Bones and its structure in detail with two different form of bone formation
- 1. BONES AND ITS STRUCTURE MS. BHARTI SHARMA M.SC NURSING BEE ENN COLLEGE OF NURSING, JAMMU.
- 2. ď‚´ A bone is composed of several different tissues working together: bone or osseous tissue, cartilage, dense connective tissues, epithelium, adipose tissue, and nervous tissue. ď‚´ The entire framework of bones and their cartilages, along with ligaments and tendons, constitutes the skeletal system. ď‚´ The study of bone structure and the treatment of bone disorders is called osteology.
- 3. FUNCTIONS OF BONE ANDTHE SKELETALSYSTEM Bone tissue makes up about 18% of the weight of the human body. The skeletal system performs several basic functions: ď‚´ 1. Support:- The skeleton serves as the structural framework for the body by supporting soft tissues and providing attachment points for the tendons of most skeletal muscles. ď‚´ 2. Protection:- The skeleton protects the most important internal organs from injury. For example, cranial bones protect the brain, etc. ď‚´ 3. Assistance in movement:- Most skeletal muscles attach to bones; when they contract, they pull on bones to produce movement. ď‚´ 4. Mineral homeostasis (storage and release):- Bone tissue stores several minerals, especially calcium and phosphorus, which contribute to the strength of bone.
- 4. ď‚´5.Blood cell production:- Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis. ď‚´6. Triglyceride storage:- Yellow bone marrow consists mainly of adipose cells, which store triglycerides. The stored triglycerides are a potential chemical energy reserve.
- 5. TYPES OF BONES Bones are classified as long, short, irregular, flat and sesamoid bone. ď‚´ 1. Long Bone:- these consist of a shaft and two extremities. As the name suggests, these bones are longer than they are wide. Example: femur, tibia and fibula. ď‚´ 2. short, irregular, flat and sesamoid bones:- These have no shafts or extremities and are diverse in shape and size. Example include: ď‚´ Short bones- carpals( wrist) ď‚´ Irregular bones- (vertebrae and some skull bones) ď‚´ Flat bones- sternum, ribs and most skull bones ď‚´ Sesamoid bones- patella (knee cap).
- 7. STRUCTURE OF BONE A long bone is one that has greater length than width. A typical long bone consists of the following parts: 1. The diaphysis is the bone’s shaft or body—the long, cylindrical, main portion of the bone. 2. The epiphyses are the proximal and distal ends of the bone. 3. The metaphyses are the regions between the diaphysis and the epiphyses. In a growing bone, each metaphysis contains an epiphyseal (growth) plate, a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length. When a bone ceases to grow in length at about ages 18–21, the cartilage in the epiphyseal plate is replaced by bone; the resulting bony structure is known as the epiphyseal line.
- 8.  4. The articular cartilage is a thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation (joint) with another bone. Articular cartilage reduces friction and absorbs shock at freely movable joints.  5. The periosteum is a tough connective tissue sheath and its associated blood supply that surrounds the bone surface wherever it is not covered by articular cartilage. The periosteum also protects the bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons. The periosteum is attached to the underlying bone by perforating (Sharpey’s) fibers, thick bundles of collagen that extend from the periosteum into the bone extracellular matrix.
- 9. ď‚´ 6. The medullary cavity or marrow cavity, is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels in adults. This cavity minimizes the weight of the bone by reducing the dense bony material where it is least needed. ď‚´ 7. The endosteum is a thin membrane that lines the medullary cavity. It contains a single layer of bone-forming cells and a small amount of connective tissue.
- 10. HISTOLOGY OF BONE TISSUE ď‚´ Bone, or osseous tissue contains an abundant extracellular matrix that surrounds widely separated cells. The extracellular matrix is about 15% water, 30% collagen fibers, and 55% crystallized mineral salts. The most abundant mineral salt is calcium phosphate. It combines with another mineral salt, calcium hydroxide, to form crystals of hydroxyapatite. ď‚´ As crystals form, they combine with still other mineral salts, such as calcium carbonate, and ions such as magnesium, fluoride, potassium, and sulfate. As these mineral salts are deposited in the framework formed by the collagen fibers of the extracellular matrix, they crystallize and the tissue hardens. This process, called calcification, is initiated by bone-building cells called osteoblasts. ď‚´ Mineral salts first begin to crystallize in the microscopic spaces between collagen fibers. After the spaces are filled, mineral crystals accumulate around the collagen fibers. The combination of crystallized salts and collagen fibers is responsible for the characteristics of bone.
- 11. Four types of cells are present in bone tissue: osteogenic cells, osteoblasts, osteocytes, and osteoclasts. ď‚´ 1. Osteogenic cells - are unspecialized bone stem cells derived from mesenchyme, the tissue from which almost all connective tissues are formed. They are the only bone cells to undergo cell division; the resulting cells develop into osteoblasts. Osteogenic cells are found along the inner portion of the periosteum, in the endosteum, and in the canals within bone that contain blood vessels. ď‚´ 2. Osteoblasts cells - are bone-building cells. They synthesize and secrete collagen fibers and other organic components needed to build the extracellular matrix of bone tissue, and they initiate calcification. As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes
- 12.  3. Osteocytes - mature bone cells, are the main cells in bone tissue and maintain its daily metabolism, such as the exchange of nutrients and wastes with the blood. Like osteoblasts, osteocytes do not undergo cell division.  4. Osteoclasts - are huge cells derived from the fusion of as many as 50 monocytes (a type of white blood cell) and are concentrated in the endosteum. On the side of the cell that faces the bone surface, the osteoclast’s plasma membrane is deeply folded into a ruffled border. Here the cell releases powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying extracellular bone matrix.
- 14. ď‚´A mnemonic that will help you remember the difference between the function of osteoblasts and osteoclasts is as follows: osteoBlasts Build bone, while osteoClasts Carve out bone.
- 15. MICRO-STRUCTURE OF BONE: Compact Bone Tissue Compact bone tissue contains few spaces and is the strongest form of bone tissue. It is found beneath the periosteum of all bones and makes up the bulk of the diaphyses of long bones. Compact bone tissue provides protection and support and resists the stresses produced by weight and movement. Compact bone tissue is composed of repeating structural units called osteons, or haversian systems. Each osteon consists of concentric lamellae arranged around a central (haversian) canal. Resembling the growth rings of a tree, the concentric lamellae
- 17.  Between the concentric lamellae are small spaces called lacunae, which contain osteocytes. Radiating in all directions from the lacunae are tiny canaliculi, which are filled with extracellular fluid. Inside the canaliculi are slender fingerlike processes of osteocytes.  The areas between neighboring osteons contain lamellae called interstitial lamellae, which also have lacunae with osteocytes and canaliculi.  They are connected to the periosteum by perforating (Sharpey’s) fibers.
- 20. SPONGY BONE TISSUE ď‚´ Spongy bone tissue, also referred to as trabecular bone tissue, does not contain osteons. Spongy bone tissue is always located in the interior of a bone, protected by a covering of compact bone. It consists of lamellae that are arranged in an irregular pattern of thin columns called trabeculae. Between the trabeculae are spaces that are visible to the unaided eye. These macroscopic spaces are filled with red bone marrow in bones that produce blood cells, and yellow bone marrow (adipose tissue) in other bones. ď‚´ Spongy bone tissue makes up most of the interior bone tissue of short, flat, sesamoid, and irregularly shaped bones.
- 22. BONE FORMATION The process by which bone forms is called ossification. ď‚´ Bone formation occurs in four principal situations: ď‚´ (1) the initial formation of bones in an embryo and fetus, ď‚´ (2) the growth of bones during infancy, childhood, and adolescence until their adult sizes are reached, ď‚´ (3) the remodeling of bone (replacement of old bone by new bone tissue throughout life), and ď‚´ (4) the repair of fractures (breaks in bones) throughout life.
- 23. INTRAMEMBRANOUS OSSIFICATION Intramembranous ossification is the simpler of the two methods of bone formation. The flat bones of the skull, most of the facial bones, mandible (lower jawbone), and the medial part of the clavicle (collar bone) are formed in this way. 1 Development of the ossification center. At the site where the bone will develop, specific chemical messages cause the mesenchymal cells to cluster together and differentiate, first into osteogenic cells and then into osteoblasts. The site of such a cluster is called an ossification center. Osteoblasts secrete the organic extracellular matrix of bone until they are surrounded by it. 2 Calcification. Next, the secretion of extracellular matrix stops, and the cells, now called osteocytes, lie in lacunae and extend their narrow cytoplasmic processes into canaliculi that radiate in all directions. Within a few days, calcium and other mineral salts are deposited and the extracellular matrix hardens or calcifies (calcification)
- 24. 3 Formation of trabeculae. As the bone extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone around the network of blood vessels in the tissue. Connective tissue that is associated with the blood vessels in the trabeculae differentiates into red bone marrow. 4 Development of the periosteum. In conjunction with the formation of trabeculae, the mesenchyme condenses at the periphery of the bone and develops into the periosteum. Eventually, a thin layer of compact bone replaces the surface layers of the spongy bone, but spongy bone remains in the center. Much of the newly formed bone is remodeled (destroyed and reformed) as the bone is transformed into its adult size and shape.
- 25. ENDOCHONDRAL OSSIFICATION The replacement of cartilage by bone is called endochondral ossification. Although most bones of the body are formed in this way, the process is best observed in a long bone. It proceeds as follows: 1 Development of the cartilage model. At the site where the bone is going to form, specific chemical messages cause the mesenchymal cells to crowd together in the general shape of the future bone, and then develop into chondroblasts. The chondroblasts secrete cartilage extracellular matrix, producing a cartilage model consisting of hyaline cartilage. A covering called the perichondrium develops around the cartilage model
- 26. 2 Growth of the cartilage model. Once chondroblasts become deeply buried in the cartilage extracellular matrix, they are called chondrocytes. The cartilage model grows in length by continual cell division of chondrocytes, accompanied by further secretion of the cartilage extracellular matrix. This type of cartilaginous growth, called interstitial (endogenous) growth (growth from within), results in an increase in length. 3 Development of the primary ossification center. Primary ossification proceeds inward from the external surface of the bone. A nutrient artery penetrates the perichondrium and the calcifying cartilage model through a nutrient foramen in the midregion of the cartilage model, stimulating osteogenic cells in the perichondrium to differentiate into osteoblasts. Once the perichondrium starts to form bone, it is known as the periosteum. Near the middle of the model, periosteal capillaries grow into the disintegrating calcified cartilage, inducing growth of a primary ossification center, a region where bone tissue will replace most of the cartilage
- 27. 4 Development of the medullary (marrow) cavity. As the primary ossification center grows toward the ends of the bone, osteoclasts break down some of the newly formed spongy bone trabeculae. This activity leaves a cavity, the medullary (marrow) cavity, in the diaphysis (shaft). Eventually, most of the wall of the diaphysis is replaced by compact bone.
- 28. 5 Development of the secondary ossification centers. When branches of the epiphyseal artery enter the epiphyses, secondary ossification centers develop, usually around the time of birth. Bone formation is similar to what occurs in primary ossification centers. However, in the secondary ossification centers spongy bone remains in the interior of the epiphyses (no medullary cavities are formed here). In contrast to primary ossification, secondary ossification proceeds outward from the center of the epiphysis toward the outer surface of the bone. 6 Formation of articular cartilage and the epiphyseal (growth) plate. The hyaline cartilage that covers the epiphyses becomes the articular cartilage. Prior to adulthood, hyaline cartilage remains between the diaphysis and epiphysis as the epiphyseal (growth) plate, the region responsible for the lengthwise growth of long bones that you will learn about next.