Lecture 6 SKIN (Integumentary System) Definition: The skin or ...

Lecture 6
SKIN (Integumentary System)
Definition: The skin or integument is the largest
organ in the mammalian body and is comprised of an
epithelial covering, its derivatives (including hair,
glands, hooves, horns and claws) and its associated
connective tissue
Epidermis
Dermis
Hair Follicle
Hypodermis
(subcutaneous fat)
Sweat Glands

II. Functional Characteristics:
1. Protection: The most important function of the skin
is its effectiveness as a barrier between the internal
and external environments (guards against injury,
bacterial invasion, UV damage and desiccation).
2. Regulation of body temperature: m ediated by t he
hair coat, cutaneous blood supply and in some
animals, sweat glands.
3. Secretion: from sweat, sebaceous and mammary
glands
4. Sensory Organ: innervation of the skin provides
pain, touch, pressure and temperature sensation.
5. Communication: The skin is an important organ in
the social life of animals because it gives off odors
that govern sexual behavior and helps animals
identify each other and their territories
6.
Reflects the physiological condition of the animal: skin and coat
condition are good indicators of overall health and alterations may
reflect a variety of external and internal disease processes (endocrine
disorders, nutritional problems; i.e. Vitamin A deficiency is
characterized by very dry, hardened skin, dry lack-luster hair and hair
loss.)

III. Organization of Skin: Epidermis, Dermis and Hypodermis
Epidermis
Hair
follicle
= loose
fascia

1. Epidermis: stratified squamous epithelium of ectodermal origin
Dermal
Papillae
Epidermal Pegs

A. Epidermal Cell Types:
1) Keratinocytes: represent the majority of cells
2) Melanocytes: derived from Neural Crest, “octopus-like” cells
that produce melanin.
3) Langerhans Cells: dendritic cells
located in the stratum spinosum
4) Merkel Cells: ubiquitous cells in
the skin that couple with axon
terminals to form mechanoreceptors

A. Epidermal Cell Types:
1) Keratinocytes: represent the majority of cells
2) Melanocytes: derived from Neural Crest, “octopus-like” cells
Keratinocytes
Epidermal Melanocyte

1) Keratinocytes: represent the majority of skin cells and are
arranged in 5 layers (discussed later)
keratinocyte
dermis

2) Melanocytes: octopus like cells that produce the pigment, melanin
They don’t retain melanin but pass it on to neighboring Keratinocytes
Keratinocytes
Melanocyte
Melanin granule

Melanin Granules in Keratinocytes
Melanin Granules

Melanocytes in skin
Clinical Note: A Melanoma is a type of very aggressive and metastatic
cancer that arises from the uncontrolled mitosis and migration
of these cells. It commonly occurs in dogs with pigmented skin.
Melanomas can occur in areas of haired skin (usually benign),
where they usually form small, dark (brown to black) lumps,
but can also appear as large flat wrinkled masses. They can also occur in
the mouth, toes or behind the eye (these tend to be malignant).

3) Langrhans cells: dendritic (immune) cells located in the stratum
spinosum that play a pivotal role in induction of cutaneous immune
responses (allergic reactions). Migrate to draining lymph node->T-cells.
Cell process
Langerhans cell
Future Development
of skin patches
containing vaccines
that will stimulate
Langerhans cells
Keratinocyte

4) Merkel cells: found throughout skin, not visible with normal
stains; couple with axon terminals to form slowly adapting
mechanoreceptors. They can form malignant tumors in cats.
Immunostained Merkel cells in skin
Microvilli on surface
Merkel Cell Tumor

1. Epidermis
B. Layers
1) Stratum
Germinativum
a) Stratum Basale:
b) Stratum
Spinosum
2) Stratum Granulosum
3) Stratum Lucidum
4) Stratum Corneumoutermost
keratinized
layer of flattened,
dead cells (squames)
Single layer of cells in contact with basement membrane
Stratum
Germinativum

1. Epidermis
B. Layers
1) Stratum
Germinativum
a) Stratum Basale
b) Stratum
Spinosum
2) Stratum
Granulosum
3) Stratum Lucidum
4) Stratum Corneumoutermost
keratinized
layer of flattened,
dead, anuclear cells:
(squames) have no
distnict cytoplasmic
boundaries

Layers of the Epidermis (thick skin)
(Stratum Lucidum)
(stratum basale)
(Stratum corneum)

Three Layers of the upper epidermis
Duct
Stratum Corneum
Stratum granulosum

C.
A. Keratinization: process involving the formation of
keratin
1) Keratin Proteins: Keratin is a structural
protein that forms the cytoskeleton of all
keratinocytes. Keratins are a principle part of
the cells in the epidermis, hair, nails, feathers,
hooves, and t he enamel of teeth. There are
several subtypes of keratin proteins, some are
called "soft" keratins and others are "hard"
keratins.
a) Soft keratin – elastic, desquamates
(example: skin)
b) Hard keratin – contains more sulfur than
soft keratin; less elastic; more permanent;
resistant to degradation, does not
desquamate (examples: nails, horns, hoof)

2) Formation of Keratin:
a) Synthesis of filaments begins in
stratum basale (makes 2 of the 4
types of keratin-other 2 in spinosum)
b) Aggregation of filaments occurs in the
superficial cells of the stratum spinosum
c) Cells of st. spinosum also form “membrane
coating granules”, MCG, that later release their
lipid-rich contents into intracellular space-->
form water proof permeability barrier.
d) Keratohyalin granules (non-membrane
bound) appear in close association with the
filaments in the stratum granulosum and nonkeratin
proteins released by these granules
cause keratin filaments to associate into
thicker bundles.
e) Degradation of the nucleus and loss of cell
organelles in the most superficial layer of
stratum granulosum-->completed in stratum
lucidum and stratum corneum
f) Formation of keratin filament matrix
complex occurs in stratum corneum

Melanin and keratohyalin granules

2. Dermis (corium):layer of skin immediately deep to the epidermis
derived from mesoderm and comprised of connective tissue.
embedded in the dermis
(Reticular layer)
Dermis
Papillary layer

2. Dermis: consists of 2 layers:
A. Papillary Layer: subepithelial loose connective tissue
B. Reticular Layer: dense connective tissue deep to papillary layer;
contains epidermally derived hair, sweat and sebaceous glands
Reticular layer/dermis
elastic fibers

3. Hypodermis: subcutaneous layer (superficial fascia), technically not
part of the skin.
A. Loose and irregular connective tissue that anchors dermis
B. In healthy animals, it has large depots of fat in it

Clinical Considerations:
4. Clinical Considerations:
Hyperkeratosis: hypertrophy of the stratum corneum
[nasodigital hyperkeratosis - an ailment affecting either the
nose or foot pads (or both) of older dogs. In
hyperkeratosis, keratin - the tough, fibrous outer covering
of foot pads - grows excessively. Often, the hard, cracked
pads appear to have "keratin feathers" around their
edges.]
Squamous Cell Carcinoma: neoplasia of cells of the
stratum spinosum [Squamous cell carcinoma (SCC) is a
common tumor involving the skin and a ccounts for
approximately 15% of cutaneous tumors in the cat and 5%
of those in the dog. SCCs are usually found in
unpigmented or lightly pigmented skin.]
. Melanoma: Melanocytic tumors represent 4 to 7% of a ll
canine neoplasms and are the most common malignant
tumor of the canine oral cavity and digits [Melanoma
tumors in dogs, more than most cancers, demand
immediate attention since early recognition can lead to
more successful attempts at removal and identification of
the grade or stage of cancer. Malignant melanomas can
metastasize (spread) to any area of the body especially
the lymph nodes and lungs and present very challenging
and dangerous prospects for the dog. Cats seem much
less susceptible to melanoma tumors than dogs]
1.

IV. Accessory Structures of the Skin: Accessory structures of the skin include
hairs and two types of glands: sweat glands and sebaceous glands. Although
these structures are anchored in the dermis or hypodermis, they are in fact
derived from the epidermis.
1. Hair: hair itself is dead, but it’s produced by living keratinocytes at the
base of the hair (hair follicles) and the pigmentation in hair comes
from melanocytes (just like skin pigmentation)
A. Functions: Hair serves several functions: serves as insulation;
provides camouflage; sex recognition; social purposes.
B. Types of hair follicles:
1) Single (simple) follicle – one hair emerges from a single opening;
found in horse, cattle, pig and sheep (face, ear, distal portion of
limbs)
2) Compound follicle - several hairs emerge from a single opening.
Found in cat, dog, sheep (wool growing areas). Consists of a
long principal (guard) hair and a n umber of smaller auxillary
(wool) hairs.
Hair follicles: Definition: Cylindrical depressions in the epidermis, lined with
stratified squamous epithelium; hair fibers are located in the lumen of
the follicle.

Auxillary Hairs
Compound Hair Follicle - Cat
Principal
Hair

C. Structure of hair follicles:
1. Hair shaft: part of hair above surface of the skin; 3 parts
a) Outer Cuticle-single layer of flat keratinized cells
b) Cortex-compact dead cell layer under the cuticle
c) Medulla-central region of the shaft, cuboidal or flat cells
2. Hair Root-part of the hair below the surface
of the skin; similar structure to hair shaft
3. Hair bulb: conical mass at the base of the
hair root, covered by stratified squamous
epithelium (Germinal Matrix). Epithelium is
indented at the base of the bulb by dermal
conn. tissue the
Dermal Papilla
Hair Root
3.
1.

Hair Follicle: The structure from which the hair grows. Parts: Internal
Root Sheath, External Root sheath and at the base the Dermal Papilla and
Germinal matrix
Actively
produces
the hair
Hair Follicle
Root sheath
Hair
Follicle

Hair Follicle:
a) Internal Root
Sheath
b) External Root
Sheath

4) Hair Follicle:
a) Internal Root
Sheath
b) External Root
Sheath
Inner
Root sheath
Longitudinal section through hair shaft
Cortex

Hair with epithelial root sheaths (ORS & IRS)
Cortex
External (outer) Root Sheath
Dermal
Sheath

5) Arrector Pili Muscle: smooth muscle with origin at the epidermal/dermal junction
it inserts outside the follicle and when it contracts it functions to erect the hair
= Piloerection, in response to cold, anger or fear.

D. Cyclic activity of hair:
1) Anagen (growth period): the active phase of hair production
when cells of the hair bulb are mitotically active and the hair
grows in length (see fig. 5).
2) Catagen (period of involution): transitory period during which
cellular proliferation slowly decreases and finally ceases; hair
bulb becomes a solid mass of keratinized cells resembling a
club (club hair) and the hair detaches from the underlying
matrix and is easily removed (that’s where the hairs embedded
in the bristles of your hairbrush come from every morning).
3) Telogen (resting phase): transitional stage of the cycle where
hair bulb atrophies; chemicals released from the dermal papilla
wakens the fo llicle from its dormancy and it begins to renew
itself for activity; it then changes back to t he active anagen
stage again.
Clinical Note: Hair loss (alopecia) is a common side effect of radiation & Chemotherapy

2. Glands of Skin:
A. Sweat (sudoriferous) glands: simple, coiled tubular glands seen as
hollow more or less circular profiles in tissue sections with walls
composed of low cuboidal epithelium. Two types:
1) Eccrine Sweat Glands: small glands that are widely distributed
and produce a watery secretion; they are mainly a mechanism
for cooling; restricted to f oot pads of carnivores, frog of
ungulates and nasolabial region of ruminants and swine.
2) Apocrine Sweat Glands: larger glands with cuboidal epithelium
that produce oily and foamy secretions; most common in the
groin, axilla and scrotum of dogs and cats; most numerous and
extensive in horses. These are the most common type found in
domestic animals.
B. Sebaceous Glands: simple, (often branched) acinar glands that
open into a ha ir follicle about halfway up the shaft; they secrete
sebum, a sec retion consisting of lysed cells and accumulated
lipids containing precursors of vitamin D. Sebum gives hair its
“sheen”. The oily sebum also acts as a lubricant for the skin and
hair.

Apocrine
Sweat
glands

Compound Hair Follicle with sebaceous glands (SbG) and sweat glands (SG)
Sweat Glands

B. Sebaceous Gland

Cross Section of a Hair with a Sebaceous Gland
Sebaceous gland