lamellae are arranged in a specific order, forming complex systems called osteons—the structural units of bone. An osteon consists of 5—20 concentric cylindrical lamellae nested within one another.

In the center of each osteon runs a central (Haversian) canal. Through it, in turn, pass an artery and a vein, which branch into capillaries and reach the lacunae of the Haversian system via canaliculi. They ensure the supply of nutrients and oxygen to the cells and the removal of metabolic products, СО₂ and О₂. Each Haversian canal also contains a lymphatic vessel and nerve fibers. On the outer and inner surfaces of the bone, the bone lamellae do not form concentric cylinders but are arranged around them. These regions are penetrated by Volkmann's canals, through which blood vessels pass, connecting with the Vessels of the Haversian canals. The ground substance of compact bone consists of bone collagen, produced by osteoblasts, and hydroxyapatite; in addition, it includes magnesium, sodium, carbonates, and nitrates.

Beneath the compact bone lies the spongy bone, which is a network of thin, anastomosing bone elements called trabeculae. The trabeculae are oriented in directions that maximize the bone's resistance to loads and compression with minimal mass. Spongy bone is found in the epiphyses of long tubular bones and in short bones (vertebrae, carpal and Tarsal Bones). It is also characteristic of embryos and growing organisms.

Inside the bone, within the medullary cavity and the spaces of the spongy bone, lies the Bone Marrow. During the prenatal period and in newborns, all bones contain Red bone marrow, which primarily performs a hematopoietic function. In adults, red bone marrow is found only in the spaces of the spongy bone of flat bones (Sternum, skull bones, iliac bones), in spongy (short) bones, and in the epiphyses of long bones. The medullary cavity of the diaphyses of long bones contains yellow bone marrow. It consists of adipose tissue and degenerated reticular stroma.

Human bones vary in shape and size and occupy specific positions in the body. There are several types of bones: tubular, spongy, flat (broad), mixed, and pneumatic.

Tubular bones function as levers and form the Skeleton of the free PARTS OF THE limbs; they are divided into long bones (humerus, Femur, bones of the forearm and lower leg) and short bones (metacarpals, metatarsals, and Phalanges of the fingers and toes).

Long tubular bones have expanded ends (epiphyses) and a middle shaft (diaphysis). The region between the epiphysis and diaphysis is called the metaphysis. The epiphyses of bones are fully or partially covered with hyaline cartilage and participate in The formation of joints.

Spongy (short) bones are located in areas of the skeleton where strength must be combined with mobility (Carpal Bones, tarsus, vertebrae, sesamoid bones).

Flat (broad) bones participate in forming the cranial vault, thoracic, and pelvic cavities, perform a protective function, and provide a large surface area for Muscle attachment.

Mixed bones have a complex structure and diverse shapes. This group of bones includes vertebrae, whose bodies are spongy, while their processes and arches are flat.

Pneumatic bones contain an air cavity within their body, lined with a mucous membrane. These include the Maxilla, frontal, sphenoid, and ethmoid bones of the skull.