Acquired Nevomelanocytic Nevi (aka moles)


Nevi (or moles) are very, very common. They are generally well-circumscribed dark spots (or “papules” to use the dermatological terminology) that can appear at any time in someone’s life.

Histologically they are composed of groups of melanocytic nevus cells and can be found in the epidermis, dermis or both.

The problem with nevi is that they are pigmented and people tend to get worried about pigmented things on the skin (for good reason as melanoma can be a pretty scary disease).

Common acquired nevi are grouped into three categories (I’ll leave out congenital and dysplastic nevi for now)

  1. Junctional: the nevus cells are completely in the epidermis, just above the dermal-epidermal junction. Clinically they are <1 cm, flat or minimally elevated and dark in colour.
  2. Compound: the nevus cells are in both the epidermis and the papillary dermis (top layer of the dermis), and cross the basement membrane. Clinically they are raised, and a medium-brown colour.
  3. Dermal: the nevus cells are completely in the dermis. Clinically they are raised and almost always pigment less as the cells lose their capacity for melanization when in the dermis. They usually have telangectasia and may or may not have hair. They don’t tend to appear until the 2nd or 3rd decades of life.

Layers of the Epidermis

The epidermis is divided into five layers. From outside to inside (dermis). The stem cells are located in the stratum basale and migrate outwards in their differentiation process

  1. Stratum corneum: The outmost layer, made of dead keratinocytes with a layer of protein around them (they have undergone keratinization)
  2. Stratum lucidum: Also dead keratinocytes (there is no real distinction here other than that the poor keratinocytes have died but have not finished the keratinization process)
  3. Stratum granulosum: the keratinocytes are still on the move, by this point they have kertahyalin granules
  4. Stratum spinosum: the keratinocytes migrating up, they have nice oval nuclei
  5. Stratum basale: Single layer of proliferating columnar keratinocytes, melanocytes (pigmented cells) and Merkel cells (mechanoreceptors) also live here

Of note, Langerhans cells, which are specialized antigen-presenting cells are present in all layers of the epidermis but are mostly in the stratum spinosum.

The 5 classes of white blood cells

White blood cells are divided into 5 classes.

  1. Neutrophils (70%): These are the big guys in infection. Generally when someone has an “elevated white count” it’s due to the neutrophils (unless there’s a lymphoma grumbling along). They are the major players in fighting off bacterial infection and the main component in pus.
  2. Lymphocytes (20-60%): These are the B cells, T cells, and Natural Killer (NK) cells. When you’re looking at a blood smear, you can’t tell the difference between them.
    • T cells: cell-mediated immunity
    • B cells: humoral (antibody-mediated) immunity
    • Natural Killer cells: protect against viruses and tumours
  3. Monocytes (3-8%): Mature into macrophages and mast cells, they play a major role in mounting an inflammatory reaction. Macrophages are the “professional antigen presenting cells“, displaying foreign peptides in Class II MHC.
  4. Eosinophils (1-6%): fight against parasitic infections (back in the good old days), now they’re the bane of many people’s existence as they’re involved in asthma and allergies.
  5. Basophils (0.1%): contain many base-loving (hence baso-phil) granules containing histamine

Where diuretics work in the nephron

The nephron is composed of distinct areas that are specific to regulating different electrolytes.

An overview of nephron anatomy

Loop diuretics: blocks the sodium/potassium/chloride transporter in the ascending loop of Henle, potassium-wasting
Thiazide diuretics: blocks the sodium/chloride transporter in the distal tubule, potassium-wasting
Amiloride: directly blocks sodium channels in the collecting duct, potassium-sparing
Spironolactone: blocks the aldosterone receptors in the cortical collecting duct. This causes a decrease in sodium and water reabsorption and decreases potassium secreting (therefore is potassium-sparing)

Anatomy of a nephron

The nephron is divided into 6 distinct parts

  1. Proximal (covoluted) tubule
  2. Descending loop of Henle
  3. Ascending loop of Henle
  4. Distal (convoluted) tubule
  5. Cortical collecting duct
  6. Distal collecting duct

Each of these sections has a main function in adjusting the amount and kind of solutes in the urine. Different drugs and diuretics work at distinct areas, which is why some diuretics are potassium sparing while others (like Lasix/furosemide) are potassium wasting.

Erythrocyte growth and regulation


Red blood cells (fancily known a erythrocytes) are the simple, non-nucleated cells that transport oxygen in the body. This just outlines their development in the bone marrow (hint, they start off with a nucleus) and the major growth factor erythropoietin that stimulates their production.

By looking at the peripheral blood and bone marrow, you can work on sorting out where and what kind of disease process is going on. For example, if there are too many reticulocytes in the peripheral blood.