Exam 4 Part 2 Flashcards
The human skeleton initially consists of
Just cartilage, which is replaced by bone, except in areas requiring flexibility
Skeletal Cartilage
•made of highly resilient, molded cartilage tissue that consists primarily of water
–Contains no blood vessels or nerves
Three types of cartilage:
Hyaline
Elastic
Fibrocartilage
Hyaline
- Provides support, flexibility, and resilience
- Most abundant type; contains collagen fibers only
- Articular (joints), costal (ribs), respiratory (larynx), nasal cartilage (nose tip)
Elastic cartilage
- Similar to hyaline cartilage, but contains elastic fibers
- External ear and epiglottis
Fibrocartilage
- Thick collagen fibers: has great tensile strength
- Menisci of knee; vertebral discs
There are seven important functions of bones
Support
Prtection
Movement
Mineral and growth factors storage
Blood cell formation
Triglyveride (fat) storage
Hormone production
Bones are also classified according to one of four shapes:
Long
Irregular
Short
Fla
Axial skeleton
- Long axis of body
- Skull, vertebral column, rib cage
Appendicular skeleton
•Bones of upper and lower limb
- Girdles attaching limbs
Compact bone
–dense outer layer on every bone that appears smooth and solid
Spongy bone
–made up of a honeycomb of small, needle-like or flat pieces of bone called trabeculae
•Open spaces between trabeculae are filled with red or yellow bone marrow
Structure of short, irregular, and flat bones
–Consist of thin plates of spongy bone (diploe) covered by compact bone
–Compact bone sandwiched between connective tissue membranes
•Periosteum covers outside of compact bone, and endosteum covers inside portion of compact bone
Structure of typical long bone
–All long bones have a shaft (diaphysis), bone ends (epiphyses), and membranes
•Diaphysis: tubular shaft that forms long axis of bone
–Consists of compact bone surrounding central medullary cavity that is filled with yellow marrow in adults
•Epiphyses: ends of long bones that consist of compact bone externally and spongy bone internally
–Articular cartilage covers articular (joint) surfaces
•Between diaphysis and epiphysis is epiphyseal line
–Remnant of childhood epiphyseal plate where bone growth occurs
Membranes:
two types (periosteum and endosteum)
Periosteum:
–white, double-layered membrane that covers external surfaces except joints
- Fibrous layer: outer layer consisting of Sharpey’s fibers that secure to bone matrix
- Osteogenic layer: inner layer abutting bone and contains primitive osteogenic stem cells that gives rise to most all bone cells
- Contains many nerve fibers and blood vessels that continue on to the shaft through nutrient foramen openings
Anchoring points for tendons and ligaments
Endosteum
- Delicate connective tissue membrane covering internal bone surface
- Covers trabeculae of spongy bone
- Lines canals that pass through compact bone
- Like periosteum, contains osteogenic cells that can differentiate into other bone cells
Red marrow
–found within trabecular cavities of spongy bone and diploë of flat bones, such as sternum
- In newborns, medullary cavities and all spongy bone contain red marrow
- In adults, red marrow is located in heads of femur and humerus, but most active areas of hematopoiesis are flat bone diploë and some irregular bones (such as the hip bone)
- Yellow marrow can convert to red, if person becomes anemic
Bone markings
–Sites of muscle, ligament, and tendon attachment on external surfaces
–Areas involved in joint formation or conduits for blood vessels and nerves
–Three types of markings:
- Projection: outward bulge of bone
- Depression: bowl- or groove-like cut-out
- Opening: hole or canal in bone
Cells of bone tissue
- Osteogenic cells
- Osteoblasts
- Osteocytes
- Bone-lining cells
- Osteoclasts
Osteogenic cells
–Also called osteoprogenitor cells
–Mitotically active stem cells in periosteum and endosteum
–When stimulated, they differentiate into osteoblasts or bone-lining cells
–Some remain as osteogenic stem cells
Osteoblasts
–Bone-forming cells that secrete unmineralized bone matrix called osteoid
- Osteoid is made up of collagen and calcium-binding proteins
- Collagen makes up 90% of bone protein
Osteoblasts are actively mitotic
Osteocytes
–Mature bone cells in lacunae that no longer divide
–Maintain bone matrix and act as stress or strain sensors
- Respond to mechanical stimuli such as increased force on bone or weightlessness
- Communicate information to osteoblasts and osteoclasts (cells that destroy bone) so bone remodeling can occur
Bone-lining cells
–Flat cells on bone surfaces believed to also help maintain matrix (along with osteocytes)
–On external bone surface, lining cells are called periosteal cells
–On internal surfaces, they are called endosteal cells
Osteoclasts
–Derived from same hematopoietic stem cells that become macrophages
–Giant, multinucleate cells function in bone resorption (breakdown of bone)
Compact bone
–Also called lamellar bone
–Consists of:
- Osteon (Haversian system)
- Canals and canaliculi
- Interstitial and circumferential lamellae
Osteon (Haversian system)
–An osteon is the structural unit of compact bone
–Consists of an elongated cylinder that runs parallel to long axis of bone
•Acts as tiny weight-bearing pillars
–An osteon cylinder consists of several rings of bone matrix called lamellae
- Lamellae contain collagen fibers that run in different directions in adjacent rings
- Withstands stress and resist twisting
Central (Haversian) canal
–runs through core of osteon
•Contains blood vessels and nerve fibers
Perforating (Volkmann’s) canals
–canals lined with endosteum that occur at right angles to central canal
•Connect blood vessels and nerves of periosteum, medullary cavity, and central canal
Bone is made up of
organic (35%) and inorganic components (65%)
Organic components
•Includes osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid
–Osteoid, which makes up one-third of organic bone matrix, is secreted by osteoblasts
»Consists of ground substance and collagen fibers, which contribute to high tensile strength and flexibility of bone
Inorganic components
–Hydroxyapatites (mineral salts)
- Makeup 65% of bone by mass
- Consist mainly of tiny calcium phosphate crystals in and around collagen fibers
- Responsible for hardness and resistance to compression
Ossification (osteogenesis)
•process of bone tissue formation
–Formation of bony skeleton begins in month 2 of development
–Postnatal bone growth occurs until early adulthood
Bone remodeling and repair are lifelong
Endochondral ossification
–Bone forms by replacing hyaline cartilage
–Bones are called cartilage (endochondral) bones
–Form most of skeleton
Intramembranous ossification
–Bone develops from fibrous membrane
–Bones are called membrane bones
Endochondral ossification
–Forms essentially all bones inferior to base of skull, except clavicles
–Begins late in month 2 of development
–Uses previously formed hyaline cartilage models
–Requires breakdown of hyaline cartilage prior to ossification
–Begins at primary ossification center in center of shaft
- Blood vessels infiltrate perichondrium, converting it to periosteum
- Mesenchymal cells differentiate into osteoblasts
•Five main steps in the process of ossification:
- Bone collar forms around diaphysis of cartilage model
- Central cartilage in diaphysis calcifies, then develops cavities
- Periosteal bud invades cavities, leading to formation of spongy bone
- Diaphysis elongates, and medullary cavity forms
- Epiphyses ossify
Intramembranous ossification:
•begins within fibrous connective tissue membranes formed by mesenchymal cells
–Forms frontal, parietal, occipital, temporal, and clavicle bones
Long bones grow
lengthwise by interstitial (longitudinal) growth of epiphyseal plate
Bones increase thickness
through appositional growth
Resting (quiescent) zone
–Area of cartilage on epiphyseal side of epiphyseal plate that is relatively inactive
- Proliferation (growth) zone
–Area of cartilage on diaphysis side of epiphyseal plate that is rapidly dividing
–New cells formed move upward, pushing epiphysis away from diaphysis, causing lengthening
- Hypertrophic zone
–Area with older chondrocytes closer to diaphysis
–Cartilage lacunae enlarge and erode, forming interconnecting spaces
- Calcification zone
–Surrounding cartilage matrix calcifies; chondrocytes die and deteriorate
4.Ossification zone
–Chondrocyte deterioration leaves long spicules of calcified cartilage at epiphysis-diaphysis junction
–Spicules are then eroded by osteoclasts and are covered with new bone by osteoblasts
–Ultimately replaced with spongy bone
–Medullary cavity enlarges as spicules are eroded
Osteoblasts
beneath periosteum secrete bone matrix on external bone
Osteoclasts
- remove bone on endosteal surface
- Usually more building up than breaking down which leads to thicker, stronger bone that is not too heavy
Growth hormone
•most important hormone in stimulating epiphyseal plate activity in infancy and childhood
Thyroid hormone
•modulates activity of growth hormone, ensuring proper proportions
Testosterone (males) and estrogens (females) at puberty
•promote adolescent growth spurts
–End growth by inducing epiphyseal plate closure
•Excesses or deficits of any hormones cause abnormal skeletal growth
1.Hormonal controls
- Negative feedback loop that controls blood Ca2+ levels
- Calcium functions in many processes, such as nerve transmission, muscle contraction, blood coagulation, gland and nerve secretions, as well as cell division
- 99% of 1200–1400 gms of calcium are found in bone
- Intestinal absorption of Ca2+ requires vitamin D
Parathyroid hormone (PTH)
–produced by parathyroid glands in response to low blood calcium levels
- Stimulates osteoclasts to resorb bone
- Calcium is released into blood, raising levels
Calcitonin
–produced by parafollicular cells of thyroid gland in response to high levels of blood calcium levels
•Effects are negligible, but at high pharmacological doses it can lower blood calcium levels temporarily
Leptin
–Hormone released by adipose tissue
–May play role in bone density regulation by inhibiting osteoblasts
Serotonin
–Neurotransmitter regulates mood and sleep; also interferes with osteoblast activity
–May inhibit bone turnover after a meal, so bone calcium is locked in when new calcium is flooding into bloodstream
Wolf’s law
–states that bones grow or remodel in response to demands placed on them
- Stress is usually off center, so bones tend to bend
- Bending compresses one side, stretches other side
–Diaphysis is thickest where bending stresses are greatest
–Bone can be hollow because compression and tension cancel each other out in center of bone
- Handedness (right- or left-handed) results in thicker and stronger bone of the corresponding upper limb
- Curved bones are thickest where most likely to buckle
- Trabeculae form trusses along lines of stress
- Large, bony projections occur where heavy, active muscles attach
- Bones of fetus and bedridden people are featureless because of lack of stress on bones
Three major bone diseases:
–Osteomalacia and rickets
–Osteoporosis
–Paget’s disease
Osteomalacia
–Bones are poorly mineralized
–Osteoid is produced, but calcium salts not adequately deposited
–Results in soft, weak bones
–Pain upon bearing weight
Rickets (osteomalacia of children)
–Results in bowed legs and other bone deformities because bones ends are enlarged and abnormally long
–Cause: vitamin D deficiency or insufficient dietary calcium
•Risk factors for osteoporosis
–Most often aged, postmenopausal women
•Estrogen plays a role in bone density, so when levels drop at menopause, women run higher risk
–Men are less prone due to protection by the effects of testosterone
–Petite body form
–Insufficient exercise to stress bones
–Diet poor in calcium and protein
–Smoking
–Hormone-related conditions
- Hyperthyroidism (Thyroid hormone stimulates bone resorption)
- Low blood levels of thyroid-stimulating hormone (TSH has bone protective properties)
- Diabetes mellitus (insulin may promote bone growth)
–Immobility
–Males with prostate cancer taking androgen-suppressing drugs
Treating osteoporosis
–Traditional treatments
- Calcium
- Vitamin D supplements
- Weight-bearing exercise
- Hormone replacement therapy
–Slows bone loss but does not reverse it
–Controversial because of increased risk of heart attack, stroke, and breast cancer
Other drugs for osteoporosis:
–Bisphosphonates: decrease osteoclast activity and number
–Selective estrogen receptor modulators: mimic estrogen without targeting breast and uterus
–Denosumab
•Monoclonal antibody shown to reduce fractures in men with prostate cancer
Improves bone density in elderly
