Connective tissues

Introduction

Connective tissues are found underneath the epithelium (skin). They are called connective tissues  because, they help in connecting various parts of the body. Our body is a web of organs that requires coordination. Coordination is achieved by joining the different tissues, organs, and systems with the help of connective tissues. Connective tissue help in building  the alliance between muscles to bones, bones to bones, tissues to muscles and muscles to muscles. They form a matrix beneath the epithelial layer that can anchor various organs. Connective tissues are highly vascularised, and capable of recovering quickly from damage, therefore,thay are pivotal in tissue repair. The most common connective tissues in our body  includes blood, bone, cartilage and lymphoid tissues.

 

Classification of connective tissues

The primary classification of connective tissue depends upon its cellular composition and the toughness the tissues have. Basing this,   2 main types of connective tissues include, soft and specialized connective tissues. The connective tissues are made up of fibers and the fibers are categorized into collagenous fibers, elastic fibres, and reticular fibers.

 

 

Components of  Connective Tissue

1. Connective tissue cells

Unlike epithelial cells, cells of the connective tissues are thinly inhabited. Moreover, they are made up of the extensive extracellular matrix formed by  the protein fibres, glycoproteins, and proteoglycans. As a whole, connective tissue contains 3 principal  components: cells, fibres, and ground substance. Furthermore, the ground substance combine with the fibers to form the extracellular matrix.

2. Matrix or ground substance

The ground substance is a clear and aqueous liquid containing of glycoproteins and proteoglycans. Matrix is distributed in between the cellular and fibrous elements of the connective tissue. It displays a gel-like viscous consistency and is polyanionic. Depending upon the way the ground substance is distributed, it determines the permeability of the connective tissue layer to various solutes and proteins.

3. Collagenous Fibers

Collages can be loose or dense in nature. Based on the density of collagen fibers, they are classified into 2 types, loose and dense connective tissues. Loose connective tissues,  also called areolar connective tissue.Areolar tissues contains loosely arranged fibers, and are the most common type of collagenous connective tissue. On the other hand, dense connective tissues are  enriched with collagen fibers with sparsely distributed ground substance. The 2 type of dense connective tissues are the regular and irregular dense connective tissues. Regular tissues are characterized by the closely packed bundles of fibers in one direction, for example, tendons . On the contrary, cells in the irregular tissues scatter themselves in multiple directions as seen in the dermis.

4. Reticular Fibers

Reticular fibers or reticulin are thinly distributed fibers comprising type-III collagen secreted by reticular cells. Reticulin forms a meshwork of cells by  cross-linking to each other. They  form supporting frameworks in the liver, lymphoid organs, capillary endothelia, and muscle fibers.

5. Elastic Fibers

Elastic fibers are expandable network of proteins elastic and the  fibrillin. They form the organized  lamellar sheaths found in the endothelium (inner walls of arteries). They’re usually dense, regular and highly flexible tissues. In addition to lose and dense connective tissues some special connective tissues  such as reticular connective tissue, adipose tissue, cartilages, bones, and blood cells are also important.

 

Connective Tissue Proper- cell varieties

Fibroblasts are the most common type of connective tissue cells. They produce the collagen as well as the ground substance of the extracellular matrix. Fibroblasts synthesise enormous amount of protein that helps in building the connective tissue layer. Because of their contractile function; they are called myofibroblasts. Furthermore, the extracellular matrix of cartilage and bones are made up of  chondrocytes and osteocytes respectively. The macrophages are the connective tissue networks present in the mononuclear phagocyte system. Macrophages are derived from monocytes spreading across many organs in our body. The most common type of macrophages are the alveolar macrophages found in the lung, the microglia of the central nervous system, Kupffer cells of the liver and the reticular cells of the spleen. Macrophages are indistinguishable from fibroblasts, but can be recognized when they internalize large amounts of visible trace substances like dyes or carbon particles. Macrophages help in the phagocytosis of the foreign material along with their ability to identify and fight specific antigens. Mast cells are the granulated cells help in the cell mediated immune responses by releasing enzymes during the entry of allergens. Another essential type of connective tissue are  the adipocytes . They are also known as fat tissues . Two types of fat tissues are white adipose tissues and the brown adipose tissues. Adipocytes contain a centrally located nucleus with the cells that can grow up to 100 microns. They also contain a vacuole, and the nucleus. The 2 fundamental functions of adipose tissue are thermal insulation and energy provision in the form of reservoirs of fats.

 

Cartilage, blood and bone tissues

The cells of cartilages are called chondrocytes made up of tough fibers spread in between the sparingly distributed matrix . The cells in a cartilage are spread within spaces called lacunae. Perichondrium is the outer capsule of cartilage made up of highly fibrous, organized, dense connective tissues. Cartilaginous tissues  are found in the  ear lobe, tip of the nose and many other flexible parts. Cartilages found at the tip of long bones play an important role in the vertical  growth of children. They are subdivided into 3 types ; hyaline cartilage, fibrocartilage and elastic cartilage. Hyaline cartilage contains a matrix composed of type II collagen and chondromucoprotein. It is found in the nose, tracheal rings, as well as the joints (where the ribs enter into the sternum). On the other hand, fibrocartilage is characterized by the orderly arrangement of type I collagen fibers. They are found in the symphysis pubis, and the regions where tendons attach to bones. Elastic cartilage comprising very rich elastic fibers  made up of type-II collagen. It is found in the auricle of the ear and the epiglottis of the respiratory system. Furthermore, bone tissues are formed by osteocytes that forms the bulk of our body weight. Bones contain lamellar circles surrounding the growth plates. The innermost region of a long bone has space that is filled with bone marrow. The outermost layer is the periosteum –a thin covering layer of all bones. Blood is the only liquid connective tissue that connects every cell of our body as it travels to each and every corner of our body. Blood tissue is further divided into white blood cells, red blood cells and platelets.

 

Functions of connective tissues

The fundamental function of connective tissues is that, they support and connect 2 or more closely situated structures. They are otherwise divided into tendons and ligaments. The tendons help in attaching the muscles to bones, whereas ligaments are the flexible fibrous connective tissue help in connecting 2 bones or cartilages (in the form of a joint). Another important function of connective tissue is the protection of organs by their fibrous capsules. The strong bones help to  protect many visceral organs. Bones are the basis of skeletal system containing specialized cells. Blood participatesin the transportation of fluid, nutrients, waste, and chemical across the body. Connective tissues are inseparable from the immune system as they form many immune cells such as macrophages.

 

Further reading

1         

 Tissues in our body

2

Structure and functions of bone tissue

3

Adipose tissues

4

Epithelial tissues

5

Structure and functions of cells

 

Check your understanding

  1. The only liquid connective tissue in our body is................................

  2. Type of connective tissue that connects bone to muscles is........................

  3. Name any 2 parts of our body made up of cartilaginous tissue.

  4. Explain the components of connective tissues.

  5. What are the functions of Mast cells and where are they found in our body?

  6. Name any 4 specialized connective tissues in our body.

 

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