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Skin Biology

Introduction by Graeme Ward, Managing Director ActiveSignal Ltd

The main part of this article was written by an Indian writer called Malvi. Actually, that is all I know about him as he seems to like to stay anonymous. This is the best skin biology description that I have found. For Skin Marvel™ we are just as interested in the chemistry of the skin. It contains hundreds of millions of cells all of which are little factories, working away and communicating with each other. Two processes are particularly relevant. The ‘main ingredient’ of the skin is water. A new born baby’s skin is 70% water; an obese person’s is 40% water. Most of us are somewhere in between. Some of this water is inside the cells, helping the factory process, but most of it is in between cells. This water is sodium rich. The water inside the cells is potassium rich. There is a constant interchange of sodium and potassium through special channels in the cell membrane. Two potassium atoms are pumped into the cell and three sodium atoms are pumped out and this goes on at a speed of millions of exchanges every second. This is how energy is created; you could call it the spark of life. The other important process in the factory is the manufacture of collagen fibril. This is the fibrous protein that, in effect, holds our skin together. It is made by the body in six stages, the first four inside the cell after which it is excreted and then the final two stages are outside the cell. Wikipedia has a good description of this if you would like to know more.

We need the electrolytes (sodium, potassium and many others) and the amino acids and peptides that are used to make collagen. To enable this we have to maintain homeostasis, a proper balance between water and electrolytes, inside cells and outside, between sodium and glucose and between acid and alkali. Skin Marvel promotes homeostasis by sending millions of messages every second as it travels through your body. These messages stimulate the cells to release blocked and over conserved elements and make the system work as efficiently as possible. Apart from promoting a healthy skin, Skin Marvel can, if not stop, certainly slow down the process of ageing of the skin.

What Is Ageing?

Ageing is the process of growing old. It is a continuous time dependent and multifactorial phenomenon of reduction in size and number of cells and also reduction in the rate of many organic functions both at cellular and molecular levels. Skin is the largest and the most superficial organ of the body which is exposed to infection, disease and injury. It also shows the signs of ageing the earliest. Skin functions that have shown to decline are cell replacement, injury response, barrier function, sensory perception, immune and vascular responsiveness, thermoregulation, sweat production, sebum production and vitamin D production.

The Skin

Skin is the largest organ of the body in terms of surface area and weight. It covers all the external surface of the body. It protects the body, helps maintain a constant body temperature and provides sensory information about the surrounding environment. Of all the bodies organs skin is most exposed to infection, disease and injury.

Structure of the Skin

Skin consists of 2 main parts the superficial, thinner portion is the epidermis and the deeper thicker part is the dermis. Deep to dermis is the subcutaneous layer which is not a part of the skin. The subcutaneous layer serves as a storage depot for fat and contains large blood vessels which supply the skin with nutrients. It also contains nerve endings called Pacinian corpuscles that are sensitive to pressure.

Human skin diagram

Figure 1 – The structure of skin – diagram shows different layers of skin

Epidermis

It consists of 4 principle cells:

  1. Keratinocytes – They make up 90% of the epidermis layer of the skin and they produce the protein keratin. The protein protects the skin and underlying tissues from heat, microbes and chemicals. It also produces lamellar granules which release a water repellent sealant.

    keratinocyte

    Figure 2 – keratinocytes as seen under a microscope

  2. Melanocytes – They make up 8% of the epidermis. It produces a pigment melanin which contributes to the colour of the skin and absorbs damaging ultraviolet light.

    melanocytes

    Figure 3 – Melanocytes

  3. Langerhans Cells – They arise from red bone marrow and migrate to the epidermis. They participate in immune response against microbes that invade the skin and these cells are easily damaged by ultraviolet light.

    Langerhans cells

    Figure 4 – Langerhans Cells

  4. Merkel Cells – They are usually located in the deepest layer of the epidermis. These cells are in contact with the flattened process of a sensory neuron structure called tactile disc. Merkel cells and tactile disc together detect different aspects of touch sensation.

    Merkel cells

    Figure 5 – Merkel Cells

Layers of the Epidermis

Epidermis of the skin usually has 4 layers but few areas where exposure to friction is greatest e.g. fingertips, palms, soles, etc. have 5 layers. The layers are:

  1. Stratum Basale – It is the deepest layer of epidermis and has a single layer of keratinocytes. It is also known as stratum germinativum.
  2. Stratum Spinosum – It is superficial to the stratum basale. It provides both strength and flexibility to the skin. This layer consists of 8‐10 layers of keratinocytes.
  3. Stratum Granulosum – It is middle layer of the epidermis. It consists of protein called keratohyalin which converts tonofilaments into keratin. This layer consists of 3‐5 layers of flattened keratinocytes. Also present in the Keratinocytes are membrane enclosed lamellar granules which release lipid rich secretion. This secretion fills the space between cells of stratum granulosum, stratum lucidum and stratum corneum. They act as a water repellent sealant that helps retard loss of body fluids and entry of foreign materials.
  4. Stratum Lucidum – This layer is present only in those areas which are prone to friction i.e. in thick skin. It consists of large amount of keratin and thickened plasma membrane. The layer is made up of 3‐5 layers of flattened dead keratinocytes.
  5. Stratum Corneum – This layer consists of 25‐30 layers of flattened dead keratinocytes. These are continuously shed and replaced by cells from the deeper strata. This layer consists of mostly keratin and lipid secretions from lamellar granules which make this layer an effective water repellent barrier. Multiple layers of dead cells protect the deeper layers from injury and microbial invasion.

Layers of epidermis

Figure 6 – Layers of epidermis: [B] = Stratum Basale, [S] = Stratum Spinosum, [G] = Stratum Granulosum, [C] Stratum Corneum
Dermis

Dermis is composed mainly of connective tissues containing collagen and elastic fibres. Cells present in dermis include:

  1. Fibroblast – They provide the structural framework for many tissues and play a critical role in wound healing. They are also responsible for synthesizing the dermal proteins.

    Fibroblasts

    Figure 7 – Fibroblasts

  2. Macrophages – They are also called as big eaters as their role is to phagocytose cellular debris and pathogens.

    macrophages

    Figure 8 – Macrophages

  3. Adipocytes – They are cells specialized in storing energy as fat.

    adipocytes

    Figure 9 – Adipocytes

Based upon the tissue structure dermis is divided into 2 regions:

  1. Papillary region – It is a superficial part of the dermis. The surface area of the papillary region is greatly increased by small finger-like projection called dermal papillae. Some dermal papillae contain tactile receptors called corpuscles of touch or Meissner corpuscles these are nerve endings that are sensitive to touch. Also present in dermal papillae are free nerve endings which initiate signal which gives rise to sensation of warmth, coolness, pain, tickling and itching.
  2. Reticular region – It is the deeper part of dermis. In this region bundles of collagen fibres are interlaced in a net like manner. Adipose cells, hair follicles, nerves, sebaceous glands, sweat glands occupy the space between the fibres. Combination of collagen and elastic fibres in reticular region is responsible for providing the skin with strength, extensibility and elasticity.

Dermal Matrix Components

  1. Collagen - is a primary structural component of the dermis and the most abundant protein found in humans. It is responsible for conferring the strength and support to human skin.
  2. Elastin - it is a protein in the connective tissue that is elastic and allows many tissues in the body to resume their shape after stretching or contraction. It helps the skin to return to its original position when it is pinched or poked.
  3. Glycosaminoglycans - It is a constituent of the dermal skin along with collagen and elastin and is responsible for conferring the outward appearance of the skin. They are composed of polysaccharides, with repeating disaccharide units attached to a core protein. They exhibit the capacity to bind water up to 1000 times their volume. The most important of the glycosaminoglycan family is hyaluronic acid. Others include dermatan sulphate, chondroitin sulphate. They render normal skin plump, soft and hydrated and maintain proper water and salt balance. Hyaluronic acid is found at the periphery of collagen and elastin fibres and where these fibres intersect. It is found in all connective tissues and is produced by fibroblast and keratinocytes in the skin. Hyaluronic acid is localized not only in the dermis but also in the epidermal intercellular spaces.