Overview of the Skin

·         Whats the largest organ in the body? Its NOT the skin! Though it plays a very important role in protecting us, thermoregulation, and maintaining hydration.

o        Musculo-skeletal system is heavier

o        Lung has more surface area

 Structure of the Skin 

·         Stratum Corneum: the outer most layer composed of dead cells (no nuclei) which are stacked like “pancakes” or (a basket weave appearance) and filled with keratin

·         Stratum Malpighii is comprised of all the living layers of cells below the S. corneum. Its named so bc it is name after an Italian anatomist (the best kinds of anatomist ;o) ) named Malpighi. Comprised of:

·         Stratum Granulosum: cells that are going to give rise to the s. corneum – are becoming filled with keratin.

·         Stratum Spinosum: contain keratinocytes which will migrate up to become cells of the S. granulosum.

o        Have prominent desmosomes so are called “prickle cells”

o        Migration is through intracellular actin fibers which anchor on desmosomes. These tonofilaments has NOTHING to do with keratinazation.

·         Stratum Basale: this layer will proliferate through mitosis to regenerate cells which will move up layers and become squamous cells of the spinosum. Only about 1 of 25 cells are actively proliferating, the rest remain on reserve.

§         Neural Crest Cells called Melanocytes are found w/in the S. Basale and contain melanin (made from tyrosine) stored in melanosomes

·         Melanosomes are sent along cytoplasmic processes and are taken up in the lysosomes of the basal cells and some lower keratinocytes, thus portraying color. Remember: there is no difference between races regarding the number of melanocytes, however the activity level of the melanocytes will differ.

·         Below the S. Malpighii is subcutaneous fat.

 Basic Histopathological Changes:

• Acanthosis: excess Malpighii layer
• Hyperkeratosis: thickening of the stratum corneum which is too dense, NOT too thick
  • Histologically you will see a solid mass, not the basket weave appearance of the normal s. corneum
  • Can be present w/ or w/o parakeratosis

• Parakeratosis disruption in maturation of the s. corneum, nuclei are still present.  This is often a sign of disease states
  •  (ie: scaling: cells are immature and don’t adhere well to each other, resulting in droplets of water or space b/t them.)
  • In paraketosis, the S. granulosum is either smaller, hypogranulosis or absent, agranulosis.                     
• Dyskeratosis: (fig 6): the premature keratinization of the S. Malpighii layer.
• Acantholysis: cells in the S. malpighii are separated from each other.

Secondary Processes

• Secondary processes are present especially in inflammatory processes and can confuse diagnoses.
• Lichen Simplex Chronicus
  • Often in association with pruritic dermatosis.
  • Rubbing of the skin, often due to pruritus (itchiness), will cause papillomatosis which is an increase in length of dermal papillae.
  • This will cause thickening of the “superficial layers” of the skin causing everything from the mid dermis up to become hypertrophied, which is known as lichenification.
  • this often makes the skin more itching, causing more rubbing, etc.
  • Excoriation are lesions produced with a nail (when scratching) which can ulcerate and may also be apparent. 
• Post Inflammatory Hyperpigmentation
  • Increased activity of pigment cells results in producing and exporting too many melaonosomes, resulting in a brown discoloration of the skin, and is reversible w/in months
  • Pigment incontinence: Following chronic inflammation, or inflammation which causes destruction of the basement membrane, melanin may drop into the dermis and be taken up into macrophages (melanophages), resulting in a blue discoloration, which can last for years.
• This is due to the Tyndall effect. When light passes through a colloidal suspension, short wave lengths will be scattered resulting in a the appearance of a blue color
• This also happens with the sky. When your little cousins ask at Thanksgiving why the sky is blue, you can tell them its really black but explain the tyndall effect makes it appear blue, so they don’t fail out of kindergarten.  
• The blue discoloration can often cause confusion to physicians who may mistake it for a melanoma, when really it could just be a dilated blood vessel deep in the dermis.
• Leukoderma
  • Some melanocytes which are stimulated by inflammation will stop making pigment, resulting in white areas of hypopigmentation in areas of hyperpigmentation, or erythematous areas due to inflammation.

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