Epithelium/Glands Flashcards
Where is Epithelium Found
- Skin (epidermis)
- Gut
- Body cavities (pleural, peritoneal and pericardial cavities)
- Blood vessels (epithelium that coves blood vessels = endothelium)
Any substance entry in the human body needs to go through epithelium cells!
Definition of Epithelium
As a sheet of closely apposed cells, separating a space (lumen) from underlying tissue.
Or as a gland: invaginated epithelium cells.
Embryonic Origin of Epithelium
• Mainly from Ectoderm and Endoderm, but also Mesoderm
– Ectoderm– Skin and its appendages (hair and sweat glands), mammary glands
– Endoderm–Liver, pancreases, GI and respiratory tracts
– Mesoderm–Reproductive system, Blood vessels and body cavities
Basic Functions of Epithelium
“PASS C”
• Protection (Skin) • Absorption and Transcellular transport (intestinal epithelium) • Secretory (glandular epithelium) • Sensation (olfactory neuroepithelium) • Contractibility (myoepithelial cells)
Function of Oral Mucosa
Protecton, Sensation, and Secretion
Four Common features of Epithelia
- Tight: Close apposition (#1 feature of epithelium)
- Polarity (unique because they have this polarity due to a basement membrane)
- Avascular (No blood vessels WITHIN epithelial tissues - while the vessels themselves are MADE of epithelium, they are not found running through it; it IS in connective tissue though)
- Renewal (Can grow back (ex. Skin every 7 days-ish, we get new skin cells on the top layers)
Classification of Epithelia
A. Based on Number of cell layers:
1. Simple epithelia: one layer
2. Stratified epithelia: have two or more cell layers
3. Pseudostratified epithelia: one layer with
stratified appearance
B. Based on Shape of superficial cells
1. Squamous cells: flattened
2. Cuboidal cells: heights = width.
3. Columnar cells: “taller” cylinder-shaped
4. Transitional: changing shape (Distended vs
relaxed)
C. Based on Basic Function of cells
1. Covering or lining Epithelia 2. Glandular Epithelia (Goblet cells)
Simple Squamous Cells
Human Simple Squamous Epithelium can be found in the lung air sacs (alveoli)
they are very thin because we need gas exchange to occur, so there is no need for multiple layers of cells
same type of epithelium makes up our blood vessels for a similar functional reason
Kidney glomeruli also function have simple squamous epithelium as its function is to filter and exchange
Kidney Tubules
Simple Cuboidal Epithelium
nuclei are centered in cubiodal and they are square shaped
Digestive Tract
Simple Columnar Epithelium
nuclei are towards the basal surface and the cell body is much longer
Uterus or Bladder
Transitional epithelium - because it can change its shape (stretch and relax); usually at the free surface
Stratified Squamous
Common for stratified cells to be squamous in oral mucosa, esophagus, and skin, but rare to see stratified cuboidal and columnar; however, they do exist in ducts of salivary glands and conjunctiva of the eye and certain large excretory ducts.
Goblet Cells
mucus-secreting cells are commonly found in columnar and pseudostratified epithelium
unicellular glands
easily detectable through Periodic-Acid- Schiff (PAS)
Respiratory Tract
Pseudostratified ciliated columnar epithelium (move particles over the free surface in areas like the trachea and nose!)
Esophagus
Stratified Squamous epithelium
Surfaces of Epithelial Cells
- Apical surface/free surface: exposed to the lumen or the outside enviornment
- Basal surface: place of attachment for the deepest layer of cells (or bottom of the cells in simple epithelium) this defines the polarity of the epithelial cell; contributes to the basement membrane
- Lateral surface: between two of the adjacent epithelial cells
Microvilli
A free surface specialization
small finger-like cytoplasmic
projections emanating from the free surface of the cell into the lumen.
Idv. microvilli are only seen under electron microscope
Collectively, microvilli
borders appear as brush-like or striated by light microscopy.
Increase intervillar spaces and facilitate absorption/
secretion*
Cilia
long, motile, hair-like emanating from the apical cell surface
Main function: motility and can sweep things out and propell substances over the surface of the epithelium via rapid rhythmic oscillations or sensations
Much longer so they can been seen under a light microscope
Core of Microtuble called AXONEME is what allows for the mobility
EXAMPLES) trachea, bronchi, oviduct, vestibular apparatus
Axoneme
Specific microtubule structure inside cilia that allows for mobility
In cross section, it appears like a wheel with 9 doublets making a circle with two singlets in the middle.
Referred to as the 9+2 structure
(9x2 = 18 = 2 = 20 total)
Stereocilia
- closely related to microvilli
- branched
•Increase surface area and facilitating the movement of molecules (SPERM)
•found at the epididmyis
Keratinization
Protection from Abrasion or Desiccation
Keratin is a specialized protein, so when layers of dead cells accumulate at the free surface, the dead cells become “keratinized” with this specific protein which thickens the apical surface and protects lower, living levels.
EXAMPLE: The palm of the hand and heels of our feet are covered by a thick stratified squamous keratinized epithelium.
Lateral (Intercellular) Specializations
Junctions
- Seals (occluding junctions)
- Adhesion (adhesive or anchoring junctions)
- Communication (gap junctions)
Occluding junctions (Zonula Occludens)
Impermeable barriers! which prevents the flow of materials between the cells
Membranes of adjoining cells are basically fused
Causes ridges and grooves of the plasma membranes of adj. cells
Adhesion (adhesive or anchoring junctions)
Requires microfilaments
Between adjacent cells (Zonula Adherens and Desmosomes)
Adhesion between epithelial cells to the basal
lamina (hemidesmosomes)
2b. ZA: microfilaments, apical
(belt desmosome)
2c. Desmosomes (Macula Adherens)
Tight Junctions (Zonula adherens)
Cause belt desmosomes and the link has these microfilaments
it’s like a zipper that brings the two cells together whereas desmosomes are more like buttons
Desmosome
accumulation of intracellular intermediate filaments inserting into the plaque of each cell
Junction Complex
includes
a) zonula occludens
b) zonula adherens
c) desmosomes
all together to make it REALLY tight
Communicating Junctions
“gap junctions”
allows for cell to cell communication
6 connexins make a connexon which is a pore in which materials can pass through to get between cells
Basal Lamina
Extracellular material between the epithelial basal surface and underlying connective tissue (reticular lamina)
Also found between adjacent epithelial cells
Dense layer of material (20-100 nm thick) which is only visible by electron microscopy
Composed mainly of type IV collagen, laminin and heparin sulfate (a proteoglycan)
Lamina is polymerized and anchors to the cell surface, then nidogen and heparan sulfate make bridges at the bottom of the cell surface
It is very complex!
Basal membrane function
Barrier: Filtration –Kidney
Cell-to-cell interactions:
Attachment to connective tissue
Receptors (Integrins) attach to BL components
Compartmentalization or separation
Define the Polarity
Tissue scaffolding: development and
regeneration
Basement Membrane
a thicker structure visible by light microscopy which is composed of either: two fused basal laminae or a basal lamina and a reticular lamina
Carcinoma/Adenocarcinoma
focal basal cell layer disruptions
Basically, cancer cells are confined within the epithelial cells - once the cells can destroy the basement membrane however, they can get into other tissues and are exposed to blood vessels which can carry the cancer and spread it to other tissues
Metaplasia
“change in form” is the reversible replacement of one differentiated cell type with another mature differentiated cell type
EXAMPLE: Pleomorphic adenoma (PA) a common salivary gland tumor with extensive squamous metaplasia and keratin cyst formations
Definition of a Gland
Formed by epithelial cells that secrete a fluid
of different composition than blood or intercellular fluid
Secreted substances can be ions, secretory polypeptides or proteins, lipids or glycoproteins.
Common Secreted Substances
- proteins (pancreas)
- lipids (adrenal, sebaceous glands)
- complexes of carbohydrates and proteins (salivary glands)
- all three substances (mammary glands)
- Ion (sweat glands, low synthetic activity)
Secretory Granules
– Molecules secreted are generally stored in the glandular epithelia
– in small membrane-bound vesicles first
Gland Secretion Methods
– Exocrine glands: retains connection (duct) to surface
– Endocrine glands: ductless; loose connection to surface
Secretion Pathways
- Constitutive: “make and release;” no signaling molecules so they’re constantly making and secreting fluids
- Regulated: concentrate and store; signaling for releasing (Neural/endocrine)
Exocrine Gland Classifications
- Unicellular: goblet cells, in lining of intestine and respiratory tract
- Multicellular: several types characterized by arrangement of glandular cells and organization of ducts
Multicellular Gland Development
• Covering epithelia with subjacent connective tissue
– proliferation and invasion
• further differentiation
– penetrate the underlying connective tissue
– Maintain or loss a
connection with the surface epithelium
If they still maintain contact with the surface, the will become a duct gland (EXOCRINE) and the bottom portion will become a specialized secretory portion; and through the duct, the material will be released
If the contact is not maintained, then you will see a gland embedded in the connective tissue but will intervene with blood vessels form either cords or follicles.
Structure of Multicellular Exocrine Glands
- Simple glands: one unbranched ducts
- Compound glands: more than one duct that branches
repeatably.
Simple Glands
Secretory portion may be tubular, coiled tubular, branched tubular, or acinar (Flask-shaped)
Compound Glands
Secretory portion may be tubular, acinar or tubuloacinar, but has to have MULTIPLE DUCTS to be compound
Merocrine
exocytosis w/o loss of cytoplasm; in the secretory portion, we can see vesicles being made and moving to the apical surface of the cell/get released through the duct
When the secretory granule is being made, it moves to the surface, and DOCKS onto the cytoplasm of the epithelial cells; then PRIMING occurs, in which the vesicle membrane fuses with the cytoplasmic membrane and they will open and exit the secretory material into the apical surface when signaled.
This process is regulated by cAMP and Ca2+, two important secondary messengers cell signaling.
most common form of secretion.
Sweat Glands
• Eccrine is specifically used to designate
merocrine secre7ons from sweat glands
• Eccrine glands are the major sweat glands of
the human body, found in virtually all skin.
• produce a clear, odorless substance,
consis7ng primarily of water and NaCl
(smelliness from sweating actually comes for the bacteria on the ski, not sweat itself! :D)
Holocrine
Whole cell is discharge as opposed to just secretory material
EXAMPLE: Sebaceous Gland
Sebaceous Glands
- Simple branched acinar - Pear-shaped saccules - Short ducts - Holocrine - Discharge Sebum
found in the mucosa of the cheek
Apocrine
secretory product and a portion of the
cytoplasm
EXAMPLE: Mammary Glands and specialized sweat glands
Major Salivary Glands
1) Parotid
2) Submandibular
3) Sublingual
Serous Secretory Cells
• Serous producing cells- watery secretions; usually intensely stained with Eosin; nuclei usually rounded or oval. • Merocrine secretion
•Glycoproteins: N-linked to the Beta-amide of asparagine
REMEMBER GYLCOPROTEINS SECRETED BY SEROUS CELLS ARE N-LINKED WHEREAS MUCOUS CELLS ARE O-LINKED*
Mucous Secretory Cells
Mucous producing cells- viscous, slimy secretions; appear empty by routine H/ E; nuclei often flattened against base of cell
Glycoproteins: :
O-linked to the hydroxyl groups of serine or threonine
Serous Demilune
Some cells in salivarfy glands form a crescent moon shape - so the inside is mucous and the outside is serious (giving it a kind of full but mostly empty appear under the microscope)
Two Types of Duct Cells
Intercalated:
flatter and cuboidal shaped and function primarily as a conduit for saliva, but also modify its content of electrolytes and secretes immunoglobin A
Striated:
more columnar (called low columnar) and gets the name "striated" because these cells contain a lot of mitochondria and the way they are aligned, it makes it look striated when stained. Also involved in the modification of primary saliva
Contractile
constricting function in which the myoepithelial cells covering the lining on the surface of ducts can be contracted and help push the salivary secretions out
