Anatomy April

The skin is the largest organ in our body, it not only protects us from invading pathogens (Viruses, bacteria, etc.) but in changes in temperature and ultraviolet radiation. It maintains the balance of fluids of your body through sweating and synthesizes vitamin D from the sun. It also accounts for about 3-5kg of your body weight. The skin is also connected to the outside world with many different sensory receptors contained in the layers of your skin. We will also discuss the function and clinical relevance. This article will explore the anatomy of the skin including its layers and components.The skin is also called the integument and consists of the epidermis and dermis, two neighbouring layers. Below these layers and the skin, we have the subcutaneous (sometimes called hypodermis) layer which contains fat and blood vessels. As you probably know the skin is constantly shedding and being replaced by epithelial cells which are constantly dividing.

The epidermis is the top layer of your skin and is flooded with melanocytes (this cell synthesizes melanine, the pigment that gives skin its colour), Langerhans cells (dendritic cells which originate in bone marrow and ingest the unwanted invading pathogens), as well as Merkel cells (pressure sensing receptors located on the border of the epidermis and dermis and combine with nerve endings). It is also made up of Stratified Squamous epithelial tissue where multiple layers set onto of each other like bricks. The bulk of it is made up of Keratinocytes which are the building block of the tough protein, keratin. Keratin gives structure, durability and waterproofing of the skin, hair and nails. It contains no blood vessels and relies on the dermis bellow for blood supply, nutrients and waste extraction.

The epidermis is different in every part of your body, for example, your thick skin located on your palms and base of your feet has five epidermis layers. Comparing this to your thin skin with just four. As seen on the left of the diagram the five main layers are illustrated, the stratum lucidum is the layer missing in thin skin. The deeper you go through the younger the cells in the epidermis, regeneration happens at the lowest levels of the epidermis and then the younger cells raise up and then die off. This process is due to the epidermis being epithelial so it is Avascular, which means that all the oxygen and nutrients that its cells need, must be supplied from the dermis below containing the blood vessels.

502_Layers_of_epidermis.jpg

The dermis on the other hand only has three layers; the first and uppermost layer being the papillary layer. This layer contains a thin sheet of areolar connective tissue that’s riddled with little peg-like projections called dermal papillae. A Dermal Papillae is a little bump that pushes through your epidermis and allows for friction between objects making it possible for you to hold things, they are also responsible for your fingerprints. Below this layer is the reticular layer which takes up 80% of the dermis and is made up of dense but unusual connective tissue. The final layer of the dermis is the hypodermis, not the most important of layers it is mostly just fatty tissue, specifically adipose connective tissue and it helps anchor your skin. Your blood vessels and nerve endings are spread through the top two layers of the dermis (the ones pictured in the diagram below).

506_Layers_of_the_Dermis.jpg

Many of the jobs the skin completes are not visible to the naked eye, however there is one job the skin does that we see on a day to day basis and is quite obvious; healing. If you get a cut that only lightly scrapes the epidermis and goes no further down than that, then you won’t see any blood and the wound will heal quite quickly. However, if you get a cut that is deep enough that you begin to see blood, that means that the cut has gone down to the dermis (because there are no blood vessels in the epidermis). Wounds always heal from the inside out and from the edges inward. In a healthy person it works this way; within seconds to minutes of an injury, blood vessels will constrict to reduce bleeding. Platelets (sticky blood cells) flood the area and aggregate into clumps. Clotting factors soon come on the scene, joining with the platelets to form a clot (have you ever heard the saying “The first clot is the best clot” this is true). Meanwhile, white blood cells (macrophages) come through, scavenging for infectious invaders. Over the next few days, macrophages also make growth factors to help repair the wound.

The clots turn into scabs, and, underneath, fibroblast cells produce collagen, a protein that connects tissues together. In a weeks-long process, the collagen creates new capillaries and the skin on the edges of the wound gets thicker and starts stretching under the scab. The skin might look reddish and start to itch—a normal part of healing, the nerve cells send itch signals in response to the perceived movement of the new skin coming in.

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