The Vertebrate Skeleton

by Sidney H. Reynolds

With the exception of the clavicle[4] all the bones of the trunk and limbs, together with a large proportion of those of the skull, are preformed in the embryo in cartilage, and are grouped as cartilage bones; while the clavicle and most of the roofing and jaw-bones of the skull are not preformed in cartilage, being developed simply in connection with a membrane. Hence it is customary to draw a very strong line of distinction between these two kinds of bone; in reality however this distinction is often exaggerated, and the two kinds pass into one another, and as will be shown immediately, the permanent osseous tissue of many of those which are generally regarded as typical cartilage bones, is really to a great extent of periosteal origin. The palatine bone, for instance, of the higher vertebrates in general is preceded by a cartilaginous bar, but is itself almost entirely a membrane bone.

Before describing the development of bone it will be well to briefly describe the structure of adult bone and cartilage.

The commonest kind of cartilage, and that which preforms so many of the bones of the embryo, is hyaline cartilage. It consists of oval nucleated cells occupying cavities (lacunae) in a clear intercellular semitransparent matrix, which is probably secreted by the cells. Sometimes one cell is seen in each lacuna, sometimes shortly after cell-division a lacuna may contain two or more cells. The free surface of the cartilage is invested by a fibrous membrane, the perichondrium.

Bone consists of a series of lamellae of ossified substance between which are oval spaces, the lacunae, giving rise to numerous fine channels, the canaliculi, which radiate off in all directions. The lacunae are occupied by the bone cells which correspond to cartilage cells, from which if the bone is young, processes pass off into the canaliculi. It is obvious that the ossified substance of bone is intercellular in character, and corresponds to the matrix of cartilage.

Bone may be compact, or loose and spongy in character, when it is known as cancellous bone. In compact bone many of the lamellae are arranged concentrically round cavities, the Haversian canals, which in life are occupied by blood-vessels. Each Haversian canal with its lamellae forms a Haversian system. In spongy bone instead of Haversian canals there occur large irregular spaces filled with marrow, which consists chiefly of blood-vessels and fatty tissue. The centre of a long bone is generally occupied by one large continuous marrow cavity. The whole bone is surrounded by a fibrous connective tissue membrane, the periosteum.

The development of bone.

Periosteal ossification. An example of a bone entirely formed in this way is afforded by the parietal. The first trace of ossification is shown by the appearance, below the membrane which occupies the place of the bone in the early embryo, of calcareous spicules of bony matter, which are laid down round themselves by certain large cells, the osteoblasts. These osteoblasts gradually get surrounded by the matter which they secrete and become converted into bone cells, and in this way a mass of spongy bone is gradually produced. Meanwhile a definite periosteum has been formed round the developing bone, and on its inner side fresh osteoblasts are produced, and these with the others gradually render the bone larger and more and more compact. Finally, the middle layer of the bone becomes again hollowed out and rendered spongy by the absorption of part of the bony matter.

Endochondral ossification[5]. This is best studied in the case of a long bone like the femur or humerus. Such a long bone consists of a shaft, which forms the main part, and two terminal portions, which form the epiphyses, or portions ossifying from centres distinct from that forming the shaft or main part of the bone.

In the earliest stage the future bone consists of hyaline cartilage surrounded by a vascular sheath, the perichondrium.

Then, starting from the centre, the cartilage becomes permeated by a number of channels into which pass vessels from the perichondrium and osteoblasts. In this way the centre of the developing shaft becomes converted into a mass of cavities separated by bands or trabeculae of cartilage. This cartilage next becomes calcified, but as yet is not converted into true bone. The osteoblasts in connection with the cavities now begin to deposit true endochondral spongy bone, and then after a time this becomes absorbed by certain large cells, the osteoclasts, and resolved into marrow or vascular tissue loaded with fat. So that the centre of the shaft passes from the condition of hyaline cartilage to that of calcified cartilage, thence to the condition of spongy bone, and finally to that of marrow. At the same time beneath the perichondrium osteoblasts are developed which also begin to give rise to spongy bone. The perichondrium thus becomes the periosteum, and the bone produced by it, is periosteal or membrane bone. So that while a continuous marrow cavity is gradually being formed in the centre of the shaft, the layer of periosteal bone round the margin is gradually thickening, and becoming more and more compact by the narrowing down of its cavities to the size of Haversian canals. The absorption of endochondral and formation of periosteal bone goes on, till in time it comes about that the whole of the shaft, except its terminations, is of periosteal origin. At the extremities of the shaft, however, and at the epiphyses, each of which is for a long time separated from the shaft by a pad of cartilage, the ossification is mainly endochondral, the periosteal bone being represented only by a thin layer.