Skeletal System– Anatomy of a Long Bone

Long bones are longer than they are wide and include such specific bones as the femur (thigh bone), radius, and phalangeal bones (finger and toe bones).  All long bones have the same basic structure that we can look at with a simple drawing…

long-bone-drawing
Drawing of a long bone (unlabeled)
long-bone-drawing-labeled
Long bone drawing (labeled)

Here is a bit of information for each of the structures labeled above…

Articular cartilage:  Articular cartilage is usually found at the end of the long bone as part of a synovial joint (where the bone articulates with another bone).  Articular cartilage is usually made of hyaline cartilage (in the knee joint there is also fibrocartilage)

Proximal epiphysis:  The long bone is wider at its ends…  we call these ends epiphyses.  The epiphysis that is closer to the torso is called the proximal epiphysis.

Spongy bone (cancellous bone, trabecular bone):  Bone comes in two basic tissue types:  Spongy bone and Compact bone (more about this in “bone tissue”).  Where the bone tissue has many spaces in it (making it look a bit like a sponge) we call it spongy bone.  Spongy bone can also be called cancellous bone or trabecular bone (these terms refer to the small spicules or pieces of bone that make up the tissue).  The spaces here help make the bone lighter, while the arrangement of the spicules of bone keep the bone strong (kind of like the steel girders in a building are arranged to give the building strength).  The spaces in spongy bone usually contain red bone marrow (sight of blood cell formation).

Compact bone (cortical bone):  Compact bone is dense bone tissue and it is found along the diaphysis (shaft) of the long bone and the outside of the epiphyses of a long bone.  There are no grossly visible spaces in compact bone like we see in spongy bone.  Compact bone is also called “cortical bone” because it makes up the cortex (outside part) of all healthy bones.

Epiphyseal plate or Epiphyseal line: These two terms are not the same thing!!   The epiphyseal plate is made of actively growing hyaline cartilage and it is where the long bone grows in length in a child.  Once the child is done growing these epiphyseal plates fuse (turning completely into bone tissue) and become epiphyseal plates.  The epiphyseal plates fade with time as the bone remodels itself (bone is always remodeling itself–  it is an dynamic and ever changing tissue).

Diaphysis:  Also called the shaft of the bone the diaphysis is the part of the bone between the two epiphyses.  It is the long part of the long bone and most of it is made of compact bone.

Medullary cavity:  This is a cavity inside of the diaphysis.  The medullary cavity is also called the marrow cavity as it contains mostly yellow bone marrow.  Yellow bone marrow is adipose (fat) tissue.  So, bones store energy as well as support the body.

Distal epiphysis:  The distal epiphysis is the widened area of the long bone at the end that is furthest from the torso of the body.  The distal epiphysis also has articular cartilage and contains spongy bone (although on the drawing these were not included).

Up next:  Bone tissue

Skeletal System and Joints

01-entire-skeleton-front-01-labeled-axial-yellow-shaded
Human Skeleton with major bones labeled (axial bones are shaded yellow)
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Human Skeleton Front unlabeled

The skeletal system includes all of the bones of the body as well as the ligaments, cartilage, and other connective tissues that connect these bones and allow for flexible movement.  It includes specific bones such as the femur, tibia, and humerus (there are 206 different bones in the average adult human).

We have a skeletal system for support, to allow for flexible movement (muscles do the moving, but the bones and joints allow for movement), and for the storage of minerals like calcium and phosphate as well as energy storage (most long bones store fat in a marrow cavity (medullary cavity)).

This series of blogs will cover (click each for details)…

Integumentary System — Receptors

Receptors Associated with Skin

One of the functions that skin performs is that of sensory input to the brain, letting us know a lot about the world around us as well as how the skin is doing.  In order to perform this function the skin has receptors that detect touch, pressure, pain, and heat.

03.05 Skin model 01 receptors in skin labeled
Receptors of the Integumentary System (click the image for a larger version)

Here is a list of the known skin receptors and what they detect:

Tactile corpuscles (Meissner’s corpuscles):  These receptors detect light touch at the skin surface.  Basically when something touches the skin surface these receptors detect the vibrations of that touch and send signals to the brain.  Tactile corpuscles are found very close to the surface of the skin, just below the epidermis and usually inside of dermal papillae.  Tactile corpuscles are most concentrated in thick skin (especially at the tips of the fingers).  These receptors are also quick adapting, meaning that if the stimulus remains constant these receptors stop sending signals within a few moments.  You can experience this sensory adaptation by placing a small coin on the back of your hand and letting it sit there, you will feel it touch you when you place it there, but that feeling will go away pretty quickly.

One way to easily recognize tactile corpuscles is to look for an oval structure that has straight lines across it.  Take a look at what they look like under a microscope…

03.27 Thick skin 450X with Meisner's corpuscle at pointer 02
Tactile Corpuscle at pointer (thick skin 450X magnification)
03.28 Thick skin 450X with Meisner's corpuscle at pointer
Tactile corpuscle at pointer (thick skin 450X magnification)

Lamellar Corpuscles (Pacinian Corpuscles)

Lamellar corpuscles detect deep pressure and vibration.  These are important for detecting the pressures involved when you pick up an object for example, and they may also be important for our sense of an objects surface texture.  Tactile corpuscles are found deep in the dermis or in the adipose tissue that underlies the dermis.  They are generally round in shape and have multiple layers (kind of like an onion).  Here are a few images of lamellar corpuscles under the microscope…

03.35 Thick skin 100X pacinian corpuscle 02
Lamellar Corpuscle in center of image (thick skin 100X magnification)

 

03.36 Thick skin 450X pacinian corpuscle 02
Lamellar Corpuscle (thick skin 450X magnification)

 

03.34 pacinian corpuscle 100X
Lamellar Corpuscle in a pancreas (pancreas 100X magnification)

Hair Follicle Receptors (hair plexus -or- root hair plexus) 

Hair follicle receptors are nerve endings that wrap around the base of a hair follicle and are stimulated by movement of the hair.  If something bends or vibrates the hairs on your forearm for example you can feel it (if an ant is crawling on your arm, or if wind blows over the surface of your arm).  These hair follicle receptors contribute to our sense of touch.

I don’t have any histology images of hair follicle receptors, but the image of the model above shows where they are and what they basically look like.

Free Nerve Endings

Free nerve endings are sensitive to changes in temperature (we sense this as warm and cold) and to stimuli that we sense as pain (those are the two stimuli that I have my students remember, but you may also need to know that there are different types of free nerve endings including some that detect touch and pressure).  Free nerve endings that sense pain have receptors for chemicals released from damaged tissues/cells and they also send signals if they are damaged themselves.  One example of a chemical that can stimulate these receptors is the chemical found in hot peppers called capsaicin; this chemical binds to receptors on pain sensing nerve endings and causes them to send pain/heat signals to the brain.  Pain receptors are also called nociceptors and lastly they do not adapt well at all (this is at least part of why pain is hard to ignore).

I don’t have histology images of free nerve endings, but the image of the skin model above should give you an idea of what they look like.

Bulbous Corpuscle (Ruffini)

Bulbous corpuscles are oblong receptors that are stimulated by stretch and pressure.  Bulbous corpuscles are also very slow to adapt, so these are important for sensing sustained pressure/stretch.  They appear to be important for sensing how firmly to hold objects as well as sensing when our grip is slipping.

I don’t have any histology of bulbous corpuscles, but here is a link to one if the first images of one of these receptors from Wikipedia…

Ruffini image

Note that in that image you can see some light colored fibers (those are collagen fibers) and you can see some very dark stained fibers (the stain is selective for nervous tissue, so the dark stained areas are nerve fibers of the bulbous corpuscle receptor)

Bulboid corpuscle (Karause’s end bulb)

These are rounded receptors found in the upper dermis that detect heat loss (cold).

Again I have no histology images of this receptor, so look at the image of the model above to get an idea of what these look like.

Not Just for Skin

Another good thing to know (for later when you cover senses) is that most of these receptors (and some other specialized receptors) are found all over inside the body as part of what we call general senses (sense of touch, stretch, pressure, pain, and hot & cold).  Inside joints like the knee for example we have receptors for stretch, pressure, and pain and these let our brain know the position our limbs (proprioception) as well as when a joint gets damaged.  Muscles, bones, tendons and ligaments also contain receptors for the same basic reasons.  Even our viscera (internal organs like the stomach, intestines, etc.) have general receptors so that we have an idea of what is going on with them.

As always if you have any questions or comments please add them below.

 

Integumentary System — Glands

Anatomy & Physiology of Glands found in skin:  

Glands found in skin include:  eccrine and apocrine sweat glands (sudoriferous glands), sebaceous glands, ceruminous glands, and mammary glands.  All of these are exocrine glands (they produce a product and deliver it to the surface of an epithelium– in this case the epidermis).

03.04 glands of the skin

03.08 Skin model-1 a glands

Sweat Glands:

  • Eccrine sweat glands:  These are simple tubed glands that produce a very watery sweat.  The major function of eccrine sweat glands is to help cool us down.  The ducts of eccrine sweat glands lead to the surface of the skin.  Eccrine sweat glands are found over most of the body, including the palms of our hands and soles of our feet.  Eccrine sweat glands are merocrine glands in that the secretory cells release their product by exocytosis.
    • What is sweat?  Sweat from eccrine sweat glands is mostly water (around 99% water), but it also contains some sodium chloride, urea, ammonia, lactic acid, and other substances.  Sweat is somewhat acidic (pH around 5) which contributes to our skin’s defense against bacteria (bacteria don’t like acidic conditions).
  • Apocrine sweat glands:  These sweat glands are larger than eccrine sweat glands, and they produce a more oily sweat into hair follicles.  Apocrine sweat contains pheromones (scent molecules) and because they are secreted into a hair follicle and thus onto a hair the pheromones get out into the air more efficiently.  Apocrine sweat glands are triggered to release sweat when we are emotionally excited, the firing of these glands is part of sexual arousal for example.  In most people apocrine sweat glands are found in only a few regions including the groin and axillary regions.  Apocrine glands secrete by apocrine secretion, meaning that a portion of the apical surface of the secreting cells actually pinch off in order to secrete the product, so part of the cell is actually lost in the process.

Sebaceous glands

  • Sebaceous glands:  Sebaceous glands secrete an oily substance called sebum.  The function of this substance is to condition skin and hair.  When we bathe we remove sebum with soap and water and therefore often need to apply conditioner to our hair and lotion to our skin.  Our skin and hair would probably be healthier if we only showered once or twice a week, but of course then we would smell more.  Sebaceous glands secrete by holocrine secretion meaning that the secreting cells literally rupture in order to secrete the sebum, so the entire cell is lost in secretion (more cells are being made at the basal surface of the gland’s epithelium).

Ceruminous glands

  • Ceruminous glands:  These glands are found in the outer ear canal and they produce cerumen or ear wax.  Cerumen itself is mostly made of long chain fatty acids and cholesteral molecules (these are more solid and wax like at room temperatures).  Cerumen functions to keep the tympanic membrane (ear drum) conditioned as well as the skin that lines the outer ear canal, it also has antibacterial properties.  Ear wax is actually made up of cerumen, sebum, dead skin cells, and often also contains guard hairs released from the wall of the ear canal.
    • Different people make different ear wax:  Some people make a more soft/sticky ear wax (wet type) while others produce ear wax that is more crusty (dry type).  People that produce the dry type of ear wax also sweat less.  This may have developed as an adaptation to living in colder climates.  If your ancestors came from East Asia you might be a dry type ear wax producer.
    • Cerumen impaction: In many people ear wax accumulates faster then it drains and this can lead to a cerumen impaction, a build up that is great enough to block the ear canal and/or press on the tympanic membrane, interfering with hearing and possibly causing other discomfort.  When I worked as a medical assistant I found the most effective way to deal with this was by using a large syringe (20 to 30cc) with an attached 1 to 1.5 cm rubber tube (usually from a butterfly needle).  I used warm water in the syringe to irrigate the outer ear canal until the cerumen impaction washed out.  If the impaction did not come out with several flushes I would very carefully use a plastic looped end probe to gently pull it out.  I would only use warm water because cold water is uncomfortable and can cause acute dizzyness, and I would only use a looped end probe while also looking in the canal so that I could be absolutely sure not to touch the tympanic membrane (the ear drum is very sensitive and delicate).

Mammary Glands

  • Mammary glands:  These glands produce milk in order to feed our offspring.  It may seem strange to discuss them when talking about skin, but there is a good reason for that.  Evolutionarily speaking, mammary glands are modified sweat glands.  These glands are also found in the hypodermal layer in both males and females in what is called the mammary fat pad (just superficial to the pectoral muscles).  In females the mammary glands and nipples are more developed and only female humans develop more fat tissue in the mammary region (with the exception of human males with gynocomastia).  Mammary glands secrete by apocrine secretion, meaning that a portion of the apical surface of the secreting cells actually pinch off in order to secrete the product, so part of the cell is actually lost in the process.  I’ll discuss mammary glands more when I cover reproduction.
    • Why do we think that mammary glands are modified sweat glands?  One piece of evidence that mammary glands are modified sweat glands comes from our friend the platypus.  This strange little creature does not have nipples and feeds its young from milk pads that consist of mammary glands with ducts leading to the skin surface (much like sweat glands).  The milk is then lapped up off of mom’s skin by the baby platypus.

That’s it for glands found in and under the skin.  Have any questions or comments?  Please leave them below.

Integumentary system — Nails

Anatomy & Physiology of Nails:  

Fingernails and toenails function to protect and support the ends or our digits.

Anatomy of nails (click on image for larger version):

03.05 fingers 013

03.05 fingers 013 labeled

03.05 fingers 013 drawing

03.05 fingers 013 drawing labeled

03.05 finger section drawing

03.05 finger section drawing labeled

 

Growth of nails (and why we don’t have claws):

Fingernails and toenails grow much in the same way that hair does.  In the nail matrix, which is at the proximal end of the nail just deep and a bit proximal to the lanula, there are keratinocyte stem cells dividing and giving rise to the cells that will become part of the nail.  The cells that are produced quickly fill with keratin (lots of keratin) and then die and become part of the nail.  So, nails are made of dead highly keratinized cells.  As the nail grows it moves toward the tip of the digit along the nail bed.  Cells are added to the nail along the length of the nail bed, they are simply added faster at the proximal end where the nail matrix is (that is why our nails grow out towards the end of our digits instead of up away from our digits the way the claws of cats, dogs, and other animals do… leading to claws).

Integumentary — Hair

The Anatomy & Physiology of Human Hair:

For anatomy related to hair see the labeled images here

Hair is found almost everywhere on the human body except on our eyelids, palms of our hands, bottoms of our feet, and parts of our genitals.

03 hair Fedor_Jeftichew_portrait 02

Fedor Jeftichew worked in side shows and circuses as “Jo Jo the dog faced boy.”  He had hypertrichosis

Vellus hair:  Over most of our body hair does not grow very long, but you can still see little hairs even on your forehead and earlobes if you look close.  These very short hairs are called vellus hair.  There is an interesting disorder called hypertrichosis in which even the vellus hair grows long.  This allows us to better see that hair grows almost everywhere on the human body.

Terminal hair:  This is the type of hair that grows on the top of the head, the eyebrows, and in the adult axillary and groin regions.  Body and facial terminal hair is also called androgenic hair because it is responsive to the hormone testosterone that signals it to grow longer starting a puberty.

Lanugo:  This is a type of very fine hair seen only on the fetus during the last few months of development.

Functions of hair: The major functions of hair include sensory function, protection from UV radiation, reduce friction, heat retention, gender identification, and communication.

  • Sensory function:  Each hair grows out of an oblique tube called a hair follicle.  At the base of each hair follicle is a hair plexus, a sensitive collection of nerve fibers that detect when the hair moves.  This is part of our sense of touch, it is part of how we might feel a bug crawling on us, a loved one’s soft touch, or wind blowing over our skin.
  • Protection from UV radiation:  Any hair growing in great enough amounts and with adequate pigmentation (melanin pigments) is also effective at blocking UV radiation.  This is especially true for the top of your head.
  • Reduce Friction:  In adults any area where skin rubs against itself during normal walking or running has adequate hair to decrease friction between the rubbing skin, unless we have removed the hair by shaving or waxing.  Hair reduces friction by rolling between the two layers of skin.  People who remove this hair often have to compensate for the loss of friction reduction by using clothing, lotions, powders, or they just deal with the occasional chafing that occurs.
  • Heat retention:  Hair makes a good insulator by trapping air close to our skin.  In order for this to work the hair has to be long enough, because of this fact the only area of the body where this works well (for most of us) is the top of the head.  Goose bumps (caused by the activation of arrector pili muscles) occur when we are cold and they have the effect of making our hair stand up more straight…  this helps to trap more air close to our skin and insulate us, this is at least somewhat effective on most of us on areas like our forearms and legs, but to be really effective the hair must long enough.
  • Gender identification:  Men and women have major differences in terms of body hair (androgenic hair) growth.  Men tend to grow much more body hair, and men grow hair on their faces, while women do not grow facial hair (at least not usually), and have less body hair.  This difference on body hair growth makes gender identification easier and more efficient and we usually help this along by identifying our with our hair style and clothing choices.
  • Communication:  There are a few ways in which we communicate with our hair.  The general appearance of a person’s hair tells you a few things about them.  If there hair is well kept, clean, and healthy appearing there is a good chance that that person is well fed, healthy, and that they generally take care of themselves.  This is useful, along with gender identification, when we are searching for potential mates.  Another way that hair is used for communication is seen in our eyebrows.  We can move our eyebrows and those movements are a major part of facial expressions.  Lastly, hair in our groin and axillary regions help to get signaling molecules called pheromones into the air.  Pheromones are scent molecules that we release, especially when sexually aroused.

Straight vs Curly vs Kinky Hair:

The amount of natural curl to a person’s hair depends on the cross sectional shape of the hair shaft.  Straight hair has a very round cross section, and the less round the hair’s cross section (the more oval or flattened) the more curly or kinky the hair, and also the lower the cross sectional area of the hair (the hair is thinner).  Hair that is very curly or kinky is also more easily broken due to its thinner nature (less cross sectional area) compared to straight hair.  Due to being oval and thus more flattened in cross section kinky hair also has a greater outer surface area to its volume, this likely leads to a more rapid loss of moisture, allowing curly and kinky hair to dry out faster.  Thus, kinky hair likely requires more moisturization.  Knowing these characteristics of different hair can be helpful in understanding how different types of hair have different care needs.

Histology — Wound Healing

Wound Healing Example

Wound healing occurs in three major stages:  hemostasis phase, inflammatory phase, proliferative phase, and maturation/remodeling phase.  We use these phases to help us learn what is happening during wound healing, but in reality they overlap quite a bit.  Here is a brief description of each of these stages.

Hemostasis Phase:  This phase occurs very quickly and directly after injury, it involves blood leaking from damaged vessels in the area of the would and the coagulation of that blood.  Coagulated blood has the effect of preventing further bleeding, releasing chemical signals that help initiate inflammation and healing, and creating an initial framework that can help hold the damaged tissue together.  A blood clot at the skin surface also forms a scab when it dries, the scab is natures band-aid, it creates a barrier that keeps the healing tissue from drying out while helping to prevent aliens from getting into the healing tissue.

Inflammatory Phase:  The inflammatory phase starts right after injury and may last a few days, it is mediated by chemicals released during blood clotting and also released by damaged cells in the tissue.  Inflammation involves the dilation of blood vessels in the tissue (brings more nutrients and heat) as well as an opening up of spaces between the cells of blood vessel walls that allows plasma proteins and white blood cells (WBCs) to enter the tissue from the blood stream.  This leads to an increase of fluid in the tissue, the presence of active clotting proteins and antibodies in the tissue, and activated phagocytic WBCs in the tissue.  The clotting proteins prevent or at least slow any aliens from leaving the area while antibodies bind known aliens and tag them for phagocytosis, the increase in fluid in the tissue causes a shift of more fluid entering the lymphatic system and gives the WBCs room to phagocytize debris and any aliens.  Chemicals released during this phase stimulate cells in the damaged tissue to divide, leading to the next phase.

Proliferative Phase:  The proliferative phase begins as the inflammation subsides and may last a few weeks, it involves cells in the tissue dividing in order to replace dead cells, produce more fibroblasts (cells that produce collagen and other ECM components), and produce new capillary blood vessels to replace damaged ones and supply nutrients to the healing tissue.  Macrophages phagocytize the initial blood clot components.  Granulation tissue may form and grow to fill space if there is a large gap between parts of the tissue.  Collagen fibers are laid down during this phase, and wound contraction also happens during this phase.

Maturation/Remodeling Phase:  The maturation/remodeling phase begins a few to several weeks after injury and is the longest phase, sometimes lasting years.   This phase involves first the development of and then the long term replacement of fibrotic tissue scarring.  The amount of scarring that occurs depends on the size of the initial wound and any disruption of the wound during healing (this and the risk of infection is why you should not pick at a scab); other factors can contribute to scarring such as genetics (some people are just more prone to develop keloid scarring for example).  The continuous remodeling that occurs for years explains why scars tend to fade over time.

Histology — The Light Microscope

Light microscopes come in many varieties but they all have the same basic components.  Here is an image of a light microscope with its parts labeled…  (click on it for a larger image)

02.01.02 Microscope with labels

Here is a detailed list with descriptions of the function of each part…

  1. Eyepiece: The eyepiece functions as the last lens that light passes through before you see it.  Most eyepieces also contain a pointer and/or a reticle.
    1. The pointer functions to point out specific things that are being viewed (an instructor or student may place the pointer on a specific cell so that another person can see exactly which cell is being looked at or asked about).
    2. The reticle appears as a small ruler with lines and numbers (the units of the reticle depend on the magnification being used).
  2. Arm: The arm of the microscope functions as a very stable connection between the viewing parts of the microscope with the base and stage of the microscope.  The arm, along with the base, are also used to carry the microscope
  3. Mechanical stage: The mechanical stage has knobs at the side of the stage that are used to move the slide being viewed in two dimensions  (up-down and left-right) as viewed through the microscope.  Attached to the mechanical stage are the stage clips.  These clips hold the slide firmly in place.
  4. Course focus adjustment knob: This is the larger of the two focus knobs.  When the course focus knob is turned it brings the objective lens and the specimen closer together or farther apart, helping to focus the image.  This knob should only need to be used when first bringing the specimen into focus under the smallest objective lens…  at higher magnification this knob moves the focus to fast.
  5. Fine focus adjustment knob: This is a smaller knob that moves the objective lens and specimen more slowly in order to nudge focus smaller amounts and perfect the focus.
  6. Condenser: The condenser is a lens below the stage through which light passes before it moves through the specimen.  The condenser can be adjusted with an adjustment knob.  Once a specimen is in focus the condenser can be adjusted to improve the view.
  7. Base: The base gives a stable platform for the entire microscope.
  8. Light source: The light source enough light to see the specimen well.
  9. On/off/dimmer switch: This switch allows us to turn the light source on and off, and also allows us to adjust the level of light being sent through.
  10. Diaphragm adjustment lever: This lever is used to adjust the amount of light passing through the diaphragm.
  11. Stage: The stage gives a stable platform for the specimen to be viewed.
  12. Objective lenses: The objective lenses magnify the image (along with the eyepiece).  Most light microscopes have three or four objective lenses in order to view specimens at different levels of magnification.
  13. Rotating nosepiece: This device allows us to move from one magnification (one objective lense) to another.

 

Integumentary System — Race is a cultural construct

Taking a scientific perspective human “races” really do not exist.  There is only a relatively small amount of genetic variation in our entire species, and most of that variation is seen within what historically think of as “races.”  Human “races” did not develop genetically, they developed culturally due to an unfortunate tendency of humans to view other human groups as just that…  as “other” and that “other” as something to be suspicious of, or worse.  The tragedies that have followed this very human miss-perception  include race based slavery, holocausts, and wars. In reality we are all one human race and there are actually few differences between us as a whole.  For a more complete analysis of what “race” is …   AAA statement on race

One of the major things that I myself take from this is that, as one human race, everything we humans have done belongs to all of us, every one of us.  The down side to this is that slavery, holocausts, and wars belong to us all.  The up side is that great art works, scientific innovations, agricultural advancements, social advancements, technical advancements, medical advancements, etc.  they also belong to all of us.  It’s empowering to think that the good things that great people like Gandhi, Sappho, Einstein, Martin Luther King Junior, Ibn al-Haytham, Issac Newton, and many others have done belong to all of us.  I believe that as a people and as individuals we need to own both the good and bad that we humans have done and be thoughtful about the decisions we make going forward.

Let us keep moving forward knowing that we belong to one human race.  Let us challenge and help each other to plan and to achieve a better world for ourselves…  for all of us.

 

Integumantary System — Skin Color Effectors

Factors that Effect Skin Color

#1  Melanin:

The major effector of human skin color (and hair color) is the pigment melanin.  Melanin is produced by melanocytes and transferred to keratinocytes.  There are several types of melanin, but here are the major three types.  The type of melanin people make along with the amount they make gives them a base skin color.

  • Black eumelanin — has a darker color.  People with very dark skin produce this type of melanin in larger quantities.  Very protective against UV radiation.
  • Brown eumelanin — a bit lighter in color than black eumelanin.  Good protection against UV radiation.
  • Phoemelanin — this melanin gives us red hair and very light skin.  Does not protect from UV radiation, in fact it appears to increase the effects of UV radiation.

Melanocytes are the cells that make melanin, and we all basically have the same number of melanocytes.  The differences in base skin color that we see are mostly due to different types of melanin, how much melanin is produced, and how fast the melanin is broken down.  In many of us some melanocytes are more active than others…  this is the source of freckles.  Exposure to UV radiation also makes melanocytes more active, that is why tanning works.

Albinism is caused by a lack of melanin production.

So, why do humans have so many different shades of skin color?  It has to do with our need for UV radiation to make vitamin D verses the damage that UV radiation can do to our skin.  If your ancestors lived in an area near the equator where UV exposure is the greatest then they had more than enough UV radiation for vitamin D production and would need darker pigmentation to protect their skin from excess UV radiation.  If your ancestors lived in an area further away from the equator with less UV exposure then darker pigmentation would have been lost in order make enough vitamin D with little UV radiation.  Because there is a gradient of UV radiation hitting our planet’s surface as we move from the equator towards the poles there is also a gradient of human skin color.  That variation also increases as peoples of different skin colors meet, mingle, and reproduce together.

When we study skin color and melanin ideas of human “races” often come up.  Here is a short blog about human “races” from a scientific and anthropological perspective.  Race is a cultural construct

Comparing the effects of UV radiation on different skin colors:

(images from Pixabay.com)

03 human 01 girl-444692_1920

The young girl pictured above has darkly pigmented skin probably due to lots of black eumelanin pigments.  This puts her at low risk for sunburn, skin cancer, and other damage that UV radiation can do to our skin, including fewer wrinkles and skin sagging as she gets old.  The only downside to this pigmentation would be if her family moved to an area with less UV exposure and no vitamin D supplementation, in that case she could have problems with bone development due to lower calcium levels.

03 human 02 girl-649011_1920

The above little girl has pigmentation that is a bit lighter probably due to a lower amount of brown melanin production.  She would still be pretty well protected from UV radiation, but a bit more susceptible to sunburn, skin cancer, and skin aging due to UV radiation.  If she lived in an area with little UV exposure she would probably need vitamin D supplements.

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The lighter skinned girl above would be at low risk for vitamin D deficiency but at much higher risk for sunburn, skin cancer, and sagging/wrinkling skin with aging.  Assuming her hair pigmentation is natural she is likely capable of tanning which would give her more protection from UV radiation.  However, exposing herself to UV radiation in order to get the tan would cause UV damage to her skin.

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The boy above has very pale pheomelanin pigmentation.  His pheomelanin pigmentation puts him at even higher risk for UV damage, but if he lived in an area with very little UV radiation exposure he would still be able to make plenty of vitamin D.  People with this pigmentation should really avoid UV radiation exposure.

Sunblock pros and cons:

Sunblocks can of course helps decrease the dangers of UV exposure, but they tend to make us think we can stay out in the sun—  our time in the sun should still be limited.  The lowest risks for skin cancer involve zero exposure to UV radiation.  That of course is near impossible, so instead I advocate a thoughtful approach to the risks involved.  Above all, make sure that children do not get severe sunburns as this leaves them at very high risk for skin cancer in their later years.

 

Other factors that effect skin color

  • Blood:  Skin is a bit see through and we can see the blood inside of a person’s skin as a pinkish or reddish color depending on how dilated the blood vessels under the skin are.  Blushing is probably the most obvious example of how blood effects skin color.  Cyanosis: Blood becomes a brighter red with more oxygen and darker red when oxygen levels are lower (blood is never blue).  This difference in blood color can change our skin color as well.  When a person’s blood oxygen level drops their skin can appear a bit blue… often appears in lips and nailbeds first.  Cyanosis can also occur when we get cold, due to a decrease in blood flow to the skin.
  • Bilirubin:  Bilirubin is a pigment produced when red blood cells are broken down (think of the yellowing that occurs during the healing of a bruise).  It is the job of the liver to eliminate this pigment and if the liver is not able to eliminate enough bilirubin a person may appear yellowish or jaundiced…  this yellow coloration is often most visible in the whites of the eyes.  Yellowing of the skin due to bilirubin is called jaundice.
  • Carotene pigments:  Carotene pigments are made by plants and have an orange or yellow color (carrots are orange due to carotene).  It is rare, but it is possible that a person can eat a lot of carotene and cause themselves to have an orange or yellow color.  I once knew a body builder who was taking “tanning pills” that contained carotene pigments and it turned his skin orange.

 

A&P to the point