SKELETAL SYSTEM: THE BONE TISSUE

 Skeletal system includes the entire framework of bones and their cartilages along with tendons and ligaments.

 A bone is composed of several different tissues such as osseous tissue, dense connective tissue, cartilage,  epithelium, nervous tissue and adipose tissue.  Osteology is the study of bone structures and their disorders.

 

Structure of bone:  A common bone consists of following parts

      Gerard J Tortora Derrickson B. Principles of anatomy & physiology. India: Wiley; 2014. 

Diaphysis: (Dia- growing between) It is the  long, cylindrical portion of bone, commonly known as shaft or body of the bone.

Epiphyses: 2 ends of the bone, the proximal end and distal end.

Metaphyses are the parts that lies between the diaphysis and epiphysis. The Metaphysis (singular) contains an epiphyseal growth plate of a  hyaline cartilage. This allows the diaphysis to grow in length.

At the age of 18-21 , this cartilage is replaced by bone known as epiphyseal line.

 Articular cartilage is a thin layer of hyaline cartilage which covers the part of the epiphysis where the bone articulate with the other bone. This cartilage reduces friction and absorbs shock at freely movable joints. 

Repair of articular cartilage is limited because it lacks blood vessels and perichondrium.

Periosteum is a tough connective tissue sheath and is associated with the blood supply that surrounds the bone surface where articular cartilage is absent. It is composed of outer fibrous layer of irregular connective tissue and inner osteogenic layer. These cells help in increasing bone width not length.

Periosteum also helps in protecting bone, repair of fracture, nourishes the bone and acts as attachment point for the ligaments. 

Medullary cavity: It is also known as marrow cavity. It is a hallow cylindrical cavity within the diaphysis filled with yellow bone marrow and blood vessels. This tubular design will provide maximum strength with minimum weight.

Endosteum, a thin membrane that lines te medullary cavity. It contains a single layer of bone forming cells and connective tissue.

HISTOLOGY OF BONE: bone or osseous tissue consists of abundant extracellular matrix that widely separated cells. It contains 15% of water, 30% collagen fibres and 55% crystallised mineral salts. Calcium phosphate is the most abundant mineral salt, it combines with other mineral salts to form crystals of hydroxyapatite. These crystals merge with the calcium carbonate and ions such as  magnesium, flouride, potassium and sulphate. This process of  formation of crystals and tissue hardening of bone tissue is known as Calcification. 

Types of bone cells: 4 types of cells

1. Osteogenic cells

2. Osteoblasts

3. Osteocytes 

4. Osteoclasts

Osteogenic cells are the unspecified bone cells derived from the mesenchyme. They are the only cells that undergo cell division and develop into osteoblasts. Osteoblasts are generally found in the inner layer of periosteum, endosteum and in the blood vessel canals of the bone. 

Osteoblasts are the bone building cells. The components required to build the extracellular matrix of bone are synthesized by the osteoblasts and initiate Calcification. The osteoblasts  with the extracellular matrix confine in their secretions to become osteocytes.

Osteocytes, the mature bone cells and maintain bone metabolism such as exchange of nutrients and wastes wit the bone. 

Osteoclasts are derived from the fusion of 50 monocytes and are found in the endosteum.  These cells participate in the resorption process by secretion of lysosomes and acids. The breakdown of  extracellular matrix of bone  is required for the healthy development, conversation and reconstruction of bone. Osteoclasts also helps in regulating blood calcium levels.

Osseous Tissue: Based on the size and distribution of the spaces in the bone tissue, classified as Compact bone tissue and Spongy bone tissue.

1. Compact Bone Tissue  is the strongest bone tissue and contains very less spaces. This tissue forms the diaphysis region of the bone. It provides protection and support to the bone. It resists the stress produced by the weight and movements. 

Osteons / Haversian systems are the structural units of compact bone tissue. An osteon consists of Central canal which is surrounded by  concentric lamellae (same as the growth rings of tree). The concentric lamellae are mineralised extracellular matrix circular structures that grow in diameter and are covered by blood vessels, lymphatics and nerves within the central canal. Between the concentric lamellae small spaces called lacunae are located which contains osteocytes. Lacunae extends as small canals called Canaliculi (small canals ) filled with extracellular fluid. The Canaliculi  connect lacunae with central canal, forming an intricate, miniature system of interconnected canals throughout the bone. This framework provides many  ways  for nutrients and oxygen for osteocytes and removal of wastes. Interstitial lamellae is the region present between the osteons and also have lacunae, osteocytes and canaliculi. 

Picture from Tortora G, Derrickson B. Principles of anatomy & physiology. India: Wiley; 2014.

              Perforating canals or volkmann's canals allows the blood vessels, lymphatic vessels and nerves from the periosteum to the compact bone and connect with the vessels of medullary cavity, periosteum and central canals. The entire outer and inner circumference of the shaft of a long bone are lamellae called circumferential lamellae, line the outer and inner surface respectively.

2. Spongy Bone Tissue : It is also called as trabecular or cancellous bone tissue, does not contain osteons instead it contains trabeculae ( irregular patterns of lamalle ) with macroscopic spaces. These spaces are filled with the red and yellow bone marrow. The bone marrow contains blood vessels that provide nourishment to the osteocytes.

Spongy bones forms the inner bone tissue of short, sessamoid and irregularily shaped bones while in long bones it forms the core of the epiphyses and forms a variable narrow rim bordering the medullary cavity of the diaphysis. Compact bone always covers  and protect the spongy bone tissue. 

Spongy bone tissue is light and reduces the overall weight on the bone. This light weight feature allows easy and ready movement when pulled by the skeletal muscle. The red bone marrow is protected by the trabecular lamellae.

 

References:

Tortora G, Derrickson B. Principles of anatomy & physiology. India: Wiley; 2014.