Introductory Anatomy: Bones

Dr. D.R.Johnson, Centre for Human Biology

Anatomists talk about both bone and bones. The former is a type of connective tissue made up of cells suspended in a matrix: the collagenous matrix in bone just happens to be heavily impregnated with minerals. You will learn about bone cells elsewhere, but here is a picture of a cast of one, just to prove they exist . This osteocyte has characteristic long processes which run through the bone putting it in touch both with other cells and with blood vessels and nerves. Bones are discrete organs made up of bone tissue, plus a few other things.
The main misconception about bones then, is that they are made up of dead tissue. This is not true, they have cells, nerves, blood vessels and pain receptors. Bone constituents, organic and inorganic matrix and cells all turn over at a fairly rapid rate. If we treat a bone with various solvents we can remove the inorganic matrix and leave the flexible collagen. Or we can burn a bone and leave a hard brittle residue.
The true structure of bone lies somewhere between these images. In tensile strength bone is rather like cast iron, although around 1/3 of the weight, in bending stress it behaves like steel, although only half as strong and in compression it can withstand the forces exerted by a running man (equivalent to a dead weight of 270kg). Even in standing the compressive force on the hip joint, which you might expect to be half the bodyweight on each side, is multiplied by a factor of around six by muscular pull, since we are not in equilibrium when standing.

Determination of shape

The shape and structure of bones is governed by many factors, genetic, metabolic and mechanical. Genetic determination of primary shape can be demonstrated by organ culture of bone rudiments, which subsequently grow into recognisable bones, i.e. roughly the finished shape in all major respects. Fine tuning is by muscular action. The muscles are active in utero, although it is difficult to isolate their effect at this stage. After birth, however, and up to adolescence there is a correlation between activity and growth. this is seen in reverse if we look at people who are bedridden, or who have paralyses (such as poliomyelitis).
Metabolic factors are also important: calcium, phosphorous, vitamins A,C and D and the secretions of the pituitary, thyroid, parathyroid adrenals and gonads are all involved. Dwarves and giants are controlled by aberrant hormones, but there is much variation in normal height. Absence of adequate supplies of vitamin D may lead to rickets, and absence of calcium in the diet to week bone liable to fracture.

Function

1. As a lever. The bones of the upper and lower limbs pull and push, with the help of muscles.
2. As a calcium store. 97% of the body's calcium is stored in bone. Here it is easily available and turns over fast. In pregnancy the demands of the fetus for calcium require a suitable diet and after menopause hormonal control of calcium levels may be impaired: calcium leaches out leaving brittle osteoporotic bones.
3. Protective? This is often quoted in books: in fact protection against outside forces is rarely needed, and if it is we usually wear a cycling helmet, or a crash hat, or a hard hat. Or sit in a very strong structure like a formula 1 carbon fibre tub or a Volvo. So the bone can't be that good. In practice these are exceeded by the almost continuous large forces exerted by our own muscles. Respiratory movements need ribs. If a thigh bone or a humerus fractures the pull exerted by the muscles, even though not in active use, will be enough to overlap or otherwise displace the broken ends and we need considerable force, traction, to reduce the fracture i.e. to un-overlap the bits so that they can be lined up. The force exerted by the masticatory muscles is sufficient to support the bodyweight.
4. As a marrow holder. This is secondary to production of maximum strength for minimum weight: the cavities produced in unstressed areas (like the holes in the tubes of a bicycle frame) are used for marrow, or in some places (mastoid) just for air storage. The saving is small in man but considerable in an elephant. Occurrence of bone in two main forms, compact and cancellous. Both can be seen in our old lady's vertebra. That section was produced like this. Around the outside is a layer of strong, hard, heavy compact bone. In the middle is a branching network of cancellous or trabecular bone which usually, like iron filings, follow lines of force. Marrow sits in the interconnecting cavities between these plates or rods of bone.

Origin of bone is again in two main forms. Some bone (in broad terms almost everything except the top of the skull) is preformed in cartilage - replacement or endochondral bone. Details will come in histology lectures. In the skull and one or two other places, however, bone forms direct in membranous connective tissue - membrane bone.
Look at the history of the skeleton to see why. Calcified skeletal tissues replaced silicacious in the Cambrian period, presumably because physiological changes either in the beasts or the oceans in which they lived allowed retention of Ca ions. Brachiopods, nautiloids, trilobites gradually converted. Later the first vertebrates had bony scales embedded in their skin - those around the mouth incidentally form the primitive basis of teeth. In some lines these scales fused to form bony carapaces. These carapaces are retained over our heads as skull vaults. Later the rest of the skeleton, vertebrae etc., which were cartilaginous also became bony. This explains the distribution and origins of membrane and cartilaginous bone. The surviving membrane bones, notably in the head and part of the clavicle (a later invention made up of 2 fused bones, one membranous one cartilaginous) are bits of dermal shield.
Whether in membrane or cartilage centres of ossification marked by the appearance of calcified matrix appear over a long period of time, some in embryonic life, others in fetal and yet others well into the postnatal growing period. Many bones ossify from one centre, others from a group, of which one, the primary centre of ossification, is usually central and early, and others, secondary centres, later and often peripheral.

Classification of bones

The skeleton is made up of many bones which change in proportion between man and his close relatives but are easily recognisable. The easiest way to classify bones is by shape.

Long bones Typical of limbs, and a good place to start. They consist of a central, usually hollow, tubular region, the diaphysis linked to specialised ends (epiphysis) by a junctional region (metaphysis). Look at the shaft first. Tubular, a bit like a bicycle frame tube. Galileo was the first to write sensibly about this, noting that a hollow tube was stronger, weight for weight than a solid rod, and that the dimensions had to be related to body weight rather than area: so the bones of an elephant have to be proportionally broader than those of a man. In some bones we can see adaptations for specific forces. For example the wing bones of vultures and other large birds have strengthening that makes them very like bridges: it is a sobering thought that the first vulture predates the first girder bridge by some millions of years. The diaphysis has layers of bone arranged like plywood for strength. The cavity is filled with bone marrow (red and active in children, yellow, fatty and inactive in adults). The shaft walls are made of compact hard bone, and thickest in the middle where forces are greatest. If these forces are too great the shaft may fracture. Young bones have less calcium and are pliable, so fracture raggedly and partially (greenstick): older bones will fracture transversely or spirally according to force applied. Fractures usually heal spontaneously, albeit rather slowly in some cases, but the broken surfaces need to be manipulated into the right place and may need to be held with casts, pins or wires.
Towards the ends of the shaft the marrow cavity tends to be wider and filled with trabecular bone, arranged along lines of force which has a skeletal function in its own right and supports the marrow.
The ends of the bone are specialised to allow growth with as little loss of strength as possible. The epiphysis permits this, but looks complicated. Lets go back to the fish long bone and try to understand the logic underlying the structure. Growth must occur both at the end of the shaft (A) and over the surface of the joint (b). The fish accomplishes this by ending the bone with a simple plug of cartilage. This works until the bone has to bear weight, when the large, floppy cartilage becomes an embarrassment. The first modification, seen in the chelonia (turtles and tortoises) is that the surface becomes curved (marrow cavities first seen in amphibians). The new radially arranged cartilage is still strong but less bulky. The next problem is when the end is no longer hemispherical: the structure is then less strong, and continual reshaping is necessary with growth. These problems can be overcome by producing a secondary centre of ossification, already mentioned, - forming another lump of strong bone in the epiphysis. This is popular, occurring in at least seven groups of land vertebrates. We see secondary centres at the ends of most long bones, often more than one per end if the shape is complex. These ultimately fuse with the main shaft in a process known as closure of the epiphysis. Which strengthens the bone but ends the possibility of growth. This is a useful for forensic/medical purposes, but does not occur in a very regular way.

Short bones

Short bones are found in the wrist and ankle, carpals and tarsals respectively. They have no shaft, as they do not increase dramatically in size in one dimension during growth, and tend to be cuboidal in shape. They are rather like a Malteser in construction, with cancellous bone in the centre and a hard outer shell of compact bone.

Flat bones

Flat bones like those of the cranium or the scapula are sandwiches of spongy bone between two layers of compact bone. They are usually curved, so we can refer to an inner and outer table with diploe between them. These diploe, especially in the skull, may become pneumatised, i.e. filled with air. A ring of facial sinuses around the nose may become infected, leading to sinusitis.

Irregular bones

Any bones which don't fit these arbitrary categories (bones of the face, vertebrae) are referred to as irregular.

Sesamoid

Sesamoid bones are interesting because they occur in tendon, especially where a tendon turns a corner, and is thus exposed to friction. We shall come across these again when we talk about muscles.

Surface markings of bone.

We can often glean clues about what is going on around a bone from its surface. In places, like joint surfaces, the bone will be covered with smooth articular cartilage. This falls off in preparation but leaves the underlying bone smooth too. Bone is constantly growing or being reshaped, and this takes place on the surface. At high magnification we can see, in a dried bone, what it was up to the point of death. This picture shows a hole for a blood vessel, a foramen. Around roughly half its diameter the collagenous bone is rough, the other half smooth. The rough is resorbing bone, being eaten by large osteoclasts which leave pits and the smooth is depositional, bone being formed. This indicates that the foramen was on the move as the bone grew. Other areas also show deposition and resorption: these would be building up and hollowing out respectively. On a macroscopic scale these effects can be seen as points of attachment to the bone - of ligaments, tendons or the fibrous insertions of muscles. All these structures transmit forces, and demand a well organised junction. Any part of this structure which has deposited calcium will appear as a bit of bone. Within the bone we often see rows of trabeculae or thick ropes of collagen, Sharpey's fibres running across the marrow cavity to insert in the cortical bone opposite. Blood vessels and nerves similarly have canals.

The various lumps for fixing things to have different names according to shape, usually derived from a dead language. There are lots of these, but common ones are:

• lumps and bumps
• process
• spine - if sharp
• tubercle - if rounded
• cornu - if horn shaped
• hamulus - if hooked
• crest - ridge
• line - low ridge
• depressions and holes
• sulcus - groove
• canal - tunnel
• foramen - hole
• fossa - depression
• articular surfaces
• facet - if small
• condyle - if rounded
• epicondyle - if near a condyle
• trochlea - if pulley shaped

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