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Bone Tissue • Bone is made up of several different tissues working together: bone, cartilage, dense connective tissue, epithelium, various blood forming tissues, adipose tissue, and nervous tissue • Each individual bone is an organ • The bones, along with their cartilages, make up the skeletal system Functions of Bone • Supporting & protecting soft tissues • Attachment site for muscles making movement possible • Storage of minerals, especially calcium & phosphate (mineral homeostasis) • Blood cell production occurs in red bone marrow (hemopoiesis & EPO) • Energy storage in yellow bone marrow • Gives pirates something to put on their flags Anatomy of a Long Bone • Diaphysis = shaft (compact bone) • Epiphysis = one end of a long bone (spongy bone) • Metaphyses are the areas between the epiphysis and diaphysis and include the epiphyseal plate in growing bones • Articular cartilage over joint surfaces acts as friction reducer & shock absorber • Medullary cavity = marrow cavity Anatomy of a Long Bone • Endosteum = lining of marrow cavity • Periosteum = tough membrane covering bone but not the cartilage Fibrous layer = dense irregular connective tissue Osteogenic layer = bone cells & blood vessels that nourish or help with repairs Anatomy of a Flat Bone • Internal and external surfaces formed of compact bone • Middle layer is spongy bone and red bone marrow • Fractures may break the compact bone layer but leave the spongy bone relatively unharmed • Bone is a type of connective tissue as seen by widely spaced cells separated by matrix • Matrix of 25% water, 25% collagen fibers & 50% crystalized mineral salts • ~85% hydroxyapatite, a crystalized calcium phosphate salt • Mineral salts are deposited in a framework of collagen fibers, a process known as mineralization or calcification • Hardness of bone depends on mineral salts, flexibility depends on collagen fibers (“like fiberglass”) General Histology of Bone Bone Cells • Osteogenic Cells: Undifferentiated cells can divide to replace themselves & can become osteoblasts found in inner layer of periosteum and endosteum • • • Osteoblasts: Form matrix & collagen fibers but can’t divide Osteocytes: Mature cells that no longer secrete matrix, now trapped in the matrix they secreted Osteoclasts: Huge cells from ~50 fused monocytes (WBC) function in bone resorption at surfaces such as endosteum Compact or Dense Bone • • • Looks like solid hard layer of bone Makes up the shaft of long bones and the external layer of all bones Resists stresses produced by weight and movement Compact or Dense Bone • Compact bone is arranged in units called osteons or Haversian systems • Osteons contain blood vessels, lymphatic vessels, nerves, and osteocytes along with the calcified matrix • Osteons are aligned in the same direction along lines of stress, which can slowly change as the stresses on the bone change Histology of Compact Bone • • • • Osteon is concentric rings (lamellae) of calcified matrix surrounding a vertically oriented blood vessel Osteocytes are found in spaces called lacunae Osteocytes communicate through canaliculi filled with extracellular fluid that connect one cell to the next cell Interstitial lamellae represent older osteons that have been partially removed during tissue remodeling Structure and Histology of Spongy Bone • • • • • • No Osteons Generally does NOT contain osteons Latticework of thin plates of bone called trabeculae oriented along lines of stress Provides strength with little weight Spaces in between struts of trabeculae are filled with red marrow where red blood cells develop Central canals not needed because no osteocyte is far from the marrow Found in epiphyses of long bones and inside flat bones such as the hipbones, sternum, sides of skull, and ribs Blood and Nerve Supply of Bone • Periosteal arteries Supply periosteum • Nutrient arteries Enter through nutrient foramina Supply compact bone of diaphysis & red marrow • Metaphyseal & epiphyseal arteries Supply red marrow & bone tissue of epiphyses Bone Formation • All embryonic connective tissue begins as mesenchyme • Bone formation is termed osteogenesis or ossification and begins when mesenchymal cells provide the template for subsequent ossification • Two types of ossification occur Intramembranous ossification is the formation of bone directly from or within fibrous connective tissue membranes Flat bones of the skull Thin layers of compact bone with spongy bone in the middle Endochondrial ossification is the formation of bone from hyaline cartilage models, forming most of the bones in the body Two growth centers: primary & secondary Two types of growth: interstitial & appositional Intramembranous Bone Formation 1) 2) 3) 4) Mesenchyme condenses into sheets Osteoblasts secrete matrix & become osteocytes, periosteum forms Trabeculae form, creating spongy bone Calcification converts surfaces to compact bone Endochondral Bone Formation 1) 2) 3) 4) 5) 6) Cartilage model forms Formation of periosteum and the primary ossification center in the diaphysis Vascular invasion, formation of medullary cavity Formation of secondary ossification center in epiphysis Epiphyseal plate forms, bone grows in length from this plate Epiphyseal plate closes to form epiphyseal line, bone stops growing 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, forming the epiphyseal line • Growth in length stops at age ~25 • Zone of reserve cartilage Anchors growth plate to bone • Zone of cell proliferation Rapid cell division (stacked coins) • Zone of cell hypertrophy Mitosis stops, cells enlarge • Zone of calcification Cartilage matrix calcifies • Zone of bone deposition Osteoclasts break down lacunae Osteoblasts & capillaries move in to create bone over calcified cartilage Zones of Growth in Epiphyseal Plate Bone Growth in Thickness • Only by appositional growth at the bone’s surface • Periosteal cells differentiate into osteoblasts and form bony ridges and then a tunnel around periosteal blood vessel • Concentric lamellae fill in the tunnel to form an osteon Factors Affecting Bone Growth • Nutrition Adequate levels of minerals and vitamins Calcium and phosphorus for bone growth Vitamin C for collagen formation Vitamin K and Vitamin B12 for protein synthesis • Sufficient levels of specific hormones During childhood need insulinlike growth factor (IGF) Promotes cell division at epiphyseal plate Human growth hormone (hGH), thyroid hormones, & insulin Sex steroids at puberty Estrogen and testosterone, stimulate sudden growth and modifications of the skeleton to create the male and female forms Testosterone Hormonal Abnormalities in Bone Growth • Oversecretion of hGH during childhood produces giantism • Oversecretion of hGH during adulthood results in acromegaly From the Greek akros = high and megalos = large Individuals with acromegaly have unusually large facial features, large hands, large feet, and wide tooth spacing Usually caused by a benign tumor of the pituitary gland Today acromegaly can be treated with drugs or surgery • Undersecretion of hGH or thyroid hormone during childhood produces short stature • Both men or women that lack estrogen receptors on cells grow taller than normal estrogen is responsible for closure of growth plate Hormonal Abnormalities in Bone Growth • Worked as a journalist The Brute Man Jungle Captive House of Horrors The Creeper • • • • Rondo Hatton (1894 - 1946) Suffered from acromegaly “Star” of “fright films” of the 1930’s & 1940’s Immortalized by the Rondo Hatton Classic Horror Awards • Originally given a bit part in a movie because of his looks • Inducted into his HS football team’s Hall of Fame Hormonal Abnormalities in Bone Growth • • • Richard Kiel (1939 -) “Jaws” in James Bond films Also in Happy Gilmore & other films Hormonal Abnormalities in Bone Growth Mineral Homeostasis • • Bone is a storehouse for calcium and phosphate Phosphate is important in many physiological processes • Needed for ATP Component of DNA, RNA Forms phospholipid bilayer The body’s pH buffering system Calcium is one of the most important regulatory elements in the body Needed for muscle contraction Needed for release of neurotransmitter from synaptic end bulbs Excess blood calcium is hypercalcemia (muscle weakness) Insufficient blood calcium is hypocalcemia (muscle spasms) Mineral Homeostasis • Calcitriol is a form of Vitamin D produced by the skin, liver, & kidney Raises blood calcium by promoting absorption in small intestine & from bones • Calcitonin is secreted by the thyroid gland Lowers blood calcium by stimulating osteoblasts & inhibiting osteoclasts • Parathyroid hormone (PTH) is secreted by the parathyroid glands Raises blood calcium by stimulating osteoclasts & promoting resorption Mineral Homeostasis • Hypercalcemia is excess Ca2+ in the blood • Hypocalcemia is insufficient Ca2+ in the blood • Regulation of blood calcium provides a good demonstration of negative feedback loops Bone Fractures Repair of a Fracture 1) Formation of fracture hematoma Damaged blood vessels produce clot in 6-8 hours, bone cells die Inflammation brings in phagocytic cells for clean-up duty New capillaries grow into damaged area 2) Formation of fibrocartilagenous callus Fibroblasts invade the fracture site & lay down collagen fibers Chondroblasts produce fibrocartilage to span the broken ends of the bone Repair of a Fracture 3) Formation of bony callus Osteoblasts secrete spongy bone that joins 2 broken ends of bone Lasts 3-4 months 4) Bone remodeling Compact bone replaces the spongy bone in the bony callus Surface is remodeled back to normal shape Exercise And Bone Tissue • Within limits, bone has the ability to alter its strength in response to mechanical stress by increasing deposition of mineral salts and production of collagen fibers (Wolff’s Law) • Removal of mechanical stress leads to weakening of bone through demineralization (loss of bone minerals) and collagen reduction Reduced activity while in a cast Astronauts in weightless environment Bedridden person • Weight-bearing activities, such as walking or moderate weightlifting, help build and retain bone mass • Decreased bone mass resulting in porous bones • Those at risk White, thin, menopausal females Athletes who are not menstruating due to decreased body fat & decreased estrogen levels People allergic to milk or with eating disorders whose intake of calcium is too low Smoking, drinking, & family history increase risk • Prevention or decrease in severity Adequate diet, weight-bearing exercise, & estrogen replacement therapy (for menopausal women) Behavior when young may be most important factor Osteoporosis