Bone Tissue

Other Factors Affecting Bone Homeostasis

20 or more hormones, vitamins and growth factors influence bone homeostasis. Minerals needed for remodeling Ca2+ - matrix PO4- - matrix magnesium - needed for osteoblast function manganese - needed for lamellae formation Vitamins needed for remodeling - D, C, A, B12 C - maintains matrix of connective tissues and for collagen synthesis A - controls activity, distribution, coordination of osteoblasts and osteoclasts during development B12 - for osteoblast metabolism and activity

Matrix of Osseous Tissue

25% water 25% organic matter: protein fibers collagen, glycosaminoglycans, proteoglycans & glycoproteins 50% Inorganic matter 85% hydroxyapatite (crystallized calcium phosphate salt) 10% calcium carbonate other minerals (fluoride, sulfate, potassium, magnesium) Combination provides for strength & resilience Osteogenesis imperfecta (brittle bone disease) Rickets disease (bone too soft)

haversian canal

vertical canal

structure of long bones

long tubular shaft, expanded ends for articulation at the joints, act as levers for the skeletal muscles to act upon

spongy bone and stress

provides strength with little weight, trabeculae develop along lines of stress

structure of round, irregular and short bones

same set-up for round irregular and short bones

dwarfism

Achondroplastic Dwarfism short stature but normal-sized head and trunk Result of spontaneous mutation when DNA is replicated mutant allele is dominant Growth Hormone Deficiency lack of growth hormone leads to short stature but with normal proportions

Epiphyseal Plates

At the end of adolescence, epiphyseal plate chondrocytes divide less often and the remaining hyaline cartilage is replaced by bone tissue.

secondary center of ossification

Begin to form in the epiphyses near time of birth Same stages occur as in primary ossification center result is center of epiphyseal cartilage being transformed into spongy bone Hyaline cartilage remains on joint surface as articular cartilage and at junction of diaphysis & epiphysis forming the epiphyseal plate

osteogenisis

Begins at 4 weeks of development Two types: Intramembranous ossification: uses a fibrous membrane model Endochondral ossification: uses a hyaline cartilage model

Osteoporosis - Treatment

Calcium and vitamin D supplements Increased weight-bearing exercise Hormone (estrogen) replacement therapy (HRT) slows bone loss Natural progesterone cream prompts new bone growth

bone tissue

Bone tissue or osseous tissue, is a dynamic tissue that continuously remodels itself. Bone tissue is a specialized connective tissue with a matrix hardened by calcium phosphate minerals

calcium homeostasis

Bones are important for Ca2+ homeostasis bone tissue is the main reservoir for Ca2+ ions in the body (500-1000 times more calcium is in bone than in the rest of the tissues) blood levels are regulated very tightly by the endocrine system bone serves as a "buffer" to prevent sudden changes in blood Ca2+ levels too much blood Ca2+ (hypercalcemia) - heart stops too little blood Ca2+ (hypocalcemia) - breathing stops 2 hormones are primarily involved in Ca2+ homeostasis Parathyroid Hormone (PTH) from the parathyroid glands, increases blood calcium levels Calcitonin from the thyroid gland decreases blood calcium levels

Bone Growth and Remodeling

Bones grow and remodel themselves throughout life Ex: bones grow and reshape to allow growth of brain Cartilage grows by both appositional & interstitial growth Bones increase in length by interstitial growth of epiphyseal plate during puberty anabolic steroids may cause premature closure of growth plate producing short adult stature

Appositional Bone Growth

Bones increase in width by appositional growth osteoblasts from the perisoteum lay down matrix in layers parallel to the outer surface osteoclasts dissolve compact bone lining the medullary cavity. If one process outpaces the other, bone deformities occur (osteitis deformans or Paget's disease)

rickets

Bones of children are inadequately mineralized causing softened, weakened bones. Bowed legs and deformities of the pelvis, skull, and rib cage are common. Caused by insufficient calcium in the diet, or by vitamin D deficiency.

Calcitriol (Activated Vitamin D)

Calcitriol behaves as a hormone (blood-borne messenger) Function: increases blood levels of calcium: stimulates intestine to absorb calcium, phosphate & magnesium weakly promotes urinary reabsorption of calcium ions promotes osteoclast activity to raise blood calcium concentration to the level needed for bone deposition

canalculi

Canaliculi house cytoplasmic extensions from osteocytes, so they are in contact for transport and communication.

early embyronic skeleton

Composed of: Fibrous connective tissue (mesenchyme) Hyaline cartilage

Hormonal Regulation of Bone Growth During Youth

During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone (hGH) An overabundance of growth hormone during childhood or adolescence gives rise to gigantism. It is a very rare condition that usually results from a tumor of the cells that produce growth hormone.

Hormonal Regulation of Bone Growth During Youth (2)

During puberty, testosterone and estrogens: Initially promote adolescent growth spurts Cause masculinization and feminization of specific parts of the skeleton Later induce epiphyseal plate closure, ending longitudinal bone growth Longitudinal growth ceases and the epiphyses/diaphysis fuse at the epiphyseal plates. females at 18 years; males at 21 years the clavicle is the last bone to stop growing around 25

ectopic calcification

Ectopic calcification is abnormal calcification in soft tissues. May occur in lungs, brain, eyes, muscles, tendons or arteries

mineral deposition

Mineralization is crystallization process in which calcium, phosphate and other ions are removed from blood plasma and deposited in bone tissue. Steps of the mineralization process: osteoblasts produce collagen fibers that spiral along the length of the osteon in alternating directions fibers become encrusted with minerals hardening matrix ion concentration must reach the solubility product for crystal formation to occur & then positive feedback forms more

Osteoporosis

Most common bone disease Bones lose mass & become brittle due to loss of both organic matrix & minerals risk of fracture of hip, wrist & vertebral column lead to fatal complications such as pneumonia widow's (dowager's) hump is deformed spine Postmenopausal white women at greatest risk by age 70, average loss is 30% of bone mass ERT slows bone resorption, but best treatment is prevention -- exercise & calcium intake (1000 mg/day) between ages 25 and 40 Therapies to stimulate bone deposition are still under investigation

osteporosis (2)

Most common bone disease in which bone reabsorption outpaces bone deposit spongy bone of the spine is most vulnerable occurs most often in postmenopausal women bones lose mass and become brittle due to loss of both organic matrix and minerals bones become so fragile that sneezing or stepping off a curb can cause fractures

cells of osseous tissue (2)

Osteoclasts develop in bone marrow by the fusion of 3-50 of the same stem cells that give rise to monocytes found in blood Bone resorption in pits called resorption bays (matrix destruction)

cells of osseous tissue (1)

Osteogenic cells reside in endosteum, periosteum or central canals arise from embryonic fibroblasts and become only source for new osteoblasts multiply continuously & differentiate into amitotic osteoblasts in response to stress or fractures Osteoblasts form organic matter of matrix Osteocytes are osteoblasts that have become trapped in the matrix they formed cells in lacunae connected by gap junctions inside canaliculi signal osteoclasts & osteoblasts about mechanical stresses

Primary Center of Ossification

Primary center of ossification forms in cartilage model chondrocytes near the center swell to form primary ossification center matrix is reduced & model becomes weak at that point cells of the perichondrium produce a bony collar cuts off diffusion of nutrients and hastens their death At 3 months of age, primary marrow space formed by periosteal bud osteogenic cells invade & transform into osteoblasts osteoid tissue deposited and calcifies into trabeculae meanwhile osteoclasts work to enlarge the primary marrow cavity

Mineral Resorption

Process of dissolving bone and releasing minerals into the blood performed by osteoclasts "ruffled border" hydrogen pumps in the cell membrane secrete hydrogen ions into the space between the osteoclast & the bone chloride ions follow by electrical attraction hydrochloric acid with a pH of 4 dissolves bone minerals an enzyme (acid phosphatase) digests the collagen Dental braces reposition teeth, creating greater pressure on the bone on one side of the tooth and less on the other side increased pressure stimulates osteoclasts; decreased pressure stimulates osteoblasts to remodel jaw bone

Intramembranous Ossification

Produces flat bones of skull & clavicle Steps of the process mesenchyme condenses into a sheet of soft tissue transforms into a network of soft trabeculae osteoblasts gather on the trabeculae to form osteoid tissue (uncalcified bone) calcium phosphate is deposited in the matrix transforming the osteoblasts into osteocytes osteoclasts remodel the center to contain marrow spaces & osteoblasts remodel the surface to form compact bone mesenchyme at the surface gives rise to periosteum

Intramembranous Ossification (2)

Produces flat bones of skull & clavicles Steps of the process: mesenchyme condenses osteoblasts form osteoid tissue calcium phosphate is deposited osteoclasts remodel the center osteoblasts remodel the surface surface mesenchyme gives rise to periosteum

calcitonin

Secreted by C cells of the thyroid gland when calcium concentration rises too high Released when calcium blood levels are too high Function: calcitonin "tones it down"! reduces osteoclast activity by as much as 70% in 15 minutes Increases calcium uptake from the blood and deposition into bone by increasing the number and activity of osteoblasts Antagonist to PTH

parathyroid hormone

Secreted by the parathyroid glands found on the posterior surface of the thyroid gland Released when calcium blood levels are too low Function: increases blood levels of calcium: binds to osteoblasts causing them to release osteoclast-stimulating factor that stimulates osteoclast multiplication & activity decreases calcium excretion in the urine by the kidneys promotes calcitriol synthesis in the kidneys inhibits collagen synthesis and bone deposition by osteoblasts Injection of low levels of PTH can cause bone deposition

bone marrow

Soft tissue that occupies the medullary cavity of a long bone or the spaces amid the trabeculae of spongy bone Red marrow looks like thick blood mesh of reticular fibers and immature cells hemopoietic means produces blood cells found in vertebrae, ribs, sternum, pelvic girdle and proximal heads of femur and humerus in adults Yellow marrow fatty marrow of long bones in adults Gelatinous marrow of old age yellow marrow replaced with reddish jelly

functions of skeletal system

Support - framework for the body Protection of internal organs Movement Blood formation in red bone marrow Mineral reservoir

Endosteum

The endosteum is a thin membrane which consists of connective tissue Lines the medullary cavities Highly vascular Contains: Osteoblasts Osteoclasts No fibrous layer

ends of long bones

The epiphysis is coated with compact bone filled with cancellous bone. Bone marrow fills the spaces between the trabeculae. The epiphyseal line is a plate of compact bone which is a remnant of the hyaline cartilage which allows the diaphysis to grow into adulthood. It stays cartilaginous until the end of puberty.

Periosteum- inner layer

The inner layer is called the osteogenic layer. It adheres to the bone surface. It contains: Osteoblasts bone forming cells Osteoclasts Bone remodeling cells

Periosteum - Outer Layer

The outer fibrous layer is a dense irregular connective tissue which contains fibroblasts, blood vessels and nerve fibers. Contains collagen fibers continuous with tendons (called perforating - Sharpey's fibers) that penetrate into bone matrix

periosteum

The periosteum is a double layered membrane which covers the external surface of bone with the exception of articular surfaces. Supplies nutrition to the bone and senses torque and stress as a response to injury. It also provides an attachment point for muscles.

the skeletal system

The skeletal system is made of bones which are composed of several tissues: Bone tissue itself Non-specialized fibrous connective tissue lining the bone tissue endosteum (inner lining) perisoteum (outer lining) Blood (another specialized connective tissue) Bone marrow Articular cartilage between the bones

The Metaphysis

Transitional zone between head and shaft of a developing long bone 1. Zone of reserve cartilage is layer of resting cartilage 2. Zone of cell proliferation where chondrocytes multiply forming columns of flat lacunae 3. Zone of cell hypertrophy shows hypertrophy 4. Zone of calcification shows mineralization between columns of lacunae 5. Zone of bone deposition where chondrocytes die and each channel is filled with osteoblasts and blood vessels to form a Haversian canal & osteon

Response to Mechanical Stress

Wolff's law - a bone grows or remodels in response to the forces or demands placed upon it Observations supporting Wolff's law include: Long bones are thickest midway along the shaft (where bending stress is greatest) Curved bones are thickest where they are most likely to buckle. Trabeculae form along lines of stress (see slide on spongy bone and stress) Athletes or manual laborers have greater bone density & bone mass. Large bony projections form where strong, active muscles attach

endochondral ossification

begins in the second month of development. is the ossification method for all bones below base of the skull (except clavicles). uses hyaline cartilage models for bone development. begins in the primary ossification center

volkman's canal

horizontal canal

Structure of flat bones

no marrow, Blow to the skull (a flat bone) may fracture outer layer and crush cancellous bone, but not harm inner compact bone.

spongy bone structure

sponge like appearance, formed by rods and plates called trabeculae, spaces filled with red bone marrow