The skin is the largest organ of the human body and comprises almost 8% of total body mass.
It is a versatile structure with a wide range of functions – though its exact composition varies across different regions of the body’s surface.
The skin generally acts as a vital barrier between external environment and internal body contents. It protects us against chemical, mechanical, thermal, osmotic, and UV damages, as well as certain bacterial invasions.
Other functions include playing a role in the synthesis of Vitamin D and regulation of body temperature. Additionally, it is a major sensory organ for touch, pain, temperature, and other stimuli.
In this article, we will focus on the 3 key skin layers and discuss the specific tasks of each.
Gross Structure
Fig. 1: The skin consists of 3 major layers: epidermis, dermis, and hypodermis (from outermost to innermost).
While the skin composition differs throughout the body, the three layers that are consistently present are the epidermis, dermis, and hypodermis. Let us examine these layers in more detail.
Epidermis: The Physical Barrier
The epidermis is the most superficial layer of the skin and is formed largely (about 90%) by layers of keratinocytes undergoing terminal maturation. This process involves increased keratin production and migration towards the external surface (known also as cornification). Simply put, when cells here die, they are replaced with new ones from the skin layers below.
Nevertheless, there are also several non-keratinocyte cells that inhabit the epidermis, such as:
- Melanocytes: produces melanin and forms pigment.
- Langerhans cells: antigen-presenting dendritic cells.
- Merkel cells: sensory mechanoreceptors.
The epidermis forms an effective deterrent against environmental damage like UV radiation, excessive heat and water loss, and pathogenic bacteria.
Dermis: The Connective Tissue
The dermis is connected to the epidermis with a highly-corrugated dermo-epidermal junction.
Various cell types and structures can be found in this layer, such as:
- Blood vessels and nerves
- Skin appendages: includes hair follicles, nails, sebaceous and sweat glands. They are actually derived from the epidermis but grow into the dermis.
- Fibroblast cells: synthesise collagen and elastin.
- Mast cells: contain histamine granules and are essential to the immune system.
The dermis is made up of two layers, that is, the superficial papillary layer and the deeper reticular layer. The latter is thicker, consisting of collagen and elastin fibres that provide the skin flexibility and strength.
Hypodermis: The Protective Padding
This innermost skin layer is known as the hypodermis. It is a major body store of adipose tissue and can vary in size between individuals depending on the amount of fatty tissue present.
The fat aids in the insulation of the skin by monitoring heat loss and gain.