Bone Tissue Structure and Function PDF

Chapter 6.2 Describe inorganic and organic functions of the extracellular matrix of the tissue bone. - constitutes about 65% of the total weight of bone and is primarily made up of **calcium salts**, especially as part of hydroxyapatite crystals (Ca₁₀(PO₄)₆(OH)₂). - Other minerals suc...

Chapter 6.2 Describe inorganic and organic functions of the extracellular matrix of the tissue bone. - constitutes about 65% of the total weight of bone and is primarily made up of **calcium salts**, especially as part of hydroxyapatite crystals (Ca₁₀(PO₄)₆(OH)₂). - Other minerals such as phosphorus, bicarbonate, potassium, magnesium, and sodium salts are also present. - This mineral composition is crucial as it imparts hardness and strength to bone, allowing it to withstand compressive forces. - constituting about 35% of the total weight of bone, is primarily composed of **collagen fibers (type I)**, which provide tensile strength and flexibility. - The organic matrix, also known as **osteoid**, contains proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins like osteocalcin. - Proteins and proteoglycans play a significant role in recruiting calcium ions and binding different components of the matrix together, while collagen fibers help resist twisting and tensile forces. Both the organic and inorganic matrix are essential for maintaining bone strength, as illustrated by the brittleness of bones when the organic matrix is removed, and the flexibility when the inorganic matrix is lacking. Explain the functions of the three main cell types in bone tissue. **Functions of the Three Main Cell Types in Bone Tissue** 1. **Osteoblasts**: - Function: **bone formation**. They synthesize and secrete the organic components of the bone matrix (osteoid) mineralize matrix - Location: Found in the inner periosteum and endosteum. - They mature into osteocytes once they are surrounded by the matrix they produce. 2. **Osteocytes**: - Function: These are mature bone cells that maintain the bone matrix, regulate mineral content, and communicate with other bone cells. They play a crucial role in bone remodeling and respond to mechanical stress. - Location: Reside in small cavities called lacunae, surrounded by bone matrix. 3. **Osteoclasts**: - Function: **bone resorption**, breaking down bone tissue by secreting hydrogen ions and enzymes that dissolve both the inorganic and organic components of the bone matrix. This process is crucial for bone remodeling and calcium homeostasis. - Location: Found on the bone surface in depressions called Howship\'s lacunae and originate from the fusion of mononuclear precursors in the bone marrow. Describe the microscopic structure of compact bone and the components of the osteon. **Microscopic Structure of Compact Bone and Osteons** - **Compact Bone**: - Organized into structural units called **osteons** which are cylindrical structures that run parallel to the long axis of the bone. - Each osteon consists of concentric layers called **lamellae** surrounding a central canal (Haversian canal), which contains blood vessels and nerves. - The collagen fibers within adjacent lamellae have alternating orientations, providing resistance to torsional (twisting) forces. - **Components of the Osteon**: - **Central Canal**: Contains blood vessels and nerves. - **Lamellae**: Layers of bone matrix that enhance strength; arranged concentrically. - **Lacunae**: Small cavities within the lamellae that house osteocytes. - **Canaliculi**: Tiny channels that connect lacunae, allowing for nutrient and waste exchange between osteocytes and blood vessels, facilitating communication. Describe the microscopic structure of spongy bone. - **Spongy Bone**: - Composed of a network of interconnecting **trabeculae** (branches of bone) that form a porous structure, making it lighter than compact bone. - Trabeculae provide support and house marrow but do not contain osteons. - Like compact bone, it contains **lamellae**, but they are organized in parallel or irregular arrangements. - Osteocytes are found in lacunae within the trabeculae and connect via canaliculi to receive nourishment from the surrounding bone marrow rather than central blood vessels. Differentiate between the processes of intramembranous and endochondral ossification. **Intramembranous Ossification** Intramembranous ossification is a process through which certain flat bones, such as most of the skull and the clavicles, develop from mesenchymal tissue. This type of ossification occurs within a membrane of embryonic connective tissue and involves the following steps: **Steps of Intramembranous Ossification:** 1. **Development of Osteoblasts**: Mesenchymal cells in the primary ossification center differentiate into osteogenic cells and then into osteoblasts. 2. **Secretion of Organic Matrix**: The osteoblasts secrete an organic matrix (osteoid), which soon starts to calcify. As the matrix becomes mineralized, the osteoblasts become trapped and mature into osteocytes. 3. **Formation of Early Spongy Bone**: The osteoblasts continue depositing new bone, laying down trabeculae of spongy bone. This early spongy bone forms interconnecting networks of bone tissue. Some surrounding mesenchyme differentiates into the periosteum. 4. **Formation of Compact Bone**: Osteoblasts in the periosteum lay down compact bone on the outer layers of the developing bone, leading to a structure where the inner layer is spongy bone, and the outer layer is compact bone. **Endochondral Ossification** Endochondral ossification is a process that forms most of the bones in the body, particularly long bones, from a cartilage model. This process begins in the fetal period and involves the following key steps: **Steps of Endochondral Ossification:** 1. **Differentiation of Chondroblasts**: Chondroblasts in the perichondrium differentiate into osteoblasts. The perichondrium becomes vascularized with blood vessels. 2. **Formation of the Bone Collar**: - **Step 2a**: Osteoblasts start by building a bone collar around the diaphysis (shaft) of the hyaline cartilage model. - **Step 2b**: The internal cartilage begins to calcify, and the chondrocytes die as they lose their nutrient supply, leading to the formation of cavities within the model. 3. **Replacement of Cartilage with Bone**: - In the primary ossification center, osteoblasts invade the cavities and replace calcified cartilage with early spongy bone. The formation of blood vessels and osteoblasts also contributes to the development of the medullary cavity. - Secondary ossification centers develop in the epiphyses (ends of the bones). 4. **Completion of Ossification**: - As marrow cavities continue to expand, the remaining cartilage is gradually replaced by bone. The growth plates (epiphyseal plates) remain until puberty when they close, and the articular cartilage at the ends of bones persists throughout life. **Summary:** Endochondral ossification involves the replacement of a cartilage model with bone, starting from the outside and forming a medullary cavity. This process is responsible for the formation of most bones in the human body, particularly long bones. It begins with the differentiation of chondroblasts and culminates in the replacement of the cartilage model with spongy and eventually compact bone. **Key Differences Between Intramembranous and Endochondral Ossification** 1. **Type of Bone Formed**: - Intramembranous ossification forms flat bones (e.g., cranial bones, clavicles). - Endochondral ossification forms long and short bones (e.g., femur, humerus). 2. **Model Used**: - Intramembranous ossification uses a fibrous membrane (mesenchyme) as the model. - Endochondral ossification uses a hyaline cartilage model. 3. **Order of Bone Formation**: - In intramembranous ossification, spongy bone forms first, followed by compact bone. - In endochondral ossification, compact bone forms first before spongy bone develops from the inside out. 6.4 - **Definition:** Longitudinal growth is the process by which long bones increase in length. - **Location:** This growth primarily occurs at the epiphyseal plate, a layer of hyaline cartilage located near the ends of long bones. - **Mechanism:** It involves the division of chondrocytes not osteocytes (cartilage cells) in the epiphyseal plate. The process can be broken down into distinct zones within the plate: 1. **Zone of Reserve Cartilage:** Contains inactive cells that can be activated for growth. 2. **Zone of Proliferation:** Active chondrocytes divide, contributing to bone length. 3. **Zone of Hypertrophy:** Chondrocytes enlarge and mature. 4. **Zone of Calcification:** Chondrocytes die, and their matrix becomes calcified. 5. **Zone of Ossification:** Osteoblasts replace the calcified cartilage with bone. - **Duration:** Longitudinal growth continues until the epiphyseal plates close, typically occurring in late adolescence or early adulthood (18-25). **Appositional Growth:** - **Definition:** Appositional growth is the process by which bones increase in width. - **Location:** This growth occurs at the outer surface of the bone, beneath the periosteum (a dense layer of vascular connective tissue). - **Mechanism:** Osteoblasts located between the periosteum and the bone surface lay down new bone on the outer surface, while osteoclasts in the medullary cavity resorb bone on the inner surface. This process leads to the formation of new circumferential lamellae. - Unlike longitudinal growth, appositional growth occurs in all bones and is important for maintaining bone strength as they adapt to stress and mechanical loads. - **Outcome:** As bone width increases, the medullary cavity enlarges, allowing bones to maintain a proportionate strength and weight. - **Duration:** Appositional growth can continue throughout life, dependent on physical activity, mechanical stress, hormonal influences, and nutritional factors. **Hormones Influencing Bone Growth** 1. **Growth Hormone (GH):** - **Source:** Produced by the anterior pituitary gland. - **Functions:** - Stimulates chondrocyte division in the epiphyseal plate, promoting longitudinal growth. - Activates osteogenic cells and enhances the activity of osteoblasts, particularly in the zone of ossification. - Directly triggers appositional growth through stimulation of periosteal osteoblasts. - **Clinical Relevance:** Excess production before epiphyseal closure can lead to gigantism; after closure, it can result in acromegaly. 2. **Testosterone:** - **Source:** Produced by the testes in males. - **Effect on Bone Growth** - Promotes appositional growth, leading to denser and wider male bones. - Increases the mitotic rate of chondrocytes in the epiphyseal plate, contributing to rapid longitudinal growth during puberty. - Accelerates the closure of epiphyseal plates after height increases, thus limiting further growth in length. **Summary** In summary, longitudinal and appositional bone growth are crucial processes for bone development, occurring through distinct mechanisms and at different anatomical sites within the bone. Hormones like growth hormone, testosterone, and estrogen significantly influence these growth processes, affecting both the timing and extent of height and width that bones achieve throughout development. 6.5 **Bone Resorption and Bone Deposition** **Bone Resorption**: - Bone resorption is the process through which osteoclasts break down bone tissue. Osteoclasts are specialized cells that secrete hydrogen ions and enzymes to dissolve the mineralized bone matrix (hydroxyapatite) and degrade the organic components (collagen and other matrix proteins). - This process releases calcium ions and other minerals into the bloodstream, which can then be utilized by the body. - Resorption is crucial for the maintenance of bone density and regulation of serum calcium levels. **Bone Deposition**: - Bone deposition is the process of forming new bone tissue, primarily carried out by osteoblasts. These cells synthesize and secrete bone matrix components, including collagen and other proteins that form the organic matrix, and facilitate the crystallization of inorganic minerals. - Osteoblasts also promote the operational phase of calcification, leading to the hardening of bone. This process is vital for bone growth, remodeling, and repair after injuries. **Role of Hormones and Vitamin D in Bone Remodeling and Calcium Homeostasis** **Calcitonin**: - Calcitonin is secreted by the thyroid gland when calcium levels in the blood are elevated. It helps to lower blood calcium levels by inhibiting osteoclast activity, thus reducing bone resorption and promoting bone deposition. **Parathyroid Hormone (PTH)**: - PTH is secreted by the parathyroid glands in response to low blood calcium levels. It acts to increase blood calcium concentration by stimulating osteoclast activity (increasing bone resorption), enhancing calcium reabsorption in the kidneys, and promoting intestinal absorption of calcium via its action on vitamin D. **Vitamin D**: - Vitamin D is synthesized in the skin through UV light exposure and can also be acquired through diet. It plays a vital role by facilitating the intestinal absorption of calcium, thus promoting bone deposition and maintaining calcium homeostasis. In the absence of sufficient vitamin D, calcium absorption is greatly reduced, leading to potential bone diseases such as rickets (in children) or osteomalacia (in adults). **General Process of Bone Repair** The process of bone repair, especially following a fracture, generally occurs in the following steps: 1. **Hematoma Formation**: Immediately after bone injury, blood vessels rupture and a hematoma (blood clot) forms around the fracture site, leading to inflammation and an influx of immune cells to the area. 2. **Soft Callus Formation**: Within a week, fibroblasts and chondroblasts migrate to the hematoma area, resulting in the formation of a soft callus made of collagen and cartilage. This soft callus provides initial stability to the fracture. 3. **Bone Callus Formation**: Osteoblasts then start to lay down new bone, forming a hard callus around the fracture site. This callus is primarily made up of primary bone, which is less durable than secondary bone. 4. **Remodeling**: Over several months, the bone callus is remodeled, with primary bone being replaced by stronger secondary bone, restoring the original shape and strength of the bone. This process may continue long after the initial healing phase. **Summary** Bone remodeling is a dynamic process involving the continuous formation and resorption of bone, regulated by various hormones and nutritional factors. Hormones like calcitonin and PTH work antagonistically to regulate calcium ion homeostasis in the body. Vitamin D is crucial for calcium absorption and deposition into bone. In the event of injury, the complex healing process restores bone integrity through a series of stages, enabling the repair and remodeling of bone. Proper nutrient intake and mechanical loading are fundamental to maintaining healthy bone density and effective repair.