BMED 2801 – Lecture 9 – Epithelium
Classification of Epithelium
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Based on arrangement and shape of cells that form the surface layer, then any apical surface specializations are added
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Unrelated to function (morphological not functional)
Simple – one cell layer thick
Stratified – two or more cell layers
Squamous – cell width is greater than cell height
Cuboidal – cell width = depth = height; nucleus sits in the middle of the cell
Columnar – cell height is greater than cell width; nucleus us tall and elongated, sitting towards the basal surface
SIMPLE
SQUAMOUS
Very flat nuclei (thin line is cytoplasm
STRATIFIED SQUAMOUS SIMPLE CUBOIDAL
SIMPLE COLUMNAR

Pseudostratified Columnar Epithelium
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All cells rest on the basement membrane but do not all reach the surface
Found in the trachea and epididymis
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Basal cuboidal cells + columnar cells
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Basal cells (steam cells) differentiate into mature cells to balance cell turnover
PSEUDOSTRATIFIED COLUMNAR CILIATED EPITHELIUM
Transitional
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True stratified
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Cuboidal
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Impermeable to salts and water
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Accomodates distension
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Found in kidney minor calyses, ureter, bladder, urethra
LM – cuboidal, dome-shaped apical surface
EM – plaques = plasma membrane folds + actin filaments e.g. The walls of the bladder require pliability and elasticity. As the bladder fills, the cuboidal cells stretch out, appearing like squamous cells. The extremely strong, well functioning junctions keep the cells together.

Endothelium and Mesothelium
Both are simple, squamous epithelium
Endothelium lines vessel walls (tubular structures with a lumen, but not exposure to the surface).
Mesothelium lines the walls and covers organs of the thoracic, pericardial and abdominal cavities. Macroscopically it is seen as a smooth, shiny surface.
Functions of Epithelium
Need movement, therefore don’t want lots of layers.
Against physical AND chemical.
Will always be at the ends of bodily caivities (where it is exposed to the outside)
Cell height reflects secretory/absorptive activity.
Squamous epithelium indicates high rates of turnover.
Stratification correlates with impermeability.
The greater the number of layers, the greater the impermeability of the epithelium

Skin
Composed of three layers
*No matter where on the body, there are always three layers. The thickness of each varies
1. Epidermis – stratified squamous epithelium
2. Dermis – dense, irregular connective tissue; provides tension and tensile strength to skin and epidermis; contains lots of fibres
3. Hypodermis – fatty cell layer, loose connective tissue
Receptor
What’s associated with the skin?
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Sweat glands
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Hairs
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Sebaceous glands
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Nails
Function of Skin
Protection Physical, chemical, biological
Pain, pressure, touch, temperature through sensory axonal contacts
Absorption
Excretion
Hydration
Vitamin D within the skin
The number of receptors varies over skin, especially for touch
Lipid-soluble substances
Sweat, oils
Prevents water loss through barrier that top cells create

Heat regulation
Epidermis Layer of Skin
Thick skin has 5 layers (i.e. palms, soles)
Thin skin has 4 layers and hairs
The 5 layers of thick skin: s.basale
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Single layer of small stem cells and keratinocytes (which will eventually produce keratin)
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Mitotically active
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can see pale unwound chromatin in nucleolus
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Rest on basal lamina
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Connected via desmisomes (very strong)
Large nucleus
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Intense pink staining, slightly granular cytoplasm

s. spinosum
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Several cells deep
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Cytoplasmic processes (spines)
 ladder-like borders between cells
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Connected via desmosomes at the spines, seen as projections
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Nucleus and nucleolus are very obvious (active cells)
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Number of layers vary, ~3 – 20 s. granulosum
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Cytoplasm contains keratohyaline granules (stain very blue)
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Stains strongly with haematoxylin
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Granular appearance
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Beginning of waterproofing
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Mechanical protection

s. lucidum
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Thin, not easily seen
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Cytoplasm is filled with keratin
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Single layer
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The closer to the surface contains more keratin and hyelin s. corneum
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Thickness varies
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No nucleus or organelles, they are non-functioning apoptotic cells (dead)
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Cytoplasm is filled with keratin
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Plasma membrane thickening and glycolipid coating (produced by cells in granulosum)
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Flaky skin on surface
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Final layer of waterproofing
Cells of the Epidermis
Keratinocytes
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Epithelial derived cells
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Lots of free ribosomes
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Contain kertain filaments
(tonofilaments)
Melanocyte
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Pigment cell (melanin)
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Round and plump
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Long dendritic processes
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Located in the s.basale and scattered among s. spinosum layer
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Pale staining in electron microscopy
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Same number of cells present no matter skin colour, but the extent of cytoplasmic extensions varies and the amount of melanin
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Melanin is transferred to keratinocytes via endocytosis
Tyrosine  dihydroxyphenylalanine  melanin
*M – Melanocyte; *K - Keratinocyte

Melanin is needed for other biological functions in the body e.g. sleep functions can be affected by melanin functioning (i.e. if always in the sun or darkness).
*In the autonomic recessive disease phenylketonuria, the enzyme to produce tyrosine is missing, and melanin cannot be produced. A change in the diet to get the right amino acids will prevent a build up of toxins as a result.
Langerhan cell
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Antigen-presenting
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Phagocytic
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Long dendritic processes
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Rough nuclear shape
Merkel cell
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Found in the s. basale
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Modified and differentiated epithelium cell
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Contain cytoplasmic granules
(catecholamines) are neurotransmitters
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Desmosomal contact to other cells
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Involved in touch sensations - basal plasma membrane contacts with nerve fibres (mechanoreceptor)
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Very active cells
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very pale euchromatin
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Tucked in between stem cells