Bone Histology Tutorial

BONE HISTOLOGY TUTORIAL

OBJECTVES:

By the end of this tutorial students should know: 1. Basic classification of bone, based on its gross and microscopic structure. 2. Haversian system and its structure 3. Difference between spongy and compact bone, and woven and lamellar bone. 4. Types of bone cells. TUTORIAL OUTLINE

BONE • Bone is a variety of connective tissue characterized by deposition of mainly salts of calcium phosphate with collagenous fibers and a large amount of water in its matrix. • The three basic components of connective tissue i.e., cells, fibers and ground substance form the organic part of bone. • Calcium is the inorganic part of ground substance. • Bone is hard, its hardness is due to the inorganic minerals. Bone serves as a storage site for calcium and phosphate.

TYPES OF BONE There are three kinds of mature bone:

 Compact bone: also called dense bone or cortical bone.  Spongy bone: also called cancellous bone, trabecular bone or medullary bone.  Woven Bone.

STRUCTURE OF A BONE A long bone e.g.., the femur consists of: • Centre piece, the shaft (diaphysis) and a thickened head (epiphysis) at each end. • Bone is covered with a tough, strong membrane, the periosteum which is richly supplied with blood vessels. • Beneath the periosteum is a layer of compact bone which is thicker in the shaft than in the two heads. • The shaft encloses a hollow, the marrow cavity, which is lined with a thin soft membrane known as the endosteum. • The marrow cavity contains a soft tissue richly supplied with fat cells and blood corpuscles, the yellow marrow.

• The epiphysis of a long bone consist of spongy (or cancellous) bone covered with a thin layer of compact bone. • This is made up of bony bars (or trabeculae) arranged in such a way that they are able to resist any force which is applied upon the bone. • Between the bars are many tiny cavities filled with a red marrow which contains numerous red blood corpuscles in different stages of development

MOLECULAR STRUCTURE

MATRIX: • The matrix is the major constituent of bone, surrounding the cells. • It has inorganic and organic parts.

INORGANIC PART OF BONE MATRIX • The inorganic matter is mainly crystalline mineral salts and calcium, present in the form of Hydroxyapatite. • The matrix is initially laid down as unmineralized osteoid (manufactured by osteoblasts). • Mineralisation involves osteoblasts secreting Vesicles.

ORGANIC PART OF BONE MATRIX • The organic part of matrix is mainly Type I collagen. • Other factors present include: • glycosaminoglycains, osteocalcin and osteonectin. One of the main things that distinguishes the matrix of a bone from that of another cell is that the matrix in bone is hard.

MICROSCOPIC STRUCTURE OF COMPACT BONE • Under the microscope dense, compact bone shows a definite and a characteristic pattern of arrangement. • The ground substance of bone is arranged in concentrated layers (lamellae) round the small canals. • The canals called Haversian canals run parallel to the long axis (shaft) of the bone. • Haversian canals of different osteons are interconnected with one another and the marrow cavity via Volkmann's canals and contain a blood vessel, a nerve and a lymph vessel. • Volkmann's canals provide major route of blood vessels from marrow cavity to the Haversian canals. • Each Haversian canal is surrounded by concentric layers of bone matrix (called lamallae) and concentric rings of bone forming cells (osteoblasts). • Once bone cells have been completely surrounded by the hard bone matrix, they are called osteocytes. • The osteocytes are embedded in fluid-filled cavities within the concentric lamellae. • These cavities are known as lacunae and occur at regular intervals in these concentric layers of bone tissue. • The lacunae are connected to one another and to the Haversian canals by a system of interconnecting canals known as canaliculi. • Each Haversian canal, its concentric lamellae, lacunae with osteocytes and canaliculi forms a long cylinder and is called a Haversian system or osteon, the structural & functional unit of bones.

LOCATION OF BONES • Compact bone and spongy bone are found in specific locations. • In long bones, most of the thickness of the diaphysis is made of compact bone, with a small amount of spongy bone facing the marrow cavity. • The ends (epiphyses) of long bones, however, consist mostly of spongy bone covered with a shell of compact bone. • The flat bones of the skull have a middle layer of spongy bone sandwiched between two relatively thick layers of compact bone. STRUCTURES OF LONG BONE • The marrow surface of compact bone, and the spicules of spongy bone, are lined by an (often single) layer of cells called the endosteum (endosteal cells). • Like the periosteal cells, these endosteal cells are also osteoprogenitor cells, capable of becoming osteoblasts. (The two names periosteal cells and endosteal cells refer to their different locations, both function as osteoprogenitor cells).

SPONGY BONE • Spongy bone is composed of bone spicules, also called trabeculae, of varying shapes and sizes. • The spaces between the spicules are filled with marrow. • The composition of spongy bone (cells and matrix) is the same as that of compact bone. • In spongy bone, however, the lamellae of collagen are not arranged concentrically around a central canal, but run parallel to one another. • Osteocytes sit in lacunae between lamellae.

WOVEN AND LAMELLAR BONE • Bone can also be either woven or lamellar (layered). • Woven bone is weak, with a small number of randomly oriented collagen fibers, during periods of repair or growth. • After a break, woven bone quickly forms and is gradually replaced by slow-growing lamellar bone on pre-existing calcified Hyaline Cartilage through a process known as "bony substitution”. • Non-lamellar form of bone is normally found in rapidly growing areas of embryonic or developing bone, and in healing fractures. • Most woven bone is replaced by lamellar bone during remodeling. • The process of bone remodeling occurs continually in the embryo, fetus and the adult.

TYPE OF BONE CELLS • Osteoblasts secrete the collagen fibres and ground substance (matrix) of bone and are responsible for the calcification of the matrix. • When an osteoblast is completely surrounded by matrix, it is called an osteocyte. • Osteocytes are responsible for maintaining the matrix, and can both secrete and resorb matrix. • Another type of cell called the osteoclast is responsible for the resorption of bone. • These large, multinucleated cells arise from monocytes. They help to break down bone matrix.