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April 2017
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WHAT IS THE SKIN?

The skin is an organ made up of various tissues which cover the human body, protect it and let it interact with the outside world.

The total surface area of an adult’s skin varies with age, height and sex, but it is an average of 1.5-1.8m², and the thickness of it differs in different areas of the body, but on average it is 1.5-2mm thick for an adult on the palm of the hands and feet, whilst the skin on the scalp can get up to 4mm thick.

The skin is the heaviest organ in the body. There are grooves, folds (interpapillary grooves and structural and movement folds) and protruding features (which may be temporary, like goose bumps, or permanent, like interpapillary ridges) present on its surface and their ‘pattern’ differs depending on various factors, such as elasticity, the skin’s attachment to the musculoskeletal system and the layout of skin appendages, that is, hair and nails. These latter features are closely connected to the skin and, together, they make up a functional anatomical entity. There are also orifices present on the skin which may be clearly visible (like the follicular ostium) or not visible to the naked eye (such as sweat pores).

The colour of the skin is the result of a number of components: the red component is caused by the blood circulating in the area under the skin; the grey component is related to the keratin structure in the stratum corneum; and the black component, which is quite accentuated, is caused by the presence of melanin.


Structure


The skin is made up of 3 types of tissue:


epidermis


dermis

(which make up the skin)


hypodermis

Epidermis

This is the part of the skin that has direct contact with the outside world, thus its main function is to protect. It is made up of 4 layers: basale, spinosum, granulosum and corneum, and the palms of the hands and soles of the feet have a fifth layer (lucidum). The epidermis grows up from the basale cells towards the stratum corneum through a process of keratinisation that lasts 3-4 weeks and which ends with the formation of keratin (packs of cells that create bumps on the skin’s surface).

Stratum basale or germinativum This is made up of cells set out in formation along the borderline with the dermis and which are responsible for the renewal of epidermis cells, which occurs through mitosis. Periodically, each cell divides into two and the newly born cells are pushed towards the surface and form the stratum spinosum.
Stratum spinosum This is formed by various congregations of cells which have many faces (polyhedral) and which are flatter compared to the cells in the stratum basale. These cells are also separated by an intercellular substance. As they mature, spinosum cells move towards the surface of the epidermis.
Stratum granulosum This is the transition stratum between the stratum spinosum and corneum. Its thickness varies from 1-4 rows of cells, although it is thicker on the palms of the hands and soles of the feet. It contains a substance (keratohyalin) which is indispensable in the process of keratinisation.
Stratum corneum This is the layer that has direct contact with the outside world and its thickness varies depending on the part of the body. Keratinisation finishes in the stratum corneum: the cells do not have nuclei here and the end products of skin renewal are removed as bumps which appear on the skin’s surface.

Dermis

The dermis is located under the epidermis, which it works very closely with because it sustains it, nourishes it and provides a place for the epidermis appendages, that is, glands and hairs. The dermis is 2-3mm thick and is made up of 2 parts: the papillary and reticular dermis. The former, which is made up of papillae and the subpapillary stratum, has a very active metabolic life because it is so close to the epidermis, whilst the latter can be considered as a supporting stroma. The dermis is made up of: collagen, which ensures the robustness of the skin, elastin, which makes it elastic, and the fundamental substance which is made from mucopolysaccharides which work like cement. Blood vessels, nerve fibres and skin appendages are also present in the skin, the latter being sweat glands, hair follicles, hairs, sebaceous glands and the hair muscles. There is a dense network of lymphatic vessels in the dermis too and they are directed towards the hypodermis. The epidermis and dermis are united via dermal papillae, that is, slightly pointed pieces of connective tissue that poke out of the dermis and penetrate the epidermis. The blood capillaries lead to the end of the papillae and they are the source of nutrition for the epidermis, which is not vascularised. The surface pattern of the skin depends on how the connective fibres are laid out and how thick they are, and this results in a precise papillae layout. The surface pattern of the skin is so different from one person to another that it is used to identify people, for example, with finger prints.

Hypodermis

This tissue is located under the dermis and it is quite fatty. The function of this tissue is to cushion and isolate, and it contains calories that can be used when we stop eating. The hypodermis is rich in nerves and blood vessels. The structure and development of the hypodermis depend on the different parts of the body, age, sex, nutrition and individual hormonal influences. Over time, this fatty tissue becomes thinner and it leaves the skin flaccid and wrinkled.

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