The skin is the largest organ of the human body, comprising approximately 17% of the body weight. An average adult would have a skin total surface area of around 1.7 square meters. The thickness of the skin varies depending on the site where it resides, ranging from 3mm to 6mm. The skin is the most visible and exposed organ of the body, and hence it is the first line of defense of the body against the harsh environment.
The structure of the skin comprises of three main layers, namely the epidermis, the dermis and the hypodermis. There are also skin appendages that play their own respective roles in the skin. Each make-up of each layer will be described below.
The epidermis is the most dynamic layer among all the layers of the skin. Despite being the thinnest layer, it is actually the most important layer among all. The epidermis consists of 5 sub-layers, that will be listed below:
(a) Stratum germinativum (basal cell layer) – This is the deepest layer composed of keratinocytes. They continually divide to supply new cells to the upper layers. Melanocytes are also present in this layer, providing protection from harmful ultra-violet radiation.
(b) Stratum spinosum (squamous cell layer)
(c) Stratum granulosum (granular cell layer)
(d) Stratum lucidum
(e) Stratum corneum (horny cell or keratin layer) – This is the top layer that acts as the main barrier for the skin. It has variable thickness, which contributes to the different thickness of the skin, for example the palms versus the forearms. This layer is continually shed, and the average turnover rate is 28 days for normal skin. This layer is not present in mucous membranes such as the nose and the eyes.
The dermis layer comprises of matrix substance supporting the epidermis. It sits between the epidermis and the hypodermis. This layer also holds many appendages, and the features of this layer are outlined below.
(a) Collagen – This provides tensile strength to the skin layer. This layer grows weaker as we grow older, hence that is why the skin is thinner and easier to tear.
(b) Elastin – This renders elastic properties to the skin, enabling it to bend and stretch.
(c) Ground substance – This is responsible for the water-retaining properties of the skin.
(d) Blood vessels – Blood vessels are important to supply nutrients to the epidermis, as well as plays a role in temperature regulation.
(e) Nerves – Nerves enables us to respond to touch, pressure, pain and heat. Pacinian corpuscles within the skin is a type of mechanoreceptor, responsible for sensitivity to deep pressure touch and high frequency vibration.
The hypodermis layer is basically a subcutaneous fatty layer, made up of loose connective tissue. It is the deepest layer of the skin, below the dermis. This layer provides fat for insulation, pliability and storage for nutrients.
There are a number of skin appendages that are important to the skin. A handful of important ones are briefly described below.
(a) Eccrine glands – These glands produce sweat onto the surface of the skin especially the palms, soles and the armpit for a cooling effect.
(b) Sebaceous glands – These glands produce sebum which lubricates and is water repellent. It also has mild antibacterial activity.
(c) Apocrine glands – These are scent glands (odiferous) found in the armpit, anogenital regions and the forehead. These glands are more important in lower animals for sex functions.
(d) Hair follicles – These are tactile, protective and insulating.
(e) Erectores pilorum – These are tiny smooth muscle fibers attached to the hair follicles, causing “goose-bump” reaction to cold or fright. These exist in most mammals.