Skin Anatomy

Skin is made up of three layers, some of which contain more specialized cells:

1. Epidermis: is the outer layer of skin. It is made up of 5 layers. From bottom to top they are: stratum basale; stratum spinosum; stratum granulosum; stratum licidum and stratum corneum. The epidermis varies in thickness from .05 to 1.5 mm.



More information about each of these layers can be found here.

The epidermis contains 3 types of specialized cells. Melanocytes which are cells that produce the pigment melanin; Langerhans cells, a type of dendritic cell involved in immune function and Merkel's cells which are associate with sensory touch.

2. Dermis: consists of 3 types of tissue found in two layers; the upper papillary layer and the lower reticular layer . Collagen, elastic tissue and reticular tissue.



The reticular layer (RD) of the skin gives it strength and elasticity. It is also the home to glands and hair follicles. The papillary layer (PD) contains the vascular network important in regulating heat flow from the skin.

Specialized cells in the dermis include: hair follicles, sweat (sebaceous) and scent (apocrine) glands associated with hair follicles; sweat glands (eccrine) not associated with hair follicles, blood vessels and nerves and sensory nerve cells (touch) called Meissner's and Vater-Pacini corpuscles.

3. Subcutaneous tissue: a layer of fat and connective tissue containing blood vessels and nerves.

More articles and resources concerning skin anatomy can be found here.

Helminth Infection of the Skin

Swimmer's itch is Cercarial Dermatitis. It results when bird (usually ducks, geese) schistosome eggs hatch and the resulting larvae infect a particular type of snail. Cercaria, from infected snails, are released into the water waiting to infect an unsuspecting duck. When the cercaria infect humans (the wrong host) a skin eruption occurs because the bird schistosome cercaria cannot develop further.



Cercarial dermatitis in a swimmer.

Another type of helminth skin infection that can be found in the USA is cutaneous larval migrans.




Cutaneous larval migrans results when roundworms that normally infect dogs or cats infect a human instead. The roundworm cannot develop normally because it is in the wrong host. Thus it wanders around under the skin.

Ectoparasites

Top to bottom: human flea (Pulex), eyelash mite (Demodex) and a louse (Pediculus)

Here is a great medical entomology website. An ectoparasite is a parasite that lives on or in the skin rather than inside the body. Humans can become infested with many different ectoparasites. Some are pretty much harmless (head lice) and others can transmit disease (ticks, fleas, mites). Some ectoparasites live in the skin. For instance the scabies mite.



Learn more about scabies (mites) here, it's something you could possibly see. You can visit these websites to learn about head lice, ticks that spread Lyme Disease, body lice, and dust mites (not an ectoparasite but a cause of allergy). Here is a paper that discusses eyelash mites (demodex) and rosacea.

Some diseases spread by ectoparasites include: Babesiosis (ticks), Plague (fleas) and scrub typhus (mites).

Helminths

Helminths are multicellular worms. The parasitic worms are found in several classes. The trematodes (flukes) and cestodes (tapeworms) contain the flatworms. The nematodes are the roundworms.

A good introduction to helminths can be found here. You can find plenty of pictures of helminths at this website if you click around.

Some important helminth diseases of man include Schistosomiasis, Oncocerciasis, Lymphatic Filariasis, Echinococcosis and Taeniasis. Some roundworm infections found in the USA include Pinworm (Enterobius), Whipworm (Trichuris), Ascariasis, Hookworm and Strongyliasis.

Protozoa

A selection of parasitic protozoa from the UK Natural History Collection. From left to right: Entamoeba histolytica which cause amoebic dysentary; Trypanosoma brucei transmitted by Tetsi flies and causes sleeping sickness; Babesia sp in Red Blood Cells transmitted by ticks; Balantidium coli a very large protozoan of pigs that rarely causes disease; and then two ciliates of cattle both of which are symbionts and cause no disease.

Protozoa are unicellular, eukaryotic,usually motile organisms that are more "animal-like" than "plant-like". More than you need to know about parasitic protozoa can be found here and here.

Protozoa have complex life cycles, some requiring two different hosts for reproduction and/or transmission. You can see some animations of protozoan life cycles at this WHO website. Below is a dipiction of the malaria life cycle which takes place in a mosquito and a human.





Malaria parasites in red blood cells.

You can find out more about protozoa and parasites in general at this CDC website.

The most important protozoan diseases include: Malaria, Leishmaniasis, Trypanosomiasis (African and American), and Amoebiasis. Protozoal infections in the USA are more likely to be Toxoplasmosis, Giardiasis, Babesiosis and Trichomonas.

Bacteria

Bacteria are unicellular, prokaryotic, microscopic organisms. They are found in many different shapes including rods, spheres and spirals.

For a short introduction to bacteria go here.

There is lots of information about bacteria, bacterial pathogenesis, bacterial genetics etc at this great website.

Everything you want to know about bacterial infections can be found at this CDC website.



LMT's should be aware of this website concerning skin diseases. Just click on the links to see pictures.

Viruses

Rabies virus.

Viruses are small infectious particles that can only replicate inside of cells. The particle is made up of a protein capsid coat containing either RNA or DNA. Viruses are generally considered to be non-living.

There are many good websites that provide all the information you need to know about viruses. All the Virology on the WWW is a great place to start. Then visit The Big Picture Book of Viruses. Want to learn more about specific viral infections...then go here.


Influenza virus

Prion Diseases

Prions are small proteinaceous infectious particles. They are non-living and lack DNA. Prion diseases are often called Spongiform encephalopathies because diseased brains often appear with large vacuoles. Prion diseases are usually rapidly progressive and are always fatal.

I highly recommend looking at this single page for more information about prions.

A free, full-text review on prions by Stanley Prusiner can be found here.

More about Creutzfieldt-Jacob disease here.

Go here for the CDC Prion Disease page.

Some info on Alzheimer disease here.

Genetic Disorders

The University of Utah has provided an excellent website on genetics. There are animations and interactive projects to reinforce your learning. Make use of them!

Injured (mutated)DNA or abnormal chromosomes can result in genetic disease.
First, refresh your memory on mutations.
What is a mutation?
How do mutations occur?
How do mutations cause genetic disorders?

To learn more about genetic disorders go to this University of Utah website. Notice the distinction between single gene disorders, chromosome abnormalities and multifactorial disorders.

You should understand the genetic basis of sickle cell disease, PKUand cystic fibrosis.

Be sure to understand genetic traits characterized as:

Autosomal dominant

Autosomal recessive

Sex-linked (or X-linked)

To learn more about disorders caused by abnormal chromosomes go to another page at the same website.

You should understand the genetic basis of Down Syndrome, Turner Syndrome, and Klinefelter Syndrome.

Another great place to learn some genetics is at the Cold Spring Harbor DNA Learning Center. Here is the page for "Your Genes, Your Health".

There is also an excellent, easily understood, source of information on understanding genetic conditions at the National Library of Medicine. Your taxes paid for it, make use of it.