Development of Skeletal System of animals
The skeletal system is a mesodermal derivative. Bone a always develops by a transformation of the embryonic or adult connective tissue.
When development of bone takes place in cartilage, the method of ossification is termed enchondral ossification.
When it occurs in membrane the process is termed intramembranous ossification but essentially the process is the same in both.
Bone matrix is deposited the activity of specialised connective tissue cells called osteoblasts.
The only difference between the two types of ossification is that the former occurs at first in cartilage which is destroyed – replaced and substituted by bone. Hence bones formed in this way are also known as substitution bones e.g. long bones of limbs. The flat bones of the skull are membrane bones.
DEVELOPMENT OF INTRAMEMBRANOUS OSSIFICATION
In an area where bone is going to develop, the mesenchymal cells are connected with one another by their processes and delicate bundles of white fibres run in all directions between them. In between the cells and bundles is a semifluid intercellular substance
At one or more central points, ossification begins by appearance of osteoblasts, which promptly deposit bone matrix in the form, of acidophil bars or spicules, which unite a meshwork of trabeculae. As the matrix is laid down some cells remain imprisoned in the meshes as osteoblasts or bone producing cells, which takes up calcium, salts from the blood and deposit the same in the network of acidophil bars, thus converting them into plates of bone. Thus Ossification starts from the centre and proceeds towards the periphery.
As this proceeds, the surrounding mesenchyme furnishes more and more of membrane for ossification and this results in an increase in the size of bone. The osteoblasts at the periphery arrange themselves in a continuous layer and these also produce parallel plates of bone at the periphery. The mesenchyme in the region forms a fibrous membrane enclosing the bone the periosteum.
Bone formed in this process has a spongy character and consists of irregular plates of bone, which branch and unite with one another and in the meshes is enclosed embryonic connective tissue rich in blood vessels forming the red marrow.
DEVELOPMENT OF ENCHONDRAL OSSIFICATION
Most of the bones in the body are preceded by a temporary cartilaginous model of the same shape as the definite bone and the chief peculiarity of the method is the preliminary destruction of the cartilage and its replacement by bone, which develops exactly as in intramembranous ossification. So bones formed by this method are called replacement or substitution bones.The bones of the appendicular skeleton are all substitution bones.
In the hyaline cartilage, which is to be substituted by bone, the cartilage cells enlarge in the centre and become arranged in characteristic radial rows and some lime is deposited in their matrix. The cartilage cells and part of the matrix then disintegrate forming primary morrow cavities. This destruction is accompanied by a simultaneous invasion of this region by the vascular connective tissue, rich in cells derived from the perichondrium.
These cells perform the function of osteoblasts of depositing lime salts in the cartilage matrix and this results in the formation of bars of calcified matrix. Bone thus formed is spongy in nature consists of network of bony plates enclosing the primary marrow spaces containing blood vessels and connective tissue corpuscles and this constitutes the red marrow. In a progressive manner the cartilage undergoes invasion, destruction and replacement by spongy bone.
This spongy bone at the centre is destroyed and absorbed by giant cells called osteoclasts. Thus a large marrow cavity is created in the centre of the bone. Simultaneously the perichondrium at the periphery functions as periosteum and deposits, by the activity in the centre and periosteal bone at the periphery, its destruction and absorption by osteoclasts at the centre goes on, resulting in increased dimension of the marrow cavity and circumferential growth of bone.
The centre of ossification appearing in the shaft of a long bone of the foetus in the primary centre of ossification. Sometime between birth and puberty, centres of ossification appear at the epiphyses also. At the junction of the shaft and the epiphyses there are two plates of cartilages, one above and the other below termed the epiphyseal cartilages.
These supply more and more of cartilage for the shaft and the epiphyses for formation of new bone and this process results in an increase in length of the bone. When the bone has reached its adult, size, both in its length circumference, the epiphyseal cartilages are ossified and further growth stops.
Reconstruction takes place in periosteal bone to form the Haversian systems and the outer and inner circumferential lamellae. The osteoclasts destroy and remove periosteal bone forming Haversian canals. Osteoblasts arrange themselves around them and deposit their exoplasm in concentric layers which are later ossified by deposition of calcium salts.
Some of the osteoblasts are enqulfed in the lamellae and occupy lacunae and canaliculi. Others withdraw and arrange themselves at the periphery and deposit periosteal lamellae. The cells towards the medullary cavity form the endosteal lamellae. Thus through reconstruction, compact bone is formed outside enclosing cancellated or spongy bone inside, around the medullary cavity.
DEVELOPMENT OF BONES OF THE SKULL
The earliest indication of the skull is a mass of dense mensenchyme enveloping the cranial end of the notochord and it extends forwards into the nasal region.
Laterally it becomes continuous with the neuro-cranium, the mesoderm covering and protecting the brain and interiorly with the mesodermal cores of branchial arches.
Chondrification sets in during the seventh week and it is confined chiefly to the base of the skull-the basioccipital and sphenoidal cartilages.
The chondrocranium hence refers to the base and neurocranium to the roof and lateral walls.
Ossification of chondrocranium begins during the third month and forms the occipital, sphenoid ethmoid and pelorus and mastoid parts of the temporal bone.
The parietals, quamous and tympanic parts of temporal, vomer nasal, lacrimal, malar are all developed in membrane.
The first branchial arch divides into an upper maxillary and a lower mandibular process.
The Meckel’s cartilage is the mesenchymal core of the – mandibular process which becomes enveloped by surrounding mesenchyme and finally disappears except at the proximal portion where it is transformed into the sphenomandibular ligaments, malleus and incus. The adult mandible develops in membrane.
The maxillary process undergoes similar degenerations and the mesoderm surrounding the original cartilage develops into the maxilla and palatine bones.
DEVELOPMENT OF VERTEBRAL COLUMN
The primitive axial support of all vertebrates is the notochord or chordadorsalis. It is a transient structure in mammals and consists of a cylindrical rod of mesodermal cells from the Sessel’s pocket to the tip of the tail.
The notochord degenerates soon except at the intervertebral discs within which it persists as the swollen mucoid nuclei pulposi.
The axial skeleton differentiates from the mesenchyme that traces its origin to serially arranged pairs of mesodermal segments – the somites.
The ventro-medial part of a somite develops into the sclerotome and cells proliferate from this mass and migrate towards the notochord. The sclerotomes are destined to from the vertebrae and ribs.
Each sclerotome presents and anterior loose and a posterior dense part.
The anterior part of a sclerotome fuses with the posterior part of the sclerotome in front of it, and these reconstituted sclerotomes form the primordia of the definite vertebra.
- From each of these sclerotomes growth takes place in three directions
- medially to surround the notochord to from the body,
- above to form the neural arch.
- ventrolaterally to provide costal processes or primordia of ribs.
Following this blastemal stage, chondrification sets in during the seventh week and ossification during the tenth week. When the atlas forms its body, it is soon appropriated by the axis as the dens.
In the thoracic region, the costal processes enlarge and from the ribs. Here the original union of costal process with the vertebrae is replaced by joints (costo-central and costotransverse articulations). Chondrification and ossification follow to form the ribs. The costal processes remain diminutive and are fused with the transverse processes in the cervical and lumbar regions.
The sternum arises from a pair of mesenchymal bands on the ventrolateral aspects of the body wall. These unite progressively from before backwards and obtain connection with the ribs and cartilages.
The mesenchyme of the sternum undergoes chondrification at nine weeks and ossification at about five months. The segmentation of the sternum is a feature secondarily acquired.
