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Spongy Bone

Spongelike 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 weighttrabeculae develop along bone’s lines of stress

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Spongy Bone Structure and Stress

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Bone Marrow

In medullary cavity (long bone) and among trabeculae (spongy bone)Red marrow like thick blood

reticular fibers and immature cellsHemopoietic (produces blood cells)in vertebrae, ribs, sternum, pelvic girdle and proximal heads of femur and humerus in adults

Yellow marrowfatty marrow of long bones in adults

Gelatinous marrow of old ageyellow marrow replaced with reddish jelly

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Intramembranous Ossification

Condensation of mesenchyme into trabeculaeOsteoblasts on trabeculae lay down osteoid tissue (uncalcified bone)Calcium phosphate is deposited in the matrix forming bony trabeculae of spongy boneOsteoclasts create marrow cavityOsteoblasts form compact bone at surfaceSurface mesenchyme produces periosteum

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Intramembranous Ossification 1

Produces flat bones of skull and clavicle.

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Intramembranous Ossification 2

Note the periosteum and osteoblasts.

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Stages of Endochondral Ossification

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Endochondral Ossification 1

Bone develops from pre-existing modelperichondrium and hyaline cartilage

Most bones develop this processFormation of primary ossification center and marrow cavity in shaft of model

bony collar developed by osteoblastschondrocytes swell and diestem cells give rise to osteoblasts and clastsbone laid down and marrow cavity created

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Primary Ossification Center and Primary Marrow Cavity

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Secondary ossification centers and marrow cavities form in ends of bone

same processCartilage remains as articular cartilage and epiphyseal (growth) plates

growth plates provide for increase in length of bone during childhood and adolescenceby early twenties, growth plates are gone and primary and secondary marrow cavities united

Endochondral Ossification 2

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Secondary Ossification Centers and Secondary Marrow Cavities

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The Metaphysis

Zone of reserve cartilage = hyaline cartilageZone of proliferation

chondrocytes multiply forming columns of flat lacunae

Zone of hypertrophy = cell enlargementZone of calcification

mineralization of matrixZone of bone deposition

chondrocytes die and columns fill with osteoblastsosteons formed and spongy bone is created

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Fetal Skeleton at 12 Weeks

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LAB

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Pelvic Girdle

Girdle = 2 hip bonesPelvis = girdle and sacrumSupports trunk on the legsand protects visceraEach os coxae is joined tothe vertebral column at thesacroiliac jointAnteriorly, pubic bones are joined by pad of fibrocartilage to form pubic symphysis

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Pelvic Inlet and Outlet

False and true pelvis separated at pelvic brimInfant’s head passes through pelvic inlet and outlet

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Os Coxae (Hip Bone)

Acetabulum is hip joint socketIlium

iliac crest and iliac fossagreater sciatic notch contains sciatic nerve

Pubisbody, superior and inferior ramus

Ischiumischial tuberosity bears body weightischial spinelesser sciatic notch between ischial spine and tuberosityischial ramus joins inferior pubic ramus

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Comparison of Male and Female

Female lighter, shallower pubic arch( >100 degrees), and pubic inlet round or ovalMale heavier, upper pelvis nearly vertical, coccyx more vertical, and pelvic inlet heart-shaped

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Femur and Patella (Kneecap)

Nearly spherical head and constricted neck

ligament to fovea capitis

Greater and lesser trochanters for muscle attachmentPosterior ridge called linea asperaMedial and lateral condyles and epicondyles found distallyPatella = triangular sesamoid

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Tibia

Tibia is thick, weight-bearing bone (medial)Broad superior head with 2 flat articular surfaces

medial and lateral condylesroughened anterior surface palpated below patella(tibial tuberosity)distal expansion = medial malleolus

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Fibula

Slender lateral strut stabilizes ankleDoes not bear any body weight

spare bone tissue

Head = proximal endLateral malleolus = distal expansionJoined to tibia by interosseous membrane

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The Ankle and Foot

Tarsal bones are shaped and arranged differently from carpal bones due to load-bearing role of the ankleTalus is most superior tarsal bone

forms ankle joint with tibia and fibulasits upon calcaneus and articulates with navicular

Calcaneus forms heel (achilles tendon)Distal row of tarsal bones

cuboid, medial, intermediate and lateral cuneiforms

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The Foot

Remaining bones of foot are similar in name and arrangement to the handMetatarsal I is proximal to the great toe (hallux)

base, shaft and head

Phalanges2 in great toe

proximal and distal3 in all other toes

proximal, middle and distal

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END OF LAB

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Bone Growth and Remodeling

Bones increase in lengthinterstitial growth of epiphyseal plateepiphyseal line is left behind when cartilage gone

Bones increase in width = appositional growthosteoblasts lay down matrix in layers on outer surface and osteoclasts dissolve bone on inner surface

Bones remodeled throughout lifeWolff’s law of bone = architecture of bone determined by mechanical stresses

action of osteoblasts and osteoclastsgreater density and mass of bone in athletes or manual worker is an adaptation to stress

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Dwarfism

Achondroplasticlong bones stop growing in childhood

normal torso, short limbs

spontaneous mutation during DNA replicationfailure of cartilage growth

Pituitary lack of growth hormonenormal proportions with short stature

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Mineral Deposition

Mineralization is crystallization processosteoblasts produce collagen fibers spiraled the length of the osteonminerals cover the fibers and harden the matrix

ions (calcium and phosphate and from blood plasma) are deposited along the fibers ion concentration must reach the solubility product for crystal formation to occur

Abnormal calcification (ectopic)may occur in lungs, brain, eyes, muscles, tendons or arteries (arteriosclerosis)

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Mineral Resorption from Bone

Bone dissolved and minerals released into blood

performed by osteoclasts “ruffled border”hydrogen pumps in membrane secrete hydrogen into space between the osteoclast and bone surfacechloride ions follow by electrical attractionhydrochloric acid (pH 4) dissolves bone mineralsenzyme (acid phosphatase) digests the collagen

Dental braces reposition teeth and remodel bone

create more pressure on one side of the toothstimulates osteoclasts to remove bonedecreased pressure stimulates osteoblasts

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Calcium and Phosphate

Phosphate is component of DNA, RNA, ATP, phospholipids, and pH buffers

~750 g in adult skeletonplasma concentration is ~ 4.0 mg/dL2 plasma forms: HPO4

-2 and H2PO4-

Calcium needed in neurons, muscle contraction, blood clotting and exocytosis

~1100g in adult skeletonplasma concentration is ~ 10 mg/dL

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Ion Imbalances

Changes in phosphate levels = little effectChanges in calcium can be serious

hypocalcemia is deficiency of blood calciumcauses excitability of nervous system if too low

muscle spasms, tremors or tetany ~6 mg/dLlaryngospasm and suffocation ~4 mg/dL

with less calcium, sodium channels open more easily, sodium enters cell and excites neuron

hypercalcemia is excess of blood calciumbinding to cell surface makes sodium channels less likely to open, depressing nervous system

muscle weakness and sluggish reflexes, cardiac arrest ~12 mg/dL

Calcium phosphate homeostasis depends on calcitriol, calcitonin and PTH hormone regulation

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Carpopedal Spasm

Hypocalcemia demonstrated by muscle spasm of hands and feet.

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Hormonal Control of Calcium Balance

Calcitriol, PTH and calcitonin maintain normal blood calcium concentration.

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Calcitriol (Activated Vitamin D)

Produced by the following processUV radiation and epidermal keratinocytes convert steroid derivative to cholecalciferol - D3liver converts it to calcidiolkidney converts that to calcitriol (vitamin D)

Calcitriol behaves as a hormone that raises blood calcium concentration

increases intestinal absorption and absorption from the skeletonincreases stem cell differentiation into osteoclastspromotes urinary reabsorption of calcium ions

Abnormal softness (rickets) in children and (osteomalacia) in adults without vitamin D

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Calcitriol Synthesis and Action

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Calcitonin

Secreted (C cells of thyroid gland) when calcium concentration rises too highFunctions

reduces osteoclast activity as much as 70%increases the number and activity of osteoblasts

Important in children, little effect in adults

osteoclasts more active in childrendeficiency does not cause disease in adults

Reduces bone loss in osteoporosis

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Correction for Hypercalcemia

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Parathyroid Hormone

Glands on posterior surface of thyroidReleased with low calcium blood levelsFunction = raise calcium blood level

causes osteoblasts to release osteoclast-stimulating factor (RANKL) increasing osteoclast populationpromotes calcium resorption by the kidneyspromotes calcitriol synthesis in the kidneysinhibits collagen synthesis and bone deposition by osteoblasts

Sporatic injection of low levels of PTH causes bone deposition

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Correction for Hypocalcemia

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Other Factors Affecting Bone

Hormones, vitamins and growth factorsGrowth rapid at puberty

hormones stimulate osteogenic cells, chondrocytes and matrix deposition in growth plategirls 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

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Fractures and Their Repair

Stress fracture caused by traumacar accident, fall, athletics, etc

Pathological fracture in bone weakened by disease

bone cancer or osteoporosis

Fractures classified by structural characteristics

break in the skinmultiple pieces

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Types of Bone Fractures

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Healing of Fractures 1

Normally 8 - 12 weeks (longer in elderly)Stages of healing

fracture hematoma (1) - clot forms, then osteogenic cells form granulation tissuesoft callus (2)

fibroblasts produce fibers and fibrocartilage

hard callus (3)osteoblasts produce a bony collar in 6 weeks

remodeling (4) in 3 to 4 months spongy bone replaced by compact bone

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Healing of Fractures 2

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Treatment of Fractures

Closed reductionfragments are aligned with manipulation and casted

Open reductionsurgical exposure and repair with plates and screws

Traction risks long-term confinement to bedElectrical stimulation used on fractures

if 2 months necessary for healing

Orthopedics = prevention and correction of injuries and disorders of the bones, joints and muscles

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Fractures and Their Repairs

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Osteoporosis 1

Bones lose mass and become brittle (loss of organic matrix and minerals)

risk of fracture of hip, wrist and vertebral columncomplications (pneumonia and blood clotting)

Postmenopausal white women at greatest risk

by age 70, average loss is 30% of bone massblack women rarely suffer symptoms

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Osteoporosis 2

Estrogen maintains density in both sexes (inhibits resorption)

testes and adrenals produce estrogen in menrapid loss after menopause, if body fat too low or with disuse during immobilizaton

TreatmentERT slows bone resorption, but increases risk breast cancer, stroke and heart diseasePTH (parathyroid hormone) slows bone loss if given daily injection

Forteo increases density by 10% in 1 yearmay promote bone cancer

best treatment is prevention -- exercise and calcium intake (1000 mg/day) between ages 25 and 40

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Spinal Osteoporosis