Connective tissue, also called supporting tissue is one of the four basic tissue types that fill the spaces between organ and tissues and provides them with structural and metabolic support. This tissue type is the most abundant, widespread and varied of all tissue types in the body.
Connective tissues are composed of three basic components:
- Cells
- Ground substance
- Fibres (Collagen, reticular, elastic)
The connective tissue cells are derived from precursor cells in primitive supporting tissue called mesenchyme.
They are widely separated from each other than in other tissue types (loosely packed within the extracellular cellular matrix).
The common cell types, found in connective tissues are fibroblasts, adipocytes and cells of the immune system include mast cells, macrophages, all types of WBCs and plasma cells.
In addition to those cells, specialized connective tissues contain specialized cells.
For instance, cartilage contains chondroblasts and chondrocytes; bone tissue contains osteoblast, osteocytes and osteoclasts; blood contains erythrocytes (red blood cells), white blood cells and thrombocytes (platelets).
Fibroblast cells are the most common cell type, which secretes fibrils and other elements of the extracellular matrix.
They are ovoid or spindle- shaped cells with pale staining cytoplasm.
Ground substance is a colourless, transparent material that fills the space between cells and fibres.
It is a complex mixture of glycoproteins and proteoglycans. Depending on the function and position of the tissue, the consistency of the ground substance varies from liquid to gel-like to a solid (calcified).
The combined mix of fibres and ground substance is called the extracellular matrix. It determines the physical properties of the tissue.
Connective tissue fibres are long, slender protein polymers.
There are two types of fibres, found in connective tissues; collagen (including reticulin, which was formerly considered a separate fibre type) and elastin.
Collagen fibres are the most abundant fibre type, which made of collagen type I.
They are strong, flexible parallel fibres, which resist stretch and provide mechanical support to the tissues.
Collagen fibres are found in the reticular dermis of the skin, ligaments, tendons, aponeurosis, dura of the brain and the capsule of organs.
Classification
Connective tissue can be divided into two types based on the cells present in the tissue and structure of extracellular matrix:
- Connective tissue proper
- Specialized connective tissue.
Connective tissue proper further divide into loose connective tissue and dense connective tissue. There are two subtypes of dense connective tissue based on the arrangement of the fibres; dense regular and dense irregular.
Specialized connective tissue includes adipose, blood, bone, cartilage and lymphoid tissue.
Connective Tissue Proper
Loose connective tissue
Loose connective tissue also called areolar connective tissue is distributed generally through the body.
The matrix is semisolid with many fibroblasts and some adipocytes, mast cells and macrophages widely separated by most abundant collagen fibres and few reticular fibres.
Fibroblasts produce fibres in connective tissue; macrophages phagocytes bacteria, dead or dying cells; mast cells produce heparin and histamine.
Locations:
- The submucosa of the respiratory and digestive tract.
- Surrounds all blood vessels and nerves
- Fill spaces between organs
- Papillary dermis
Functions:
- Support, packing, repair, protection of nerves, lymphatic and blood vessels.
- Defence against the invasion of foreign bodies.
- Diffusion of O2, nutrients and metabolites.
Dense Connective tissue
This tissue type contains more fibres and fewer cells than loose connective tissue.
According to the arrangement of fibres, there are two subtypes of dense connective tissue: dense regular connective tissue & dense irregular connective tissue.
Dense regular connective tissue
This tissue is composed of parallel, densely packed sheets, cords of fibres with little matrix and the cells are densely packed.
Location:
- Ligaments ( connect bones and supporting organs of the abdominal cavity)
- Tendons ( attach muscles to bones)
- Aponeurosis (which links muscles)
- Dura of the brain
- Capsules of organs (spleen, adrenal gland)
Dense irregular connective tissue
Dense irregular connective tissue contains collagen fibres randomly interwoven, forming a mesh- like network resistant to distension in all direction.
Location:
- Skin ( reticular dermis of the skin)
Specialized Connective Tissue
Adipose tissue
Adipose tissue is a specialized connective tissue to store neutral fat (triglycerides). This tissue is composed of fat cells (adipocytes), containing large fat globules, in a matrix of areolar tissue.
There are two types of adipose tissue depending on how lipids are distributed within the cell:
- White adipose tissue
- Brown adipose tissue
White adipose tissue
This type of adipose tissue makes up 20–25% of body weight in adults with a normal body mass index but can reach more than 50% in obesity.Â
This contains adipocytes, rounded cells with a single lipid droplet in the cytoplasm. The gross appearance of white adipose tissue is yellow coloured.
Location:
- Loose connective tissue
- Surrounding organs (kidney, adrenal glands)
- Subcutaneous tissue (deep layers of the skin)
- Breast
Function:
- A storage site for lipids
- Surrounds and protects internal organs from shock
- Invoves in in body temperature regulation.
Brown adipose tissue
This variant of adipose tissue contains multiple fat droplets, surrounding the centrally positioned nucleus.
Brown adipose tissue has a more extensive capillary network than white adipose tissue. This tissue is rich in mitochondria, accounts for the brown colour of brown adipose tissue macroscopically.
Location:
- It is found particularly in newborn mammals and some hibernating animals.
- Only small amounts are found in human adults, mainly around the adrenals and around great vessels in retroperitoneal fat.
Function:
- It is specialized for the generation of heat
- Body temperature regulation.
Cartilage
Cartilage is a specialized connective tissue that a component of the skeletal system.
The cells of cartilage tissue are chondroblasts and chondrocytes. Chondrocytes produce a large amount of the extracellular matrix, composed of proteoglycan, collagen fibres and elastin fibres, and are isolated in small lacunae within the extracellular matrix.
The consistency of the matrix is semi-rigid. Cartilages are avascular, so the cells are nourished by diffusion through the matrix and it doesn’t contain nerves also. Â
There are three types of cartilage tissues based on their matrix characteristic; Hyaline, fibro and elastic cartilage.
Hyaline cartilage
This is the most common type of cartilage. In this cartilage, single or small aggregates of chondrocytes in lacunae embedded in an amorphous matrix of ground substance, reinforced by collagen fibres.
Location:
- Nasal septum
- Larynx
- Tracheal rings
- Articular surfaces
- Sternal ends of the ribs
Function:
- Resists compression and provide flexible supports.
- Articular surfaces – provide a smooth surface for joints to move over.
Fibrocartilage
Fibrocartilage has characteristics intermediate between cartilage and dense fibrous supporting tissue.
This is a tough, slightly flexible supporting tissue, which is composed of dense masses of white collagen fibres in a matrix similar to that of hyaline cartilage with widely dispersed cells.
Location:
- Intervertebral discs
- Some articular cartilages
- Pubic symphysis
- Ligaments
- In the connection of some tendons to bone.
Function:
- Withstand stresses
Elastic cartilage
Histological features of elastic cartilage are similar to hyaline cartilage. This flexible tissue is composed of numerous bundles of branching, yellow elastic fibres lying in the solid matrix with chondrocytes lying between the fibres.
Location:
- Pinna and external auditory canal
- Epiglottis
- Part of the laryngeal cartilages.
- Walls of the Eustachian tubes.
Function:
- It provides strength, elasticity and maintains the shape of certain structures, e.g. the pinna or lobe of the ear, the epiglottis, etc.
Bone
Bone tissue is a specialized connective tissue, consists of bone cells and an extracellular matrix, which is composed of type I collagen and calcium hydroxyapatite.
Cells in bone tissue are osteoblasts, osteocytes and osteoclasts. Osteoblasts are the cells that produce the bone matrix and osteoclasts are phagocytic cells, capable of absorbing the bone matrix.
Inactive osteoblasts called osteocytes. Osteoblasts along with osteocytes and osteoclasts involve in the remodelling of bones.
Bone tissue is classified into two types according to the density of the matrix ; compact bone and spongy bone.
Compact bone
Compact bone is made of closely packed osteons or Haversian systems, microscopic cylinders of bone matrix with osteocytes in concentric rings (lamellae) around central Haversian canals.
The central canals contain blood vessels, lymphatics and nerves. Osteocytes are located in spaces, between the rings of the matrix are called lacunae. The Haversian canals are interconnected with each other and the periosteum by Volkmann’s canals.
Location:
- Under the periosteum
- In the diaphysis of long bones
Spongy bone
Spongy bone also called cancellous or trabecular, looks rather like a sponge with its visible holes or cavities is less dense than compact bones.
Spongy bone made up of trabeculae, a three-dimensional latticework of bony processes.
The cavities in spongy bone often filled with redbone marrow and blood vessels.
Location:
- Ends of the long bones (epiphyses)
Blood
Blood is considered a specialized connective tissue that transports blood cells and dissolved substances throughout the body via blood vessels.
Blood consists of cells and a fluid matrix, called plasma, and no fibres.
The blood plasma, which is about 52% to 62% of the total blood volume in the body contains dissolved salts, nutrients, hormones, gases and waste products.
The blood cells, which make up 38% to 48% of the total blood are erythrocytes (red blood cells), white blood cells including neutrophils, eosinophils, basophils, monocytes and lymphocytes, and thrombocytes (platelets).
Each type of blood cell has specific functions.
Red blood cells are involved in the transport of oxygen and carbon dioxide.
White blood cells defend the body against infections, destroying pathogens by phagocytosis, the production of antibodies, or other chemical methods, and provide immunity to some diseases.
Platelets prevent bleeding; the process of blood clotting involves platelets. Â
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