Relevant to this topic, but to be fair – that guy’s face is basically drenched in sebum
So we move on to the intergumentary system, or put simply – the Skin!
The skin is basically the organ that makes boys swoon to girls, the organ that determines love at first sight, the organ that can decide to say f*ck you and pop out a pimple right before an important presentation or an equally important first date! Or, for some people, just a pimple is considered a good day. When your skin goes full Sparta and goes into sebaceous overdrive, you will probably end up like the guy in the video above. Not the muscular guy.
HISTOLOGY OF THE SKIN
Reference mostly from Gartner’s Histology, and Junqueira’s Atlas of Histology.
In a summary:
The skin is the largest organ in our body. Aside from playing a major role in our body’s defense as the first line defense against pathogens, UV radiation, producing vitamin D, temperature regulation, etc. It also has receptors – mechanoreceptors that can stimulate nerves near the skin and therefore plays a role in receiving inputs from external and internal sources. They also make us look fabulous.
The skin itself is composed of two main layers – the outer epidermis, and the inner dermis. In between the two is a membrane called the basal membrane or the basement membrane. Be sure to not make a mistake of confusing the dermis as the basal membrane like I stupidly did in class in front of the doctor teaching. Below the dermis is another layer – termed the hypodermis, which contains a lot of adipose tissue in overweight people, but is generally NOT considered a part of the skin.
EPIDERMIS
The avascular epidermis gets nutrients from the capillary network of the dermis. It consists of four different epithelial cell types. It consists of four different epithelial cell types – all of which are stratified squamous keratinized epithelial cells which have differentiated themselves – which are:
1. keratinocytes – which make up like most of the epidermis. Secretes keratins. Each layer (strata) has different forms of keratinocytes, and may also secrete different kinds of keratin proteins. For instance – (and this is hopefully just good for us to know and won’t come out on exams, or else dr. Najeeb help us all), keratins 5 and 14 are produced mainly in the stratum basale, keratins 1 and 10 are produced in the stratum spongiosum, and they replace the old keratin 5 and 14s in the stratum basale, pushing the old keratin cells up.
2. Langerhans cells – macrophages that live on the epithelium, scattered all across the stratum spinosum. Functions to phagocytose antigens, and brings said antigens to be presented into nearby lymph nodes to help initiate the adaptive immunity process.
3. Melanocytes – which secretes melanin, is derived from the neural crest and usually located in the deep layer of the epidermis. Melanin are essentially pigments, gives us skin tones, and protects us from UV radiation. They are produced in small granules called melanosomes where production of melanin from tyrosine takes place with the help of they enzyme tyrosinase. So basically, tyrosine are turned into melanin with the help of tyrosinase. Melanin then move into the tip of the melanocytes by stimulation of UV rays, and then the melanin containing melanocytes gets “nipped” off, and their melanin contents gets released outside while the outer melanocyte shell gets degraded.
4. Merkel cells, which have mechanoreceptors and nerve endings that forms the Merkel cell-neurite association.
There are two types of epidermis: thick epidermis which has 5 layers, and thin epidermis which only has 3. The layers of the epidermis are as follows:
- Stratum corneum – the most superficial layer, and the thickest, which mainly consist of dead keratinocytes, which are called squames, and are filled with keratin fragments.
- Stratum lucidum – a transparent layer of cells whose organelles are killed by lysosomal actions. So basically they are dead and are literally just cells filled with a special substance called eleidin. Which makes them transparent I guess. Essentially nonexistent in thin epidermis. (they kinda exist but their cells don’t form thick, distinct layers as they do in thick skin, so histologists decide its not a layer)
- Stratum granulosum – creates a lipid barrier that makes the epidermis waterproof from the inside and out. The lipid barrier is created by membrane-coating granules which transport out (exocytose) their contents out into the junction between the stratum granulosum and the stratum lucidum. Again, essentially nonexistent in thin epidermis.
- Stratum spinosum – composed of several layers of polyhedral cells which can also undergo mitosis, but the outer parts of the cells cannot. Also has a lipid barrier.
- Stratum basale – the innermost part of the epidermis, composed of a single layer of cuboidal-to-low-columnar shaped cells, which sits right on the basement membrane. Actively undergoes cell division, where the newer cells push out their older predecessors into the upper surfaces.
The outer four layers stated above have desmosome type cell junctions, aside from the stratum basale which form hemidesmosomes with the underlying basal lamina.
DERMIS
Below the epidermis lies the dermis, which consists of two layers – the outside, relatively looser papillary layer and the deeper, denser reticular layer. Both layers contain a dense irregular fibroelastic connective tissue.
The papillary layer, wherein lies the basement membrane, forms “ridges” called dermal ridges or dermal papillae that point outwards towards the direction of the epidermis. Consists of type 3 collagen and elastic fibers, with the occasional anchoring fibrils that attaches the basement membrane with this layer. Contains capillary loops in which nutrients can diffuse out into the epidermis. Also contains special nerve endings such as Meissner tactile receptors and Krause thermoreceptors, and specialized nerve endings that function as nocireceptors.
The reticular layer is a much denser connective tissue, which consist of type 1 collagen with thick elastic fibers, with the help of dermatan sulfate rich ground substance. Usually has the deeper portions of sweat glands, sebaceous glands, hair follicles and their respective arrector pilli muscles (which cause them to become erect, stimulated by peritrichial nerve endings), also contains plenty of blood and lymph vessels. But most notably, they also have specialized nerve endings called Paccinian and Ruffini corpuscles that respond to deep pressure and high temperatures, respectively.
SKIN APPENDAGES – GLANDS, HAIR, AND NAILS
The skin has two major types of glands, sebaceous glands which secrete an oily substance called sebum, and sweat glands which secrete sweat.
Sebaceous glands are simple cuboidal epithelial cells, differentiated into glands, which are holocrine glands – which means they commit honorable seppuku, killing themselves and exploding, spreading their oily, sebum contents all over the place through hair follicles. The sebum itself contains fat and cellular debris and keeps the skin and hair soft and waterproof. These glands are present everywhere except the palms and feet, and are stimulated by androgens during puberty – thus explaining why people get more acnes during puberty.
Oh right. Overproduction of these things into the hair folicles may cause clogging and can also predispose into an infection which can usually cause acnes.
Sweat glands…secrete sweat. They are usually eccrine/merocrine glands, which means they spit out their contents through the means of exocytosis, but they can also act as apocrine glands under the influence of certain hormones. The secretory unit consists of the duct and myoepithelial cells. The myoepithelial cells consist of dark and clear cells, where dark cells release a more mucousy secretion, while clear cells secrete a more serousy secretory product. The sweat itself is more or less isoosmotic with the blood plasma, but the cells of the duct conserve sodium, chloride and potassium.
Sweat glands can also differentiate, forming cerumen releasing glands in the ear, mammary glands in breasts, and Moll glands in the eyelids, specialized sweat glands for the eyelids.
Hair follicles functions consist of temperature control, shielding and filtration of pathogens. From hair follicles, out comes hair. Usually hair folicles extend all the way into the hypodermis. Hair folicles are covered by a special basement membrane termed a glassy membrane, which is surrounded by dermally derived connective tissue membrane. The hair itself consists of the hair root, where the core of the hair root consists of cells known as a matrix, where the mitotic activity there is responsible for hair growth.
Hair grows around 2-3mm/week, and occurs in three phases: anagen, catagen and telogen. Anagen is where the growth period happens, catagen is where the hair bulb shrinks and degenerates, and the telogen is where the hair follicle stops growing until the hair falls out where new hair replaces it.
Nail plates are thick plates of horny keratin (that sounds so wrong im sorry). Each nail plate lies on the nail bed, and grows from the nail matrix outside.
EFFLORESCENCE – AKA DERMATOLOGIC EXAMINATION AND COMMON SKIN LESION
Effluorescence is the act of evaluating a skin lesion as objective as possible. The evaluation consists of determining the type of lesion, the characteristics, its arrangement and how the lesion is distributed.
Skin lesions are primarily divided into two major types: primary lesions and secondary lesions. Primary lesions are lesions that just happen on their own due to a disease or an abnormal state, secondary lesions are primary lesions that have progressed and do not form by themselves.
To avoid overcomplicating things, I will try to define each type of lesion as simply as possible:
- Primary Lesions
- Macules = flat,lesions with no palpable alterations (doesn’t become more rough or tender). In general, can be caused due to vasodilatation, bleeding, or pigmentation change. A large macule (>1cm in diameter) is sometimes referred to as a patch.
- Hyperemia is usually just caused by local vasodilatation, and some inflammatory changes. It loses color when you press it.
- Haemorrhage/purpura is when bleeding is involved, and does NOT lose color when you press it. It comes in three flavors:
- Petechiae is when the lesion is <2mm in diameter,
- ecchymoses is when the lesion is >3mm in diameter,
- and the third is called a vibises which is somewhere between the two.
- Changes of skin pigmentation consist of hyperpigmentation (>>color), hypopigmentation (<<color), and depigmentation (loss of pigments/melanin)
- Papules = elevations on the skin which can be palpated, usually less than 1mm in diameter. Usually caused due to hyperplasia of the epithelial cells, lipid deposits, or clumps of benign or malignant cells.
- Plaques = essentially larger papules (usually >1mm in size), or can also manifest as accumulations of papules in a small cluster.
- Vesicles = lesions that contains clear fluid, usually feels solid and raised (<1cm in diameter)
- Bulla/blisters = larger vesicles (>1cm in diameter)
- Pustules = vesicles that has pus or cellular debris inside of them
- Nodules = Rounder looking lesions with a diameter of usually more than 0,5mm. Contains solid tissue. Depending on the layer where the lesion originates, it can be divided into 5 types: epidermal, epidermal-dermal, dermal, dermal-subdermal, and subcutaneous.
- Cysts = basically nodules that is usually filled with liquids or semisolids
- Wheals/Urticaria = defined as a transition between a papule and a plaque. Usually red and edematous, also usually comes with some degree of elevation.
- Teleangiectasia = visible dilated capillaries
- Comedo/Blackheads = more or less a hair follicle that is enlarged and contains lots and lots of dead keratinocytes. Usually black in appearance and difficult to get rid of.
- Macules = flat,lesions with no palpable alterations (doesn’t become more rough or tender). In general, can be caused due to vasodilatation, bleeding, or pigmentation change. A large macule (>1cm in diameter) is sometimes referred to as a patch.
2. Secondary Lesions
- Crusts = dried exudates which may have been blood or serous/purrulent material.
- Excoriation = You know how when you scratch an itch in your skin then you get that red wound from overscratching your skin? Yea that thing’s called an excoriation.
- Lichenification = when the skin thickens because its overexposed to stress i.e. you scratching it too much, thus causing the skin to thicken.
- Scaling = flat plate or a lamella of the stratum corneum, essentially theres another layer on your skin.
- Exfoliation = the splitting of the stratum corneum into scales or sheets
- Fissures = a clear, linear split or gap in the skin surface
- Ulcers = a hole in the skin due to impairment of nutrients in the skin. Usually extends from the epidermis to a part of the dermis.
- Keratoderma = a thickening of the keratin layer of the skin, usually due to a congenital keratin overproduction disease
- Scar= essentialy a wound that had undergone repair and causing a white, shiny lesion.
- Atrophy = when your skin get wrinkled and thinner due to age, burns, or side effects of chronic topical corticosteroids.
I guess that’s about it about the basics of the skin. I sincerely hope you did not sweat too much by the sheer volume of the things you are going to learn in this system.
To close this post off, I just want to say that the skin – contrary to popular belief – is not as simple as it seems. It is actually really, really complex and a lot of respect has to be given to dermatologists. The underlying mechanisms of skin diseases can also be a manifestation of other systemic diseases – for instance, skin lesions in patients with TB. Also, since the skin is basically one wide ass barrier against pathogens, the clinical manifestations are huge, because of how easily the pathogen can reach the skin, and how many kinds of pathogens that can easily contact the skin, bust through their defenses and become presented into nearby APCs so inflammation can take place.
Bottom line is – do not underestimate the Intergumentary system. It is hard.
Also, don’t forget to wash your face so you don’t end up like the guy in the video at the start.
#medschoolishard
#notsponsoredbypondsmens
#saynotoacnes
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