Histology of the Bone

By:Dr Mohammed Faez

Objective of The Lecture

• To know about definition of the bone.• To master the basic structure of

the bone.• To know about bone matrix and

bone cells.• Illustrate and differentiate the three

type of bone cells.• To know about the periosteum &

endosteum.

Bone

• Bone is a dense, semirigid, porous, calcified connective tissue forming the major portion of the skeleton.

• It consists of a dense organic matrix and an inorganic, mineral component.

Bone• Bone is a specialized

connective tissue composed of intercellular calcified material, the bone matrix, and three cell types: osteocytes, osteoblasts and osteoclasts

• All bones are lined on both internal and external surfaces by layers of tissue containing osteogenic cells endosteum on the internal surface and periosteum on the external surface.

Bone Functions

• Protects vital organs • Supports soft tissue• Movement• Mineral storage • Blood cell production

Microscopic structure of compact bone

• The structural unit of Compact bone is the osteon,or haversian system.

Each osteon• Is an elongated cylinder • Oriented parallel to the • Long axis of the bone.

Microscopic structure of compact bone

Osteon System: • A central (Haversian)

canal with concentric rings (lamellae) of bone matrix running lengthwise.

• Very strong!

Microscopic structure of compact bone

• Central, or haversian canal carries blood vessels and nerves to all areas of the bone.

• Canaliculi tiny canals that radiate outward from the central canals to each lacunae space.

• Volkmann’s Canals: canals that run at right angles to the central canals and perforate the shaft of the bone.

Microscopic structure of compact bone

Osteon

Central Canal w/ blood vessels, nerves

Lacunae w/ bone cells

Compact bone structure

Compact bone structure

Spongy Bone

• Spongy bone contains trabeculae and spicules giving it a honeycomb appearance.

• Trabeculae: are irregularly arranged and contain lamellae and osteocytes, but contain no osteons as they receive nutrients from the marrow tissue.

Spongy bone histology

Bone Matrix• 25% Water• 25% Protein or organic

matrix– 95% Collagen Fibers– 5% Chondroitin Sulfate

• 50% Crystalized Mineral Salts Hydroxyapatite (Calcium Phosphate) Other substances: Lead, Gold, Strontium, Plutonium, etc.

• Combination provides strength and rigidity– Laid down by osteoblasts

Bone Matrix

• If mineral removed, bone is too bendable• If collagen removed, bone is too brittle

Bone Cells

1. Osteoblasts: Bone generating cells “B” means building

2. Osteocytes: Mature bone cells, spider shaped and maintain bone tissue

3. Osteoclasts: Bone destroying cells “C” means chewing

Osteoblasts

• Osteoblasts are responsible for the synthesis of the organic components of bone matrix (type I collagen, proteoglycans, and glycoproteins).

• Osteoblasts depends on deposition of the inorganic components of bone.

Osteoblasts• Osteoblasts are exclusively

located at the surfaces of bone tissue, side by side, in a way that resembles simple epithelium.

• When they are actively engaged in matrix synthesis, osteoblasts have a cuboidal to columnar shape and basophilic cytoplasm.

• When their synthesizing activity declines, they flatten, and cytoplasmic basophilia declines.

Osteoblasts

Osteocytes

• Osteocytes, which derive from osteoblasts, lie in the lacunae situated between lamellae of matrix.

• Only one osteocyte is found in each lacuna.

• Lacunae: spaces occupied by osteocyte cell body

Osteocytes

Osteocytes

Osteoclasts

• Osteoclasts are very large and branched motile cells.

• Dilated portions of the cell body contain from 5 to 50 (or more) nuclei.

• Osteoclasts are derived from the mononucleated cells; (engulf bony material).

• Active osteoblasts stimulate osteoclast activity.

Osteoclasts

Resorption of bone• Ruffled border: where

cell membrane borders bone and resorption is taking place.

• H ions pumped across membrane, acid forms, eats away bone.

• Release enzymes that digest the bone.

Osteoclasts

Bone Resorption

Periosteum

• It consists of an outer layer of collagen fibers and fibroblasts.

• Bundles of periosteal collagen fibers, called Sharpey's fibers, penetrate the bone matrix, binding the periosteum to bone.

Periosteum

Endosteum

• It lines all internal cavities within the bone and is composed of a single layer of flattened osteoprogenitor cells and a very small amount of connective tissue.

• The endosteum is therefore considerably thinner than the periosteum.

Periosteum & Endosteum

• The principal functions of periosteum and endosteum are nutrition of osseous tissue and provision of a continuous supply of new osteoblasts for repair or growth of bone.

Periosteum & Endosteum