THE GENERAL DOCTRINE ABOUT BONES

The general doctrine about bones

Tissues and organs

In human body they distinguish some levels of the organization:

1. molecular level object of the biochemistry

2. subcellular level object of the histology

3. cellular level object of the histology

4. tissues level object of the histology

5. organs object of the anatomy

6. systems object of the anatomy

7. organism object of the anatomy

connective tissue - less elasticity - wrinkles

cartilage - less elasticity - degenerative changes such as osteo-arthritis with associated changes. Costal cartilages often replaced by bone: bony spurs develop in unusual places.

bone - becomes brittle, reduced in size with less activity

muscle - ditto, plus deposition of fat.

heart and blood vessels - arteries become tortuous, walls become furred up with atheroma. Left ventricle is enlarged as consequence of load. Veins often varicose.

nervous system - Often small strokes will cause paralysis and reduction in size of some muscles

The human body is composed of 206 bones that perform five main functions:

1)                 support

2)                 protection

3)                 body movement

4)                 blood cell formation

5)                 storage of inorganic salts and lipid

II. Cells in Osseous tissue: osteocytes, osteoblasts, osteoclasts, osteoprogenitor, cells.

III. Cellular matrix - 1/ calcium hydroxyapatite, 2/ collagenous fibers

Long bone structure

Diaphysis -- shaft

epiphysis -- bone ends

epiphyseal line

nutrient foramen

medullary cavity

V. Types of bone:

a. lamellar or compact bone

1. contains osteons composed of concentric lamellae

2. each osteon has an osteonic canal (Haversian canal) which has blood vessels and nerves

3. osteocytes found within lacunae

4. canaliculi connect osteocytes and function to diffuse nutrition to the osteocytes

5. communicating (also called perforating or Volkman's) canals, connect adjacent osteons, and carry blood vessels.

6. the diaphyses, or shafts of long bones, are composed mostly of lamellar bone

b. Cancellous, trabecular, or spongy bone

1. does not contain haversian canal systems (osteons)

2. nerves and blood vessels run randomly through the loose meshwork of bone.

Structure and Physical Properties of the bone. Bone is one of the hardest structures of the animal body; it possesses also a certain degree of toughness and elasticity. Its color, in a fresh state, is pinkish-white externally, and deep red within. On examining a section of any bone, it is seen to be composed of two kinds of tissue, one of which is dense in texture, like ivory, and is termed compact tissue; the other consists of slender fibers and lamellæ, which join to form a reticular structure; this, from its resemblance to lattice-work, is called cancellous tissue. The compact tissue is always placed on the exterior of the bone, the cancellous in the interior. The relative quantity of these two kinds of tissue varies in different bones, and in different parts of the same bone, according as strength or lightness is requisite. Close examination of the compact tissue shows it to be extremely porous, so that the difference in structure between it and the cancellous tissue depends merely upon the different amount of solid matter, and the size and number of spaces in each; the cavities are small in the compact tissue and the solid matter between them abundant, while in the cancellous tissue the spaces are large and the solid matter is in smaller quantity.

Bone during life is permeated by vessels, and is enclosed, except where it is coated with articular cartilage, in a fibrous membrane, the periosteum, by means of which many of these vessels reach the hard tissue. If the periosteum be stripped from the surface of the living bone, small bleeding points are seen which mark the entrance of the periosteal vessels; and on section during life every part of the bone exudes blood from the minute vessels which ramify in it. The interior of each of the long bones of the limbs presents a cylindrical cavity filled with marrow and lined by a highly vascular areolar structure, called the medullary membrane.

Periosteum.The periosteum adheres to the surface of each of the bones in nearly every part, but not to cartilaginous extremities. When strong tendons or ligaments are attached to a bone, the periosteum is incorporated with them. It consists of two layers closely united together, the outer one formed chiefly of connective tissue, containing occasionally a few fat cells; the inner one, of elastic fibers of the finer kind, forming dense membranous networks, which again can be separated into several layers. In young bones the periosteum is thick and very vascular, and is intimately connected at either end of the bone with the epiphysial cartilage, but less closely with the body of the bone, from which it is separated by a layer of soft tissue, containing a number of granular corpuscles or osteoblasts, by which ossification proceeds on the exterior of the young bone. Later in life the periosteum is thinner and less vascular, and the osteoblasts are converted into an epithelioid layer on the deep surface of the periosteum. The periosteum serves as a nidus for the ramification of the vessels previous to their distribution in the bone; hence the liability of bone to exfoliation or necrosis when denuded of this membrane by injury or disease. Fine nerves and lymphatics, which generally accompany the arteries, may also be demonstrated in the periosteum.

Marrow.The marrow not only fills up the cylindrical cavities in the bodies of the long bones, but also occupies the spaces of the cancellous tissue and extends into the larger bony canals (Haversian canals) which contain the bloodvessels. It differs in composition in different bones. In the bodies of the long bones the marrow is of a yellow color, and contains, in 100 parts, 96 of fat, 1 of areolar tissue and vessels, and 3 of fluid with extractive matter; it consists of a basis of connective tissue supporting numerous bloodvessels and cells, most of which are fat cells but some are marrow cells, such as occur in the red marrow to be immediately described. In the flat and short bones, in the articular ends of the long bones, in the bodies of the vertebræ, in the cranial diploë, and in the sternum and ribs the marrow is of a red color, and contains, in 100 parts, 75 of water, and 25 of solid matter consisting of cell-globulin, nucleoprotein, extractives, salts, and only a small proportion of fat. The red marrow consists of a small quantity of connective tissue, bloodvessels, and numerous cells, some few of which are fat cells, but the great majority are roundish nucleated cells, the true marrow cells of Kölliker. These marrow cells proper, or myelocytes, resemble in appearance lymphoid corpuscles, and like them are ameboid; they generally have a hyaline protoplasm, though some show granules either oxyphil or basophil in reaction. A number of eosinophil cells are also present. Among the marrow cells may be seen smaller cells, which possess a slightly pinkish hue; these are the erythroblasts or normoblasts, from which the red corpuscles of the adult are derived, and which may be regarded as descendants of the nucleated colored corpuscles of the embryo. Giant cells (myeloplaxes, osteoclasts), large, multinucleated, protoplasmic masses, are also to be found in both sorts of adult marrow, but more particularly in red marrow. They were believed by Kölliker to be concerned in the absorption of bone matrix, and hence the name which he gave to them osteoclasts. They excavate in the bone small shallow pits or cavities, which are named Howships foveolæ, and in these they are found lying. but the great majority are roundish nucleated cells, the true marrow cells of Kölliker. These marrow cells proper, or myelocytes, resemble i n appearance lymphoid corpuscles, and like them are ameboid; they generally have a hyaline protoplasm, though some show granules either oxyphil or basophil in reaction. A number of eosinophil cells are also present. Among the marrow cells may be seen smaller cells, which possess a slightly pinkish hue; these are the erythroblasts or normoblasts, from which the red corpuscles of the adult are derived, and which may be regarded as descendants of the nucleated colored corpuscles of the embryo. Giant cells (myeloplaxes, osteoclasts), large, multinucleated, protoplasmic masses, are also to be found in both sorts of adult marrow, but more particularly in red marrow. They were believed by Kölliker to be concerned in the absorption of bone matrix, and hence the name which he gave to them osteoclasts. They excavate in the bone small shallow pits or cavities, which are named Howships foveolæ, and in these they are found lying.

Vessels and Nerves of Bone. The bloodvessels of bone are very numerous. Those of the compact tissue are derived from a close and dense network of vessels ramifying in the periosteum. From this membrane vessels pass into the minute orifices in the compact tissue, and run through the canals which traverse its substance. The cancellous tissue is supplied in a similar way, but by less numerous and larger vessels, which, perforating the outer compact tissue, are distributed to the cavities of the spongy portion of the bone. In the long bones, numerous apertures may be seen at the ends near the articular surfaces; some of these give passage to the arteries of the larger set of vessels referred to; but the most numerous and largest apertures are for some of the veins of the cancellous tissue, which emerge apart from the arteries. The marrow in the body of a long bone is supplied by one large artery (or sometimes more), which enters the bone at the nutrient foramen (situated in most cases near the center of the body), and perforates obliquely the compact structure. The medullary or nutrient artery, usually accompanied by one or two veins, sends branches upward and downward, which ramify in the medullary membrane, and give twigs to the adjoining canals. The ramifications of this vessel anastomose with the arteries of the cancellous and compact tissues. In most of the flat, and in many of the short spongy bones, one or more large apertures are observed, which transmit to the central parts of the bone vessels corresponding to the nutrient arteries and veins. The veins emerge from the long bones in three places (Kölliker): (1) one or two large veins accompany the artery; (2) numerous large and small veins emerge at the articular extremities; (3) many small veins pass out of the compact substance. In the flat cranial bones the veins are large, very numerous, and run in tortuous canals in the diploic tissue, the sides of the canals being formed by thin lamellæ of bone, perforated here and there for the passage of branches from the adjacent cancelli. The same condition is also found in all cancellous tissue, the veins being enclosed and supported by osseous material, and having exceedingly thin coats. When a bone is divided, the vessels remain patulous, and do not contract in the canals in which they are contained. Lymphatic vessels, in addition to those found in the periosteum, have been traced by Cruikshank into the substance of bone, and Klein describes them as running in the Haversian canals. Nerves are distributed freely to the periosteum, and accompany the nutrient arteries into the interior of the bone. They are said by Kölliker to be most numerous in the articular extremities of the long bones, in the vertebræ, and in the larger flat bones.

Minute Anatomy. A transverse section of dense bone may be cut with a saw and ground down until it is sufficiently thin.

If this be examined with a rather low power the bone will be seen to be mapped out into a number of circular districts each consisting of a central hole surrounded by a number of concentric rings. These districts are termed Haversian systems; the central hole is an Haversian canal, and the rings are layers of bony tissue arranged concentrically around the central canal, and termed lamellæ. Moreover, on closer examination it will be found that between these lamellæ, and therefore also arranged concentrically around the central canal, are a number of little dark spots, the lacunæ, and that these lacunæ are connected with each other and with the central Haversian canal by a number of fine dark lines, which radiate like the spokes of a wheel and are called canaliculi. Filling in the irregular intervals which are left between these circular systems are other lamellæ, with their lacunæ and canaliculi running in various directions, but more or less curved (Fig. 73); they are termed interstitial lamellæ. Again, other lamellæ, found on the surface of the bone, are arranged parallel to its circumference; they are termed circumferential, or by some authors primary or fundamental lamellæ, to distinguish them from those laid down around the axes of the Haversian canals, which are then termed secondary or special lamellæ.

The Haversian canals, seen in a transverse section of bone as round holes at or about the center of each Haversian system, may be demonstrated to be true canals if a longitudinal section be made. It will then be seen that the canals run parallel with the longitudinal axis of the bone for a short distance and then branch and communicate. They vary considerably in size, some being as much as 0.12 mm. in diameter; the average size is, however, about 0.05 mm. Near the medullary cavity the canals are larger than those near the surface of the bone. Each canal contains one or two bloodvessels, with a small quantity of delicate connective tissue and some nerve filaments. In the larger ones there are also lymphatic vessels, and cells with branching processes which communicate, through the canalculi, with the branched processes of certain bone cells in the substance of the bone. Those canals near the surface of the bone open upon it by minute orifices, and those near the medullary cavity open in the same way into this space, so that the whole of the bone is permeated by a system of bloodvessels running through the bony canals in the centers of the Haversian systems.

The lamellæ are thin plates of bony tissue encircling the central canal, and may be compared, for the sake of illustration, to a number of sheets of paper pasted one over another around a central hollow cylinder. After macerating a piece of bone in dilute mineral acid, these lamellæ may be stripped off in a longitudinal direction as thin films. If one of these be examined with a high power of the microscope, it will be found to be composed of a finely reticular structure, made up of very slender transparent fibers, decussating obliquely; and coalescing at the points of intersection; these fibers are composed of fine fibrils identical with those of white connective tissue. The intercellular matrix between the fibers is impregnated by calcareous deposit which the acid dissolves. In many places the various lamellæ may be seen to be held together by tapering fibers, which run obliquely through them, pinning or bolting them together; they were first described by Sharpey, and were named by him perforating fibers.

The Lacunæ are situated between the lamellæ, and consist of a number of oblong spaces. In an ordinary microscopic section, viewed by transmitted light, they appear as fusiform opaque spots. Each lacuna is occupied during life by a branched cell, termed a bone-cell or bone-corpuscle, the processes from which extend into the canaliculi.

The Canaliculi are exceedingly minute channels, crossing the lamellæ and connecting the lacunæ with neighboring lacunæ and also with the Haversian canal. From the Haversian canal a number of canaliculi are given off, which radiate from it, and open into the first set of lacunæ between the first and second lamellæ. From these lacunæ a second set of canaliculi is given off; these run outward to the next series of lacunæ, and so on until the periphery of the Haversian system is reached; here the canaliculi given off from the last series of lacunæ do not communicate with the lacunæ of neighboring Haversian systems, but after passing outward for a short distance form loops and return to their own lacunæ. Thus every part of an Haversian system is supplied with nutrient fluids derived from the vessels in the Haversian canal and distributed through the canaliculi and lacunæ.

The bone cells are contained in the lacunæ, which, however, they do not completely fill. They are flattened nucleated branched cells, homologous with those of connective tissue; the branches, especially in young bones, pass into the canaliculi from the lacunæ.

In thin plates of bone (as in the walls of the spaces of cancellous tissue) the Haversian canals are absent, and the canaliculi open into the spaces of the cancellous tissue (medullary spaces), which thus have the same function as the Haversian canals.

Chemical Composition. Bone consists of an animal and an earthy part intimately combined together.

The animal part may be obtained by immersing a bone for a considerable time in dilute mineral acid, after which process the bone comes out exactly the same shape as before, but perfectly flexible, so that a long bone (one of the ribs, for example) can easily be tied in a knot. If now a transverse section is made the same general arrangement of the Haversian canals, lamellæ, lacunæ, and canaliculi is seen.

The earthy part may be separately obtained by calcination, by which the animal matter is completely burnt out. The bone will still retain its original form, but it will be white and brittle, will have lost about one-third of its original weight, and will crumble down with the slightest force. The earthy matter is composed chiefly of calcium phosphate, about 58 per cent. of the weight of the bone, calcium carbonate about 7 per cent., calcium fluoride and magnesium phosphate from 1 to 2 per cent. each and sodium chloride less than 1 per cent.; they confer on bone its hardness and rigidity, while the animal matter (ossein) determines its tenacity. Ossification.Some bones are preceded by membrane, such as those forming the roof and sides of the skull; others, such as the bones of the limbs, are preceded by rods of cartilage. Hence two kinds of ossification are described: the intramembranous and the intracartilaginous.

According to their structure bones are dividing into long bone, short bone, flat bone, irregular bone, pneumatized bone, sesamoid bone. Bone is covered by periosteum. An individual bone is composed of bones tissue, cartilage, fibrous connective tissue, and blood and nerve tissue. Because most of the bone is made of inorganic salts a bones seems nonliving, but the bone is a system of tissues that form together and the bones make the skeletal system.

On long bones, such as those in the arms and legs the large expanded portions are called epiphyses. These are the regions of the bones that articulate with other bones. The shaft of the bones between the epiphyses is called the diaphysis. Except of the articular cartilage that covers the epiphyes the entire bones is covered in a tough, vascular tissue called periosteum. Periosteum fibres interlock with fibres of tendons and muscles that are connected to the bone. The wall of the diaphysis is composed of a strong, tightly packed, and resistant to bending tissue called compact bone. The epiphyses, on the other hand, are formed mostly by spongy bone. Spongy bone is made of many small bone plates that have irregular interconnected spaces that help keep bones light but very strong. The compact bone in the diaphysis of long bones forms a tube or channel called the medullary cavity. This cavity is continuos through the length of the diaphysis and then fades into the spongy bone. The medullary cavity is filled with a special type of soft connective tissue called marrow.

Bones usually developed from separate centers of ossification termed epiphyses, and consist of cancellous tissue surrounded by thin compact bone. The medullary canal and the spaces in the cancellous tissue are filled with marrow. The long bones are not straight, but curved, the curve generally taking place in two planes, thus affording greater strength to the bone. The bones belonging to this class are: the clavicle, humerus, radius, ulna, femur, tibia, fibula, metacarpals, metatarsals, and phalanges.

Short Bones. Where a part of the skeleton is intended for strength and compactness combined with limited movement, it is constructed of a number of short bones, as in the carpus and tarsus. These consist of cancellous tissue covered by a thin crust of compact substance. The patellæ, together with the other sesamoid bones, are by some regarded as short bones.

Flat Bones. Where the principal requirement is either extensive protection or the provision of broad surfaces for muscular attachment, the bones are expanded into broad, flat plates, as in the skull and the scapula. These bones are composed of two thin layers of compact tissue enclosing between them a variable quantity of cancellous tissue. In the cranial bones, the layers of compact tissue are familiarly known as the tables of the skull; the outer one is thick and tough; the inner is thin, dense, and brittle, and hence is termed the vitreous table. The intervening cancellous tissue is called the diploë, and this, in certain regions of the skull, becomes absorbed so as to leave spaces filled with air (air-sinuses) between the two tables. The flat bones are: the occipital, parietal, frontal, nasal, lacrimal, vomer, scapula, os coxæ (hip bone), sternum, ribs, and, according to some, the patella.

Irregular Bones. The irregular bones are such as, from their peculiar form, cannot be grouped under the preceding heads. They consist of cancellous tissue enclosed within a thin layer of compact bone. The irregular bones are: the vertebra, sacrum, coccyx, temporal, sphenoid, ethmoid, zygomatic, maxilla, mandible, palatine, inferior nasal concha, and hyoid.  Surfaces of Bones.If the surface of a bone be examined, certain eminences and depressions are seen. These eminences and depressions are of two kinds: articular and non-articular. Well-marked examples of articular eminences are found in the heads of the humerus and femur; and of articular depressions in the glenoid cavity of the scapula, and the acetabulum of the hip bone. Non-articular eminences are designated according to their form. Thus, a broad, rough, uneven elevation is called a tuberosity, protuberance, or process, a small, rough prominence, a tubercle; a sharp, slender pointed eminence, a spine; a narrow, rough elevation, running some way along the surface, a ridge, crest, or line. Non-articular depressions are also of variable form, and are described as fossæ, pits, depressions, grooves, furrows, fissures, notches, etc. These non-articular eminences and depressions serve to increase the extent of surface for the attachment of ligaments and muscles, and are usually well-marked in proportion to the muscularity of the subject. A short perforation is called a foramen, a longer passage a canal.  

Development of Bone

Bone formation is due to the osteoblasts which are specialized mesenchymal cells. Osteobiasts secrete an intercellular substance, the osteoid, which consists initially of soft around substance and collagen fibers. Osteobiasts develop into osteocytes, the definitive bone cells. At the same time multinucleated osteoclasts develop, cells connected with resorbing and remodelling bone. We distinguish direct or Intramembranous ossification from Indirect or chondral ossification.

Intramembranous ossification, osteogenesis membranacea is the development of bone from connective tissue. The latter contains many mesenchymal cells which develop via osteobiasts into osteocytes. At the same time osteoclasts develop and collagen fibers also appear. The original bone is fibrous and it is subsequently remodelled into lamellar bone. The skull cap, the facial bones and the clavicles develop as membranous bones. Preformed cartilaginous skeletal parts are necessary for chondral ossKlcatlon, osreogenesis cartilaginea when they become replaced by bone. Growth is possible only as long as cartilage still remains. The prerequisites for replacement bone formation are chondroclasts, differentiated connective tissue cells, which remove cartilage and enable the osteobiasts to form bone. Two types of replacement bone formation are recognized endochondral and perichondral.

Endochrondral ossification begins within cartilage, and occurs near the epiphyses. Just before ossification begins the mass is entirely cartilaginous, and in a long bone, which may be taken as an example, the process commences in the center and proceeds toward the extremities, which for some time remain cartilaginous. Subsequently a similar process commences in one or more places in those extremities and gradually extends through them. The extremities do not, however, become joined to the body of the bone by bony tissue until growth has ceased; between the body and either extremity a layer of cartilaginous tissue termed the epiphysial cartilage persists for a definite period.

The first step in the ossification of the cartilage is that the cartilage cells, at the point where ossification is commencing and which is termed a center of ossification, enlarge and arrange themselves in rows. The matrix in which they are imbedded increases in quantity, so that the cells become further separated from each other. A deposit of calcareous material now takes place in this matrix, between the rows of cells, so that they become separated from each other by longitudinal columns of calcified matrix, presenting a granular and opaque appearance. Here and there the matrix between two cells of the same row also becomes calcified, and transverse bars of calcified substance stretch across from one calcareous column to another. Thus there are longitudinal groups of the cartilage cells enclosed in oblong cavities, the walls of which are formed of calcified matrix which cuts off all nutrition from the cells; the cells, in consequence, atrophy, leaving spaces called the primary areolæ.

At the same time that this process is going on in the center of the solid bar of cartilage, certain changes are taking place on its surface. This is covered by a very vascular membrane, the perichondrium, entirely similar to the embryonic connective tissue already described as constituting the basis of membrane bone; on the inner surface of this that is to say, on the surface in contact with the cartilage are gathered the formative cells, the osteoblasts. By the agency of these cells a thin layer of bony tissue is formed between the perichondrium and the cartilage, by the intramembranous mode of ossification just described. There are then, in this first stage of ossification, two processes going on simultaneously: in the center of the cartilage the formation of a number of oblong spaces, formed of calcified matrix and containing the withered cartilage cells, and on the surface of the cartilage the formation of a layer of true membrane bone. The second stage consists in the prolongation into the cartilage of processes of the deeper or osteogenetic layer of the perichondrium, which has now become periosteum. The processes consist of bloodvessels and cells osteoblasts, or bone-formers, and osteoclasts, or bone-destroyers. The latter are similar to the giant cells (myeloplaxes) found in marrow, and they excavate passages through the new-formed bony layer by absorption, and pass through it into the calcified matrix. Wherever these processes come in contact with the calcified walls of the primary areolæ they absorb them, and thus cause a fusion of the original cavities and the formation of larger spaces, which are termed the secondary areolæ or medullary spaces. These secondary spaces become filled with embryonic marrow, consisting of osteoblasts and vessels, derived, in the manner described above, from the osteogenetic layer of the periosteum.

Thus far there has been traced the formation of enlarged spaces (secondary areolæ), the perforated walls of which are still formed by calcified cartilage matrix, containing an embryonic marrow derived from the processes sent in from the osteogenetic layer of the periosteum, and consisting of bloodvessels and osteoblasts. The walls of these secondary areolæ are at this time of only inconsiderable thickness, but they become thickened by the deposition of layers of true bone on their surface. This process takes place in the following manner: Some of the osteoblasts of the embryonic marrow, after undergoing rapid division, arrange themselves as an epithelioid layer on the surface of the wall of the space. This layer of osteoblasts forms a bony stratum, and thus the wall of the space becomes gradually covered with a layer of true osseous substance in which some of the bone-forming cells are included as bone corpuscles. The next stage in the process consists in the removal of these primary bone spicules by the osteoclasts. One of these giant cells may be found lying in a Howships foveola at the free end of each spicule. The removal of the primary spicules goes on pari passu with the formation of permanent bone by the periosteum, and in this way the medullary cavity of the body of the bone is formed.

The number of ossific centers varies in different bones. In most of the short bones ossification commences at a single point near the center, and proceeds toward the surface. In the long bones there is a central point of ossification for the body or diaphysis: and one or more for each extremity, the epiphysis. That for the body is the first to appear. The times of union of the epiphyses with the body vary inversely with the dates at which their ossifications began (with the exception of the fibula) and regulate the direction of the nutrient arteries of the bones. Thus, the nutrient arteries of the bones of the arm and forearm are directed toward the elbow, since the epiphyses at this joint become united to the bodies before those at the opposite extremities. In the lower limb, on the other hand, the nutrient arteries are directed away from the knee: that is, upward in the femur, downward in the tibia and fibula; and in them it is observed that the upper epiphysis of the femur, and the lower epiphyses of the tibia and fibula, unite first with the bodies. Where there is only one epiphysis, the nutrient artery is directed toward the other end of the bone; as toward the acromial end of the clavicle, toward the distal ends of the metacarpal bone of the thumb and the metatarsal bone of the great toe, and toward the proximal ends of the other metacarpal and metatarsal bones.

Parsons groups epiphyses under three headings, viz.: (1) pressure epiphyses, appearing at the articular ends of the bones and transmitting the weight of the body from bone to bone; (2) traction epiphyses, associated with the insertion of muscles and originally sesamoid structures though not necessarily sesamoid bones; and (3) atavistic epiphyses, representing parts of the skeleton, which at one time formed separate bones, but which have lost their function, and only appear as separate ossifications in early life.

Epiphyses are found at the ends of long bones, whilst the shafts are called diaphyses. Pericnondrai ossification, which originates in the perichondrium. The epiphyslal disk (growth plate), which is necessary for growth in length, forms a layer between the epiphysis and the diaphysis. That part of the shall adjacent to the epiphysial disk is called the melaphysis and develops fiist on an endochondral basis. Within the epiphysial cartilage, the processes of ossification occur in separate zones. First, in the epiphysis is the zone of the capping, hyaline cartilaginous material, which has not been influenced by bone formation. Next to this area of 'resting cartilage' is the zone or cartilage cell columns, the growth zone. Here cartilage cells divide and so increase in number. The next layer, which lies nearer to the shaft, is the zone of large vesicular cartilage cells, in which calcification is occurring. This is contiguous with the zone of cartilage destruction, where cartilage is broken down by chondroclasts and replaced by bone-forming osteobiasts. A cartilage remnant persists, which enables endochondral bone and perichondral bone to be distinguished in the diaphysis. It Is secondarily replaced by perichondral bone. Endochondral bone is destroyed by the immigrant osteoclasts. In crease in thickness in the region of the diaphysis is brought about by deposition of new bony materal on the outer surface beneath the cellular layer of the periosteum. The bone marrow cavity becomes larger as a result of bone destruction. Hormones regulate all growth processes.

Bone and joint health is a concern for almost everyone over the age of 30. Studies show that bone density is determined by the mid-twenties for both men and women and that bone mineral loss occurs naturally as we age. This is also true for the building blocks that make-up our joints and connective tissue.

Flexibility: Stretching can increase flexibility. Bone strength: Weight-bearing exercise strengthens bones and helps prevent osteoporosis. Exercising on a regular basis can help build your bones, but the kind of exercise is what makes a difference. There are basically two kinds of exercise - aerobic and weight-bearing. Some aerobic exercises, like swimming and bicycling outdoors or on a stationary bike, are certainly good for you, but they don't do much for your bones.

The ones that help build and strengthen bone are the weight-bearing kind. Weight-bearing exercises, some of which can be aerobic, are those that force you to put weight, and therefore stress, on your muscles and bones. Weight-lifting walking, hiking, and step aerobic classes are all activities that require your muscles to work against gravity. For weights you can even use soup cans and water bottles.See the list below for more good examples of this kind of exercise. Weight-bearing exercises that may be right for you: gardening, stair climbing, chair exercises, tennis, walking, weight lifting erobics, dancing.

Ideally, you should do some kind of weight-bearing exercise on a regular basis. In addition, weight-bearing exercise stimulates the formation of new bone. Exercise strengthens the muscles that pull or tug on bones, an action that keeps bones strong. Exercise improves balance, strength, and co-ordination, which reduces the risk of falling and breaking a bone. Talk to your doctor about the best weight-bearing exercise for you. In general, exercise is a good thing. However, if you already have osteoporosis, or if you have any other medical conditions, some activities may not be good choices for you. Have a discussion with your doctor or other healthcare provider about the exercises and activities that would be best for you, especially before starting any new exercise programme.

When you start any new programme, start slowly and build gradually. If you develop any pain, check with your doctor or other health care provider immediately. Exercise to prevent falls. Exercise counts in fall prevention. You've probably heard about the benefits of exercise. It helps make your bones stronger, improves your overall health, and can even brighten your outlook. But did you know that exercise might reduce your risk of falling by improving your balance, muscle strength, and co-ordination? It may even help you avoid a serious injury if you do fall. Tips for developing an exercise programme that works for you

1. Talk to your healthcare provider before you start. While the right exercise program offers great benefits, the wrong exercises can lead to injury or serious illness. Discuss your exercise plan with your healthcare provider and keep in mind these special precautions:

2. Anyone more than age 40 should have a thorough medical exam before beginning an exercise programme. If a woman has a significant amount of bone loss, some exercises may actually increase her risk of fracture. For example, sit-ups and toe touches increase the risk of fracture in women with osteoporosis of the spine. A woman at high risk for heart disease may need a stress test before starting an exercise programme.

Tai chi (a popular exercise using gentle, slow movements to relax muscles): This improves balance, flexibility, and state of mind.

Weight-bearing, low-impact aerobic exercises such as walking, dancing, and climbing stairs: These increase muscle strength and co-ordination, improve balance, and make bones stronger, without putting too much stress on joints and muscles.

Consider one of the most natural forms of exercise - walking. This is an easy, effective way to strengthen muscles, increase bone strength, and improve overall health.