Skeletal Cartilages
- Basic Structure, Types, and Locations
- Skeletal cartilages are made from cartilage, surrounded by a layer of dense irregular connective tissue called the perichondrium.
- Hyaline cartilage is the most abundant skeletal cartilage, and includes the articular, costal, respiratory, and nasal cartilages.
- Elastic cartilages are more flexible than hyaline, and are located only in the external ear and the epiglottis of the larynx.
- Fibrocartilage is located in areas that must withstand a great deal of pressure or stretch, such as the cartilages of the knee and the intervertebral discs.
- Growth of Cartilage
- Appositional growth results in outward expansion due to the production of cartilage matrix on the outside of the tissue.
- Interstitial growth results in expansion from within the cartilage matrix due to division of lacunae-bound chondrocytes and secretion of matrix.
Classification of Bones
- Axial skeleton - bones of the skull, vertebral column, and rib cage
- Appendicular skeleton - bones of the upper and lower limbs, shoulder, and hip
- Classification by Shape
- Long bones - longer than they are wide (e.g., humerus)
- Short bones - Cube-shaped bones of the wrist and ankle
- Seasmoid bone develop within a tendon (e.g., patella)
- Flat bones - thin, flattened, & bit curved (sternum, most skull bones)
- Irregular bones - complicated shapes (vertebrae; hip bones)
- Wormian bones tiny bones in between major skull bones
- Support - form the framework that supports the body and cradles soft organs
- Protection - provide a protective case for the brain, spinal cord, and vital organs
- Movement - provide levers for muscles
- Mineral storage - reservoir for minerals, especially calcium and phosphorus
- Blood cell formation - hematopoiesis occurs within the marrow cavities of bones
Bone Markings are bulges, depressions, and holes that serve as:
- Joint surfaces
- Fossa - shallow, basinlike depression
- Condyle - rounded articular projection
- Epicondyle - raised area above a condyle
- Head -bony expansion on a narrow neck
- Muscle & ligament attachment
- Trochanter - large, blunt, irregular surface
- Tuberosity - rounded projection
- Process - any bony prominence
- Crest - narrow, prominent ridge of bone
- Line - narrow ridge of bone
- Fovea - small pit like indent
- Tubercle - small rounded projection
- Ramus - armlike bar of bone
- Spine - sharp, slender projection
- Passageways for vessels, etc.
- Foramen - round or oval opening through a bone
- Fissure - narrow, slitlike opening
- Meatus - canal-like passageway
- Space within a bone - Sinus
Structure OF A LONG BONE (gross anatomy)
- Long bones consist of a diaphysis and an epiphysis
- Diaphysis -Tubular shaft that forms the axis of long bones
- Composed of compact bone that surrounds the medullary cavity
- Yellow bone marrow (fat) is contained in the medullary cavity
- Epiphyses - Expanded ends of long bones
- Exterior is compact bone, and the interior is spongy bone
- Joint surface is covered with articular (hyaline) cartilage
- Metaphyses are the areas between the epiphysis and diaphysis and include the epiphyseal plate in growing bones.
- Bone Membranes
- Periosteum - double-layered protective membrane
- Outer fibrous layer is dense regular connective tissue
- Inner osteogenic layer is composed of osteoblasts and osteoclasts
- Richly supplied with nerve fibers, blood, and lymphatic vessels, which enter the bone via nutrient foramina
- Secured to underlying bone by Sharpey's fibers
- Endosteum - delicate membrane covering internal surfaces of bone
Structure of Short, Irregular, and Flat Bones
- Thin plates of periosteum-covered compact bone on the outside with endosteum-covered spongy bone (diploë) on the inside
- Have no diaphysis or epiphyses
- Contain bone marrow between the trabeculae
Location of Hematopoietic Tissue (Red Marrow)
- In infants
- Found in the medullary cavity and all areas of spongy bone
- In adults
- Found in the diploë of flat bones, and the head of the femur and humerus
Microscopic Structure of Compact Bone
- Haversian system, or osteon - the structural unit of compact bone
- Lamella - weight-bearing, column-like matrix tubes composed mainly of collagen
- Haversian, or central canal -containing blood vessels and nerves
- Volkmann's canals - channels lying at right angles to the central canal, connecting blood and nerve supply of periosteum to the Haversian canal
- Osteocytes - mature bone cells
- Lacunae - small cavities in bone that contain osteocytes
- Canaliculi -hairlike canals that connect lacunae to each other & central canal
Chemical Composition of Bone
- Organic
- Osteoblasts - bone-forming cells
- Osteocytes - mature bone cells
- Osteoclasts - large cells that resorb or break down bone matrix
- Osteoid - unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen
- Inorganic - hydroxyapatites, or mineral salts
- Sixty-five percent of bone by mass
- Mainly calcium phosphates
- Responsible for bone hardness and its resistance to compression
Osteogenesis and ossification - the process of bone tissue formation, which leads to:
- The formation of the bony skeleton in embryos
- Bone growth until early adulthood
- Bone thickness, remodeling, and repair
- Formation of the Bony Skeleton
- Begins at week 8 of embryo development
- Intramembranous Ossification - bone develops from a fibrous membrane - bone develops from a fibrous membrane
- Formation of most of the flat bones of the skull and the clavicles
- Stages of Intramembranous Ossification
- Fibrous connective tissue membranes are formed by mesenchymal cells
- An ossification center appears in fibrous connective tissue membrane
- Bone matrix is secreted within the fibrous membrane
- Woven bone and periosteum form
- Bone collar of compact bone forms, and red marrow appears
- Endochondral Ossification
- Begins in the second month of development
- Uses hyaline cartilage "bones" as models for bone construction
- Requires breakdown of hyaline cartilage prior to ossification
- Stages of Endochondral Ossification
- Formation of bone collar
- Cavitation of the hyaline cartilage
- Invasion of internal cavities by the periosteal bud, and spongy bone formation
- Formation of the medullary cavity; appearance of secondary ossification centers in the epiphyses
- Ossification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal plates
Postnatal Bone Growth
- Bone Growth in length
- Epiphyseal plate or cartilage growth plate
- cartilage cells are produced by mitosis on epiphyseal side of plate
- cartilage cells are destroyed and replaced by bone on diaphyseal side of plate
- Between ages 18 to 25, epiphyseal plates close.
- cartilage cells stop dividing and bone replaces the cartilage (epiphyseal line)
- Growth in length stops at age 25
- 4 Zones of Growth in Epiphyseal Plate
- Zone of resting cartilage
- anchors growth plate to bone
- Zone of proliferating cartilage
- rapid cell division (stacked coins)
- Zone of hypertrophic cartilage
- cells enlarged & remain in columns
- Zone of calcified cartilage
- thin zone, cells mostly dead since matrix calcified
- osteoclasts removing matrix
- osteoblasts & capillaries move in to create bone over calcified cartilage
Long Bone Growth and Remodeling
- Growth in length - cartilage continually grows and is replaced by bone as shown
- Remodeling - bone is resorbed and added by appositional growth as shown
- Appositional Growth of Bone
- Osteoblasts beneath the periosteum secrete bone matrix, forming ridges that follow the course of periosteal blood vessels.
- As the bony ridges enlarge and meet, the groove containing the blood vessel becomes a tunnel.
- The periosteum lining the tunnel is transformed into an endosteum and the osteoblasts just deep to the tunnel endosteum secrete bone matrix, narrowing the canal
- As the osteoblasts beneath the endosteum form new lamellae, a new osteon is created. Meanwhile new circumferential lamellae are elaborated beneath the periosteum and the process is repeated, continuing to enlarge bone diameter.
Hormonal Regulation of Bone Growth During Youth
- During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone
- During puberty, testosterone and estrogens:
- Initially promote adolescent growth spurts
- Cause masculinization and feminization of specific parts of the skeleton
- Later induce epiphyseal plate closure, ending longitudinal bone growth
- In adult skeletons, bone remodeling is balanced bone deposit and removal, bone deposit occurs at a greater rate when bone is injured, and bone resorption allows minerals of degraded bone matrix to move into the blood.
- Control of Remodeling
- The hormonal mechanism is mostly used to maintain blood calcium homeostasis, and balances activity of parathyroid hormone and calcitonin.
- Nutrition - Ca, P, Mg, vitamins, A, C and D.
- In response to mechanical stress and gravity, bone grows or remodels in ways that allow it to withstand the stresses it experiences.
- Trabeculae form along lines of stress
- Large, bony projections occur where heavy, active muscles attach
- Wolff's law - a bone grows or remodels in response to the forces or demands placed upon it
- Observations supporting Wolff's law include
- Long bones are thickest midway along the shaft (where bending stress is greatest)
- Curved bones are thickest where they are most likely to buckle
- Hematoma formation
- Torn blood vessels hemorrhage
- A mass of clotted blood (hematoma) forms at the fracture site
- Site becomes swollen, painful, and inflamed
- Fibrocartilaginous callus forms
- Capillaries grow into the tissue and phagocytic cells begin cleaning debris
- Osteoblasts & fibroblasts migrate to fracture & begin reconstructing bone
- Fibroblasts secrete collagen fibers that connect broken bone ends
- Osteoblasts begin forming spongy bone
- Osteoblasts furthest from capillaries secrete an externally bulging
- Bony callus formation
- New bone trabeculae appear in the fibrocartilaginous callus
- Fibrocartilaginous callus converts into a bony (hard) callus
- Bone callus begins 3-4 weeks after injury, and continues until firm union is formed 2-3 months later
- Bone remodeling
- Excess material on the bone shaft exterior and in the medullary canal is removed
- Compact bone is laid down to reconstruct shaft walls
- Nondisplaced - bone ends retain their normal position
- Displaced - bone ends are out of normal alignment
- Complete - bone is broken all the way through
- Incomplete - bone is not broken all the way through
- Linear - the fracture is parallel to the long axis of the bone
- Transverse - the fracture is perpendicular to the long axis of the bone
- Compound (open) - bone ends penetrate the skin
- Simple (closed) - bone ends do not penetrate the skin
Homeostatic Imbalances of Bone
- Osteomalacia and Rickets
- Osteomalacia includes a number of disorders in adults in which the bone is inadequately mineralized.
- Rickets is inadequate mineralization of bones in children caused by insufficient calcium or vitamin D deficiency.
- Osteoporosis
- Group of diseases in which bone reabsorption outpaces bone deposit
- Spongy bone of the spine is most vulnerable
- Occurs most often in postmenopausal women
- Bones become so fragile that sneezing or stepping off a curb can cause fractures
- Factors that contribute to osteoporosis include a petite body form, insufficient exercise or immobility, a diet poor in calcium and vitamin D, abnormal vitamin D receptors, smoking, and certain hormone-related conditions.
- Treatment
- Calcium and vitamin D supplements
- Increased weight-bearing exercise
- Hormone (estrogen) replacement therapy (HRT) slows bone loss
- Natural progesterone cream prompts new bone growth
- Statins increase bone mineral density
- Paget's disease is characterized by excessive bone deposition and resorption, with the resulting bone abnormally high in spongy bone. It is a localized condition that results in deformation of the affected bone.
Developmental Aspects of Bones: Timing of Events
- The skeleton derives from embryonic mesenchymal cells, with ossification occurring at precise times. Most long bones have obvious primary ossification centers by 12 weeks gestation.
- At birth, most bones are well ossified, except for the epiphyses, which form secondary ossification centers.
- Throughout childhood, bone growth exceeds bone resorption; in young adults, these processes are in balance; in old age, resorption exceeds formation.
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