BIO 291 Chapter 6 Bones Students PDF

Chapter 6 Bone Function of 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 Blood cell production – hematopoiesis (or hemopoiesis) occ...

Chapter 6 Bone Function of 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 Blood cell production – hematopoiesis (or hemopoiesis) occurs within the red marrow cavities of bones Mineral storage – reservoir for minerals, especially calcium and phosphorus Triglyceride storage – Yellow bone marrow consists primarily of adipose tissue. Gross Anatomy of Bones: Bone Textures Compact (lamellar) bone – dense outer layer Spongy (cancellous or cancellate) bone – honeycomb of trabeculae filled with red bone marrow The trabeculae are not randomly arranged, but develop along the bone’s lines of stress. lamellar – consisting of thin concentric layers cancellous [Latin: cancellus, latticework] trabecula – [Latin: little beam] a structural element resembling a small beam or crossbar Structure of Long Bones Diaphysis -tubular shaft that forms the axis of long bones -composed of compact bone surrounding a medullary cavity -yellow bone marrow (fat) is contained in the medullary cavity (in adults) Structure of Long Bones Epiphyses -expanded ends of long bones -exterior is compact bone, and the interior is spongy bone -enlarged to strengthen the joint and to provide added surface area for the attachment of tendons and ligaments -joint surface is covered with articular (hyaline) cartilage Structure of Long Bones Metaphyses -region where the diaphysis joins the epiphyses -in growing bones, includes the epiphyseal plate -in mature bone, includes the epiphyseal line Structure of Long Bones – Blood Supply Bone Membranes – periosteum and endosteum 1. Periosteum – double-layered outer membrane, consisting of a dense, fibrous outer layer, to which tendons and ligaments attach, and a more delicate, inner layer capable of forming bone. n outer fibrous layer is dense irregular connective tissue proper n inner osteogenic layer contains osteoblasts and osteoclasts n richly supplied with nociceptors (nerve fibers that signal pain), blood, and lymphatic vessels, which enter the bone via nutrient foramina n secured to underlying bone by collagen fibers called Sharpey’s fibers n does not cover articular surfaces Bone Membranes – periosteum and endosteum 2. Endosteum – delicate membrane covering internal surfaces of bone covers the trabeculae of spongy bone and the canals that pass through compact bone contains both osteoblasts and osteoclasts endo = witihin + oste = bone Structure of Short, Irregular, and Flat Bones n thin plates of periosteum-covered compact bone on the outside with endosteum- covered spongy bone on the inside n often contains red bone marrow between the trabeculae  Bone Marrow n Yellow marrow n in the medullary cavity of long bones (in adults) n consists of adipose tissue n contains some red marrow cells (hematopoietic cells) n Red marrow n found in the spongy bone of the skull, ribs, sternum, vertebrae, and most of the pelvis and within the proximal epiphyses of the femur and humerus (in adults) n consists primarily of hematopoietic cells within a reticular connective tissue proper n produces the formed elements of blood (erythrocytes, leukocytes, platelets) At birth, all bone marrow is red. As growth slows with age, more and more of it is converted to yellow bone marrow. Distribution of red bone marrow (in the adult) shown in red Organic Components of Bone Osteoblasts – bone-forming cells Osteocytes – matured osteoblasts that maintain the tissue Osteoclasts – large, multinucleated cells that resorb (break down) the bone matrix Osteoid – unmineralized bone matrix secreted by osteoblasts the protein component of the bone matrix composed of proteoglycans and collagen makes up about 1/3rd of the bone matrix imparts a degree of flexibility to the bone, making the bone less brittle Inorganic Component of Bone Mineral salts makes up 50% of the bone matrix Mainly calcium phosphate + calcium hydroxide deposited into the framework formed by the osteoid in a process called calcification (or mineralization) responsible for bone hardness and ability to resist compression Microscopic Structure of Bone: Compact Bone Haversian system (or osteon) – the structural unit of compact bone lamella – weight-bearing, column-like tubes of bone matrix rich in collagen (Haversian canal) (Volkmann’s canal) Microscopic Structure of Bone: Compact Bone Osteocytes – matured osteoblasts that maintain the bone matrix Lacunae – small cavities in the bone at the junctions of the lamellae Canaliculi – hairlike canals that connect lacunae to each other and to the central canal The osteocytes are connected to each other through the canaliculi by gap junctions. This allows nutrients and wastes to be relayed between cells throughout the osteon. Microscopic Structure of Bone: Cancellous Bone Bone Development n Osteogenesis (also called ossification) – the process of bone tissue formation, leading to: n initial formation of the bony skeleton n bone growth until early adulthood n bone remodeling and repair Formation of the Bony Skeleton begins at week 6 of embryonic development Intramembranous ossification – bone develops from a fibrous membrane forms most of the flat bones of the skull, the mandible, and the clavicles Endochondral ossification – bone forms by replacing a hyaline cartilage model forms all the bones of the skeleton below the base of the skull (except the clavicles) Note: Osseous tissue may sometimes form in the lungs, brain, eyes, muscles, tendons, arteries, and other organs. Such abnormal calcification of tissues is referred to as ectopic ossification. Fibrodysplasia ossificans progressiva – Harry Eastlack Postnatal, Longitudinal Bone Growth n Bones grow in length at the epiphyseal plate. n The cartilage in the region of the epiphyseal plate abutting the epiphysis continues to grow. n Osteoclasts and osteoblasts move in from the diaphysis and ossify the matrix to form bone. Functional Zones in Longitudinal Bone Growth n An epiphyseal plate can be divided into four functionally distinct zones: A. Quiescent (or resting) zone – non- proliferating cartilage that serves to anchor the epiphyseal plate to the epiphysis B. Proliferating zone – cartilage cells abutting the epiphysis undergo mitosis, pushing the epiphysis away A B C D from the diaphysis C. Hypertrophic zone – older cells below the proliferating zone hypertrophy, the matrix begins to deteriorate, and the cartilage cells die D. Calcification (or Osteogenic) zone – new bone formation occurs (“resting”) Appositional Bone Growth n Even though bones stop growing in length in early adulthood, they can continue to increase in thickness or diameter throughout life in response to stress from increased muscle activity or increased weight bearing. The increase in diameter occurs by appositional growth.  Appositional Bone Growth Central canal of osteon Periosteal ridge Artery Periosteum Penetrating canal 1 Osteoblasts beneath 2 As the bony ridges 3 The periosteum 4 As the osteoblasts the periosteum enlarge and meet, lining the tunnel is beneath the endosteum secrete bone matrix, the groove transformed into an form new lamellae, a new forming ridges that containing the endosteum and the osteon is created. follow the course of blood vessel osteoblasts just Meanwhile new periosteal blood becomes a tunnel. deep to the tunnel circumferential lamellae vessels. endosteum secrete are elaborated beneath bone matrix, the periosteum and the narrowing the canal. process is repeated, continuing to enlarge bone diameter. Bone Growth is Hormonally Regulated n During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone. n During puberty, testosterone and estrogens: n initially promote adolescent growth spurts. n cause masculinization and feminization of specific parts of the skeleton. n later induce epiphyseal plate closure, ending longitudinal bone growth. Bone Remodeling n Remodeling – an active process in which bone is resorbed and deposited n repairs microfractures n reshapes bones in response to use and disuse n helps maintain Ca2+ and phosphate homeostasis n Spongy bone is replaced every 4-5 years. n Compact bone is replaced about every 25 years. Bone Resorption n accomplished by osteoclasts n involves secretion by osteoclasts of: n hydrochloric acid that converts calcium salts into soluble forms n lysosomal enzymes that digest organic matrix n forms resorption bays – grooves formed by osteoclasts as they break down bone matrix n dissolved matrix is transcytosed across the osteoclast’s cell where it is secreted into the interstitial fluid and then into the blood Response to Mechanical Stress n The shape of a bone can be altered to provide the proper support against the stresses applied. n Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon it n The thickness of the bone is remodeled and trabeculae form along lines of stress. n Large, bony projections (e.g. trochanters of the femurs) occur where large, active muscles attach.  Response to Mechanical Stress n Collagen fibers will preferentially be laid down so as to follow lines of tension and compression. Importance of Ionic Calcium (Ca2+) in the Body n Ca2+ is necessary for: n transmission of nerve impulses n muscle contraction n blood coagulation (clotting) n secretion by glands and neuroendocrine cells n cell division Ca2+ Homeostasis [Greek: hypo-, below] n Hypocalcemia – a deficiency of blood Ca2+ [Greek: hyper-, above] n can result from a variety of causes including vitamin D deficiency, diarrhea, excessive vomiting, thyroid tumors, underactive parathyroid glands, or pregnancy and lactation n Hypercalcemia - an excess of blood Ca2+ n rare Maintaining Ca2+ Homeostasis Calcitonin inhibits Ca2+ reabsorption in the kidneys, increasing Ca2+ excretion into the urine. Calcitonin inhibits osteooclasts and increases the number and activity of osteoblasts. PTH increases numbers and activity of osteoclasts, stimulating Ca2+ release from the bones. PTH = parathyroid hormone Calcitrol increases the PTH stimulates absorption of PTH stimulates Ca2+ reabsorption in the release of Ca2+ from the kidneys, decreasing Ca2+ excretion into diet. the urine. (calcitrol) Maintaining Ca2+ Homeostasis Calcitonin inhibits Ca2+ reabsorption in the kidneys, increasing Ca2+ excretion into the urine. Calcitonin inhibits osteooclasts and increases the number and activity of osteoblasts. PTH increases numbers and activity of osteoclasts, stimulating Ca2+ release from the bones. PTH = parathyroid hormone Calcitrol increases the PTH stimulates absorption of PTH stimulates Ca2+ reabsorption in the release of Ca2+ from the kidneys, decreasing Ca2+ excretion into diet. the urine. (calcitrol) Maintaining Ca2+ Homeostasis Calcitonin inhibits Ca2+ reabsorption in the kidneys, increasing Ca2+ excretion into the urine. Calcitonin inhibits osteooclasts and increases the number and activity of osteoblasts. PTH increases numbers and activity of osteoclasts, stimulating Ca2+ release from the bones. PTH = parathyroid hormone Calcitrol increases the PTH stimulates absorption of PTH stimulates Ca2+ reabsorption in the release of Ca2+ from the kidneys, decreasing Ca2+ excretion into diet. the urine. (calcitrol)