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Lecture 1 -
Introduction to Dermatology: Structure, Function, and Terminology
I. The Dermatologic
Exam
A. History
1.  Signalment
It is important to note age, breed, and sex as these factors may be important
in recognizing the predilection towards specific dermatologic conditions.
2.  History
concerning the skin lesion
a.  The
lesion(s)
Obtain information concerning the location of initial lesion, initial
appearance, rate of progression, pattern of progression, degree of
pruritis, location of pruritis, and contagion to other animals or
humans in household.
b.  Other
influencing factors
Question the owner about ectoparasite control, travel history, seasonal
influences, environmental influences (indoor, outdoor, bedding, etc.), and
the effect of diet or medications on the lesions.
3.  Presence
of other underlying disease: obtain a thorough history to rule-out disease in
other organ systems.
Must be able to answer the question: "Is this dermatologic condition the
primary problem, or does my patient have underlying systemic disease with a
dermatologic manifestation?"
*Q: What types of systemic diseases may have cutaneous manifestations?
B. Physical exam
1.  General
appearance
Does your patient appear to be a healthy animal with a dermatologic complaint
or is there overt evidence of generalized poor general health?
2.  Dermatologic
exam
a.  From
a distance: determine the overall distribution of the skin lesions.
b.  Up
close and personal
More closely examine the regionally affected areas. Evaluate body regions of
the skin (face, ears, hair, ventrum, mucocutaneous junctions, nail beds, and
digits) during your exam. *Q: Why is it important to evaluate these
specific areas?
c.  Hair:
note texture (dry, oily), ease of epilation, presence of alopecia, and degree
of dryness. *Q: What might easily epilated hairs imply?
d.  Skin:
note thickness, pigmentary characteristics, sebum production, elasticity,
heat production, and configuration of skin lesions.
e.  Evaluate
if the skin lesions are primary or secondary.
3.  Thorough
physical exam: to evaluate for underlying disease.
C. Diagnostic Evaluation
1.  The
"core" tests: tests routinely performed in the evaluation of the
patient with a dermatologic complaint.
a.  Skin
scrapings:
i.  Indications:
primarily used to detect mites (Demodex, Sarcoptes, etc.).
ii.  Technique:
moisten scalpel blade with small amount of oil, grasp skin between fingers,
squeeze, and scrape. The process is not painful, but should be deep enough to
cause capillary bleeding. *Q: Is it possible to have false negative
results?
b.  Cytology
i.  Indications:
used to assess characteristics of discharge or mass lesion (inflammatory,
neoplastic, cystic, etc).
ii.  Technique:
collect sample via aspiration (22G needle and 12 cc syringe) or via scraping
of exudative skin lesion (mix with small amount of saline on slide, air dry,
and stain with DiffQuik stain).
c.  Dermatophyte
test media (DTM) culture
i.  Indications:
diagnosis of dermatophytosis (see lecture 3).
ii.  Technique:
see lecture 3
d.  Trichogram:
microscopic examination of hairs
i.  Indications:
may provide clues as to cause of alopecia. Normal animals exhibit hairs in
various stages of the growth cycle (anagen, telogen). Patients with endocrine
disease or systemic illness often have a predominance of hairs in telogen,
while those with other causes of hair loss often have a mixture of hairs in
telogen and anagen phase.
ii. Technique: use hemostat to grasp hair close to skin surface and pluck
(note ease of epilation). Put hair on slide with mineral oil and coverslip.
View under 40X. Telogen bulb (photo on right) is spear shaped, rough and
non-pigmented. Anagen bulb (photo on left) is expanded and often pigmented.
e.  Skin
biopsy
i.  Indications:
to evaluate for disease of unknown etiology, suspected neoplasia,
immune-mediated disease, or disease that is non-responsive to therapy.
ii.  Technique:
gently clip or cut hairs, do NOT prep skin surface, infiltrate local
lidocaine and use either 6 mm biopsy punch (press firmly and rotate punch in
one direction - not back and forth), or elliptical skin incision technique.
Gently remove sample without crushing.
2.  Additional
tests
The following tests may be helpful in the diagnosis of specific dermatoses
(as indicated by signalment, history, physical exam and dermatologic exam
findings - see later lectures): minimum data base (CBC, profile, UA),
endocrine testing, bacterial culture, biopsy, lymph node aspirates, immune
testing, and serology.
D. Morphology of skin lesions
1.  Primary
lesion
Primary skin lesions develop as a direct reflection of underlying disease and
may suggest a specific dermatosis.
2.  Secondary
lesion
Secondary skin lesions evolve from primary lesions (through degeneration,
trauma, etc.) or result from artifacts induced by the patient or the client
(licking, scratching, secondary infection, medications, etc.).
II.
Terminology
A. Primary lesions
1.  Macule:
a flat circumscribed spot up to 1 cm in diameter characterized by a change in
color.
2.  Patch:
a macule > 1 cm in size.
3.  Purpura:
a type of macule caused by bleeding into the skin.
4.  Papule:
a small, solid elevation of the skin up to 1 cm in diameter. Many are pink or
red in color.
5.  Plaque:
a larger flat-shaped elevation formed by the coalition of papules.
6.  Nodule:
a circumscribed solid elevation > 1 cm in diameter that usually extends
into the dermis or subcutis.
7.  Tumor:
neoplastic enlargement.
8.  Cyst:
an epithelial-lined cavity containing fluid or solid material.
9.  Pustule:
a small, circumscribed elevation of the epidermis filled with pus.
10. Abscess:
a demarcated fluctuant lesion resulting from the dermal or subcutaneous
accumulation of pus.
11. Wheal:
a sharply circumscribed raised lesion consisting of edema that tends to
resolve quickly.
12. Vesicle:
a sharply circumscribed elevation of the epidermis filled with clear fluid.
13. Bulla: a vesicle > 1 cm in diameter.
B. Secondary lesions
1.  Scale:
an accumulation of loose fragments of the horny layer of the skin.
2.  Epidermal
collarette: a type of scale arranged in a circular rim of loose keratin
flakes - represents the remnants of a vesicle, bullae, or pustule.
3.  Crust:
occurs when dried exudate, serum, pus, blood, etc. adheres to the surface of
the skin and hairs.
4.  Scar:
an area of fibrous tissue that has replaced normal tissue after injury.
5.  Excoriation:
linear erosion (abrasion) of the skin induced by scratching or self-trauma.
6.  Erosion:
a shallow ulcer (loss of the superficial part of epidermis, does not
penetrate dermis).
7.  Ulcer:
loss of the epidermis with exposure of the underlying dermis.
8.  Lichenification:
a thickening and hardening of the skin which follows chronic skin
inflammation - note exaggerated skin markings.
9.  Hyperpigmentation:
increased melanin in epidermis and sometimes dermis. Often occurs with
chronic inflammation, post-trauma or in association with endocrine disorders.
10. Comedo:
a dilated hair follicle filled with cornified cells and sebaceous debris.
11. Fissure:
linear cleavage (cracks) in the epidermis or dermis caused by disease or
injury.
12. Erythema:
reddening of the skin.
13. Follicular
cast: an accumulation of keratin and sebaceous material adherent to a hair
shaft. May be a primary (vitamin-A responsive dermatoses, primary seborrhea)
or secondary (demodex, dermatophytes) skin lesion.
Goals of Lecture 1 - Dermatology introduction
References (for all dermatology lectures): Muller &
Kirk's Small Animal Dermatology 5th edition 1995. Kirk's Current Veterinary
Therapy XII. Bonagura. W.B. Saunders Co. 1996. WB Saunders Co. Selected
excerpts from ACVIM 1995 and 1996 proceedings, and JAVMA, JIVM, and Compendium
1992-2006. Small Animal Dermatology Secrets: Karen L. Campbell 2004. Skin
Diseases of the Dog and Cat Richard Harvey, Patrick McKeever 2003. Small
Animal Dermatology A Color Atlas and Therapeutic Guide Linda Medleau, Keith
Hnilica 2001. Supplemental information also provided courtesy of Dr. Danny
Scott and Dr. Miller, Cornell University.
1.  What
3 areas should your history taking of the dermatologic patient cover? (A .1.
2. 3.) - generalities only. Be able to recognize the components of a complete
dermatologic exam. (B.2. all).
2.  What
are the 5 basic "core" tests utilized in the evaluation of a
patient with a dermatologic complaint? Be able to describe indications and
technique for these tests (C.1.a. through e.)
3.  How
does the root of a hair plucked in telogen differ from the root of a hair
plucked in anagen? How is this information helpful in the diagnostic work-up
of the patient with alopecia?
4.  What
is the difference between a primary and secondary skin lesion? (be able to
define these terms)
5.  Be
familiar with the definitions of specific primary and secondary skin lesions
(II.)
Appendix
I.
General functions of the skin
A. Barrier and protector
The skin provides a barrier to the outside world and
houses all the internal organs. It acts to protect against physical insults,
chemical insults, and pathogenic microorganisms.
B. Sensory organ
The skin is the largest organ in the body. It senses
heat, cold, pain, pruritis, touch, and pressure.
C. Temperature regulation
The skin plays an important role in temperature
regulation via modifying cutaneous blood flow, sweat gland production, and
the elaboration of a hair coat.
D. Immunosurveillance
The Langerhans cells, lymphocytes, and keratinocytes of
the skin play an important role in the immunologic defense of the body.
E. Vitamin production
Under the influence of sunlight, the skin produces
vitamin D3 from precursors in the skin.
F. Electrolyte regulation
The skin acts as a reservoir of water, electrolytes,
fat, and other substances. It also has limited excretory capabilities via
apocrine, sebaceous, and sweat glands.
G. Hair: the skin produces pigmentation and keratinized
structures including hair and nails.
II.
Anatomy and physiology: hair
A. Hair growth: occurs in cycles - not continuously.
There are three phases of hair growth: anagen (growth
phase), telogen (rest phase), and catagen (stage in between).
B. Anatomy
1.  Structure
a.  Hair
shaft: composed of an inner medulla, middle cortex, and outer cuticle.
b.  Hair
follicle: serves to anchor the hair shaft in the dermis.
The anagen hair follicle extends into the deep dermis. The lowest part of the
hair follicle is the dermal hair papilla.
c.  Dermal
hair papilla: hair grows from a layer of cells that covers the papilla.
2.  Types
of hair: the normal haircoat consists of primary and secondary hairs.
a.  Primary
hairs: coarse guard hairs or outercoat.
b.  Secondary
hairs: fine hairs or undercoat.
In cats, the secondary hairs are more numerous than primary hairs. In dogs,
the ratio varies depending on the type of haircoat.
3.  Color
of haircoat: genetically determined and occasionally temperature-dependent.
In some cats (Siamese, Himalayan, etc.) there is a temperature dependent
enzyme that converts melanin precursors into melanin. Warm temperatures =
light colored hair, cool temperatures = dark colored hair.
4.  Structures
associated with individual hairs
a.  Arrector
pili muscles
These muscles are innervated, play a role in thermoregulation, and contract
in response to catecholamines, producing piloerection.
b.  Glandular
structures: see later.
C. Factors which influence hair growth
1. General
The hair cycle is controlled by the photo-period,
ambient temperature, hormones, nutritional status, and other factors.
Anagen
(hair growth) is stimulated by thyroid hormones and inhibited by excess
glucocorticoids and estrogens.
2. Malnutrition: poor quality haircoat
Hair is predominantly protein and malnutrition has a
profound effect on the quality of the haircoat - often resulting in a dull,
brittle or thin haircoat.
3. Illness: excess shedding
During ill health, the anagen phase is often shortened
and most hairs are in telogen. These hairs are lost more easily - and thus
the ill animal is often observed to have "increased shedding."
Severe
illness may result in "telogen defluxion," a condition whereby many
hairs synchronously enter telogen and are shed together.
4. Microscopic appearance of hair root (see above)
5. Hair regrowth: no new hair follicles are formed
after birth.
Hair growth cycles occur due to the repeated induction
of the hair follicle.
Hair
grows until it reaches its preordained length (genetically determined), then
enters the resting phase, then is shed.
Regrowth
of a normal or short haircoat after shaving may take 3 - 4 months (or longer
in long-coated breeds).
III.
Anatomy and physiology: skin
A. Overview
In general, skin thickness decreases dorsally to
ventrally on the trunk and proximally to distally on the limbs.
Skin
consists of 3 specific layers: epidermis, dermis, and subcutis. This dermis
constitutes the bulk of skin thickness in any one area.
B. Epidermis: outermost layer of the skin
1.  Cell
types
Consists of four basic cell types: keratinocytes (about 85% of cells -
produce keratin), melanocytes (produce melanin), Langerhans' cells (important
in cutaneous immunosurveillance), and Merkels cells.
2.  Cell
layers: from inner to outer
a.  Basal
layer: separates epidermis from dermis and serves to replenish the epidermal
cells.
b.  Spinous
layer: consists of daughter keratinocytes and Langerhans' cells.
c.  Granular
layer: flattened cell layer.
d.  Clear
layer: compact layer of anuclear keratinized dead cells.
e.  Horny
layer (stratum corneum): outer layer of keratinized tissue that is constantly
being shed. These tightly packed cells are infiltrated by an mixture of sebum
and sweat that helps to form a physical barrier.
3.  Function
of the epidermis
The epidermis produces the protein keratin which acts as a major barrier
between the animal and the environment.
4.  Epidermal
turnover
The average turnover time from basal layer to granular layer in dogs is 22
days (although turnover time in seborrheic Cocker spaniels can be as short as
7 days).
Various hormones influence epidermal proliferation, differentiation, and
keratinization.
5.  Resident
microflora
There is a normal population of resident microflora on the skin that helps to
prevent colonization by pathogenic organisms.
The single most important factor causing overgrowth of microorganisms on the
skin is increased skin moisture.
C. Basement membrane
This layer functions to anchor the epidermis to the
dermis, acts as a barrier, and helps with wound healing. It consists
primarily of collagen, proteoglycans, and other substances.
D. Dermis
1.  Anatomy
The dermis is composed of fibers (collagenous fibers most numerous), ground
substance, cells, lymphatics, small blood vessels, and epidermal appendages.
2.  Function
The dermis allows for the diffusion of nutrients and electrolytes to the
upper avascular epidermal layer and acts to maintain and repair the skin.
E. Glandular structures
1.  Sebaceous
glands
a.  Location:
found throughout haired skin - ducts open into the hair shaft.
b.  Function
Produce an oily secretion (sebum) which spreads over the stratum corneum and
helps to retain moisture as well as has some antimicrobial action.
Secretion of sebum is stimulated by androgens and inhibited by
glucocorticoids and estrogens.
2.  Apocrine
sweat glands
a.  Location:
as for sebaceous glands.
b.  Function:
role still debated - probably have pheromonal and antimicrobial properties.
3.  Eccrine
sweat glands
a.  Location:
found only in the footpads - duct opens directly onto surface of footpad.
b.  Function:
these glands sweat in response to agitation and likely play a role in
thermoregulation and heat dissipation.
c.  In
general, dogs and cats do not have the ability to sweat as an effective means
of temperature control as can humans.
F. Subcutis (hypodermis)
This is the deepest and thickest layer of the skin. It
contains fat and connective tissue and serves primarily as protection
(padding for the body), insulation, and as an energy reserve.
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