Bone
I. General characteristics
§ Like cartilage:
consists of cells living in lacunae, surrounded by matrix they produce (matrix is infiltrated with inorganic salts, making it rigid and inflexible)
outer fibrous CT- periosteum
§ Unlike cartilage:
highly vascular
complex architecture overall
constantly remodeled and reshaped in response to stress and mineral needs
§ Functions
Support, attachment, leverage, mineral storage, protection of hematopoietic tissues within marrow spaces
II. Basic structure
A. Grossly visible features
Marrow cavity
§ soft CT fills central cavities of long bones and all spaces between bony spicules/trabeculate
§ marrow classified as red or yellow
§ red marrow filled with sinusoid, reticular tissue, hematopoietic tissue (blood cells formed)
§ yellow marrow is old, inactive, filled with fat
Spongy or cancellous bone
§ Consists chiefly of trabeculae, with large spaces between them
Compact bone
§ Primarily bone substance with spaces
§ Transition from spongy to compact bone that is intermediate in appearance
Most bone contains variable amounts of outer compact and inner spongy bone
B. Microscopic organization (both compact and spongy bone)
Osteocytes and related features
§ Body of mature bone cell is imprisoned within matrix in lacuna
§ Long cytoplasmic processes radiating outward like tentacles
§ Each process contacts similar processes of another cell
§ Nutrients and metabolite diffuse from one cell to another around processes (unlike car substances cannot diffuse through matrix)
§ Each cell process imprisoned in channel in matrix called canaliculus
§ Matrix is travered by numerous canalicui that radiate out from lacunae
§ Osteocyte and processes vaportized, but lacunae and canaliculi remain
§ Osteocyte derived from osteoblast (arise from osteoprogenitor cell)
§ Osteoblast responsible for production of organic matrix of bone, imprisoned in matrix and called osteocyte
§ Osteocyte does not produce matrix, but maintains it
§ Both osteoblast and osteocyte are post-miotic
Bone matrix
§ Organic material-osteoid
95% type I collagen, GAGs (small amount of chondroitin 4-sulfate, chondoitin 6-sulfate,keratin sulfate), glycoprotein
§ Inorganic material
Hydroxyapatite crystals of calcium (99% of body’s Ca++ storage), phosphorous, carbonate, magnesium, fluoride, sulfate; 50% of dry weight
§ BONE CT
Cell- osteoprogenitor cells, osteoblasts, osteocytes
Matrix- of fibers type I collagen
Ground substance- organic and inorganic components
C. Details of compact bone
Haversian systems (osteons)
§ Osteocytes organized around vascular units in way to maximize access to blood supply
§ Arrangement of osteocytes into haversian system in interior of compact bone
§ Cylindrical structure at center which runs a canal filled with blood vessels
§ Osteocytes in lacunae arranged in progressively larger concentric circles around canal
§ Cross sections, HS appears with central canal/Haversian canal surrounded by 4-20 concentric lamellae
§ Small thread like canaliculi can be seen traversing lamellae forming connections among all of lacunae and central canals
§ Interstitial fluid from cap in Hav canals percolates around osteocyte processes inside canaliculi
§ In this way nutrients are carried out to the most peripheral cells of system.
§ Each HS is surrounded by amorphous cement line.
§ Volkman’s canal -Large transverse channels cut across HS and connect adjacent Hav canals; contain blood vessels
§ HS contantly being remodeled
Other features
§ Interspersed b/t complete HS are interstitial lamellae (remnants of concentric lamellae previously comprising osteon that were worn away in remodeling process.
§ At outer periphery of compact bone, several layers of compact bone are oriented parallel to free surface called circumferential lamellae.
D. Details of spongy bone
§ Contains lamellae, but less regularly arranged and osteons are infrequent since this type of bone has little mass and consists of peninsulas of bone (trabeculae: specules) that project into marrow cavity with its nutritive vasculature
§ Multinucleate osteoclast found in Howship’s lacuna (depression in trabeculae)osteoclast responsible for dissolution of bone
§ Activity increase by parathyroid hormone PTH and diminished with calcitonin
§ Osteoclast arises from marrow derived monoctyes which fuse together to form large multinucleated cell with abundant eosinophilic cytoplasm.
§ Belongs to mononuclear phagocyte system.
§ Osteoclast seals itself around central area of bone and form ruffled border where enzymes and acid are released
§ Calcium phosphate in bone is dissolved and taken up into osteoclaste cytoplasm the diffused into vasculature
E. Periosteum and endosteum
Periosteum
§ CT surrounding perimeter of bone, merges with surrounding CT
§ Inner osteogenic layer- generation of underlying bone
§ Outer fibrous layer-dense irregular CT that merges with surrounding CT
Endosteum
§ Thin layer of cells that line inner part of bone along marrow cavity
§ Purely osteogenic in function (no fibrous component)
§ Osteogenic layers of periosteum and endosteum comprosed of osteoprogenitor cells and osteoblasts
§ Osteoblasts are responsible for matrix production turn into osteocyte (in actively growing bone)
§ Osteoprogenitor cells are squamous, spindly cells that are more immature than osteoblasts and can evolve into osteoblasts or chondroblasts
§ With proper stimulus, osteoprogenitor cells become osteoblsts in highly vascularized regions, or chondroblasts in avascular regions
§ Osteoprogenitor cells are a population of bipotential stem cells that give rise to either bone or cartilage
§ Endosteum and periosteum of quiescent bone consists of osteoprogenitor cells; in actively growing bone, these layers would contain large numbers of osteoblastst
F. Remodeling
§ Bone is dynamic tissue
§ Compensatory deposition or resorption of compact bone or realignment of bony trabeculae are normal processes
§ Compensatory mechanisms exits to insure total bone mass remains constant
§ Compact bone is always being resorbed and replaced with new compact bone:
Osteoclasts from marrow move into area of compact bone to be replaced- absorb bone in resorption tunnels
Different from Hav. Canals by irregular outlines and osteoclasts lining borders
Tunnels contain blood vessels, osteoprogenitor cells, and osteoblasts which proceed to lay down concentric lamellae from outside to inside of tunnel where central canal is left
Osteons develop through progressive filling in of tunnel with successive lamellae of bone
§ Growth of bone across its diameter is another type of remodeling (during youth)
§ For a bone in increase in diameter, new bone is laid down by periosteum on the outside of the preexisting compact bone, while a compensatory widening of the marrow cavity must occur to prevent bone from being too heavy
§ Osteoclasts widen the marrow cavity by reabsorbing compact bone from inner aspect of cortex, forming trabeculae in process
§ This is why HS (compact bone) may be seen in trabeculae (spongy bone)
§ IMPORTANT: anytime bone grows, it is by laying down of new bone on preexisting surfaces (appositional growth only); osteocytes do not divide or lay down new matrix. Cartilage unlike bone can grow appositionally or interstially.
III. Osteogenesis
A. Intramembraneous bone formation
§ Second month of embryotic life, osteoprogen cells and osteoblast develop from mesenchymal CT
§ Osteoblasts lay down osteoid à becomes calified
§ Irregular bony plaes and trabeculae form around primitive marrow spaces
§ Later, compact bone forms externally
§ Occurs in frontal and parietal bones, parts of mandible, maxilla, occipital, and temporal bones
B. Endochondral bone formation- form around and with cartilage perimordium that has general shape it will eventually assume.
Formation of primary ossification center
§ Beginning at 2 months, bone begins to replace hyaline cartilage primordial
§ Occurs in dyaphysis (center of cartilage)
§ Chgs in local envirn stim oseoprogen cell of perichondrium to make osteoblast instead of chondroblasts
§ Bony collar surround cartilage model
§ Perichondrium is now periosteum
§ Bone is laid down on outer surface from now on (causing expansion of diameter)
§ Chondroboyes in center of cartilage degenerate, reaborb matrix, lacunae enlarged
§ Ca++ deposits form in matrix and cartilage becomes calcified
§ Blood vessesl and osteoprogen cells from periosteum invade center of cartilage and enlarged lacunar spacesàmarrow spaces
§ Osteogen cellsàosteoblasts lining calcified cartilage
§ They lay down osteoidà well developed endosteum
§ Calification turns osteoid into mineralized bone and osteo lastsare immobilized as osteocytes
§ Calcified cartilage is NOT osteoid
Formation of secondary ossification center
§ After birth, distal ends of cartilage primorida (epiphyses) undergo ossification sep from diaphysis
§ Central chondrocytes degenerate, large spaces form, cartilage matrix calcifies
§ Osteogenic vascular buds break into ends, converting center into bone marrow spaces lined by endosteum.
§ Proximal and distal epiphyses ossify at different times after birth
§ Process results in formation of 1o and 2o ossification centers formed in diaphysis and epiphyses
§ Critical zone of catilage ramins throughout childhood and puberty = epiphyseal disk/plate
IV. Endochondral bone growth
§ Epiphyseal disk necessary for bone to attain final length
§ Bone lengtheningvposit of new bone on caldified cartilage remnants
§ Hyaline cartilage of epiphyseal disk of lengthening bone is no homogenous.
§ 5 distinct zones:
A. Zone of reserve cartilage (quiescence, resting zone)
§ Little mitosis of syn of new cartilage
B. Zone of cartilage cell multiplication (proliferative zone)
§ High mitotic activity and synthesis of new matrix. Cells align into longitudinal rows
C. Zone of cartilage cell hypertrophy
§ Cartilage b/t adjacent cells become thinner as cells and lacunae enlarge
D. Zone of cartilage calcification
§ Cartilage matrix becomes calcified: cells die and blood vessels form marrow cavity invade forming channels. Chondroclasts bibble at calcified cartilage, reducing the “peninsulas” of calcified cartilage in some areas to “islands”
E. Zone of ossification
§ Osteoblasts align spicules of remaining calcified cartilage and lay down osteoid
§ Epiphyesal plate maintained thru childhood by constant expansion of cartilage disk, and constant replacement onf inner suface of disk with bone.
§ Process is a race b/t cartilage and bone
§ Even thru childhood: cartilage growth resp for rapid and constant extension of long bones of body
§ At puberty (increase levels of sex hormones) bone growth accelerates
§ After few years bone on either side of epiphyseal plate meets in center of plate, leaving epiphyseal line or synostosis
V. Summary concepts
§ Intramembrneous or endochondral ossification (converting mesenchyme or cartilage to bone) restricted to perinatal period.
§ Endochondral bone growth refers to lengthening process of long bones requiring cartilaginous epiphyesal plate and ceasing at childhood.
§ Remodeling and growth/expansion of bone occurs thru life via appositional depositin of bone on endosteal and periosteal bony surfaces without presence of cartilage.
VI. Hormones affecting bone growth
A. Parathyroid Hormone (PTH)
§ Secreted by parathyroid glands
§ Maintains serum calcium by increasing dissolution of bone and transferring calcium from bone to blood
1. activation of existing osteoclasts, nibble away bone matrix and release calcium into circulation
2. proliferation of osteoclasts -slower (days-weeks)
§ PTH on feedback system: hypocalcemia counteracted by increase rate of seretion of PTH
§ Large amounts of PTH can cause bone destruction
B. Calcitonin
§ Secreted by pararfollicular cells in thyroid gland
§ Reduces bone resorption and opposes effects of PTH
§ Lowers blood calcium levels
§ Decreases osteoclast activity
C. Growth Hormone (GH)
§ Secreted by anterior pituitary
§ GH enhances sysn of all CT
§ Acts thru somatomedin (liver) to cause proliferation of chondrocyte in epiphyseal disk
§ Responsible for height increases
§ Excess in childhood produces giants, lack produces midgets
D. Androgens and Estrogens
§ Sex steroids (ovaries, testes, adrenal glands)stim osteoblast activity and deposition of bony matrix
§ Bone growth results in closure of epiphysis and increase thickness of all bones
E. Other
§ Weight bearing or use
§ Short period of weightlessness or prolonged bed rest can cause bone demineralization
§ Lack of weight bearing exercise over long period of time w/ low estrogen levels in women results in osteoporsis and pathologic fractures
§ Much of loss of inorganic bone mass is irreversible
Tags: bone, bone demineralization, bone lengthening, cancellous bone, Chondroclasts bibble, chondroitin, cytoplasmic processes, Epiphyseal disk, Howship's lacuna, Hydroxyapatite crystals, inorganic salts, lacunae, marrow, mononuclear phagocyte system, osteoblasts, Osteocytes, Osteogenesis, osteoporsis, osteoprogenitor cells, pathologic fractures, PTH, spongy bone, trabeculae
