The two layers of the skin consist of the epidermis and dermis. The epidermis is the uppermost part of the skin, which we can see all over our bodies. The epidermis and dermis are composed of their own layers, which we will discuss in a bit. Below the dermis is the hypodermis, which consists of adipose tissue. Below the hypodermis is the subcutaneous layer, which is the location site for many of our injectable drugs, such as insulin. Between the layers there are many hair follicles, which make up the pilosebaceous unit, consisting of hair, sebaceous gland, apocrine and eccrine sweat glands, and the arrector pili muscle. The pilosebaceous unit is particularly important in thermoregulation and electrolyte homeostasis - sweat glands function to release NaCl and H2O from the body, producing a cooling effect and maintaining electrolyte balance.
Before we get into the functions of the skin, I will go back and summarize the various layers in the epidermis and dermis. The epidermis is composed of 5 or 6 layers, depending on the type of skin. These layers include (from bottom up): Stratum basale, spinosum, granulosum, lucidem (only on thick skin - soles of feet, palms of hands), and the stratum corneum. These sit on the basement membrane, which connects the dermis to the epidermis. The stratum basale consists of merkel cells and cuboidal cells. The stratum spinosum contains melanocytes, responsible for color of the skin, and langerhans cells, which are antigen-presenting cells involved in the immune response. The stratum corneum, the uppermost layer, is also the thickest layer of the skin.
The dermis is made up of many types of cells: fibroblasts, which make up the extracellular matrix, including collagen, mast cells, sensory nerve fibers, and capillaries. The dermis is the area of the skin containing the nerves and blood supply for the skin. Different sensations which are felt on the skin are pressure, pain, and temperature.
The functions of the skin are as follows: 1)protection/barrier for the underlying tissues, 2)wound healing, 3)vitamin D synthesis, 4)sensation, 5)thermoregulation, and 6)secretion. The skin acts as a barrier for bacteria (by secreting its own antimicrobials), UV light, and injury. The skin is extremely efficient at wound healing, as evidenced by the quick healing of superficial cuts and scrapes. Vitamin D synthesis occurs when the sun causes the conversion of 7-dehydrocholesterol to cholecalciferol and eventually into active vitamin d, which is crucial in the regulation of calcium. The skin also detects sensations, as mentioned above, pain, temperature, and pressure. The skin also secretes sweat, antimicrobials, and sebum. Sweat helps to regulate temperature, antimicrobials help to prevent bacterial infection on the skin, and sebum acts as a lubricant and fat secretor.
Pharmacologically, the skin conditions which are included in NAPLEX prep are: acne, cold sores, dandruff, alopecia, eczema, hyperhidrosis, fungal infections, diaper rash, hemorrhoids, pinworm, lice/scabies, minor wounds, burns, poison ivy/oak/sumac, inflammation/rash, and sunscreens. I will be delving deeper into these subjects individually throughout the duration of this APPE rotation. Please refer to this GoogleDoc Folder for all NAPLEX review of skin:
https://drive.google.com/drive/folders/1fS5RbT9WIJHUFy1TrVuWnd4XeQHT7LOT?usp=sharing
Eczema (Atopic Dermatitis): NAPLEX Review
“Eczema” or atopic dermatitis, is a general term for many types of skin inflammation. Eczema is most commonly seen in children and infants, but it can occur at any age. Eczema presents dermatologically as a skin rash, which becomes crusty and scaly. The rash is red, itchy, dry, and sore - blisters may or may not develop. Common affected areas include the elbows, behind the knees/ears, face (often on the cheeks), buttocks, hands, and feet. It is important to note that certain products or environmental conditions can trigger eczema, such as allergens (in soaps, perfumes, etc), environmental irritants (pollen), stress, or changes in the weather.
Hydration is essential to reduce severity of atopic dermatitis - patients should be advised to use unscented moisturizers such as CeraVe or Eucerin, and to maintain adequate humidity in the home (which can be particularly difficult in the winter months). If necessary, pharmacological treatments can include topical steroids, short courses of oral steroids, antihistamines for itching, and finally immunosuppressant calcineurin inhibitors (if topical steroids with hydration are not adequate). In severe, refractory cases of atopic dermatitis, oral immunosuppressants such as cyclosporine and MTX, or monoclonal antibody drugs can be used. Dupilumab (Dupixent) is an FDA-approved monoclonal antibody for the treatment of moderate to severe eczema. Dupixent is administered as a biweekly subcutaneous injection; however, there are other drugs used for eczema off-label.
Resources:
1) RxPrep NAPLEX Review 2021. Chapter 39: Common Skin Conditions. Page 571.