Your skin acts as a protective barrier, ensuring your internal organs do not dry out and that you are safe from infection. The three layers of the skin are intricate, with various cell types and appendages that give the system its unique function. On the surface is the epidermis, which sits atop the dermis and hypodermis. Together these layers and their associated structures, like hair and nails, form the integumentary system.
Epidermis
The epidermis is home to four types of cells: keratinocytes, melanocytes, langerhans cells, and merkel cells. Each of their functions are shown in the graphic below:
As shown, melanocytes are the cells that produce pigmentation in your skin. Interestingly, regardless of skin tone everyone has the same number of melanocytes. The differing factors are the breadth of the cell and color the cells produce – which can be dark/brown or yellow/red. It was found that people originating near the equator had more expression of the darker melanocytes than people who lived near the north or south poles. This was primarily due to the amount of UV exposure. At the equator, UV exposure is much higher, causing the need for more pigmentation to protect the body from cancer.
The four cell types of the epidermis are housed in five unique layers. Starting at the surface is the stratum corneum, followed by the stratum lucidum, which is only found in the thick skin of your palms and feet. These layers are composed of dead cells that are easily shed and replaced. Next, the stratum granulosum and stratum spinosum are made up of living keratinocytes that arise from the deepest epidermal layer, called the stratum basale. This is a single layer of cells that continually produces the new cells that migrate up the epidermis and are eventually shed.
Dermis
Beneath the epidermis is the dermis, which is subdivided into three layers. The first layer, the papillary layer, is made up of dermal papillae, which look like small projections that create the ridges of our fingerprints and increase friction for better grip. Next, is the reticular layer, which contains numerous structures: oil glands, sweat glands, arteries, veins, and nerves. The oil and sweat glands, hair follicles, and nails are classified as skin appendages. The deepest layer of the dermis is the hypodermis, which mostly consists of fat cells and blood vessels.
Skin Appendages
Oil glands, also called sebaceous glands, are found everywhere but the palms of your hands and soles of your feet. These glands release sebum, an oily substance that softens and lubricates the skin while also protecting the skin from water loss.
Sweat glands, also known as sudoriferous glands, are widely distributed throughout the body with most people having up to three million tiny glands. When you sweat, the body is able to excrete salts and urea while also keeping your body temperature from getting too high. Sweat glands come in two types: eccrine and apocrine. Eccrine glands are found in the palms, forehead, and soles of the foot and arise from bundles in the dermis that use a tube to travel upward through the epidermis. Apocrine glands are far less numerous and only appear around the armpits and groin after puberty; producing a different type of sweat that causes body odor.
Hair and nails form through keratinization, a process in which new cells at the root harden and are pushed forward. This occurs because the arteries at the root nourish the cells. As more cells accumulate, their nourishment is cut off which leaves hard, dead cells to be pushed onto the surface of the skin. This results in the continued growth of your hair and nails.
Want to know more about the other body systems? Check out this article!
