The Skin I: Structure

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These articles are meant to inform those who are interested in the formulation of cosmetics at home as hobbyists. They’re not meant for medicine students. Plus, they don’t provide any medical advice. The idea is just to give some basic information on the structure of skin, hair and nails that could be useful for those who are learning how to make cosmetics.

Role of the skin

The skin is the widest organ of our organism and has a fundamental role as barrier between “us” and the external environment, providing protection against external environmental aggressions. Moreover, it is involved in other important functions such as permeation, metabolism, thermoregulation and sensorial function. 

Structure of the skin

There are two main types of human skin: hairy and glabrous. The only glabrous skin of our body is located at the palms and soles, whereas the rest of our body skin comprises more or less big hair follicles. 

Schematic structure of the skin

The skin consists of several layers. From the deepest layer to the surface, we have:

  • Ipodermis: it’s the deepest layer of our skin and it consists of conjunctive tissue whose function is protecting and reserving fat. 
  • Dermis: the dermis is a thick layer composed of connective tissue and is permeable. It confers mechanical protection to our body. Here we can find vessels, nerve fibers, sweat and sebaceous glands as well as the hair follicles. 
  • Epidermis: the epidermis is the upper layer of our skin and is thinner than the dermis, to which it is connected through the dermo-epidermal  junction. The epidermis itself is divided in layers, from the bottom to the surface:
    • Stratum Basale or Stratum germinativum: here the keratinocytes, the cells composing the epidermis, are generated. New keratinocytes push the older ones towards the upper layer. 
    • Stratum Spinosum: here the keratinocytes undergo a first modification, by assuming a spiny appearance because of the attachment of desmosomes to their surface. 
    • Stratum Granulosum: here we find also granules of a material called keratohyalin.
    • Stratum Corneum: finally we are in the outermost layer of the skin, the one that we actually see. Here the keratinocytes don’t have neither nuclei nor organelles anymore, they are flattened and filled with keratin and they are therefore described as cornified. These cornified cells at some point will detach and be exfoliated from the skin, as the regeneration of keratinocytes goes on from the stratum basale and new keratinocytes are pushed towards the surface. 

Other cell types

The epidermis is not only made of keratinocytes. At this level we also find other specialized cells: the melanocytes, the Merkel cells, and the Langerhans’ cells. 

Melanocytes and the pigmentary system

The component that determines the color of our skin is the pigment melanin, which is produced in the melanosomes, which are organelles typical of the specialised cells called melanocytes. 

Melanins are actually polymers of two kinds: phaeomelanins (yellow or red) and eumelanin (brown or black). Eumelanins determine the skin color. The difference in the skin color of different ethnicities depends on the amount of produced melanin. Melanin production can also be triggered by environmental factors and behaviours, like the exposure to the sun. 

Langerhans Cells

Langerhans cells are dendritic cells that are involved in the immune response (they are antigen-presenting cells during skin infections). 

Merkel cells

Merkel cells are involved in the sensory function of our skin. They have synaptic contacts with the somatosensory nerves and they function as mechanical transducers, which means, they can convert the mechanical energy into electric signals in the peripheral nervous system. 

Epidermal Appendages

Eccrine Sweat Glands

The sweat glands are found almost everywhere in our body. They are formed by a duct, that extends from the surface to deep in the epidermis, where it is rolled up into a ball. The glands secrete sweat, a colorless liquid that gets released on the skin surface. The sweat glands are important in the temperature control of our body, as the sweat evaporation has a cooling effect. 

The sweat mainly consists mainly of electrolytes, urea, amino acids. 

Apocrine Sweat Glands

They are present in restricted areas of the organisms, mainly in the axillary, anal and genital regions; they secrete a more viscous sweat compared to the eccrine glands, that is initially odourless, before the intervention of bacteria.

Hair Follicles

The hair follicles are invaginations of the epidermis. At the bottom, we find the matrix, which produces the cells that then will undergo keratinization to form the hair and are pushed towards the surface. 

Each follicle has an activity cycles with an active phase (anagen) that lasts for 1-3 years, a transition phase (catagen) and a resting phase (telogen). In this phase, the matrix does not undergo mitosis anymore. The base of the hair is keratinized and once the follicle becomes active again, it is pushed up and shed (see the upcoming article about the structure of the hair for more info). 

Sebaceous Glands

Sebaceous glands secrete sebum and they are often associated with hair follicles. The cells of this gland accumulate lipid and at some point they disintegrate and release their content into the lumen of the gland. For this reason they are called holocrine glands. 

The human sebum is composed of glycerides, free fatty acids, wax esters, squalene, cholesterol esters, cholesterol. 

Skin Types

What is normal skin?

It’s not easy to have a universal definition of normal skin. A normal skin is not dry, not oily, not mixed, not pathological, not old. 

Skin types

Classically, skin types are known to be dry, oily, mixed and sensitive. A deeper characterisation of the skin types and of how they can somehow coexhist in the same person is provided by the Baumann Skin Type Indicator. 

In this approach, parameters are compared in couples to give four basic parameters to the skin, and the combination of these four parameters results in 16 different types of skin. 

The parameters that are anaysed and compared are: dry vs. Oily skin, sensitive vs. Resistant skin; pigmented vs. Non-pigmented skin; wrinkled vs. Tight skin. 

I suggest you to read the related paper (see References below), here I will just quickly list these parameters:

  • Dry skin or xerosis: dull color, rough texture, rigdes; desquamation resulting from disruption of balance in the stratum corneum components that provide protection and water retention (ceramides/fatty acids/cholesterol; natural moisturizing factor; aquaporin-3). 
  • Oily skin: characterised by excess of sebum production. 
  • Sensitive skin: sensitive skin is prone to develop some conditions like acne, rosacea, allergies. 
  • Resistant skin: the stratum corneum in this skin type is able to protect the skin from environmental aggressions. 
  • Pigmented skin: skin characterised by melasma, solar lentigos, ephelides, post-inflammatory hyperpigmentation. 
  • Wrinkled skin: skin aging is partially genetic and partially triggered by external factors (like UV radiation) and results in skin relaxation and formation of wrinkles. 

References

McGrath and Uitto (2010). Anatomy and Organization of Human Skin. In Rook’s Textbook of Dermatology (eds Burns, Breathnach, Cox and Griffiths)

The Skin (1982), In Harry’s Cosmeticology (eds. Wilkinson and Moore)

Couturaud (2009) Biophysical Characteristics of the Skin in Relation to Race, Sex, Age, and Site. In Handbook of Cosmetic Science and Technology (eds. Paye, Barel, and Maibach)

Romani et al. (2010) Langerhans cells and more: langerin-expressing dendritic cell subsets in the skin, Immunological Reviews, 234, 120-141

Abraham and Mathew (2019), Merkel Cells: A Collective Review of Current Concepts, International Journal of Applied Basic Medical Research, 9(1), 9-13

Baumann (2008), Understanding and Treating Various Skin Types: The Baumann Skin Type Indicator, Dermatologic Clinics 26, 359-373

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