#13354 - Compare The Structure And Function Of Loose And Dense Connective Tissue - Organisation of the Body

Compare the structure and function of loose and dense connective tissue? How might these functions be compromised in disease?

The connective tissue consists of two main components; various cell types which are either migratory or resident and the extracellular matrix which is made up of fibres and macromolecules. This tissue can be subdivided into two main types depending on the relative amounts of cells, fibres and macromolecules found in the tissue. The first type is known as ‘loose connective tissue’ whereas the second type is known as the ‘dense connective tissue.’ The main structural difference between the two is that the loose connective tissue has many cells and loosely arranged fibres, whereas dense connective tissue is formed by a fewer cells but more numerous and thicker fibres. As a result of these differences the functions of both types of these tissues varies and therefore the amount and type of these connective tissues varies in different locations of the body.

Loose connective tissue is a category of connective tissue that includes areolar tissue, reticular tissue and adipose tissue. Areolar tissue is the main component that fills space in body parts and it acts as a supporting matrix for tissues that are normally under pressure or have low friction. The epithelium lies on a layer of loose connective tissue which supports it and also connects it to other tissues such as muscle. The main role of the loose connective tissue is that is provides a supporting matrix for blood vessels which supplements the epithelial tissue with nutrition, allows for the removal of waste and also enables the epithelial cells to fight infection. The loose connective tissue is also found in the hypodermis of the skin which allows the skin to move over the layer of muscle which lies underneath it. Another key role is that it sheaths lymphatic and blood vessels, muscle fibres and surrounds organs. The sheath forms a structure known as ‘fascia’ which in muscle creates a sliding and gliding environment whereas in blood vessels provides a supportive structure which also enables movement when blood passes through and also protects the pathway of blood.

The supportive function of the loose connective tissue is due to it being structurally delicate, weak and flexible. This main type of cells that are found within this tissue is fibroblasts and mast cells. Fibroblasts are resident cells with irregular branching and its main function is to secrete fibrous proteins and glycosaminoglycans that make up the extracellular matrix. In loose connective tissue, the main type of fibrous proteins that are synthesised by the fibroblasts are elastic fibres but a small amount of collagen and reticular fibres are also produced. Elastic fibres are made of the protein elastin which is made of subunits of coiled polymers of tropoelastin. The elastin fibres are then cross linked to form a 3D meshwork structure. Elastic fibres are formed when microfibrils of glycoprotiens attach to the elastin. Having a large proportion of elastic fibres allows the loose connective tissue to have elastic properties which enables it to recoil after stretching. This property is essential in blood vessels as it allows the wall of arteries to recoil after being stretched by high blood pressure. The elastic fibres also allow sheets of loose connective tissue to conform to the changing shapes of the gut. The importance of elastic fibres within the loose connective tissue is shown in the disease ‘Marfan’s syndrome’. In this disease there is a mutation in the gene coding for the microfibril glycoprotein ‘fibrillin’. As a result of this there is a loss of strength in the elastic fibres which means that the aorta has a greater tendency to rupture if there is a high blood pressure.

The other main class of cells found in the loose connective tissue are migratory and are involved in immunity. Machrophages that are found in the loose connective tissue are known as ‘hisitocytes’ and are a type of phagocytic cells which engulf and remove invading organisms. It is also releases peptide cytokines which are involved in initiating and down regulating infections. In response to pathogens, the macrophage also stimulates the entry of mast cells which synthesise and release molecules such as histamine. This chemical mediator, histamine, leads to an inflammatory response which includes increased capillary permeability and smooth muscle contractions. The mast cells also release signalling molecules such as ‘tumour necrosis a’ and this triggers white blood cells to rapidly move from the blood into the areolar tissue. The main function of neutrophils is to remove pathogens and this is down by the production of free radicals. As these cells are found in the loose connective tissue, another key function is that the loose connective tissue also plays a protective role as is protects tissues such as the epithelia from infections. In contrast to the loose connective tissue, the dense connective tissue is made up of a very small proportion of cells. The majority of its scarce cellular population is made of fibroblasts but it also contains a small amount of macrophages and mast cells.

Another type of loose connective tissue is the adipose tissue which is made up adipose cells. White adipose tissue stores triglyceride as a source of energy as each cell has a single large lipid droplet. During periods of starvation, the triglyceride is broken down to release energy. This type of tissue is found throughout the body. The other type of adipose tissue is brown adipose tissue and the cells that form this tissue contain smaller lipid droplets and numerous mitochondria. This type of tissue is mostly found in new born babies and makes up 2-5% of the body weight. When the lipid droplets are broken down heat is generated instead of ATP and this plays a crucial role in generating heat in a new born baby.

In comparison to loose connective tissue which has loosely arranged fibres, dense connective tissue is made of numerous and thicker fibres. The key function of dense connective tissue is resistance and protection which is why it is mostly located in the bone and the cartilage. The main component found in this tissue is collagen fibres. In loose connective tissue, collagen fibres are arranged in small, elongated bundles that are separated by regions that contain the ground substance. However the dense connective tissue is enriched in collagen fibres with little ground substance. Each collagen chain is made of 3 alpha chains...