Structure and Function of the Skin (Integumental System and skin)
NB//
- Integumental means ‘covering’
- T he integumental system consists of the skin, associated glands and specialized skin structures ie. Nipples, nails, hairs etc
1. Describe the structure of the skin, including its surface anatomy, histological features and variations in different parts of the body.
Skin consists primarily of two layers, the Epidermis and the Dermis.
The Epidermis, or outer surface of the skin, is composed of stratified squamous epithelium. The epidermis can be divided into 5 layers, each containing several cell types which relate to the function of skin (more later ‘I can assure you!’). Starting from the external surface of the skin, the 5 layers are the;
- Stratum corneum
- Stratum lucidum
- Stratum granulosum
- Stratum spinosum
- Stratum basale
The thickness of the epidermal layer varies around the body from roughly 0.08mm to 0.5mm (or more simply, varies in thickness from glad wrap to paper towel). Areas such as the soles of the feet or palms of the hands have a thicker epidermal layer to compensate for the constant abrasion and stress they are subject to.
The Dermis, or inner surface of the skin, can also be divided into regions. These are the Papillary and Reticular Dermis. The dermis is responsible for carrying the blood vessels, lymphatics and nerves that innervate the skin. It is important to note that the epidermis is an avascular layer, however the terminal branches of the blood vessels are in close proximity to this layer (ie in the papillary dermal layer) in order to perform various physiological functions (which will be outlined soon). The dermis is also home to collagen and elastic fibres which give the skin toughness and strength.
The associated structures of the skin are also housed within the dermis. These include hair follicles, sebaceous and sudoriferous (sweat) glands.
Skin thickness at various regions around the body is primarily determined by the dermal layer. Although this varies greatly depending on location, there is a direct difference in dermal layer thickness between the Extensor and Flexor surfaces of the body. The flexor surfaces are developed primarily for discriminating touch, and as a result have a much thinner dermal layer than that of extensor surfaces.
(For diagrams see lecture notes week 4 and week 6)
2. Discuss the structure of hair follicles, sebaceous and sweat glands.
Apart from its epidermal and dermal layer, the skin also contains numerous specialized structures. Two of these include hair follicles and sebaceous glands. Together they form what is known as a Pilosebaceous unit.
In layman’s terms, a hair follicle is the part of the skin that grows hair. It consists of the tubular in-folding of the epidermis to contain the root of the hair. The various structures of the hair follicle include;
- Papilla at the base of the follicle
- Matrix around the hair follicle – a mixture of epithelia and melanocytes (pigment producing cells)
- A Root Sheath – the epidermic coat of the hair follicle and has an internal and external component
- A hair fibre that is continuous with the root sheath and itself has a cortex and medulla region
NB// Each hair follicle has a layer of cells at its base that continually divides, pushing overlying cells upwards inside the follicle. These cells become keratinised and die, like the cells in the epidermis, but here form the hair shaft that is visible above the skin. The colour of the hair is determined by the amount and type of melanin in the outer layer of the hair shaft.
There are also other structures that are associated with the hair follicle. They are;
Sebaceous Glands
- these are glands that liberate sebum which is an oily substance composed of fat, keratin and epithelial debris, directly into the hair follicle, where it is moved along the shaft to the surface of the skin
- Glands arise from thickened out-pushing from the side of the developing follicle near the dermis
- Central cells in the gland form oil droplets within the cytoplasm
Arrector pili muscle
- a tiny bundle of muscle fibres that help the hair to become perpendicular (ie stand on end). This is also known as piloerection. Also has a secondary function to squeeze sebaceous glands to secrete sebum into hair follicle
In addition to the Pilosebaceous unit, the skin also contains Sweat Glands.
Cells of the sweat gland secrete a substance consisting of water (mostly), electrolytes (namely sodium chloride NaCl which gives it a salty taste) and metabolites into the lumen of the gland. However, sweat glands also contain Myoepithelial Cells. These are specialized epithelial cells that contract to squeeze the gland and discharge the accumulated secretions – directly onto the skin surface which has thermoregulatory and antimicrobial functions.
3. Explain the functions of skin, including its role in sensation, heat exchange and defence against trauma and infection, and relate these to microscopic structure.
Skin has a number of physiological functions and these can primarily be related to its structure. The primary functions and mechanisms behind these include;
1. Sensation
As already mentioned, the skin contains nerves. These nerves can either innervate parts of the skin to perform a particular function (efferent signals from the nervous system) or transmit information back to the nervous system (afferent signals). As skin is in contact with the external environment, certain stimuli will activate receptors in the skin which transduces this signal to allow us to sense it. The types of receptors include –
Mechanoreceptors – receptors that fire when surrounding tissue experiences a mechanical force (ie pressure, touch, stretch etc.)
Thermoreceptors – receptors that are sensitive to changes over a range of temperatures
NB// Extreme temperature changes will be perceived as pain
Nociceptors – receptors that are sensitive to pain and this can be due to tissue damage, extremes in temperature, excess mechanical stimuli, inflammation
It should be noted that each type of receptor is anatomically distinct and this largely relates to their function.
2. Heat Exchange
As the skin has such a large surface area and is in contact with the external environment, it is a huge area for heat and water exchange ie thermoregulation. It does this via the neural innervation of the blood vessels and associated specialized structures of the skin.
Blood supply
Blood vessels in the body receive Sympathetic Nervous System stimulation and this therefore allows them to be innervated in response to certain stimuli. When core body temperature drops, the SNS stimulation results in the vasoconstriction of skin arterioles and arteriovenous anastamoses, redirecting blood back to the core of the body where it is needed.
When core temperature rises, vasodilation of skin arterioles and arteriovenous anastamoses causes increased blood flow and heat loss to the external environment.
Sweat
Acetylcholine released from sympathetic nerve fibres stimulates sweat secretion from sudoriferous glands. This sweat is secreted onto the skin surface allowing for heat loss. In brief, sweat absorbs heat so when it is eventually evaporated off the skin, it takes this heat with it, effectively cooling the body.
Piloerection
Although not very common in humans and more a residual response due to mammalian ancestry, when the body is cold, innervation of the ‘arrector pili’ muscle which is attached to the pilosebaceous unit causes the hair to stand on end. This results in air being trapped within the hairs in order to maintain body temperature.
3. Defence against Trauma and Infection
The skin acts as a barrier to infection and this is largely a function of its structure. Various protection mechanisms include;
Keratinised stratified epithelium
The outer layer of the epidermis, the stratum corneum, consists of a hard layer of flattened keratin cells, which provides a protective as well as a structural cover for all the underlying layers. Also acts as a waterproofing layer.
Low moisture content and low pH
The naturally low moisture content and pH of the skin surface provide an unfavorable environment for the growth and development of micro-organisms.
Antimircobial Secretions
Sebum and sweat secretions onto skin surface has antimicrobial activity
Normal microflora
The natural indigenous flora of the skin provides natural defence against invading organisms
Pigmentation
Melanin, produced in the Melanocytes of the basal layer in the epidermis provide protection against UV radiation
Subcutaneous Fat
Provides a cushion for the underlying structures and organs against trauma, but also provides insulation to prevent heat loss.
NB// Skin also has a major role in Vitamin D synthesis.
Energy absorbed from the skin is used to synthesize vitamin D which is necessary for cell division and bone calcification.