The skin is the single largest organ that covers the entire surface of our body and consists of two layers mainly the epidermis and the dermis. Also beneath the dermis is the subcutaneous layer which is not part of the skin but separates the integument from muscles and bones, which will be included in the essay as tissue fibres are connected with those of the dermis.
The main functions of the skin are protection of the tissues and organs that lie beneath against bumps and shocks, prevents entry of microorganisms and controls entry of foreign materials, protects the body from harmful radiation from the sun, controls water loss and regulates heat loss from the body and transmits social and sexual signals to others. However this essay will demonstrate how some of these different functions of the skin contribute to the maintenance of homeostasis which is the maintenance of a constant internal environment.
The epidermis of the skin consists of stratified squamous epithelium that is made of corneocytes, packed with keratin as a result of which the layers are tough and water resistant. Dominated by keratinocytes, thin skin has four layers which are stratum corneum (SC), stratum granulosum, stratum spinosum and stratum germinativum / basal layer while thick skin has an extra layer called the stratum lucidum which lies in-between the stratum corneum and stratum granulosum. The epidermis also consists of langerhan cells, melanocytes and merkel cells.
Since the epidermis has no blood vessels or fibres, nutrients diffuse from the blood vessels located in the dermis and wastes are carried away which only happen between the cells close to the dermis which is the basal layer. These cells undergo mitosis and they move away from the dermal blood supply through the epidermis layers towards the SC as they mature in the process of keratinisation. These cells die due to lack of nutrients and cornify replacing the cytoplasm of the cells with keratin.
Keratin is a highly hydrophilic material that can bind substantial amounts of water and are highly cross-linked proteins, which is why water diffusion through keratinocytes do not occur freely. When the cells are fully cornified at the stratum corneum, it can be taken apart through the process of desquamation. Adjacent Keratinocytes are locked together by desmosome rivets, acting as a physical barrier against pathogen and antigenic compounds from underlying tissues. They also keep the skin cells intact and prevent mechanical forces mposed on the skin. In between the cells of the SC are lipid lamellae which secrete free sterols, sphingolipids and compounds such as glycoproteins (Pugliese, 2001) and are arranged as a bilayer that permits hydrophilic and hydrophobic materials to pass through the intercorneal space. The lipids also block the diffusion of water preventing loss from and entry in to the body. But at the cornified layer that is the SC, the cells are attached by weak desmosomes that are broken down by enzymes hence, can be taken apart by desquamation.
Besides, getting rid of damaged cells and cells containing environmental pollutants and bacteria, the desquamation process is also a self cleansing mechanism. The basal layer also consist of melanocytes – cells that produce melanin in response to sunlight producing more melanocytes and melanosomes- packets by which melanin is transferred to keratinocytes. They are found at a ratio of about 1:4-1:10 melanocytes to basal keratinocytes. (Pugliese, 2001, p. 50) Melanin is produced as a protective chemical mechanism which responds to skin inflammation by increasing melanocyte activity thus producing more melanin in order to heal the area.
It protects the nucleus of living cells from ultraviolet (UV) damage such as langerhan cells. Langerhan cells are immune system cells that present antigen to lymphocyte at the lymph nodes. (Schrader et. al, 2005, I-46). UV rays diminish the number of langerhan cells in the epidermis and also their competence. The other main barrier function of the epidermis is to minimise transepidermal water loss which is, ‘the total amount of water vapour loss through skin and appendages, under non-sweating condition’ (Zhai et al, 2008, p. 6). This water that escapes through the SC into the air can’t be seen or felt unlike sweat.
The permeability barrier function is due to the organisation of the lipid bilayer which holds the water between the cell envelope, lipids and lipid bilayer. Another major component that helps this barrier function is the low levels of calcium ions and potassium ions present during an increased transepidermal water loss. They enhance lamellar body secretion and lipid synthesis. (Loden et. Al, 2006), Besides, the physical, mechanical, chemical and biological barriers of the epidermis, the components of the dermis also play a major role as the second line of the skin defence mechanism.
The dermis which is the supportive tissue layer of the epidermis has two divisions that is the papillary layer and the reticular layer and is connected to the basal cells of the epidermis by means of a structure called hemidesmosomes. The papillary layer has smaller collagen fibres, dense vascular elements and greater cellularity including sensory neurones as compared to the reticular dermis. The reticular dermis is connected by interwoven adipose tissue fibres from the subcutaneous layer, which keeps the skin stable in relation to underlying tissues.
The main cells of the dermis are fibroblast that produce enzymes, collagen, elastin and other matrix proteins. It also contains hair follicle, sweat glands and oil secreting sebaceous glands, the ducts of which open on to the surface of the epidermis. All these components play a chief role in maintaining homeostasis. Sebaceous glands, attached to the hair follicles, secrete sebum an oily substance keeping the epidermis supple and protects against bacteria. It is a component of the skin ‘acid mental’.
Very strong collagen fibres that resist stretching and flexible elastin fibres that permit it, enable the dermis to tolerate limited stretching without damage. In addition collagen fibres give the skin strength and ability to spring back when stretched. It binds water, helping to keep the skin hydrated. (Leslie, 2002, p8). The network of arteries bordering the reticular layer called the cutaneous plexus travel to the epidermis, their branches of which supply blood to the hair follicles, sweat glands, and other structures of the dermis.
On reaching the papillary layer, these arteries form the papillary plexus which provides atrial blood to capillary loops that follows to the epidermis-dermis boundary. The arteries and vein are contained in the subcutaneous layer that forms the cutaneous plexus and papillary plexus in the dermis. As heat is lost through radiation, convection and conduction this rich supply of capillaries plays an important role in the thermoregulation processes, vasodilation and vasoconstriction.
In warm conditions heat is lost to the surrounding as the diameter of the skin arterioles increase (vasodilation) thus increasing the blood flow to the capillaries and leading to heat loss through the SC. Also one of the two glands that is the eccrine gland, regulates body temperature by cooling the skin surface through evaporation of the sweat produced. Conversely, external cold results in a decrease in the diameter of the arterioles (vasoconstriction) and results in reduced blood flow to capillaries in the skin. Due to the extensive blood supply the skin also acts as a blood reservoir to hard working organ.
The vessels constrict sending more blood to the general circulation. Since the internal environment of the body functions within narrow parameters for example body temperature, ranging between 36. 5 – 37. 5 degrees Celsius, it is important that the organ system within the body functions to counteract the changes from the internal and external environment for the body to perform efficiently and normally. The components of the skin and their different functions and mechanisms mentioned above, all play an important role in the maintenance of homeostasis without which we would not be able to survive.