WORLD OF SCIENCE

Te Awamutu Courier, Volume 59, Issue 4222, 11 December 1939, Page 3

WORLD OF SCIENCE

THE LATER GROWTH OF BONE. (By Prof. F. Goldby.) It was described in a former article how bone, superficially an unattractive material for study, took on a different aspect from the point of view of its formation. The changes which occur in this process are bewildering in their complexity and in the rapidity with which they succeed one another in the early weeks of embryonic life. They are, however, only a prelude to others, slower and less dramatic, which, during a long period of growth transform the bone of the infant into the mature adult structure.

A bone, after its first formation consists of a compact outer shell enclosing a mass of spongy texture.' In the meshwork of the sponge lie the blood vessels that nourish it. At its ends cartilage, or gristle, remain unossified. Such a bone will at first be very tiny, the thigh bone much less than an inch long, for example. In growing it is faced with th ree main problems—how to increase in length (and much of this increase must take place after birth when the ends of the bone are subject to the pressure and friction that will occur at joint surfaces), how to increase in girth or thickness, and how to model its shape and structure to suit the different functional needs of infancy, childhood and, finally, adult life. Of these problems that of growth in girth or thickness is the simplest and the main features of its solution were described in a previous article. Bone, because it is relatively rigid, cannot expand interstitially. Growth must depend, therefore, on the apposition of fresh lasers of bone from the out-

side, formed .actually by a membrane, the “periosteum,” with which all bones are surrounded.

So that the bone may be kept light but strong, simultaneous destruction of bone from the inside takes place, accomplished by osteoclasts, cells that are capable of eroding or digesting bone. Similar cells destroy the roots of the milk teeth so that they become loose and are shed. In this way the shaft of a bone is formed as a hollow tube, the cavity is filled with marrow. Owing to the method of growth, none or very little of the bony tissue present in the skeleton of an infant will remain as part of the adult skeleton. It will all have been removed and replaced by fresh bone during the period of growth, a considerable feat when one remembers that all the time Uiis replacement has been going on the bone is in active use and an esential part of bodily structure. It is as if a complex machine had to be kept running while radical modifications and replacements were being made in its design. The problem of growth in length is less easy to solve. Bones nearly always end in joints where they are in movable contact with other bones. Bone forming cells are exceedingly delicate and cannot survive the mechanical pressure and friction that is inevitable on a joint surface. No superficial apposition of bone on a surface is possible, and, in fact, at birth the ends of all the bones are wholly or Almost wholly cartilaginous. Cartilage has sufficient “give” in it to allow for growth from the centre, expanding outwards; for interstitial growth, in other* words. In this type of growth the growing cells are no longer on the exposed surface, but deep in the tissue where they are protected from mechanical injury, but where once more the problem of nutrition arises.

Cartilage contains no blood vessels; as it grows in size, once more the central parts become starved; once more calcium is deposited in this starved tissue; once more death ensues and the dead tissue is removed and bone deposited in its place. A secondary centre of. ossification has appeared in the cartilaginous, or “epiphysis,” of a long bone.

The bone now consists of a tubular shaft, with, at each end, masses of cartilage in which bone formation is spreading from the centre ( outwards in all directions. There must, how-

ever, be a region between this secondary centre of ossification and the shaft of the bone which is still cartilaginous,. a region which is protected, moreover, by the now bony epiphysis from the mechanical stresses to which surfaces at the ends are subjected. It is in a particularly favourable position for growth, and being cartilage, it can grow interstitially and so increase the length of the bone.

This is obviously not enough. If no more occurred we should be left with bones which were long enough, but in which the bone at the ends and in the shaft were separated by a wide gap filled with cartilage. To consolidate the bone the process of calcification of cartilage, its death, removal and replacement must be repeated. It can proceed from both ends to the cartilaginous gap, from the secondary centre of ossification in the epiphysis and from the primary ossification that has formed the shaft.

For many years the processes of destruction of cartilage and of formation of bone are just balanced by the growth of the cartilage in the gap; the end of the bone remains, therefore, separated from the shaft by a plate of growth cartilage, as it is called, and the bone increases steadily in length' but without any increases in the width of the cartilaginous gap.

At a time which varies with different bones the balance between cartilage growth and ossification becomes upset. Ossification continues, but the growth of cartilage slows down, so that finally, when the bone has reached its adult length, all the cartilage is removed and bone put in its place. The end of the bone has fused in firm bony union with its shaft, and growth in length has ceased.

The third problem, that of remodelling the shape and architecture of the growing bone, is naturally solved by methods similar to those which result in growth in thickness. Through the activity of osteoclast cells bone is constantly removed here, and by means of osteoblasts, deposited there. Quite radical changes in shape and structure are thereby effected, without at any time interfering with the active function of the bone. Nor do these processes ever cease, even in adult life. They become much slower, but when needed to repair the effects of disease or injury, they can be speeded up. Not only is the injured bone in a fracture removed and the

gap filled by now bone formation, but the irregularity is smoothed out by the balanced activity of osteoclas s and osteoblasts, so that in many cases a surprisingly perfect reconstitution of the original bone is brought about.

In time, however, as in all tissues, destructive processes begin to gain on the constructive. As we have seen, senile changes may appear at any period of life, even before birth, in tissues that are about to be replaced or which are no longer needed. It is only that in old age these changes become general, and the processes that make good the loss fall behind in the race. As far as the skeleton is concerned the thin brittle bones of senility are only the last phases in a., balanced process of growth, and decay which began many months before the infant was born.