A group of cells along with intercellular substances that perform a specific function is called tissue.
There are mainly four different types of tissues present in our body.
- Epithelial- provides covering or lining
- Muscular- helps in movement
- Neural- responds to stimuli
- Connective- supports, links and cushions
What is Connective Tissue?
Connective tissues, as the name implies, support and connect different tissues and organs of the body. They are widely distributed in every part of the body. They originate from the mesoderm (the middle germinal layer of the embryo).
Connective tissue is made up of a few cells present in the intercellular framework of protein fibres secreted by the cells, known as collagen or elastin. The cells also secrete a thin gel of polysaccharides, which together with fibres make matrix or ground substance.
The elasticity, flexibility and strength of the connective tissues are due to fibres. The function and types of connective tissues depend on the nature of the intercellular substance present.
Connective tissues contain three types of fibres: collagen, elastic and reticular
Collagen fibres are the most widespread and made up of fibrous protein, collagen. Collagen fibres are flexible and have high tensile strength (comparable to steel).
Elastic fibres form a network and can be stretched like a rubber band. They are made up of protein elastin. They retain their original shape and size once the force is removed.
Reticulate fibres consist of collagen and glycoproteins. They are thin and form a delicate network. They join connective tissues to neighbouring tissues.
There are various kinds of cells present in different types of connective tissues. They secrete different types of fibres and matrices. Fibroblasts or adipose cells are stationary and macrophages, mast cells, monocytes, lymphocytes are migrating cells.
Fibroblasts are found in developing tissues and play an important part in wound-healing. They are spindle-shaped and present between collagen fibres. They secrete tropocollagen and other substances found in the matrix.
Macrophages are also known as scavenger cells. They wander through connective tissues, clean up debris and remove bacteria and other antigens by phagocytosis.
Types of Connective Tissue
Connective tissues are divided into three groups:
Loose Connective Tissue
Loose connective tissues are present all over the body, where support and elasticity both are needed. Blood vessels, nerves and muscles, all have a loose connective tissue wrapping. They form the subcutaneous layer under the skin along with adipose tissues, attaching muscles and other structures to the skin.
The fibres and cells are loosely arranged in the semi-fluid matrix. They are found between many organs as a filling and act as a shock absorber and reservoir for salt and fluid.
Areolar Tissue: It is present under the skin and supports epithelium. It contains randomly distributed fibres, fibroblasts, mast cells and macrophages. It supports the organs present in the abdominal cavity, fills the space between muscle fibres and wraps around blood and lymph vessels.
Adipose Tissue: They are present under the skin and store fat. It acts as a shock absorber and helps in maintaining body temperature in colder environments.
White adipose tissues protect kidneys and are also found at the back of the eye, in the hump of camels, blubber of whales, etc.
Brown adipose tissue is found in infants, polar bears, penguins and other animals found in cold regions. It contains more mitochondria and generates 20 times more heat as compared to the other fat. It releases metabolic heat.
Reticular Connective Tissue: It is made up of reticular fibres. It supports the internal framework of organs such as liver, lymph nodes and spleen.
Dense Connective Tissue
In the dense connective tissue, fibroblast cells and fibres are compactly packed. Their main function is to support and transmit mechanical forces. They are somewhat less flexible than loose connective tissue. On the basis of the arrangement of collagen fibres, they are divided into two types:
Dense regular tissue: In this type of tissue, the orientation of fibres are regular. The collagen fibres are present between the parallel running bundles of fibres. The regular arrangement enhances tensile strength and poses resistance to stretching in the direction of the orientation of fibre. Examples of dense regular tissue are tendons and ligaments.
Tendons and Ligaments: Tendons attach bones to skeletal muscles. Ligaments attach two bones together.
Dense irregular tissue: There are many fibres including collagen, which are oriented irregularly or randomly. The irregular arrangement gives uniform strength in all directions. Fibres may form a mesh-like network. This type of tissue is present in the dermis of the skin.
Specialised Connective Tissue
Other than these, there are supportive connective tissues, that help in maintaining correct posture and support internal organs, e.g. cartilage and bone.
Blood and lymph are fluid connective tissues that circulate in the body and help in interaction and communication among all the organs.
Cartilage: Cartilage is mostly present in the embryonic stages and works as a supporting skeleton. Most of the cartilage is replaced by bones in adults, however, it supports some structures in adults too. In humans, cartilage is present between the bones of the vertebral column, in the external ear, nose and hands.
The cartilage consists of chondrocytes cells, which are enclosed in a hard, rubbery matrix, secreted by them. They secrete collagen fibres also, which provide additional strength. Chondrocytes lie in the cavities known as lacunae, in a group of 2-4 cells or singly. Cartilage possesses elasticity, but is firm too. They lack nerves, blood and lymph vessels.
Bones: Bone is the hardest connective tissue and helps in maintaining the shape and posture of the body, it protects internal organs. They are rich in collagen fibres and calcium, which give strength.
The cells of the bone are known as osteocytes. They are present in lacunae and secrete the matrix. There is substantial blood supply in bony tissues. The cytoplasmic extension of osteocytes makes tiny channels known as canaliculi. These channels help in communication among osteocytes and capillaries.
Spongy bone is present in the core surrounded by the compact bone. Osteons is the spindle-shaped unit present in the compact bone. Osteocytes are present in the concentric layers of the matrix in each osteon, called lamellae. Capillaries and nerves pass through a central channel known as Haversian canals. Haversian canals are surrounded by lamellae.
There is a central marrow cavity made up of spongy tissues (marrow). The yellow marrow contains fat, whereas red marrow produces blood cells.
Blood: Blood is made up of various cells present in the plasma. The blood contains red blood cells (RBCs), white blood cells (WBCs) and platelets.
RBCs have haemoglobin and transport oxygen.
WBCs form a defence system and protect from foreign antigens.
Platelets are important for blood clotting.
Plasma contains proteins, water, hormones, salts, etc. to transport to different parts of the body.
Lymph: Lymph drains into the blood and transports absorbed fat to the blood, which cannot enter the bloodstream directly. Lymph has white blood cells in the liquid matrix. They help in getting rid of toxins and waste materials. They contain WBCs, which help in fighting infection.
Connective Tissue Disorder
Connective tissue disorders may be due to mutation of genes or by inherited faulty genes. The two genetic disorders of connective tissue are epidermolysis bullosa (EB) and Marfan syndrome.
Marfan syndrome is due to defective genes producing a protein fibrillin-1. The disease is characterised by a very thin and long body. Fingers and toes are spider-like.
EB is characterised by skin oversensitivity.
The other type of connective tissue disorder is autoimmune. When the immune system of the body starts attacking healthy tissues, it is known as an autoimmune disorder.
Some of the autoimmune diseases of connective tissue are the following:
The systemic lupus erythematosus (SLE) – It is an inflammatory disorder of the skin and internal organs. The immune system of the body starts attacking its own tissues. Symptoms include fever, swollen and painful joints, mouth ulcers, hair loss, butterfly rash, etc.
Rheumatoid arthritis (RA) – It leads to the damage in joints and deformities. RA is due to the immune system attacking synovium, the membrane between joints.
Scleroderma- It is characterised by thickening and hardening of the connective tissue. It may be localised, only affecting a part of the skin or systemic, impacting vital organs.