In this video we discuss the structure of the skin, we look at the 2 different layers of skin, the epidermis and the dermis, the structure of each of these layers, and the 2 different types of skin, thick skin and thin skin.
Structure of skin
Human skin, which is also called the integument or the cutaneous membrane, is made up of 2 layers, the epidermis and the dermis, which are labeled on this model of skin. The subcutaneous layer at the bottom has also been labeled, however, it is not actually a part of the structure of skin, but it is connected to the dermis of the skin.
Let’s start by looking at the epidermis. The epidermis consists of 4 to 5 layers depending on the type of skin. Thick skin has 5 layers, and it is found in the palms of the hands, and on the soles of the feet. Thin skin has 4 layers and is what covers most of the body. The skin model we are looking at has all 5 layers.
The bottom or deep layer is called the stratum basale. It is made up of a single layer of cells attached to a basement membrane. There are 3 types of cells in the stratum basale; keratinocytes, melanocytes and tactile cells.
Keratinocytes are the most common cell in this layer, and they go through cell division to replace cells that are shed from the surface of the skin. These cells can produce a tough structural protein called keratin which strengthens the skin and makes it almost waterproof.
Melanocytes are scattered among the keratinocytes and they produce the pigment melanin in reaction to exposure to ultraviolet light. Melanin gets transferred to keratinocytes and surrounds the nucleus to protect DNA from mutating from ultraviolet radiation.
Tactile cells are also scattered among the keratinocytes, and they serve as light touch receptors.
The next layer, moving upwards in the epidermis is the stratum spinosum. This layer is made up of daughter keratinocytes made from dividing cells in the stratum basale layer below, and epidermal dendritic cells. The daughter keratinocytes connect to neighboring cells desmosomes, which are one of the ways cells connect to one another, giving them a prickly appearance. The dendritic cells are immune cells that help fight infections in the skin.
Moving upwards, the next layer is the stratum granulosum. This layer is comprised of 3 to 5 layers of keratinocytes. The process of keratinaztion begins in this layer of the epidermis. Keratinization is where the keratinocytes fill with the keratin protein metioned earlier. This process continues as the cells move upwards in the epidermis, and as it continues, the cell’s nucleus and organelles are eliminated and the cell dies.
The next layer up is the stratum lucidum. This layer is only found in the thick skin in the palms and soles of the feet. The keratinocytes in this layer are clear, flat, closely packed and have no nucleus or organelles. They are also filled with a protein called eleidin, which is eventually transformed into keratin.
The last or top layer is called the stratum corneum. This layer is comprised of dead keratinized cells. It takes about 2 weeks for a new keratinocyte to reach the stratum corneum, and it remains in this layer for about another 2 weeks before it is shed.
Now let’s look at the dermis. The dermis is comprised of connective tissue proper with collagen being the most plentiful type of fiber found throughout the dermis. The dermis also houses other structures such as blood vessels, hair follicles, sweat glands, sebaceous glands, which secrete a lubricating oil, sensory nerve endings, nail roots and arrector pili muscles which affect hair follicles.
The dermis has 2 layers; a papillary layer, and a reticular layer.
The papillary layer is the top superficial layer of the dermis, it is composed of loose connective tissue and forms bumps or projections called dermal papillae that fit with the epidermal ridges of the epidermis. The form of the dermal papillae and epidermal ridges increases the surface area of contact between the 2 layers. The dermal papillae contain capillaries that supply nutrients to the cells of the epidermis, and they contain sensory nerve endings that help monitor touch on the surface of the skin.
The reticular layer is composed of dense connective tissue and it extends from the papillary layer to the deeper subcutaneous layer. It is composed of a dense connective tissue with some elastic fibers and many bundles of collagen fibers.
The subcutaneous layer, which again is not part of the skin, is located below or deep to the dermal layer of the skin. This layer is often referred to as the hypodermis and it consists of loose connective tissue and adipose connective tissue or fat tissue. Many times it is referred to as subcutaneous fat. This layer helps to bind skin to underlying structures, acts as a cushion, protects the body, provides insulation and provides for energy storage.