Integumentary system — Layers of the skin and Thick vs. Thin skin

Layers of skin

03.02.01 layers of the skin

 

03.11 thin skin 40X major layers

Skin has three major layers (or two if your instructor does not count the subcutaneous layer).  Starting from the outer surface these major layers are the epidermis, dermis, and subcutaneous.

  • epidermis:  (keratinized stratified squamous epithelium)  This layer is the outer most major layer, it is the layer we touch and touch with.  The epidermis is made of keratinized stratified squamous epithelium.  There are no blood vessels in this layer, but some receptors extend into this layer (that’s why sometimes a paper cut can hurt bad but not bleed).  The majority of cells in this layer are called keratinocytes because they become highly keratinized as they approach the surface.  All epithelial cells make keratin by the way, but these epithelial cells make a lot more than do the epithelial cells that line your mouth for example, and the greater amount of keratin in the outer layers of the epidermis makes for a tough outer surface that resists abrasion and puncture.  This layer takes most of the abuse that we give our skin, we rub off cells from this layer basically 24 hours a day as we move and brush against things like our clothing and furniture (these flakes of skin cells make up a great deal of the dust in our homes–  gross, but it gives you an idea of how just how many skin cells you wear off every day).  There are other cell types in the epidermis including stem cells, dermal dendritic cells (Langerhans cells), melanocytes, and tactile cells (Merkel cells).  Here is the story on each of these cell types…
    • Keratinocyte stem cells:  these stem cells are found deep in the epidermis and their job is to continuously divide (by mitosis) and give rise to new keratinocytes to replace those lost off of the surface of the epidermis.  These are stem cells that are unipotent, meaning that they can only turn into one type of cell.
    • Dendritic cells (Langerhans cells):  These are a type of white blood cell.  These cells are part of the immune system.  Many aliens come to the surface of your body, from viruses and bacteria, to yeasts and other fungus, and many of these make it into the upper layers of the epidermis.  Many of these aliens are completely normal to have on your skin (normal flora), and some are even helpful–  However we don’t want them to get into the body proper or they might cause infection.  I tend to think of these Langerhans or skin dendritic cells as scouts for the immune system…  The job of a dendritic cell is to phagocitize aliens (helping to make sure they don’t get deeper into the body) and report back to the immune system, showing the immune system exactly what kinds of aliens live on the skin…  In this way the dendritic cells both prevent infection by normal flora and they allow the immune system to develop defenses against these aliens just in case a break in the skin occurs and some of these organisms get into the underlying tissues.  (We have many other defenses that will be described when we cover body defenses)
    • Melanocytes: these cells produce the pigment molecule melanin.  Melanin first evolved as a way to protect us from UV radiation from the sun.  There are several types of melanin and three are found in human skin:  black eumelanin, brown eumelanin, and pheomelanin (red).  The type and amount of these that are produced are a major contributor to our base skin color and hair colors.  If you wonder why we have such variation of skin and hair color in humans I will be adding a blog on skin color variation in humans.  Right now I’ll just say that it has to do with our historic need for UV radiation to make vitamin D combined with our need to protect ourselves from the damage that UV radiation can do to us (sun burns and skin cancer).
    • Tactile cells (Merkel cells):  These cells are a type of mechanoreceptor sensitive to light touch and feeling the texture of an object.  You have lots of these on your fingertips.

03.11 thin skin 40X major layers

Note:  In the image above we so not only the major layers of the skin, but also hair follicles, a sebaceous gland, and an eccrine sweat gland.

  • dermis:  (Mostly dense irregular connective tissue with embedded glands, hairs, receptors, etc.) The dermis layer of skin is made mostly of dense irregular connective tissue…  lots and lots of collagen fibers for strength as well as elastic fibers for resilience.  If you pull on part of your skin it will not rip due to the strength of the collagen fibers, and when you let it go it will bounce back into position due to the elastic fibers.  The dermis is also the home to most of the accessory structures -or- derivatives of skin such as hair, sweat glands, oil glands, and receptors.  There are actually two layers to the dermis, an uppermost papillary dermis, and a more dense reticular dermis.
    • Papillary dermis:  This is a thin layer of the dermis that makes contact with the epidermis, it is very vascular and it contains fewer collagen fibers with space in between…  this allows space for white blood cells to move through.  A special type of white blood cells called neutrophils patrols this papillary region just in case there is a break in the skin and aliens get in.
      • Dermal papillae:  these are projections of the dermis into the epidermis.  These papillae often contain receptors and in thick skin they give rise to friction ridges (fingerprints).  Notice that we have these friction ridges on all thick skin surfaces (not just on our fingertips).
      • 03.25 Thick skin 100X 03
      • Thick skin (100 X magnification):  Note the dermal papillae in this image and how the basic structure that they produce is reflected at the upper surface of the skin…  these are the ridges of fingerprints!!!
    • Reticular dermis:   this makes up most of the dermal layer of the skin (only the thin upper part is not included).

 

    • Dermis, epidermis, and tattoos:  When a person gets a tattoo a needle bringing ink is past through the epidermis and into the dermis repeatedly.  The process stains the connective tissue of the dermis and the epidermis with the ink.  However, the ink only stays around in the dermis… the epidermis has such a fast turn over rate of cells that the ink is lost from the epidermis as fast as the epidermis heals from the initial damage of getting the tattoo.  This means that when you look at a tattoo you are actually seeing through the epidermis and seeing the ink in the person’s dermal layer.  The ink is trapped in the extracellular matrix of the dermis, which has a very slow turnover rate.  Over time the ink will fade and also diffuse out…  this is the reason that tattoo lines get faded and a bit wider as time goes by.
  • Subcutaneous layer or hypodermis:  this is the layer under the dermis.  It may or may not be considered part of the skin (its name actually means “under the skin”), but in many locations skin structures extend into the hypodermis (you’ll notice that in my histology images of thick skin the base of sweat glands and some receptors are in the fat tissue of the hypodermis).  This layer consists of mainly adipose tissue, at least over most of the body.

 

Layers of the epidermis:

03.13 thick and thin skin labeled for epidermal layers 450X

The epidermis has five specific layers, all of these are made of keratinocytes, although other cell types are present.  Above is a labeled image of these layers in both thick and thin skin (450 X magnification) and I will list and describe each layer below…

  • Stratum corneum:  this is a layer of dead keratinized cells.  It is very much thicker in thick skin, and in the area of a callus it can be quite thick (a callus is basically just a very thick stratum corneum).  The cells in this layer are dead and are thus lacking nuclei and other organelles; from the original cell the only thing that is really left is the keratin protein that makes this a very tough layer.
  • Stratum lucedum: this layer is only found in thick skin.  It is at the bottom of the stratum corneum and it is simply a more translucent layer of these dead cells.
  • Stratum granulosum:  this layer is very visible in thick skin.  The cells here stain with a granular appearance.  The keratinocytes lose their nuclei and organelles in this layer.  One other important thing that happens at this particular level is that the cells are secreting lipids and proteins.  The functions of these secretions are still being studied, but one well known reason for them is to create a lipid barrier in the stratum granulosum, and this is a critical part of the skin’s water barrier properties…  so, this may be the most important layer for making sure we don’t dehydrate through loss of water through the skin.
  • Stratum spinosum:  in thin skin this is the thickest layer of the epidermis.  It contains keratinocytes that are moving upwards as more cells are added below.  Some of the cells here are dividing.  The cells are very actively producing keratin as them move up, and they are connecting with each other firmly by producing connections called desmosomes.  It is also in this layer that we can find dendritic cells (Langerhans cells).
  • Stratum basal (stratum germinativum):  this layer is quite thin (usually only one cell layer thick) and contains mostly keratinocyte stem cells that are continuously dividing to give rise to the rest of the epidermis.  In this layer we can also find tactile cells (Merkel cells), melanocytes, and dendritic cells (Langerhans cells)— notice that dendritic cells can be found in two layers (one of the reasons for this is that these are white blood cells that can move on their own.  Remember that dendritic cells phagocytize aliens… then they go back into the body and head for the lymphatic system where they report to immune system cells on what is living on/in the skin.

Thick vs. Thin Skin:

03.21 Thick skin 40X pacinian corpuscle

Thick skin (40 X magnification)

03.11 thin skin 40X

Thin skin (40 X magnification)

Note the differences in thick and thin skin when viewed at the same magnification.  The major reason that thick skin is called thick skin is that is has a much thicker (and therefore tougher) epidermis, especially the stratum corneum layer of the epidermis.  Note also that, at least in the slides from my lab, the thick skin has a thinner dermis layer and a much thicker subcutaneous (hypodermis) layer consisting of lobules of adipose tissue that is separated by strands of dense collagen fibers.  The adipose tissue pads the thick skin while the collagen framework it is contained in gives strength…  compare the palm of your hand to the back of your hand to observe this difference in tough padding.

Another thing to notice is that while thin skin has hair follicles and sebaceous glands thick skin does not…  we do not ever grow hair on our palms (regardless of what you parents told you about your alone time activities :).  There are however eccrine sweat glands found in both thick and thin skin, but thick skin never has apocrine sweat glands.  Apocrine sweat glands are found primarily in our axill (axillary or armpit) and groin regions.

Have any questions or comments?  Please add them below 🙂

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