The Versatile Integumentary System
Everyone sees their skin every day. However, no one pays too much attention to it except when they wish it would be prettier or that it would stop itching or the like. One look at the skin does not do it justice. On the outside, it appears to only do a couple things: hold us together, allow us to sweat, grow hair, get darker in the sun’s rays. Most people have heard that the skin is the largest organ in the body. However, very few have heard about the complexity of the skin or its system, known as the Integumentary system. Rather than just one sheet of skin all over the body, the skin contains systems within systems. These are the three main layers called the epidermis, dermis, and hypodermis. And even more than that, there are more layers within those layers, all unique and assigned to specific duties to protect and take care of our bodies.
The Layers of the Skin
The first main layer of the skin is called the epidermis. The epidermis consists of many layers of cells called “keratinized squamous epithelium”. A look at these cells reveals their “squashed” appearance, almost as if they are tiny fried eggs cooked “sunny-side up”. The first layer, which you can see by looking at your skin, is called “Stratum Corneum”, meaning, “horny layer”. It consists of up to thirty layers of dead, scaly keratinized cells. These keratinized cells make this layer very durable and resistant to abrasion, penetration, and water loss. These are the cells that exfoliate and flake off when you scratch your skin or wash with a cloth or loofa in the shower.
Sketched View of Epidermal Layers
The next layer down in the epidermis is called the “stratum lucidum”, meaning “clear layer”. This layer is only seen in thick skin, such as on the palms of the hands or the bottoms of the feet. It is thin and translucent, hence its Latin name. The epidermal cells that produce keratin, called “keratinocytes”, are densely packed with eleidin. Eleidin is converted to keratin in the stratum corneum. These cells have no nucleus or other organelles, making them unable to undergo mitosis (replicate). This area is pale, featureless, and does not have any distinct boundaries. This layer adds durability and resists strong abrasion, allowing you to walk on more rough surfaces with bare feet as opposed to walking on your bare knees.
Histologic View of Epidermal Layers
The next layer down is called the “stratum granulosum”, which means “granular layer”. This layer consists of three to five layers of flat keratinocytes. This means that it is highly involved in the production of keratin for the higher layers. The reason why it is called the “granular layer” is because it contains coarse, dark-staining karatohyalin granules, which are protein structures that may be involved in keratinization. This layer has dead skin cells at the surface packed with tough protein for protection.
Another layer down rests the “stratum spinosum”, or “spinous layer”. Numerous desmosomes, which bind cells to each other, and cell shrinkage give this layer is spiny appearance. This layer consists of keratinocytes that produce lots of keratin filaments. This causes the cells to flatten. In addition to keratinocytes, dendritic cells can be found throughout the stratum spinosum. Dendritic cells process antigen material. They are close to the external environment so if there is a breach in the epidermal layer, these cells can be activated to combat bacteria and diseases. The deepest cells here remain capable of cell reproduction, but stop replicating as they are pushed upward though the layers.
The last layer in the epidermis is called the “stratum basale”, which means “basal layer”. This is a single layer of cube-shaped (cuboidal) or column-like (columnar) stem cells and keratinocytes resting on the basement membrane. Melanocytes, which produce melanin that contributes to the color in our skin, and tactile cells are scattered among the stem cells and keratinocytes. The stem cells divide and give rise to the keratinocytes and replace lost epidermal cells. This is very necessary because we are constantly shedding dead skin cells. Although these epidermal layers are extremely important to our survival, they wouldn’t be able to do much without the next layer.
The second main layer of the skin is called the dermis. The dermis is a layer of connective tissue that is mostly composed of collagen with elastic fibers, reticular fibers, and fibroblasts. These components maintain the structural integrity and flexibility of the connective tissue. This layer is well-supplied with blood vessels, sweat glands, sebaceous glands, and nerve endings. It is also the growth-site for hair follicles and nail roots. In this dermal layer, hair follicles are surrounded by smooth muscles that contract in response to various stimuli, such as cold, fear, and touch. This is gives us the sensation that is commonly known as “goose bumps”.
Dermal Layers Viewed through Microscope
The first layer of the dermis is called the “dermal papillae”. These are finger-like extensions of the dermis. They create the friction ridges on fingertips that leave fingerprints. Dermal papillae are present all over our bodies. But, scientists don’t know why we don’t have a fingerprint-like pattern all over out skin’s surface.
The next layer is the “papillary layer”. Rich in small blood vessels, this thin layer contains areolar tissue in and near the dermal papillae. This layer allows for the mobility of leukocytes and other defense cells in case the epidermis is broken and invaded by foreign bacteria.
Lastly in the dermis is the “reticular layer”. This layer is deeper and much thicker than the other dermal layers. It consists of dense, irregular connective tissue that plays a part with our next main layer:
The third and last main layer of the skin is called the hypodermis. The hypodermis is made up of subcutaneous tissue that contains more areolar and adipose tissue than the dermis. The main function of this layer is to pad the body and bind skin to underlying tissues. Because it is highly vascular (meaning, it has many blood vessels), it can quickly absorb substances injected into it. This is why doctors use hypodermic needles to inject drugs that need to be quickly absorbed. In addition, this layer contains subcutaneous fat, which acts as an energy reservoir and thermal insulation. While both men and women have subcutaneous fat, it is 8% thicker in women in order to protect their important reproductive organs.
Cross-Section of Integumentary System
The integumentary system is not just one system – it is systems within a system. They are complex, detailed, and carry out important roles that go unnoticed by us in our everyday lives. Without these complex layers, we would be left exposed to cold, disease, foreign bacteria, hits and blows against our bodies, and left without a way to communicate or absorb the vitamin D given to us by the sun’s rays. We need every last part of the elaborate integumentary system, right down to the smallest cell.
All information was gathered from "Anatomy & Physiology: The Unity of Form and Function" by Saladin, 5th edition. Published and copyrighted by The McGraw-Hill Companies, Inc. 2010. All rights reserved.