Chapter 7: Bone Tissue PDF

Chapter 7: Bone Tissue Learning Outcomes Tissues and organs of the skeletal system Histology of bone (osseous) tissue Bone development Physiology of osseous tissue Bone disorders Osteocyte: surrounded by a calcified bone matrix  Functions...

Chapter 7: Bone Tissue Learning Outcomes Tissues and organs of the skeletal system Histology of bone (osseous) tissue Bone development Physiology of osseous tissue Bone disorders Osteocyte: surrounded by a calcified bone matrix  Functions of the Skeleton 1. Support – Bones of the limbs, etc. 2. Protection – The brain, spinal cord, heart, lungs, etc. 3. Movement – Limb movements, breathing, etc. 4. Electrolyte balance – Stores calcium and phosphate and release when needed. 5. Acid-base balance – Buffers the blood against excessive pH changes. Regulates alkaline phosphate and carbonate salts. 6. Blood formation – Red bone marrow: major producer of blood cells, including WBCs. 7. Hormone secretion – Secrete hormones that influence the secretion and action of insulin. Moderates stress response.  Bone (Osseous) Tissue Connective tissue – a matrix hardened by minerals (calcium phosphate and others) Mineralization of Calcification – the hardening process itself Continually remodels itself Bone is not the hardest substance – Tooth enamel is Shapes of Bones Long bones – levers acted upon by muscles Short bones – glide across one another in multiple directions Flat bones – protect soft organs Irregular bones  General Features of Bones Shaft (diaphysis) = cylinder of compact bone – marrow cavity (medullary cavity) lined with endosteum (osteogenic cells and reticular connective tissue) Enlarged ends (epiphyses) – enlarged to strengthen joint and attach ligaments Joint surface covered with articular cartilage Shaft covered with periosteum – outer fibrous layer of collagen Epiphyseal plate (growth plate)/line (adults) Structure of a Flat Bone External and internal surfaces – Composed of compact bone Middle layer – Spongy bone and bone marrow Skull fracture may leave inner layer of compact bone unharmed  Cells of Osseous Tissue Osteogenic cells: give rise to new osteoblasts – arise from embryonic fibroblasts (bone stem cells) – multiply continuously – location: endosteum, periosteum or central canals Osteoblasts: mineralize organic matter of matrix Osteocytes (mature cells): osteoblasts trapped in the matrix (slide 8) they formed – cells in lacunae connected by gap junctions inside canaliculi  Cells of Osseous Tissue Osteoclasts: develop in bone marrow by fusion of 3-50 blood stem cells Reside in resorption pits that they “eat” into the bone  Matrix of Osseous Tissue Dry weight = 1/3 organic and 2/3 inorganic matter Organic matter – collagen, glycosaminoglycans (carb-protein complexes), proteoglycans and glycoproteins Inorganic matter – 85% hydroxyapatite (calcium phosphate) – 10% calcium carbonate – other minerals (fluoride, potassium, magnesium) Combination provides for strength and resilience – minerals resist compression; collagen resists tension – bone adapts by varying proportions  Histology of Compact Bone  Compact Bone Osteon = basic structural unit – cylinders formed from layers (lamellae) of matrix around central canal (osteonic canal) – osteocytes connected to each other and their blood supply by tiny cell processes in canaliculi Perforating canals or Volkmann canals – vascular canals perpendicularly joining central canals 7- Blood Vessels of Bone  Spongy Bone Sponge-like appearance formed by plates of bone called trabeculae – spaces filled with red bone marrow Trabeculae have few osteons or central canals – no osteocyte is far from blood of bone marrow Provides strength with little weight – trabeculae develop along bone’s lines of stress Bone Marrow In medullary cavity (long bone) and among trabeculae (spongy bone) Red marrow like thick blood – reticular fibers and immature cells – Hemopoietic (produces blood cells) – in vertebrae, ribs, sternum, pelvic girdle and proximal heads of femur and humerus in adults Yellow marrow – fatty marrow of long bones in adults  Intramembranous Ossification Produces flat bones of skull and clavicle. Intramembranous Ossification Condensation of mesenchyme into trabeculae Osteoblasts on trabeculae lay down osteoid tissue (uncalcified bone) Calcium phosphate is deposited in the matrix forming bony trabeculae of spongy bone Osteoclasts create marrow cavity Osteoblasts form compact bone at surface Surface mesenchyme produces periosteum  Intramembranous Ossification Produces flat bones of skull and clavicle. Intramembranous Ossification Note the periosteum and osteoblasts. Stages of Endochondral Ossification  Endochondral Ossification Bone develops from pre-existing model – perichondrium and hyaline cartilage Most bones develop this process Formation of primary ossification center and marrow cavity in shaft of model – bony collar developed by osteoblasts – chondrocytes swell and die – stem cells give rise to osteoblasts and clasts – bone laid down and marrow cavity created  Endochondral Ossification Secondary ossification centers and marrow cavities form in ends of bone – same process Cartilage remains as articular cartilage and epiphyseal (growth) plates – growth plates provide for increase in length of bone during childhood and adolescence – by early twenties, growth plates are gone and primary and secondary marrow cavities united Stages of Endochondral Ossification Fetal Skeleton @ 12 weeks  Epiphyseal Growth Plate 1. Zone of reserve cartilage = hyaline cartilage 2. Zone of proliferation chondrocytes multiply forming columns of flat lacunae 3. Zone of hypertrophy = cell enlargement 4. Zone of calcification mineralization of matrix 5. Zone of bone deposition chondrocytes die and columns fill with osteoblasts osteons formed and spongy bone is created  Bone Growth and X-ray of child’s hand. Mostly still Bones increase in cartilaginous length – interstitial growth of epiphyseal plate – epiphyseal line is left behind when cartilage gone Bones increase in width = appositional growth – osteoblasts lay down matrix in layers on outer surface and osteoclasts dissolve bone on inner surface  Bone Remodeling Bones remodeled throughout life – Wolff’s law of bone = architecture of bone determined by mechanical stresses action of osteoblasts and osteoclasts – greater density and mass of bone in athletes or manual worker is an adaptation to stress  Mineral Deposition Mineralization is crystallization process – osteoblasts produce collagen fibers spiraled the length of the osteon – minerals cover the fibers and harden the matrix ions (calcium and phosphate and from blood plasma) are deposited along the fibers Abnormal calcification (ectopic) – may occur in lungs, brain, eyes, muscles, tendons or arteries (arteriosclerosis)  Mineral Resorption from Bone Bone dissolved and minerals released into blood for other uses – performed by osteoclasts “ruffled border” Dental braces reposition teeth and remodel bone – create more pressure on one side of the tooth – stimulates osteoclasts to remove bone – decreased pressure stimulates osteoblasts  Calcium and Phosphate Phosphate – Component of DNA, RNA, ATP, phospholipids, and pH buffers Calcium – Needed in neurons, muscle contraction, blood clotting and exocytosis  Calcium Regulation Calcitriol (form of Vitamin D) – Increases calcium absorption from small intestines – Increases calcium resorption from the skeleton – Binds to osteoblast stimulates cell to differentiate into osteoclast – Promotes reabsorption of calcium by kidneys  Calcitonin Regulation Two principle mechanisms: 1. Osteoclast inhibition – Within 15 min. osteoclast activity is reduced by ~70%. 2. Osteoblast stimulation – Within 1 hour the # of osteoblasts increase, depositing calcium into the skeleton.  Parathyroid (PTH) Regulation Raises calcium level by 4 mechanisms: 1. Binds to receptors on the osteoblasts, stimulates osteoclast and promotes bone resorption. 2. Promotes calcium reabsorption by kidneys. 3. Promotes the final step of calcitriol synthesis in kidneys. 4. Inhibits collagen synthesis by osteoblasts, inhibits bone deposition.  Calcium Homeostasis Blood Calcium Balance  Phosphate Homeostasis Average adult: 500 to 800 g of phosphorus 85%-90% is in bone Needed for bone strength Also component for ATP, DNA, RNA, phospholipids… Not as tightly regulated as calcium.  Other Factors Affecting Bone Hormones, vitamins and growth factors Growth rapid at puberty – hormones stimulate osteogenic cells, chondrocytes and matrix deposition in growth plate – girls grow faster than boys and reach full height earlier (estrogen stronger effect) – males grow for a longer time and taller Growth stops (epiphyseal plate “closes”) – teenage use of anabolic steroids = premature closure of growth plate and short adult stature 7-  Bone Fractures Nondisplaced One bone stays in place Displaced At least one piece is shifted out of place with the other(s) Comminuted At least one broken into three or more pieces Greenstick One bone broken on one side and the other bone bent Healing of Bone Fracture Osteoporosis Most common of bone diseases Bone density declines Becomes brittle Occurs around 40 years in age Less initial bone loss due to estrogen decrease Menopause Seen less likely in women of African decent due to greater bone density Genetic predisposition Bone Disorders