The skin is an amazing organ. It covers our whole body and acts as a first-line defense of our immune system for our body. Just imagine if we don’t have skin, it would be gross, we know! We wrote about skin facts, and today we present to you the detailed structure and function of the skin. Let us begin.
Before jumping in to the structure and function of the skin, here is what you need to know about skin (just to get knowledge).
Skin is a part of integumentary system1. It is the largest organ present in the human body*. How large? It has an average surface area of 16.1 to 21.5 square feet or 1.5 to 2 square meters!
The word cutaneous is used for skin because the Latin word “cutin” refers to the skin. The thickness of the skin varies with the body part to body part, age, etc. Men usually have thicker skin compared to women.
For one square inch of skin, there are nearly 650 sweat glands, 60,000 melanocytes (cells which create melanin pigment), over 1,000 nerve endings, 20 blood vessels. (this is just an average. Some of the body parts are more sensitive than others, and other body parts don’t sweat at all!).
Structure and Function of the Skin – Structure
Before learning the functions, let us learn the structure of the skin in our atricle ‘structure and function of the skin.’
There are three layers of the skin – epidermis, dermis, and hypodermis.
The Epidermis
The epidermis is the outermost layer of the skin. In Greek, epi means over, and derma means skin. It is made up of stratified squamous epithelium. It is the first layer of defense against microorganisms.
It doesn’t have blood vessels. The cells at the bottom layers receive oxygen more from diffusion than from the blood vessels of the dermis layer.
Merkel cells, Langerhans cells, melanocytes, and Keratinocytes are the cells that are present in the epidermis.
The epidermis is further divided into:
Stratum basale
It is also called as Stratum germinativum. Dermal papillae (plural form of Dermal papilla), are finger-like projections that arise from the dermis layer. It intensifies the connection of both the layers.
It consists of basal cells, Merkel cells (receptor cells that stimulate sensory nerves to touch), melanocytes. Basal cells present at the basale layer divide through mitosis2.
The daughter cells then move up, and while moving up, they change their shape and composition. They eventually die because there is no blood supply in epidermis’ top layers.
Keratin, a protein, is inserted, removing the cytoplasm. They reach the corneum layer and get sloughed off. This whole process is called Keratinization.
Keratin is produced by the cells present in the upper layers of the epidermis and hence are called keratinocytes.
Stratum spinosum
It has a spiny structure because of the presence of protruding cell processes that are attached to cells through desmosomes.
Cells get interlocked with each other because of desmosomes. It increases the bond between cells.
It has 10 layers of keratinocytes. The keratinocytes present in this sub-layer make the cells, and eventually, the skin relatively hydrophobic or waterproof. These cells are pushed upwards as the basal cells produce more and more keratinocytes.
It also has a kind of dendritic cell known as Langerhans cell. It works as macrophage – killing bacteria, damaged cells, foreign elements, etc.
Stratum granulosum
Keratinocytes of this sub-layer have a grainy appearance. It has only 3 to 5 layers.
The cells are flatter than earlier, have thicker cell membranes, and produce keratin and keratohyalin (a protein structure) in large quantities (as already mentioned, the cells move up and change in shape and composition).
The nuclei and other cell organelles generally die off, and the cells are filled with keratin and keratohyalin.
Stratum lucidum
It is present only in palms, digits (fingers or toes), and soles. It is a smooth and translucent layer.
Keratinocytes are usually flattened and are dead. They are densely packed with eleidin (a protein which contains lipids, derived from keratohyalin).
Eleidin is responsible for the lucid appearance of these cells. It also blocks water.
Stratum corneum
It is the outermost layer and consists of nearly 25 to 30 layers of dead cells.
The word corneum is named after the process of Keratinization or Cornification.
This sub-layer helps in protecting the inner layers from dehydration, invasion of microbes, and abrasion.
Cells are usually sloughed off, through a process known as desquamation, to accommodate new keratinocytes which are pushed up from the inner layers.
It takes nearly 4 weeks for the entire stratum corneum to get replaced.
Structure and Function of the Skin: Dermis
The Dermis
It is the second layer after epidermis. It forms the core of the integumentary system. It contains lymph vessels, nerves, blood vessels, hair follicles, sweat glands, etc.
It consists of two layers which are:
Papillary layer
It is present above the reticular layer. It is made up of loose areolar connective tissue.
Elastin and collagen fibers make a loose mesh in this layer. It is this layer that makes protrusions in Stratum basale to form finger-like projections known as dermal papillae.
Fibroblasts3, a lot of small blood vessels, and some adipocytes (fat cells) are seen in this layer.
This layer also has phagocytes (cells that kill bacteria and other microorganisms which successfully breached the epidermis).
In addition to the above cells, Meissner corpuscles (touch receptors), lymphatic vessels and nerve fibers are also seen in this layer.
Reticular layer
It is present below the papillary layer. It consists of dense and irregular connective tissue.
It is thicker than the papillary layer. It has a lot of blood vessels, a lot of nerve fibers (sympathetic4), and sensory nerves.
This layer is named after its appearance. It is reticulated or net-like because of the compact mesh of collagen and elastin fibers.
Collagen fibers give the structure and strength to the skin. They extend upwards to the papillary layer and extend downwards to reach the hypodermis. Collagen retains water and helps the skin to remain hydrated.
Elastin fibers provide elasticity facilitating the movement of the skin.
The Hypodermis
It is the last layer. It is also known as the superficial fascia or subcutaneous layer. It connects the dermis with fascia, which is the underlying fibrous tissue of muscles and bones.
It is not exactly a part of the skin, but differentiating between dermis and hypodermis is extremely difficult. It consists of loose areolar connective tissue, which has an abundant supply of blood vessels. It also contains adipose tissue (fat tissue).
The adipose tissue helps in fat storage and insulation. It also acts as a cushion and protects the internal structures from trauma.
Structure and Function of the Skin – Function
As said earlier, it acts as a first-line defense of our body. It is a tactile sense organ, meaning it reacts to touch, vibration, pressure, heat, and cold.
It helps in heat regulation. It tries to cool the body by evaporating sweat, thereby bringing down the temperature of our body.
Our skin also plays a role in excretion. Sweat contains urea. However, sweat contains 1/30th of urea present in urine. It is the secondary function of the skin.
It is water-resistant so that essential nutrients are not washed away. The top layers of the skin can absorb oxygen from the air. It absorbs ointments, gels, and other topical agents.
Skin also show our mood, physical state, and attractiveness. It is a storage center for lipids and water. It also synthesizes vitamin D.
Human body*– It is essential to note that the liver is the largest gland and skin is the largest organ of our body.
Integumentary System1 – It is a system (just like the digestive system, respiratory system, etc.) that contains skin, nails, hair, and exocrine glands like the pancreas.
Mitosis2 – It is a form of cell division that results in two daughter cells that resemble the parent cell in every aspect. The other form of cell division is meiosis.
Fibroblasts3 – Fibroblasts are a type of cell that produces collagen and stroma in animal tissues.
Sympathetic nerves4 – Nerves of the sympathetic nervous system, one of the divisions of the Autonomic Nervous System. It readies your body for any mental or physical activity. It stimulates fight or flight response.
If you have any doubts on the structure and function of the skin, feel free to drop us a message and we will try to help you. If you want us to write on topics similar to structure and function of the skin.
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