Psoriasis is a non-contagious, chronic autoimmune disease that causes skin inflammation, and is linked to other autoimmune deficiencies such as psoriatic arthritis. It can be incredibly itchy and form silver plaques on the skin that can be embarrassing and have both physical and psychological effects. Afflicted reality TV star Kim Kardashian and singer Cyndi Lauper have each spoken out on how psoriasis can have a significantly negative impact on the quality of life. Normally, the skin is divided into three layers–the epidermis, dermis, and hypodermis. The hypodermis is made of fat and connective tissue that anchors the skin to the underlying muscle.
Just above the hypodermis is the dermis, which contains nerves, sweat glands, lymph, and blood vessels. Just above the dermis is the epidermis. The epidermis itself has multiple cell layers that are composed of developing keratinocytes – which are named for the keratin protein that they’re filled with. Keratin is a strong, fibrous protein that allows keratinocytes to protect themselves from getting destroyed when you rub your hands through the sand at the beach.
Keratinocytes start their life at the lowest layer of the epidermis called the stratum basale, or basal layer, which is made of a single layer of small, cuboidal to low columnar stem cells that continually divide and produce new keratinocytes. These new keratinocytes migrate upwards, forming the other layers of the epidermis. As keratinocytes in the stratum basale begin to mature and lose the ability to divide, they migrate into the next layer, called the stratum spinosum which is about 8 to 10 cell layers thick.
The stratum spinosum also has dendritic cells lurking around, which are star-shaped immune cells constantly patrolling for invading microbesas part of the body’s immune defense system. The next layer up is the stratum granulosumwhich is 3 to 5 cell layers thick. Keratinocytes in this layer begin the process of keratinization, which is the process where the keratinocytes flatten out, and get ridof their intracellular structures and die, and in the process they create the epidermal skin barrier. Keratinization leads to development of thestratum lucidum layer which is 2 to 3 cell layers thick of translucent, dead keratinocytesthat have shed nuclei.
The stratum lucidum is only found in thickskin like on the palms and soles of the feet, because those are the areas that need extraprotection. The stratum lucidum is absent in thin skin,which covers the rest of the body, and the other layers are thinner. Finally, there’s the stratum corneum, orthe uppermost and thickest layer of the epidermis, which is like a wall of 20-30 layers. As new keratinocytes push up into the stratumcorneum, older dead cells are sloughed off forming skin flakes or dandruff.
In this way, the thickness of the epitheliumremains constant with a regulated turn-over of keratinocytes. Normally, there are microbes that live onthe surface of the skin, but when there’s a tiny break in the skin or cut, those microbeshave the ability to enter into the skin layer. At that point, the body notices these ‘foreigninvaders’ and immune cells called dendritic cells capture foreign antigens, break themdown into little fragments that they then present to T-cells. There are two different types of T Cells,cytotoxic T Cells, which directly kill infected cells, and helper T cells, that help to facilitatethe overall immune response.
So, if the dendritic cells present these fragmentsand the fragments are recognized by the T-cell, then the T-cell releases cytokines. Cytokines are small proteins used in cellsignaling – such as IL-12, IL- 23, interferon-γ, tumor necrosis factor or TNF, and IL-17, whichspecifically has been linked to chronic inflammation. This entire process of inflammation increaseskeratinocyte proliferation in the skin. This also recruits other immune cells, likeneutrophils, to the site of infection. Once the microbe is completely destroyed, the immunologic response slowly returns back to normal. In psoriasis, this immunologic response isabnormal, and it leads to excessive inflammation. Y
ou can think of it as an over-amplificationof the skin’s natural immune process that goes a bit haywire. The causes of psoriasis aren’t clear butthere’s definitely a genetic component and environmental triggers, like trauma, stress,dietary factors, smoking, or a previous infection. Regardless of how the process is triggered,once it’s underway it doesn’t shut off, resulting in chronic damage to the skin.
In the dermis, inflammation causes the bloodvessels to dilate, particularly at the border between the dermis and epidermis. This allows vessels to circulate and delivermore immune cells, particularly neutrophils, to the epidermis. In the epidermis, neutrophils collect in thestratum corneum layer. The inflammation also causes keratinocytesto proliferate excessively and mature abnormally. The increased keratinocyte proliferation thinsout the stratum basale; but thickens the other layers, particularly the stratum corneum andstratum spinosum.
In fact, as keratinocyte growth outpaces sloughingoff, the cells begin to pile up. These proliferating keratinocytes have a fewmaturation defects which are most obvious in the stratum corneum. First, the keratinocytes produce more keratin than usual causing this layer to become even thicker. Second, the keratinocytes retain their nuclei- which is called parakeratosis. Third, the keratinocytes don’t adhere toeach other properly, causing breaks in the epidermis. That weakens the skin and makes it more susceptibleto bacteria and viruses, and also gives the skin a scaly appearance. If scales are picked off, the blood vesselsin the dermis can get injured, causing localized spots of bleeding, called an Auspitz sign.
Symptoms of psoriasis can vary, and thereare a few subtypes. Plaque psoriasis is the most common. It appears as flattened areas of epidermalelevation that are inflamed and red or salmon-colored, from the underlying dilated blood vessels,and are topped with white-silvery scales. Plaques are typically itchy and found on thescalp and in tensor regions, like the elbows, groin, lower back, and knees. Guttate psoriasis appears as small, red, individualspots on the trunk and limbs.
This subset usually starts in childhood, andis sometimes triggered by an infection, like a strep throat. Inverse psoriasis appears as smooth and shinyred lesions that lack scales and typically form within skin folds like in the genitalregion, under arms or under the breasts. Pustular psoriasis appears as areas of red skin with small, white elevations of cloudy pus, formed from dead immune cells. They are usually tender and form on the handsand feet. Erythrodermic psoriasis is usually the most severe subset and appears as fire-red scales that can cover a large area of the skin. They are often extremely itchy and painful and fall off in large sheets. Psoriatic arthritis manifests as inflammation in the joints.
Nail pitting, shallow or deep holes in the nails, may appear in any subset. The diagnosis is typically based on the distributionof the skin damage, and a tissue biopsy can be done to confirm the diagnosis by looking for classic changes in the epidermal layers. Treatment for psoriasis can include moisturizersand emollients to help clear psoriatic plaques and minimize itchiness or may require topicalor systemic immunosuppressive therapies to dampen the immune response. UV phototherapy can also be an option to induceDNA damage in the keratinocytes and halt their proliferation.
New research has been targeting stress management,dietary intervention, and other immunotherapies to decrease whole body inflammation. For some individuals, there can be significantfeelings of depression or anxiety, especially when there are areas of skin affected by severepsoriasis. Psychological counselling can help sufferscope with their condition. In fact, there’s a field called psychodermatologywhich utilizes strategies like anti-anxiety medications, biofeedback, allergy and immunefunction testing, as well as cognitive behavioral therapy.
OK, so, to recap: Psoriasis is an autoimmunedisease that leads to chronic changes in the epidermis due to overstimulation of immunecells and excessive proliferation of keratinocytes that don’t mature correctly. Psoriasis most commonly causes silvery scalingof the skin and itchiness. The diagnosis can be made visually, or witha tissue biopsy. There are various oral and topical treatmentoptions available, as well as new areas of lifestyle and dietary intervention, and psychodermatologywhich addresses the relationship between emotions and physical changes in the skin.