The skin makes up 20% of the total body mass and is in fact the largest “organ system” of the human body. It protects the body against external germs, UV irradiation, prevents dehydration and preserves body heat via insulation and cools the body down via convection of heat and sweating. An integral part of skin are the hairs, sweat glands and apocrine glands that form part of it. Skin is in fact produced from the Stem Cells that reside in the Bulge area of Hair Follicles. Each hair follicle is attached to an Arector pilli muscle, which delivers Stem Cells via it’s blood supply to the hair follicle, making the Arector pilli muscle a critical component of hair and skin maintenance and restoration. The skin consist of several layers. The two basic layers are the Epidermis on the outside and then the Dermis which is located below the Epidermis. Skin thickness varies in different parts of the body and so also the various cell layers found within the Epidermis. For example, the skin of the eyelids are very thin and the skin under the foot is much thicker and this is the result of different functionalities required from skin in different parts of the body. In general the Epidermal layer is about as thin as a sheet of paper. Within the Epidermis there are five different layers based on cell differentiation. The bottom layer, Stratum Basalis is located just above the Papillary Dermis and rests on the Basement Membrane. The essential skin cells that produce the Epidermis are located in the Stratum Basalis and they are called Keratinocytes. We also find highly specialized Melanocyte cells in the Stratum Basalis, which produce Melanin that provides protection for the skin against Ultaviolet rays (UV) by spreading throughout the skin by means of small vesicles (containers) called Melanosomes. The more UV exposure experienced, the more melanin is produced and released and spread via melanosomes throughout the epidermis to protect or shield the skin cells against harmful UV irradiation. This phenomenon is known as “tanning” to the lay person. The Keratinocytes move from the Stratum Basalis upwards towards the Stratum Corneum (the outer layer of skin cells) over a period of 6 weeks on average (40 days). This is also referred to as one skin cycle. The average skin cycle in healthy skin is therefore 40 days. Above the Stratum Basalis the Keratinocytes differentiate into a different form and this is referred to as the Stratum Granulosum. Above that we get the Stratum Spinosum and then the Stratum Lucidum and finally the Stratum Corneum.The Corneocytes in the startum corneum are essentially dead cells, packed as shingles in a shield format to act as a protective layer on the outside of the skin. It takes the Keratinocytes 28 days to migrate from the Stratum Basalis to just below the Stratum Corneum and another 12 days to shed via the Corneal layer, therefore 40 days for one normal and healthy skin cycle. As we age the skin cycle slows down, taking longer. On the other hand there are conditions that speed up the skin cycle and cause excessive exfoliation of skin, eg. Psoriasis.
The Dermis is located below the Epidermis. This is a thicker layer of skin and consists of the Papillary Dermis, which is immediately below the Basement Membrane of the Epidermis. The Papillary Dermis is known as the “heart of the skin”, because it contains a lot of blood vessels and also the essential cells that produce the Dermis and possibly part of the Epidermis. The essential skin cells of the Dermis are Fibroblasts, producing Collagen and Elastin, also Angioblasts that produce blood vessels, Lipoblasts that produce fatty tissue, and the Fibroblasts are also responsible for producing Glycoaminoglycans, Hyaluronic Acid and Gliserol which are all involved in hydration and fluid retention of the skin. There are also the specialized Langerhans cells, which form part of the immuno-surveillance system and other specialized sensory cells and end organs. The main components of the Reticular Dermis are Collagen and Elastin fibers which provide strength and elasticity to the skin which are again produced by the Fibroblasts.
Below the Reticular Dermis we find the subcutaneous fat layer and below the fat layer are the muscles and below the muscles we find bone. Those are essentially the various layers that comprise the skin and the the tissues below the skin. Each hair follicle in the skin has a rich network of blood vessels and nerves that encapsulate it and connect with the surrounding skin and other hair follicles. Hair follicles and hairs are divided into three basic sizes, the smallest are Vellus hairs which are less than 30 microns in diameter and these hairs are spread all over the body, approximately 5 million and they essentially produce the skin and maintain it. We then have Terminal hairs, which are between 60 microns and 84 microns and they are located on the scalp, the arm pits (axillae) and pubic area. In-between we get Miniaturized hairs which are between 30 to 60 microns in diameter and they are mainly seen on the scalp in conditions such as Androgenetic Alopecia.
The skin is a dynamic organ which is essential for preserving the human condition. When large amounts of skin loss or destruction occur, eg. as experienced with 2nd and 3rd Degree Burns, the mortality rate is generally linked to the percentage of total skin destroyed, therefore 50% burns of 2nd and 3rd degree nature transpires into a 50% mortality rate etc. We therefore can’t stay alive without our skin protecting us and functioning normally. Furthermore, skin ages chronologically and becomes progressively thinner (atrophic) with the passage of time. This process starts between the ages of 25 to 30 years when the essential skin cells start shutting down and gradually stop performing their basic functions of maintaining, replenishing and rejuvenating the skin. It results in less blood supply to the skin, loss of sub-cutaneous fat, diminished quality of the various skin layers resulting in fragile, intolerant and weak skin in old age. In addition to chronologic ageing, the biggest factors that accelerate skin aging are Ultraviolet irradiation from sun exposure, Infrared or heat exposure from the sun, and exposure to pollutants and smoking. According to Dr. Zein Obagi, the general tendency in the population, especially amongst women is to “over-hydrate” their skin due to the “over-emphasis” and “misguided” information provided via the vast majority of skin restoration product marketing groups in the world. The Epidermis only needs around 10% hydration in general and because of over moisturization the average female epidermis is hydrated to a level of approximately 30% in studies conducted, according to Dr. Obagi and this may trigger a negative feedback mechanism that causes the essential skin cells to “shut down” prematurely and not produce the required glyco-aminoglycans and hyaluronic acid which are the natural fluid retainers and moisturizers of the skin, because of excessive exogenous moisturization. In effect, the modern emphasis via media advertising / marketing campaigns on over-hydration of skin may very well be a contributor towards accelerating skin aging.
- Obagi Skin Health Restoration & Rejuvenation, by Zein E. Obagi, M.D
- ISHRS Basics Course Lectures, Anatomy and Physiology, by Dr. W Parsley
- Hair Transplantation, 5th Edition, Unger; Shapiro