38.2 Bone
3 min read•june 14, 2024
Bones are the foundation of our skeletal system, providing structure and support. They come in various shapes and sizes, each serving specific functions. From that facilitate movement to that protect vital organs, our skeleton is a marvel of engineering.
development is a complex process that begins before birth and continues throughout life. Through intramembranous and , bones form and grow. This ongoing cycle of growth and remodeling ensures our bones remain strong and adaptable to our body's changing needs.
Bone Structure and Function
Types of human skeletal bones
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- Long bones are longer than they are wide and consist of a (shaft) and (ends) which provide leverage and facilitate movement (femur, humerus, radius, ulna, metacarpals, metatarsals)
- are roughly cube-shaped and provide stability and support in areas such as the wrist (carpals) and ankle (tarsals)
- Flat bones are thin and flat in shape, offering protection and serving as attachment points for muscles (skull bones, scapulae, ribs, sternum)
- have complex shapes and protect internal organs (vertebrae, sacrum, coccyx)
- are embedded within tendons and protect them from wear and tear (patella)
Functions of bone cells
- are bone-forming cells that secrete (unmineralized bone matrix) and differentiate into when surrounded by bone matrix
- are mature bone cells that maintain bone tissue, communicate with each other and other cells through , and respond to mechanical stress to regulate
- are bone-resorbing cells that break down and remodel bone tissue by secreting enzymes and acids to dissolve bone matrix and release minerals, working in balance with osteoblasts to maintain bone homeostasis
Bone composition and structure
- Bones are composed of both organic and inorganic components
- The organic component consists mainly of collagen fibers, providing flexibility
- The inorganic component is primarily , a calcium phosphate mineral that gives bones their strength and rigidity
- Bones are organized into structural units called , which are cylindrical structures that run parallel to the long axis of the bone
- The is a tough, fibrous membrane that covers the outer surface of bones, containing blood vessels and nerves
- , found in the cavities of bones, is responsible for producing blood cells and storing fat
Bone Development and Growth
Process of bone development
- forms flat bones:
- differentiate directly into osteoblasts
- Osteoblasts secrete osteoid, which mineralizes to form bone
- Bones grow by , adding new bone on the surface
- Endochondral forms long bones:
- Mesenchymal cells differentiate into , forming a cartilage model
- hypertrophy and die, leaving cavities in the cartilage
- Blood vessels invade the cavities, bringing osteoblasts
- Osteoblasts secrete osteoid, which mineralizes to form bone
- forms in the
- form in the
- (growth plate) between the diaphysis and epiphysis allows longitudinal growth
- Bone growth and remodeling:
- Appositional growth occurs as osteoblasts add new bone on the surface
- Resorption occurs as osteoclasts break down bone tissue
- Remodeling is the coordinated action of osteoblasts and osteoclasts to replace old bone with new bone
- Longitudinal growth continues until the epiphyseal plates close during puberty
- plays a crucial role in regulating calcium homeostasis and bone metabolism by stimulating osteoclast activity and increasing calcium reabsorption in the kidneys
Key Terms to Review (44)
Appositional growth: Appositional growth is the process by which bones increase in width through the addition of new bone tissue to their outer surface. This type of growth is crucial for bone remodeling and repair, allowing bones to adapt to changes in stress and load. Appositional growth works alongside endochondral growth, which is responsible for the increase in length of long bones.
Bone: Bone is a rigid organ that constitutes part of the vertebrate skeleton, providing structural support and protection for various organs. It also serves as a reservoir for minerals like calcium and phosphorus.
Bone marrow: Bone marrow is a soft, spongy tissue found within the cavities of bones that is crucial for the production of blood cells. It serves as the primary site for hematopoiesis, the process by which red blood cells, white blood cells, and platelets are generated, making it essential for maintaining a healthy circulatory and immune system.
Bone remodeling: Bone remodeling is a continuous process where old bone tissue is replaced by new bone tissue, allowing the skeleton to adapt to changes in stress and maintain overall bone health. This dynamic process involves the coordinated action of osteoclasts, which break down old bone, and osteoblasts, which form new bone. Through remodeling, bones can repair themselves, maintain mineral balance, and respond to mechanical forces experienced throughout life.
Bone resorption: Bone resorption is the process by which osteoclasts break down bone tissue, releasing minerals, such as calcium and phosphorus, into the bloodstream. This dynamic process is crucial for maintaining bone health, enabling the body to adapt to changes in mechanical load and providing essential minerals for various physiological functions. Bone resorption is balanced by bone formation, and an imbalance can lead to bone disorders.
Canaliculi: Canaliculi are small, hair-like canals that exist within the bone matrix, connecting osteocytes and facilitating communication and nutrient exchange between them. These microscopic channels play a crucial role in the overall health and maintenance of bone tissue, enabling the transport of essential substances and signaling molecules throughout the bone structure.
Chondrocytes: Chondrocytes are the cells found in cartilage tissue responsible for maintaining the cartilaginous matrix. They play a crucial role in cartilage formation, growth, and repair.
Chondrocytes: Chondrocytes are specialized cells found in cartilage tissue that are responsible for the maintenance and synthesis of cartilage matrix components, including collagen and proteoglycans. These cells play a crucial role in the growth and repair of cartilage, which is essential for providing support and flexibility to joints and other structures in the body. Chondrocytes are embedded within the extracellular matrix they produce, which allows them to maintain the structural integrity of cartilage.
Compact bone: Compact bone, also known as cortical bone, is the dense and solid outer layer of bone tissue that provides structural support and protection. It makes up approximately 80% of the total bone mass in the human body.
Diaphysis: The diaphysis is the central, elongated shaft of a long bone. It is primarily composed of compact bone and contains the medullary cavity filled with yellow marrow.
Diaphysis: The diaphysis is the elongated shaft of a long bone, which is primarily composed of compact bone tissue and houses the medullary cavity. This structure is crucial for the overall strength and support of the bone, allowing it to bear weight and withstand stress during movement. The diaphysis connects the two ends of a long bone, known as the epiphyses, and plays a significant role in the growth and development of bones through the process of ossification.
Endochondral ossification: Endochondral ossification is the process by which bone tissue is formed through the replacement of hyaline cartilage with bone. This process is crucial during fetal development and in the growth of long bones, enabling them to grow in length and achieve their final shape. Understanding this mechanism helps explain how skeletal structures develop and mature over time.
Epiphyseal plate: The epiphyseal plate, also known as the growth plate, is a hyaline cartilage plate located at the ends of long bones. It is responsible for longitudinal bone growth during childhood and adolescence.
Epiphyseal plate: The epiphyseal plate, also known as the growth plate, is a layer of hyaline cartilage located at the ends of long bones in children and adolescents. This structure is crucial for bone growth, allowing for the elongation of bones as new cartilage cells are produced and then replaced by bone tissue. As individuals reach maturity, the epiphyseal plate ossifies and becomes the epiphyseal line, marking the end of longitudinal growth.
Epiphyses: Epiphyses are the rounded ends of long bones, which contain spongy bone and red marrow. They are crucial for joint movement and growth during development.
Epiphyses: Epiphyses are the rounded ends of long bones, located at both the proximal and distal ends, which play crucial roles in bone growth and joint articulation. These regions contain spongy bone and red marrow, contributing to hematopoiesis, while also being covered by articular cartilage that facilitates smooth joint movement. The epiphyses serve as sites for muscle attachment and provide structural support for joints.
Flat bones: Flat bones are thin, flattened, and typically curved bones that provide protection to vital organs and serve as attachment points for muscles. They play a critical role in the skeletal system, contributing to the overall structure and function of the body by supporting and protecting organs while also allowing for muscle attachment.
Haversian canal: A Haversian canal is a microscopic channel found in compact bone. It contains blood vessels and nerves, providing nourishment and sensation to bone cells.
Hydroxyapatite: Hydroxyapatite is a naturally occurring mineral form of calcium apatite, primarily found in bone and teeth. It serves as the main structural component of these tissues, providing them with strength and rigidity while playing a crucial role in the skeletal system's overall function and health.
Intramembranous ossification: Intramembranous ossification is the process by which bone forms directly from mesenchymal tissue, without a cartilage intermediate. This type of ossification primarily occurs in flat bones, such as the skull and clavicles, where the mesenchyme differentiates into osteoblasts that create bone matrix, leading to the formation of bone directly. This process is crucial during fetal development and in the healing of certain bone fractures.
Irregular bones: Irregular bones are a category of bones that do not fit into the other classifications of bone shapes such as long, short, or flat. They have complex shapes that serve specific functions in the body, often providing support and protection to vital organs while also facilitating movement. These bones can vary greatly in size and structure, reflecting their unique roles in the skeletal system.
Lamellae: Lamellae are thin layers of bone matrix in compact bone tissue. They are arranged in concentric circles around central Haversian canals, providing structural support and strength.
Long bones: Long bones are a category of bones characterized by their elongated shape, which is longer than they are wide. These bones play a crucial role in supporting the body, facilitating movement, and serving as sites for muscle attachment, primarily found in the limbs such as the femur, humerus, and tibia. The structure of long bones includes a diaphysis (shaft) and epiphyses (ends), making them essential for both stability and mobility in the skeletal system.
Mesenchymal cells: Mesenchymal cells are multipotent stromal cells that have the ability to differentiate into a variety of cell types, including osteoblasts, chondrocytes, and adipocytes. These cells play a critical role in the formation and repair of bone and connective tissues, acting as a key source for skeletal development and maintenance.
Osseous tissue: Osseous tissue, also known as bone tissue, is a type of dense connective tissue that forms the structural elements of bones. It consists mainly of collagen fibers and inorganic mineral components, providing both strength and rigidity.
Ossification: Ossification is the process of bone tissue formation from cartilage or fibrous tissue. It occurs in two primary forms: intramembranous and endochondral ossification.
Osteoblasts: Osteoblasts are specialized cells responsible for bone formation through the synthesis and secretion of bone matrix components. They play a crucial role in the growth, remodeling, and healing of bones by producing collagen and other proteins that form the organic part of the bone matrix. In addition to forming new bone, osteoblasts are involved in mineralizing the matrix with calcium and phosphate, essential for bone strength and density.
Osteoclasts: Osteoclasts are specialized cells responsible for bone resorption, playing a crucial role in bone remodeling and maintenance. These multinucleated cells break down bone tissue by secreting enzymes and acids, allowing for the release of minerals such as calcium into the bloodstream. Their activity is essential for maintaining the balance between bone formation and resorption, influencing overall bone health.
Osteocytes: Osteocytes are mature bone cells that maintain the mineral content of the surrounding bone matrix. They are derived from osteoblasts and reside in small cavities called lacunae.
Osteocytes: Osteocytes are mature bone cells that play a critical role in maintaining bone tissue. They reside in small cavities called lacunae and extend long, hair-like processes through tiny channels known as canaliculi, allowing them to communicate with other bone cells. This communication is essential for regulating bone remodeling and maintaining the overall health and strength of the skeletal system.
Osteoid: Osteoid is an unmineralized organic matrix of bone that serves as the foundation for bone formation. It primarily consists of collagen fibers and various proteins, playing a crucial role in providing structural support and flexibility to bones before mineralization occurs. This organic component is essential for bone health and development, as it forms the scaffolding upon which minerals like calcium and phosphate are deposited to create hard bone tissue.
Osteons: Osteons are cylindrical structures that form the functional unit of compact bone. They consist of concentric layers of calcified matrix and house blood vessels and nerves.
Osteons: Osteons, also known as Haversian systems, are the fundamental structural units of compact bone. They consist of concentric layers of mineralized matrix, called lamellae, surrounding a central canal that houses blood vessels and nerves. This organization allows for efficient nutrient and waste exchange, contributing to the overall strength and resilience of bone tissue.
Osteoprogenitor cells: Osteoprogenitor cells are stem cells located in the bone that have the ability to differentiate into osteoblasts. They play a crucial role in bone growth, repair, and maintenance.
Parathyroid Hormone: Parathyroid hormone (PTH) is a key regulator of calcium and phosphate metabolism in the body, secreted by the parathyroid glands. It plays a crucial role in maintaining calcium homeostasis by increasing blood calcium levels when they drop too low, which connects it to various physiological processes involving hormone regulation, endocrine function, and bone health.
Parathyroid hormone (PTH): Parathyroid hormone (PTH) is a hormone secreted by the parathyroid glands that regulates calcium and phosphate balance in the blood and tissues. It increases blood calcium levels by stimulating osteoclast activity, increasing intestinal calcium absorption, and promoting renal calcium reabsorption.
Periosteum: The periosteum is a dense layer of vascular connective tissue that envelops the bones except at the surfaces of the joints. It serves multiple functions, including providing a protective layer, facilitating bone growth and repair, and serving as an attachment point for muscles and tendons. The periosteum is crucial for overall bone health and plays a significant role in the body’s skeletal system.
Primary ossification center: A primary ossification center is the first area in a developing bone where bone tissue begins to form, playing a crucial role in the process of endochondral ossification. This center typically appears during fetal development and is essential for the growth and maturation of long bones. It marks the transition from cartilage to bone, setting the stage for further growth and development of the skeletal system.
Secondary ossification centers: Secondary ossification centers are specific areas within bones where ossification occurs after the initial formation of bone tissue during the development of an organism. These centers play a crucial role in the growth and maturation of bones, particularly in long bones, as they contribute to the formation of the epiphyses, or ends of the bones, which eventually allow for growth in length and joint function.
Sesamoid bones: Sesamoid bones are small, round bones that form within tendons, typically in areas where tendons pass over joints. They serve to protect tendons from stress and wear, while also improving the mechanical advantage of muscles during movement. Their presence can vary among individuals, and they are most commonly found in the hands and feet, particularly the patella, or kneecap.
Short bones: Short bones are a category of bone that are approximately equal in length, width, and thickness, giving them a box-like shape. These bones provide stability and support while allowing for limited movement. Common examples include the carpals in the wrist and tarsals in the ankle, which play essential roles in facilitating various movements of the hands and feet.
Spongy bone: Spongy bone, also known as cancellous bone, is a porous type of bone tissue found at the ends of long bones and in the interiors of other bones. It contains trabeculae, which are small needle-like pieces that create a supportive framework.
Sutural bones: Sutural bones, also known as Wormian bones, are extra bone pieces that occur within the sutures of the cranial bones. They vary in number and size among individuals and are typically found in the lambdoid suture.
Trabeculae: Trabeculae are the small, often microscopic, beam-like structures that form the spongy part of bone tissue. They provide structural support and help distribute loads in the bone.