In a vertebrate embryo the endoskeleton is at first, cartilaginous, which is replaced by bone in most adult vertebrates. In cyclostomes and elasmobranchs, the skeleton does not pass beyond the cartilage stage. In higher vertebrates the endoskeleton is largely bony.

Types of Bones:

Bone exists in cancellous and compact form.

(i) Spongy or cancellous or endochondral bone consists of small pieces joined together irregularly, the small spaces between the pieces contain bone marrow.

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(ii) Compact or periosteal bone is a hard, solid mass of bone without any apparent spaces.

Long Bone Showing Compact and Spongy Parts

The broad ends of long bones are made of spongy bone underlying a layer of compact bone. The shafts of long bones are made of compact bone enclosing a marrow cavity. The skull bones have a layer of spongy bone between two layers of compact bone called inner and outer tables.

Bone Formation:

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Bone is formed in two ways, by a replacement of pre-existing cartilage with bone, called cartilage bone, and secondly by direct ossification of mesenchyme or connective tissue without an intervention of cartilage, in which case the bone is known as membrane or dermal bone. In both types of bones the method of formation is similar.

1. In formation of cartilage bones known as endochondral bone formation, the original cartilage cells become arranged in rows and then they die. The matrix becomes calcified and invaded by blood vessels and connective tissue eroding the cartilage to make intercommunicating channels between bars of calcified cartilage. The connective tissue fibroblasts become differentiated into osteoblasts.

T.S. of Dermal Bone of Skull

The osteoblasts secrete ossein of bones in concentric layers around bars of cartilage. The original perichondrium becomes the periosteum. Several osteoblasts fuse to form new cells called osteoclasts, which reconstruct the forming bone and dissolve away unwanted portions.

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2. In formation of membrane bones mesenchyme cells collect and form an interlacing network and in between the cells of the network are present colla