Chapter 6: Osseous Tissue and Bone Structure

The Skeletal System Skeletal system includes: bones of the skeleton cartilages, ligaments, and other connective tissues that stabilize the bones

Skeletal System Functions: 1. Support: framework & structure of body 2. Storage of minerals and lipids Minerals: calcium and phosphate - for osmotic regulation, enzyme function, nerve impulses Yellow marrow: triglycerides 3. Blood cell production: all formed elements - red marrow: stem cells  hematopiesis 4. Protection: surround soft tissues 5. Leverage for movement: - levers upon which skeletal muscles act

Classification of Bones Bone are identified by: shape internal tissues bone markings SHAPE: Long bones Flat bones Sutural bones Irregular bones Short bones Sesamoid bones

Shape of Bones Long Bones: Short Bones: Flat Bones: Longer than wide, consist of shaft and 2 ends e.g. bones of appendages Short Bones: Approx. equal in all dimensions e.g. carpals, tarsals Flat Bones: Thin, 2 parallel surfaces e.g. skull, sternum, ribs, scapula Figure 6–1a

Shape of Bones Irregular Bones: Sesamoid Bones: Sutural Bones: Complex shapes E.g. vertebrae, os coxa Sesamoid Bones: Seed shaped, form in tendon E.g. patella, total number can vary Sutural Bones: - Extra bones in sutures of skull

Bone Structure A bone is an organ consisting of many tissue types: Osseous, nervous, cartilage, fibrous CT, blood, etc. All bones consist of 2 types of bone tissue Compact bone: - solid, dense bone, makes up surfaces and shafts Spongy Bone/Cancellous bone: - meshy, makes up interior of bones, houses red marrow in spaces

Bone Markings Bones are not flat on the surface: Have projections, depressions, and holes for muscle attachment, blood & nerve supply Depressions or grooves: along bone surface Projections: where tendons and ligaments attach at articulations with other bones Tunnels: where blood and nerves enter bone

Bone Markings Table 6–1 (2 of 2)

Long Bones Structure Diaphysis: Medullary (marrow) cavity: Epiphysis: - Hollow shaft of compact bone Medullary (marrow) cavity: Center of diaphysis, contains yellow marrow Triglycerides for energy reserve Epiphysis: Expanded end of bone, surface of compact bone Center filled with spongy bone with red marrow in spaces Produces blood cells Figure 6–2a

Long Bones Structure Epiphyseal line or plate: Periosteum: Endosteum: Cartilage that marks connection of diaphysis with epiphysis Line: adults, narrow (aka metaphysis) Plate: thick, allows growth during childhood Periosteum: 2 layer covering around outside of bone: Outer Fibrous Layer Inner Cellular Layer Endosteum: Cellular layers, covers all inside surfaces

Articular Cartilage: Joint/Articulation: Hyaline cartilage on end where bone contacts another, no periosteum or perichondrium Joint/Articulation: - connection between two bones, surrounded by CT capsule, lined with synovial membrane Joint cavity filled with synovial fluid to reduce friction on articular cartilage

Flat Bone Structure Thin layer of spongy bone with red marrow between two layers of compact bone Covered by periosteum and endosteum Site of most hematopoiesis Production of blood cells and cell fragments that are suspended in plasma (RBC, WBC, and platelets

Characteristics of Bone Tissue Periosteum: covers outer surfaces of bones consist of outer fibrous and inner cellular layers Endosteum: Inner, cellular layer of periosteum

Bone Histology Bone = osseous tissue, supporting CT Consists of specialized cells in a matrix of fibers and ground substance Characteristics of bone: Dense matrix packed with calcium salts Osteocytes in lacunae Canaliculi for exchange of nutrients and waste Two layer periosteum, covers bone except at articular surfaces

Bone Histology Matrix = 98% of bone tissue Cells = only 2% of bone 1/3 = osteoid; organic part: Collagen fibers + ground substance Tough and flexible 2/3 = densely packed crystals of hydroxyapatite (calcium salts, mostly calcium phosphate) Hard but brittle Cells = only 2% of bone Osteocytes Osteoblasts Osteoprogenitor cells Osteoclasts

Cells located in Bones Osteocytes = mature bone cells -no cell division -located in lacunae between layers of matrix called lamellae -canaliculi link lacunae to each other and blood supply -osteocytes linked to each other via gap junctions on cell projections in canaliculi: - allow exchange of nutrients and wastes -Function 1. To maintain protein and mineral content of matrix 2. Can also participate in bone repair: -become stem cell like when broken free of lacuna

Osteocytes in lacunae Blood vessels LM X 362 Canaliculi Osteocytes in lacunae Matrix Blood vessels Central canal PERIOSTEUM Fibrous layer Cellular

Cells located in Bones Osteoblasts - Immature bone cells Perform osteogenesis: Formation of new bone matrix Produce osteoid Organic components of matrix that is not yet calcified to form bone Promote deposit of calcium salts which spontaneously form hydroxyapatite Once enclosed in lacuna by matrix, osteoblast differentiates into osteocyte and no longer produces new matrix

Cells located in Bones 3. Osteoprogenitor Cells – mesenchymal cells - bone stem cell that produces daughters - daughters become osteoblasts for repair and growth - located in endosteum and inner periosteum

Cells located in Bones 4. Osteoclasts - large, multinuclear - derived from monocytes (macrophages) - perform osteolysis = - digest and dissolve bone matrix - release minerals: 1. For use in blood or 2. Recycling during bone remodeling

Cells located in Bones Osteocyte: Mature bone cell that maintains the bone matrix Osteoblast: Immature bone cell that secretes organic components of matrix Osteoclast: Multinucleate cell that secretes acids and enzymes to dissolve bone matrix Osteoprogenitor cell: Stem cell whose divisions produce osteoblasts Osteoid Osteoblast Matrix Marrow cavity Osteoprogenitor cell Osteoclast Canaliculi Osteocyte

Homeostasis Bone building (by osteocytes) and bone recycling (by osteoclasts) must balance: more breakdown than building, bones become weak exercise causes osteocytes to build bone

How would the strength of a bone be affected if the ratio of collagen to hydroxyapatite increased? Strength increases, flexibility increases. Strength increases, flexibility decreases. Strength decreases, flexibility. decreases. Strength decreases, flexibility increases.

stable mass, but re-positioned matrix mass will not be affected If the activity of osteoclasts exceeds the activity of osteoblasts in a bone, how will the mass of the bone be affected? stable mass, but re-positioned matrix mass will not be affected more mass less mass

The difference between compact bone and spongy bone.

Structure of Compact Bone Consists of osteons: Parallel to surface Each osteon is around a central canal: Contains blood vessels and nerves Perforating canals perpendicular to osteons act to connect the osteons Osteon is built of layers of matrix secreted by osteoblasts Each layer = concentric lamella Osteocytes are located in lacunae between lamellae Ostocytes are connected to neighboring cells and central canal via canaliculi

Structure of Compact Bone Interstitial lamellae fill spaces between osteons Circumferiential lamellae run perimeter inside and out in contact with: endosteum and periosteum Compact bone is designed to receive stress from one direction Very strong parallel to osteons Weak perpendicular to osteons

Compact Bone Figure 6–5

Structure of Spongy Bone Lamellae = meshwork called trabeculae (no osteons) Red marrow fills spaces around trabeculae Osteocytes in lacunae are linked by canaliculi No direct blood supply (no central canals) Nutrients diffuse into canaliculi in trabeculae from red marrow Spongy bone make up: low stress bones Areas of bone where stress comes from multiple directions Provide light weigh strength

Bone Marrow Red Marrow: Yellow Marrow: Located in space between trabeculae Has blood vessels Forms red blood cells Supplies nutrients to osteocytes Yellow Marrow: In some bones, spongy bone holds yellow bone marrow: is yellow because it stores fat

Structure of Spongy Bone

Periosteum and Endosteum Compact bone is covered with membrane: periosteum on the outside endosteum on the inside

Periosteum Fibrous outer layer: Cellular Inner layer: Functions: - Dense irregular CT Cellular Inner layer: Osteoprogenitor cells Functions: Isolate bone from surrounding tissues Site for attachment for tendons and ligaments Route for nerves and blood vessels to enter bone Participates in bone growth and repair

Endosteum Thin cellular layer Lines medullary cavity, central canals, and covers trabeculae Consists of: osteoblasts, osteoprogenitor cells, and osteoclasts Cells become active during bone growth and repair

Endosteum Figure 6–8b

Bone Growth Begins 6-8 weeks post fertilization Continues through puberty (18-25 y) Osteogenesis = ossification = formation of bone Not calcification Hardening of matrix or cytoplasm with calcium Can happen to many tissues Two types of Ossification: Intramembranous: forms flat bones Endochondrial: forms long bones

Bone Development Human bones grow until about age 25 Osteogenesis: bone formation Ossification: Deposition of calcium salts the process of replacing other tissues with bone

The difference between intramembranous ossification and endochondral ossification.

Intramembranous Ossification Bone develops from mesenchyme or fibrous CT in deep layers of dermis Also called dermal ossification: because it occurs in the dermis produces dermal bones such as mandible and clavicle Produces skull bones There are 4 main steps in intramembranous ossification

Intramembranous Ossification: Step 1 Ossification center appears in the fibrous CT membrane Mesenchymal cells aggregate Differentiate into osteoblasts Begin ossification at the ossification center

Intramembranous Ossification: Step 2 Bone matrix (osteoid) is secreted within the fibrous membrane Osteoblasts begin to secrete osteoid, which is mineralized within a few days Trapped osteoblasts become osteocytes

Intramembranous Ossification: Step 3 Woven bone and periosteum form Accumulating osteoid is laid down between embryonic blood vessels, which form a random network Vascularized mesenchyme condenses on the external face of the woven bone and becomes periosteum around spongy bone

Intramembranous Ossification: Step 4 Bone collar of compact bone forms and red marrow appears Trabeculae just deep to the periosteum thickens, forming a woven bone collar that is later replaced with mature lamellar bone Spongy bone, consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow

Endochondral Ossification Ossifies bones that originate as hyaline cartilage Most bones originate as hyaline cartilage Cartilage grows by interstitial and appositional growth Cartilage is slowly replaced from the inside out

Endochondral Ossification Growth and ossification of long bones occurs in 6 steps

Endochondral Ossification: Step 1 Primary ossification center begins to form: Chondrocytes in the center of hyaline cartilage: Enlarge in diaphysis Surrounding matrix calcifies killing the enclosed chondrocytes die, leaving cavities in cartilage Figure 6–9 (Step 1)

Endochondral Ossification: Step 2 Blood vessels grow around the edges of the cartilage Cells in the perichondrium change to osteoblasts: Secrete osteoid Osteiod is mineralized and produces a layer of superficial bone around the shaft which will continue to grow around the diaphysis and become compact bone (appositional growth) Figure 6–9 (Step 2)

Endochondral Ossification: Step 3 Capillaries and fibroblast migrate into the primary ossification center: Blood vessels enter the cartilage Bringing fibroblasts that become osteoblasts and secrete osteoid Mineralized into rebeculae Spongy bone develops at the primary ossification center and continues to growth toward the epiphysis Figure 6–9 (Step 3)

Endochondral Ossification: Step 4 Remodeling creates a marrow cavity: Osteoclasts degrade trabeculae in the center to create the marrow cavity Bone increases in length by interstital growth of the epiphyseal plate followed by replacement of plate cartilage by spongy bone Cartilage continues to grow on epiphyseal side and is replaced by bone on diaphysis side Bone increases in diameter by appositional growth from cellular layers of peristeum Figure 6–9 (Step 4)

Endochondral Ossification: Step 5 Secondary ossification centers form in epiphyses: Capillaries and osteoblasts enter the epiphyses: creating secondary ossification centers Figure 6–9 (Step 5)

Endochondral Ossification: Step 6 Epiphyses become ossified with spongy bone Hyaline cartilage remains on articular surfaces (not calcified or ossified) Ossification continues at both 1°and 2° ossification centers until all epiphyseal cartilage has been replaced with bone  epiphyseal closure Adult bone retains the epiphyseal line Figure 6–9 (Step 6)

Endochondral Ossification Appositional growth: compact bone thickens and strengthens long bone with layers of circumferential lamellae Figure 6–9 (Step 2)

During intramembranous ossification, which type(s) of tissue is/are replaced by bone? hyaline cartilage fibrous connective tissue mesenchymal connective tissue osteoid tissue

In endochondral ossification, what is the original source of osteoblasts? de novo synthesis cells brought with via the nutrient artery cells of the inner layer of the perichondrium chondrocytes from the original model

The characteristics of adult bones.

Epiphyseal Lines Figure 6–10

Epiphyseal Lines When long bone stops growing, after puberty: epiphyseal cartilage disappears is visible on X-rays as an epiphyseal line

A child who enters puberty several years later than the average age is generally taller than average as an adult. Why? Epiphyseal plates fuse during puberty. Bone growth continues throughout childhood. Growth spurts usually occur at the onset of puberty. All of the above.

The skeletal system remodels and maintains homeostasis The skeletal system remodels and maintains homeostasis. The effects of nutrition, hormones, exercise, and aging on bone.

Bone Remodeling Bones are not static: constantly recycled and renewed 5-7% of skeleton is recycled/week Osteoclasts secrete: Lysosomal enzymes: digest osteoid Hydrochloric acid: solubilize calcium salts Osteoblasts secrete: Osteoid (organic matrix) Alkaline phosphatase: induces mineralization of osteoid - Complete mineralization takes ~1 week

Bone Remodeling Bones Adapt: Stressed bones grow thicker Bumps and ridges for muscle attachment enlarge when muscles are used heavily Bones weaken with inactivity: up to 1/3 or mass is lost with few weeks of inactivity Heavy metals can get incorporated Condition of bones depends on interplay between osteoclast and osteoblast activity

Skeleton as a Calcium Reserve Calcium is important for normal function of neurons and muscle Blood calcium: 9-11 mg/100ml If blood levels are too high: Nerve and muscle cells are non responsive If blood levels are too low: Nerve and muscle cells are hyper-excitable  convulsions, death

The Skeleton as Calcium Reserve Bones store calcium and other minerals Calcium is the most abundant mineral in the body Calcium ions are vital to: membranes neurons muscle cells, especially heart cells

Skeleton as a Calcium Reserve Calcium homeostasis depends on: Storage in the Bones Absorption in the GI Excretion at the Kidneys ** These factors are controlled by hormones to regulate blood calcium levels

If blood calcium levels Low: Parathyroid hormone (from parathyroid gland) triggers: Increase osteoclast activity - decrease storage Enhanced calcitriol action - increase absorption Decreased calcium excretion at the kidneys

If Blood Calcium levels High Calcitonin (from thyroid gland) triggers: Inhibition of osteoclast activity Increased calcium excretion at the kidneys

Nutritional and Hormone Effects on Bone Many nutrients and hormones are required for normal bone growth and maintenance: Calcium and phosphate salts Calcitriol Vitamin C Vitamin A Vitamin K and B12 Growth Hormones Thyroxin Estrogens and Androgens Calcitonin Parathyroid Hormone

Nutritional and Hormone Effects on Bone Calcium and phosphate salts - From food, for mineralization of matrix Calcitriol - From kidneys, for absorption of calcium and phosphate Vitamin C - From food, for collagen synthesis and osteoblast differentiation Vitamin A - From carotene in food, for normal bone growth in children Vitamin K and B12 - From food, for synthesis of osteoid proteins

Nutritional and Hormone Effects on Bone Growth Hormones - From pituitary gland, for protein synthesis and cell growth Thyroxin - From thyroid gland, for cell metabolism and osteoblast activity Estrogens and Androgens - From gonads, for epiphyseal closure Calcitonin - From thyroid gland AND Parathyroid Hormone From parathyroid gland, to regulate calcium and phosphate levels in body fluids Affects bone composition

Hormones for Bone Growth and Maintenance Table 6–2

Abnormalities Genetic/Physiological Abnormalities 1. Giantism: too much Growth hormone prior to epiphyseal closure, bones grow excessively large 2. Acromegaly: - too much GH after closure, bones don’t grow but all cartilage does - ribs, nose, ears, articular cartilage 3. Pituitary Dwarfism: - not enough GH, bones fail to elongate

Abnormalities Diet Related Abnormalities: 1. Scurvy: - lack of Vit. C - causes low collagen content, reduced bone mass, bones brittle 2. Osteomalacia: - lack calcitriol, osteoid produced but not mineralized, bones flexible -Called Rickets in children and leads to permanent deformity

The individual will be taller. The individual will be shorter. A seven-year-old child has a pituitary tumor involving the cells that secrete growth hormone (GH), resulting in increased levels of GH. How will this condition affect the child’s growth? The individual will be taller. The individual will be shorter. Growth of the individual will be erratic and slow. Excessive growth will be limited to axial skeleton.

Why does a child who has rickets have difficulty walking? Joints become fused, preventing movement. Bones are brittle and break under body weight. Bones are flexible and bend under body weight. Motor skills are impaired.

What effect would increased PTH secretion have on blood calcium levels? higher level of calcium lower level of calcium uncontrolled level of calcium no effect on blood calcium, PTH effects calcium in the bones

How does calcitonin help lower the calcium ion concentration of blood? by inhibiting osteoclast activity by increasing the rate of calcium excretion at the kidneys by increasing the rate of calcium uptake by intestinal cells 1 and 2

Types of fractures and how do they heal.

Fractures Fractures: Bones break in response to excessive stress cracks or breaks in bones caused by physical stress Bones break in response to excessive stress Bones are designed to heal Fractures are repaired in 4 steps

Fracture Repair: Step 1 Bleeding: produces a clot (fracture hematoma) Seals off dead osteocytes and broken blood vessels Figure 6–15 (Step 1)

Fracture Repair: Step 2 Cells of the endosteum and periosteum: Divide and migrate into fracture zone Cells of Periosteum: create external callus of fibrocartilage Cells of Endosteum: create internal callus of spongy bone Calluses stabilize the break: external callus of cartilage and bone surrounds break internal callus develops in marrow cavity Figure 6–15 (Step 2)

Fracture Repair: Step 3 Osteoblasts: replace cartilage with spongy bone Fracture gap is now filled with all spongy bone Figure 6–15 (Step 3)

Fracture Repair: Step 4 A bulge from the callus marks the fracture point Osteoblasts and osteocytes remodel the fracture for up to a year: Spongy bone is replaced with compact bone and excess callus material is removed Figure 6–15 (Step 4)

The effects of aging on the skeletal system.

Effects of Aging Bones become thinner and weaker with age 1. Osteopenia = reduction in bone mass All adults suffer in some degree Osteoclasts out-work osteoblast sex hormones in youth inhibit osteoclasts Women: 8%/decade after 40 Men: 3%/decade after 40

Effects of Aging 2. Osteoporosis = reduction in bone mass that compromises function More common in women: Over age 45, occurs in: 29% of women 18% of men Thinner bones to start Greater rate of osteopenia

(b) Spongy bone in osteoporosis (a) Normal spongy bone SEM X 25 (b) Spongy bone in osteoporosis SEM X 21

Effects of Bone Loss The epiphyses, vertebrae, and jaws are most affected: resulting in fragile limbs reduction in height tooth loss

Hormones and Bone Loss Estrogens and androgens help maintain bone mass Bone loss in women accelerates after menopause

Why is osteoporosis more common in older women than in older men? Testosterone levels decline in post-menopausal women. Older women tend to be more sedentary than older men. Declining estrogen levels lead to decreased calcium deposition. In males, androgens increase with age.

SUMMARY (1 of 2) Bone shapes, markings, and structure The matrix of osseous tissue Types of bone cells The structures of compact bone The structures of spongy bone The periosteum and endosteum Ossification and calcification Intramembranous ossification Endochondrial ossification

SUMMARY (2 of 2) Blood and nerve supplies Bone minerals, recycling, and remodeling The effects of exercise Hormones and nutrition Calcium storage Fracture repair The effects of aging