Biology Today - February 2018

Structure of mammalian bone

Mammalian bone consists of four parts :
(i) Periosteum : It is a thick and tough sheath of collagen that forms an envelop around the bone. Bundles of periosteal
collagen fibres, called Sharpey’s fibres, penetrate the bone matrix to provide a firm connection between the two. The
periosteum contains blood vessels and bone-forming cells, which produce new bone material.

Types of bone cells

Osteoblasts : Secrete collagen fibres and matrix (ground substance) of bone and are responsible for the calcification of the

matrix. They retain the ability to divide and communicate by thin cytoplasmic processes which form gap junctions.

Osteocytes : When an osteoblast is completely surrounded by the matrix, it is called an osteocyte. These are inactive bone cells

or bone forming cells. They are responsible for maintaining the matrix and can both secrete and resorb matrix.

Osteoclasts : Formed by fusion of monocytes, destroy bone matrix. They release lysosomes, organic acids and hydrolytic enzymes

to break down, the bone matrix.

Volkmann’s

canal

Haversian

canal

Lacunae

containing bone

cells

Two lamellae

of matrix

Lymphatic

vessel

Canaliculi

Artery

Vein

(A) (B)

Fig.: (A) T.S. of mammalian bone; (B) Magnified view of bone showing Haversian and Volkmann’s canal

(ii) Matrix : It is tough, containing both inorganic and organic substances. It consists of ossein protein and mineralisation occurs
by calcium phosphate salts. The Haversian canal, a characteristic feature of the mammalian bones, is present in the
matrix. It contains an artery, a vein, a lymph vessel, a nerve, and some bone cells packed in connective tissue. These canals are
interconnected by transverse channels called Volkmann’s canals. The matrix of the bone are present as layers called lamellae.
The lamellae are of four types: Haversian lamellae, interstitial lamellae, outer and inner circumferential lamellae.
(iii) Endosteum : It is present outer to the bone marrow cavity and comprises of white fibrous tissue and osteoblasts. Because of the
presence of osteoblasts in both periosteum and endosteum, bone can grow both from outside and inside,LHgrowth of bone is
bidirectional.
(iv) Bone marrow : In long bones, a cavity is present inner to endosteum called as bone marrow cavity which is filled with a soft
neurovascular connective tissue (areolar, adipose and blood vascular) called bone marrow. Bone marrow is of two types:

Yellow bone marrow

• Yellow coloured (due to predominance of adipocytes), inactive vascular tissue
  in the long bones.


• More fat cells are found.


• Produces blood corpuscles only in emergency, LH., excessive blood loss.


• With age, yellow bone marrow gradually replaces red bone marrow in the
  long bones.

Red bone marrow

• Red coloured (due to erythrocytes), active vascular tissue in
  the long bone.


• Few fat cells are present.


• Produces blood corpuscles and platelets.


• During fetal life and at birth it occurs throughout the skeleton.

Types of bone marrow