Meghan Barboza, PhD
Online Course
Components of the skin
Epidermis
Dermis
Hypodermis
Subcutaneous
Cells of the Epidermis
Keratinocytes
Melanocytes
Merkel cells
Langerhans cells
Keratinocytes
 Main component of the
epidermis
 Change in size and shape
as they mature
 Protection through production
of proteins
SB
Stratum basale or basal cell layer
 One layer of cells,
resting on BM
 (90%) keratinocytes
50% resting (G0)
 50% dividing and
leaving the basement
membrane (stem cells
for rest of epidermis)

Stratum basale or basal cell layer
 One layer of cells, on
basement membrane
 ~90% keratinocytes


50% resting (G0)
50% dividing and
leaving the BM (stem
cells for rest of
epidermis)
 up to 10%
Melanocytes
National Cancer Institute
Stratum basale or basal cell layer
 Desmosomes
 Cell-to-cell adhesion
 Hemidesmosomes
 Cell-to-basement
membrane adhesion
 Cell = keratinocyte
Defined by desmosomal presence
Stratum spinosum
 Variable number of layers
 Polyhedral-shaped cells
Stratum spinosum
 Variable number of layers
 Polyhedral-shaped cells
 Contains lamellar
granules (also called
Odland bodies)
 Filled with lipids and
enzymes
 Important for
 Barrier function
 Desquamation
Desmosome
Lamellar granule
Stratum granulosum or
Granular layer
 Variable number of layers
(usually fewer than
spinosum or corneum)
 Aggregation of
keratohyaline granules
 Filaggrin (keratin filament
aggregation)
 Lamellar body
(protein/lipids) secretion
 Hydrophobic properties
Stratum Lucidum
 Observable only in
friction areas
 Nose
 Footpads
 Fully cornified, dead,
anuclear cells
 Also accumulates
lamellar bodies
Stratum Corneum
 Outermost layer of epidermis
 Several to many layers of dead cells (corneocytes)
 Intercellular spaces are filled with lipids
 Serves to Protect
underlying layers from:
 Dehydration
 Infection
 Chemicals
 Mechanical Stressors
Keratinization
 Synthesis of proteins
 Modification of organelles
 Modification of cell size and shape
 Alteration in plasma membrane
 Formation of lipid bilayer
 Dehydration
 Synthesis of proteins
 Keratin
 Keratohyalin granules
 Filaggrin
 Matrix for aggregation of
keratin filaments
Current Opinion in Immunology
Volume 42, October 2016, Pages 1-8
Keratinization
Keratinization continues
 Modification of organelles
 Loss of Nucleus
 New synthesis
 Lamellar granules
 release their content into the extracellular
space
 Interface between stratum granulosum and
stratum corneum
 Water Barrier

Skin Shedding
Formation of lipid layer
 Resulting from the
content of lamellar
bodies
 Ceramides (waxy
lipid)
 Cholesterol
 Fatty acids
Keratinization: programmed cell death
Born to die!
Life cycle of epidermal cells
 Mitosis
 Differentiation (cornification
or keratinization)
 Exfoliation, desquamation
 Average transit ~40 days in
humans ~20 in dogs
Non-keratinocytes
Melanocytes
 Dendritic cells
 Clear cells on H & E
Melanocytes
 Dendritic cells
 Clear cells on H & E
 Responsible for melanin synthesis
 Principally eumelanin, also
 phaeomelanin (red – yellow)
 Found in
 Epidermis/dermis
 Eyes
 Brain
Merkel cells
 First discovered by namesake in
1875 on the skin of geese and ducks
and the snouts of pigs
 Clear cells at dermo-epidermal
junction, i.e., basal cell layer
 Not visible with regular stains
 In close proximity to myelinated
nerve fibers
 Mechanoreceptors
Langerhans cell
 Subset of dendritic cells (DC)
characterized due to branching
appearance
 Derived from the bone marrow
 Part of immune system: Antigenpresenting cells
 Upon stimulation they migrate from
the skin to the lymph nodes to present
antigen to T-cells
Langerhans cells
 Special stains are required to visualize LC on
histologically
Integument:
Dermis and
Hypodermis
• Great example of
many cells and
types of connective
tissue
http://www.lab.anhb.uwa.edu.au/mb140/co
repages/integumentary/integum.htm
Integument of canine claw
 CT
 Loose
 Dense
 Papillary zone
 Reticular zone
Dermis
Cells of the
dermis (usual c.t.
cast)
 Fibroblast
 Macrophage
 Mast cell
 Melanocyte
Collagen degradation
 Resistant to many proteases
except for collagenases
 Collagenase is produced by:
 Fibroblasts
 Neutrophils
 Macrophages
 Bacteria (Clostridium histolyticum)
Elastic fiber Degradation
 Reduced with
age
 Degraded by
elastases
 Neutrophils and
macrophages
Reticular fibers
 Produced by
 Reticular cells
 Location
 Around vessels,
nerves, and
epidermal
appendages
 Silver stain
Ground substance of the dermis
 Glycosaminoglycans (GAG)
 Located on surface of cells or in the extracellular
matrix
 Long unbranched polysaccharides
 Majority are linked to proteins (PG)
 Important for


Structural integrity
Cell migration
Hypodermis
• Primarily used for fat
storage
• contains
• Loose connective
tissue
• Adipocytes
• Some glands
• Hair follicle root
• Arrector pili muscle
Blue histology
Functions of the Skin
 Barrier
 Physical and immune
 Communication
 Temperature
regulation
 Secretion
 Storage
 Pigmentation
Check your Understanding
 Psoriasis is a skin disease which includes rapid and
excessive growth of keratinocytes. Why is this a problem
and how can symptoms such as redness and scaly skin
be explained by the cause?
 How do you think the transit time of keratinocytes
compares to those that do not have psoriasis
(slower/faster)?
 Why does skin have specialized immune cells?
 As a person ages their skin sags and gets wrinkles, this
is caused by aging of which cells in the dermis?