Structure and function of skin
Skin is the largest organ in the human body and it is very important. It is our external cover and its basic function is to separate our bodies from the environment. Skin is a large organ; its area is calculated to be approximately 2m2 and its weight, 4kg, which is around 6% of our total body weight. In this post we will explain to you its main functions and we will get to know a bit more about its structure.
What are the main functions of skin?
- Protection: a barrier against mechanical, chemical or toxic attacks and against pathogenic microorganisms; it is a filter against ultraviolet radiation.
- Heat regulation: protection from the heat and cold, and maintenance of body temperature.
- Fluid electrolyte balance: it maintains the balance of body fluids, as it acts as a barrier against potential water loss.
- Sensory: through receptors and nerve endings it transmits touch, pressure, temperature and pain.
- Emotional: we externalise our emotional state through our skin; we blush, we turn pale, our hair stands on end and we give off smell (pheromones).
What is our skin’s structure?
From the most superficial to the deepest, it is distributed over three layers of tissue, with a different embryological origin:
- The epidermis:
The epidermis is a flat multi-stratified and keratinised epithelium that covers the surface of the whole body. It is the layer of skin with the largest number of cells and with an extraordinary capacity to regenerate. The cells grow from the interior (basal layer) to the exterior (stratum corneum). It has a variable thickness (from 0.1 mm to 1 or 2 mm on the palms of the hands and the soles of the feet).
Types of cells:
- Keratinocytes (90%): they produce keratin, which gives cohesion to the tissue and makes the dermis waterproof.
- Melanocytes (5-10%): they synthesise melanin, which is our natural pigment and protects us from ultraviolet radiation. Skin colour does not depend on their number, but rather on their size.
- Langerhans cells (2-5%): immune defence.
- Merkel cells: sensory cells.
- The dermis or corium:
It provides a support structure to the skin and provides it with resistance and elasticity. It is basically formed of fibroelastic connective tissue. The extracellular matrix contains a high proportion of fibres, which are not very compact: collagen (>75%), elastine and reticulin. It is a vascularised tissue that functions as a support and nourishment for the epidermis. It constitutes the greatest mass of the skin and its maximum thickness is around 5 mm.
Here we will find nerve endings responsible for touch and pressure.
Skin annexes:
- Hair follicles: they basically contain hair, which is based on a keratinised structure that settles from an epidermal invagination to its depth (dermal papilla), where it receives cell nutrition.
Follicular growth phases: anagen (90% of hairs are in this phase): growth, 2-5 years; catagen: regression, 2-5 weeks; telogen: growth, 2-5 months.
The infundibulum discharges the content excreted by the glands:
- Sweat glands: they control temperature, excreting water and other substances (salt, ammonia, uric acid, urea, lactic acid). They also possess an odoriferous function, since they secrete a substance that gets contaminated quickly and generates our characteristic smell.
- Sebaceous glands: they produce lipids that help maintain the external protective layer.
- The subcutaneous tissue, also called the hypodermis or subcutis:
It is composed of a loose connective tissue and is fastened like an anchor. When these bridges are lost, it gives way to the typical wrinkles. It is formed by adipose tissue, through which vessels and nerves run. The subcutaneous tissue is used to store energy, as well as thermal insulation and a mechanical protector.
How does a dermatological treatment penetrate the skin?
Given the structural complexity of skin, we must be aware that all topical treatments and their active principles only penetrate the epidermis, specifically the more external layers (stratum corneum), and they do not pass the dermoepidermal junction nor the basal layers. This does not mean that they have no effect, because the nutritional and support contribution is very significant for these layers with higher cell replicability. With this they allow these cells to grow in a healthier manner and not suffer oxidative stress, a basic step for the dermatological effect.
In any case, if through the content or the action of the product we want it to act in deeper layers, we must use invasive techniques (injections, for example).
