BSMP 2135 Anatomy and Physiology Chapter 7
BSMP 2135 Anatomy and Physiology Chapter 7 2135 BMSP
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This 18 page Class Notes was uploaded by Marlena Trone on Sunday September 25, 2016. The Class Notes belongs to 2135 BMSP at Virginia Polytechnic Institute and State University taught by in Fall 2016. Since its upload, it has received 7 views.
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Date Created: 09/25/16
CHAPTER 7: Bone Tissue BONE • Osteology: the study of bone • skeletal system organs: bones, cartilages, and ligaments – form strong flexible framework of the body – Cartilage: forerunner of most bones • covers many joint surfaces of mature bone – Ligament: attach bone to bone – Tendons: attach muscle to bone and are typically considered part of the muscular system FUNCTIONS OF THE SKELETON • support – hold the body up, supports muscles, mandible and maxilla support teeth • protection – brain, spinal cord, heart, lungs • movement – limb movements, breathing, action of muscle on bone • electrolyte balance – calcium and phosphate ions • acidbase balance – buffers blood against excessive pH changes • blood formation – red bone marrow is the chief producer of blood cells BONES AND OSSEOUS TISSUE • bone (osseous tissue) connective tissue with the matrix hardened by calcium phosphate and other minerals • mineralization or calcification – the hardening process • individual bones consist of bone tissue, bone marrow, cartilage, adipose tissue, nervous tissue, and fibrous connective tissue • continually remodels itself and interacts physiologically with all of the other organ systems of the body – permeated with nerves and blood vessels, which attests to its sensitivity and metabolic activity – your bones are living, active, and constantly changing! SHAPES OF BONES • long bones – longer than wide – rigid levers acted upon by muscles • short bones – equal in length and width – glide across one another in multiple directions • flat bones – protect soft organs – curved but wide & thin • irregular bones – elaborate shapes that don’t fit into the other categories STRUCTURE OF A LONG BONE • Epiphyses – (Singular form = epiphysis) • Diaphysis (plural form = diaphyses) • Compact bone • Spongy bone • Nutrient foramen (plural form = foramina) • Medullary or marrow cavity – space in a long bone that contains bone marrow • articular cartilage – hyaline cartilage found at the joint surface • Periosteum – outer covering of bone • Endosteum – lines inner surface of bone STRUCTURE OF FLAT BONE • sandwichlike construction • two layers of compact bone enclosing a middle layer of spongy bone (diploe) – Periosteum lines the outside of the bone • Diploe (DIPloee) – spongy layer in the cranium – absorbs shock if a blow to the head fractures the outer layer of compact bone – marrow spaces lined with endosteum GENERAL FEATURES OF BONES • periosteum – external sheath that covers bone except where there is articular cartilage – outer fibrous layer of collagen • some outer fibers continuous with the tendons that attach muscle to bone • perforating (Sharpey’s) fibers – other outer fibers that penetrate into the bone matrix • strong attachment and continuity from muscle to tendon to bone – inner osteogenic layer of bone forming cells • important to growth of bone and healing of fractures • endosteum – thin layer of connective tissue lining marrow cavity – has cells that dissolve osseous tissue and others that deposit it • epiphyseal plate (growth plate) – area of hyaline cartilage that separates the marrow spaces of the epiphysis and diaphysis – enables growth in length – epiphyseal line – in adults, a bony scar that marks where growth plate used to be HISTOLOGY OF OSSEOUS TISSUE • bone is connective tissue that consists of cells, fibers, and ground substance • four principal types of bone cells – osteogenic (osteoprogenitor) cells, osteoblasts, osteocytes, osteoclasts • osteogenic (osteoprogenitor) cells stem cells found in endosteum, periosteum, and in central canals – arise from embryonic mesenchymal cells – multiply continuously to produce new osteoblasts • osteoblasts: bone forming cells – line up as single layer of cells under endosteum and periosteum – are nonmitotic – synthesize soft organic matter of matrix which then hardens by mineralization • osteocytes: former osteoblasts that have become trapped in the matrix they have deposited – lacunae – tiny cavities where osteocytes reside – canaliculi – little channels that connect lacunae • cytoplasmic processes of the osteocytes reach into the canaliculi – some osteocytes reabsorb bone matrix while others deposit it – contribute to homeostatic mechanism of bone density and calcium and phosphate ions CELLS OF OSSEOUS TISSUE • osteoclasts– bonedissolving cells found on the bone surface – osteoclasts develop from same bone marrow stem cells that give rise to blood cells – unusually large cells formed from the fusion of several stem cells • typically have 3 to 4 nuclei, may have up to 50 – ruffled border – side facing bone surface • several deep infoldings of the plasma membrane which increases resorption efficiency – resorption bays – pits on surface of bone where osteoclasts reside remodeling results from combined action of the osteoclasts and osteoblasts THE MATRIX • The matrix is the extracellular material in a tissue. • matrix of osseous tissue is, by dry weight, about onethird organic and twothirds inorganic matter • organic matter – synthesized by osteoblasts – collagen, carbohydrate–protein complexes, such as glycosaminoglycans, proteoglycans, and glycoproteins • inorganic matter – 85% hydroxyapatite (crystallized calcium phosphate salt) – 10% calcium carbonate – other minerals (fluoride, sodium, potassium, magnesium) • bone is a composite: combination of two basic structural materials – Hydroxyapatite and other minerals – Collagen – combines optimal mechanical properties of each component – mineral portion allows the bone to support the body weight, and collagen portion gives bone some degree of flexibility • rickets – soft bones due to deficiency of calcium salts • osteogenesis imperfecta or brittle bone disease – excessively brittle bones due to lack of protein, collagen • Bones have a hard mineral to provide structural support. Lack of calcium salts can lead to soft bones. • Bones have collagen proteins to supply a degree of flexibility to the bones. Lack of protein leads to brittle bones – Ex. Osteogenesis Imperfecta – Bones fracture very easily BONE TISSUE • osteon (haversian system) – the basic structural unit of compact bone – a central canal and its concentric lamella • circumferential lamellae inner and outer boundaries of compact bone – run parallel to bone surface • interstitial lamellae: remains of old osteons found between osteons • perforating (Volkmann) canals are transverse or diagonal passages along the length of the osteon • collagen fibers: “corkscrew” down the matrix of the lamella giving it a helical arrangement – helices coil in one direction in one lamella and in the opposite direction in the next lamella for added strength BLOOD VESSELS IN BONE • blood flow skeleton receives about half a liter of blood per minute • nutrient foramina – opening on the surface of bone tissue that allow blood vessels and nerves to enter the bone – open into the perforating canals and feed into central canals – innermost osteocytes near central canal receive nutrients and pass them along through their gap junction to neighboring osteocytes • The processes of osteocytes extend into the canaliculi towards a neighboring cell – they also receive wastes from their neighbors and transfer them to the central canal maintaining a twoway flow of nutrients and waste SPONGY BONE • Spongy bone or Cancellous bone or Trabecular bone is spongelike in appearance • spongy bone consists of: – slivers of bone called spicules – thin plates of bone called trabeculae – spaces filled with red bone marrow • few osteons and no central canals – all osteocytes close to bone marrow • provides strength with minimal weight – trabeculae develop along bone’s lines of stress BONE MARROW • bone marrow: soft tissue that occupies the marrow cavity of a long bone and small spaces amid the trabeculae of spongy bone • red marrow is hemopoietic (HEmopoyETic) tissue – hemopoietic tissue produces blood cells – composed of multiple tissues in a delicate, but intricate arrangement that is an organ to itself – in nearly every bone in a child contains red marrow – in adults, red marrow is found in skull, vertebrae, ribs, sternum, part of pelvic girdle, and proximal heads of humerus and femur • yellow marrow found in adults – most red marrow turns into fatty yellow marrow – no longer produces blood BONE DEVELOPMENT • ossification or osteogenesis: the formation of bone • in the human fetus and infant, bone develops by two methods: – intramembranous ossification – endochondral ossification INTERMEMBRANOUS OSSIFICATION bones develop within a fibrous sheet – process similar to formation of the dermis – dermal bones form by this process – Flat bones of the skull and the clavicle – embryonic connective tissue, mesenchyme, condenses into a layer of soft tissue with dense supply of blood capillaries – mesenchymal cells enlarge and differentiate into osteogenic cells – osteogenic cells differentiate into osteoblasts • these cells deposit organic matrix – osteoid tissue – mesenchyme close to the surface of a trabecula becomes denser and more fibrous, forming periosteum – osteoblasts continue to deposit minerals • producing a honeycomb of bony trabeculae • some persist as permanent spongy bone • osteoclasts resorb and remodel others to form a marrow cavity in the middle of bone – trabeculae at the surface continue to calcify until the spaces between them are filled in, converting spongy bone to compact bone – gives rise to the sandwichlike arrangement of mature flat bone • Mesenchyme becomes a network of sheets called trabeculae • Osteogenic cells form osteoblasts • Osteoblasts deposit osteoid tissue • Some osteoblasts become osteocytes • Outer mesenchyme forms the periosteum • Osteoblasts form a network of bony trabeculae – spongy bone • Calcification continues at the surfaces and the spongy bone becomes compact bone • Spongy bone persists in the middle layer ENDOCHONDRRAL OSSIFICATION • endochondral ossification: process in which bone develops from preexisting cartilage model – most bones develop by this process • mesenchyme develops into a body of hyaline cartilage in location of future bone – covered with perichondrium – cartilage model grows due to cell division of chondrocytes • perichondrium starts to produce osteoblasts • osteoblasts form a bony collar around middle of cartilage model • former perichondrium is now considered to be periosteum • chondrocytes in the middle of the model enlarge forming the primary ossification center – bone tissue replaces most of the cartilage • Blood vessels penetrate the cavity, and it becomes the primary marrow cavity • The cartilage dies and the osteoclasts enlarge the marrow cavity • A secondary ossification center forms at one or both ends of the bone • Secondary marrow cavity develops • The epiphyseal plate and the articular cartilage are hyaline cartilage that remain • In an adult bone – epiphyseal line – marrow (medullary) cavity BONE GROWTH AND REMODELING • ossification continues throughout life with the growth and remodeling of bones • bones grow in two directions: length and width • bone elongation – epiphyseal plate – a region of transition from cartilage to bone • functions as growth zone where the bones elongate • consists of hyaline cartilage in the middle with a transition zone on each side where cartilage is being replaced by bone • metaphysis is the zone of transition facing the marrow cavity • Interstitial growth The Epiphyseal plate, a region of hyaline cartilage is the site of bone lengthening. When no cartilage remains the plate “closes” and the bones can no longer lengthen. A bony region known as the epiphyseal line remains HISTOLOGY OF METAPHYSIS • zone of reserve cartilage – typical hyaline cartilage farthest from marrow cavity – shows no sign of transforming into bone • zone of proliferation – chondrocytes multiply forming columns of flat lacunae • zone of hypertrophy – chondrocyte enlargement – matrix between lacunae become very thin • zone of calcification – mineral deposited in the matrix between columns of lacunae – temporary support for cartilage • zone of bone deposition – chondrocytes die, longitudinal columns fill with osteoblasts and blood vessels, osteoclasts dissolve the calcified cartilage – osteons and spongy bone are created by osteoblasts • appositional growth bones increase in width – appositional growth occurs throughout a lifetime – deposition of new bone at the surface – osteoblasts on deep side of periosteum deposit osteoid tissue • Osteoblasts become trapped as tissue calcifies – lay down matrix in layers parallel to surface • forms circumferential lamellae over surface – osteoclasts of endosteum enlarge marrow cavity • bone remodeling occurs throughout life 10% per year – repairs microfractures, releases minerals into blood, reshapes bones in response to use and disuse – architecture of bone determined by mechanical stresses placed on it and bones adapt to withstand those stresses • remodeling is a collaborative and precise action of osteoblasts and osteoclasts • bony processes grow larger in response to mechanical stress Growth at the edges is known as appositional growth and this results in bone thickening Interstitial growth is “growth from within” and results in bones lengthening (i.e. the growth at the metaphysis) Here’s a mnemonic from a student: Apples and stitches – Apples are thick (appositional growth) Stitches are long (interstitial growth) DWARFISM • achondroplastic dwarfism – long bones stop growing in childhood • normal torso, short limbs – failure of cartilage growth in metaphysis – spontaneous mutation produces mutant dominant allele • pituitary dwarfism – lack of growth hormone – normal proportions with short stature PHYSIOLOGY OF OSSEOUS TISSUE • a mature bone remains a metabolically active organ – involved in its own maintenance of growth and remodeling – exerts a profound influence over the rest of the body by exchanging minerals with tissue fluid • disturbance of calcium homeostasis in skeleton disrupts function of other organ systems – especially nervous and muscular MINERAL DEPOSITION • mineral deposition (mineralization) a crystallization process in which calcium phosphate, and other ions are taken from the blood plasma and deposited in bone tissue – osteoblasts produce collagen fibers that spiral the length of the osteon – fibers become encrusted with minerals that harden the matrix • calcium and phosphate (hydroxyapatite) from blood plasma are deposited along the fibers • the calcium and phosphate ion concentration must reach a critical value called the solubility product for crystal formation to occur • most tissues have inhibitors to prevent this so they do not become calcified • osteoblasts neutralize these inhibitors and allow salts to precipitate in the bone matrix • first few crystalls attract more calcium and phosphate from solution • abnormal calcification (ectopic ossification) – may occur in lungs, brain, eyes, muscles, tendons or arteries (arteriosclerosis) MINERAL RESORPTION • mineral resorption – the process of dissolving bone and releasing minerals into the blood break down bone – performed by osteoclasts at the ruffled border – hydrochloric acid dissolves bone minerals • hydrogen pumps in membrane secrete hydrogen into space between the osteoclast and bone surface • chloride ions follow by electrical attraction – acid phosphatase enzyme digests the collagen MINERAL HOMEOSTASIS • calcium and phosphate homeostasis • Roles of phosphate and calcium in the body: • phosphate component of DNA, RNA, ATP, phospholipids, and pH buffers • calcium needed in neuron communication, muscle contraction, blood clotting, and exocytosis • minerals are deposited in the skeleton and withdrawn when they are needed for other purposes PHOSPHATE HOMEOSTASIS • average adult has 500 – 800 g of phosphorus • 8590% of phosphate is in the bones • normal plasma concentration is 3.5 – 4.0 mg/dl • occurs in two principal forms: 2 – HPO 4 and H 2O 4monohydrogen & dihydrogen phosphate ions) • phosphate levels are not regulated as tightly as calcium levels – no immediate functional disorders CALCIUM HOMEOSTASIS • about 1100g of calcium in adult body – 99% in the skeleton • as easily exchangeable calcium ions and more stable hydroxyapatite reserve • 18% of adult skeleton exchanged with blood each year • normal calcium concentration in blood plasma is normally 9.2 to 10.4 mg/dl – hypocalcemia blood calcium deficiency • causes excess excitability of muscle, tremors, spasms or tetany (inability to relax) – hypercalcemia blood calcium excess • sodium channels less responsive and nerve and muscle less excitable than normal (sluggish reflexes, depression) ION IMBALANCES • hypercalcemia is rare • hypocalcemia has a wide variety of causes – vitamin D deficiency – diarrhea – thyroid tumors – underactive parathyroid glands – pregnancy and lactation – accidental removal of parathyroid glands during thyroid surgery • calcium homeostasis depends on a balance between dietary intake, urinary and fecal loses, and exchanges between osseous tissue • calcium homeostasis is regulated by three hormones: – Calcitriol, calcitonin, and parathyroid hormone HORMONAL CONTROL OF CALCIUM calcitriol, calcitonin, and PTH maintain normal blood calcium concentration CALCITRIOL (ACTIVATED VITAMIN D) • Calcitriol (CALsihTRYol) – a form of vitamin D produced by the sequential action of the skin, liver, and kidneys • Principal role of calcitriol is to promote bone deposition • calcitriol behaves as a hormone – raises blood calcium concentration • increases calcium absorption by small intestine • increases calcium resorption from the skeleton – increases stem cell differentiation into osteoclasts which liberates calcium from bone • promotes kidney reabsorption of calcium ions, so less lost in urine – raises blood phosphate concentration • Increases absorption of phosphate from the small intestine • adequate calcium AND phosphate are necessary for bone deposition • abnormal softness of bones without adequate vitamin D – Rickets occurs in children and is known as osteomalacia when it occurs in adults CALCITRIOL SYNTHESIS AND ACTION • Calcitriol is produced by the following process: 1. Epidermal keratinocytes use UV radiation to convert a steroid of Vitamin D 3 2. Liver converts it to calcidiol 3. Kidneys converts calcidiol to calcitriol (most active form of Vitamin D) CALCITONIN • calcitonin secreted by the thyroid gland when calcium concentration rises too high • Calcitonin lowers blood calcium concentration in two ways: – osteoclast inhibition – osteoblast stimulation • increases the number and activity of osteoblasts • important in children, weak effect in adults – osteoclasts more active in children due to faster remodeling – deficiency does not cause disease in adults • reduces bone loss in women during pregnancy & lactation PARATHYROID HORMONE • parathyroid hormone (PTH) – secreted by the parathyroid glands • PTH primary role is to raise blood calcium levels – Secreted in response to low blood calcium levels • PTH raises calcium blood level by four mechanisms – Increased osteoclast acitvity – promotes calcium reabsorption by the kidneys, less lost in urine – promotes the final step of calcitriol synthesis in the kidneys, enhancing calcium raising effect of calcitriol – inhibits osteoblasts, inhibiting bone deposition • PTH decreases blood phosphate level by promoting urinary excretion OTHER FACTORS AFFECTING BONE • at least 20 or more hormones, vitamins, and growth factors affect osseous tissue • bone growth especially rapid in puberty and adolescence – surges of growth hormone, estrogen, and testosterone occur and promote ossification – girls grow faster than boys and reach full height earlier • estrogen stronger effect than testosterone on bone growth – males grow for a longer time and taller • anabolic steroids cause growth to stop – epiphyseal plate “closes” prematurely – results in abnormally short adult stature OSTEOPOROSIS • osteoporosis – the most common bone disease – severe loss of bone density – bones lose mass and become brittle due to loss of organic matrix and minerals – affects spongy bone the most since it is the most metabolically active – subject to pathological fractures of hip, wrist and vertebral column – Kyphosis (widow’s hump) – deformity of spine due to vertebral bone loss – excessive thoracic spinal curvature • postmenopausal white women at greatest risk – begin to lose bone mass as early as 35 yoa • by age 70, average loss is 30% of bone mass – risk factors race, age, gender, smoking, diabetes mellitus, diets poor in calcium, protein, vitamins C and D
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