Epithelial tissue

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What is a tissue?
a group of cells similar in structure and function
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What are the 4 types of tissue?
epithelial
connective
muscle
nerve
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What is epithelial tissue?
A continuous layer of closely packed cells that covers the external body surface and lines body cavities and generally marks off our insides from outsides
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Where is epithelial tissue found?
everywhere as it covers all internal and external body surfaces
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What are the 2 types of epithelial cells?
epithelia- layers of cells covering internal and external surfaces
glands (glandular epithelium) structures that produce secretions
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What are the functions of epithelial tissues?
-protection from abrasion, dehydration, destruction
-control permeability
-provide sensation (sensory epithelium) nose eyes ears
-secretions- glandular-hormones
-transport-ciliated epithelia- sweep mucus
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What are the characteristics of epithelial tissues?
-polarity-apical and basal surfaces
specialised contacts- cell junctions close to cells
-attachment-via basal lamina to connective tissue
-avascularity-replace lost cells by cell division
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What does polarity refer to?
not just the apical and basal surface but also refers to the organisation of intracellular organelles within epithelial cells
-uneven distribution of organelles in epithelial cell-located to basal side
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With polarity, what does it refer to the apical surface about?
the epithelial cells have an apical surface which is always free and can be inside or outside
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What is the basal surface?
The surface of epithelial cells which come into direct contact with the basement membrane
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What is the texture of the apical surface and what specialised structures does it contain?
not smooth and contains microvilli and cilia which increase surface area of epithelial cells and can increase by up to 20 times.
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What does increasing the surface area of the epithelial cells increase?
increases the capacity of the epithelial cells
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Where are the specialised structures in the epithelial cells found?
in the lining of the intestine or kidney tubules-absorption
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Where is cilia found?
on the apical surface and are hair like extensions of the PM and mvoe materials like mucus along surface of epithelium.
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What are cell junctions important for in specialised contacts?
essential to keep epithelial cells strongly packed together and provide epithelium with strength and impermeability
-provides strength by mechanically linking 1 epithelial cell with the next
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How is impermeability done?
by preventing the presence of any extracellular space between the cells
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What does attachment mean?
-the epithelial cells are attached via basement membrane- to underlying connective tissue
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What is the basement membrane made up of?
-two layers:
-basal lamina and reticular lamina
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What does avascularity mean?
no blood vessels in the epithelial tissue.
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Where do nutrients arrive from to the connective tissue?
by diffusion from the blood vessles
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What is the connective tissue important for?
to provide epithelial tissues with structural support and provide nutrients and oxygen to epithelial cells because connective tissues host the capillaries
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Does epithelial tissue have high or low regenerative capacity?
high
- can reproduce rapidly
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Why is regeneration essential?
to maintain tissue integrity and is achieved by cell division from stem cells
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What do stem cells in the basal layer do?
divide and migrate to outer layers
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What are stem cells?
undifferentiated cells and are self renewable (can go through cell division many times whilst staying undifferentiated)
-after division it can either stay as a stem cell or commit to differentiation
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How are epithelial tissue classified?
refer to the number of layers and the cell shape
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What is a simple epithelial cell?
when it only has 1 layer of cells
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What is a stratified epithelial cell?
more than one layer of epithelial cells
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What is a squamous shape?
when it is flat and has disc shaped central nuclei.
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What is a cuboidal shape?
a cube shape of cells
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What is columnar cells?
When the cells are tall/columnar
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What is the function of simple squamous epithelium and where is it located?
allows materials in by diffusion and filtration and is located in body cavities like lung alveoli and capillaries
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Is there any extracellular space between the cells in simple squamous epithelium?
no
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What is the function of simple cuboidal epithelium and where is it located?
secretion and absorption and lined in kidney tubules, ducts and secretory portions of small glands
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What is the function of simple columnar epithelium and where is it located?
Absorbs nutrients and secretes mucus and enzymes. It is located in the lining of digestive tract as nonciliated and ducts of some glands
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What are the 2 types of stratified squamous epithelium?
keratinized and non keratinized
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What is keratinized stratified squamous?
thick layer of keratin on top of the squamous cells. Intracellular organelles are lost and it filled up with protein keratin so it results in dead epithelial cells filled with keratin which makes it strong
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What is the function of stratified squamous epithelium?
Protects underlying tissues against abrasion, pathogens and chemical attack
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What is the function of stratified cuboidal epithelium and where is it found?
for secretion and lines lumen of mammary glands and sweat glands
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What is the function of stratified columnar epithelium and where is it located?
for secretion and protection and is in large ducts of mammary glands and urethra
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What is pseudostratified columnar epithelium?
Some cells come into contact with the apical surface but some dont and they resemble a simple columnar epithelium
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What is transitional epithelium?
Several layers of basal cells cuboidal or columnar and have a dome shape at the surface cells
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What is the function of transitional epithelium?
stretches to allow distension of urinary bladder
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What are glands?
specialised epithelial tissue structures that secrete substances
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What are released during secretion?
mucin-from goblet cells
hormones-adrenal thyroid
enzymes-pancreas
electrolytes and waste-sweat gland
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What do globular cells contain?
substances from blood and transform them chemically into secretion product
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What are prominent in glandular epithelial cells?
ER, golgi apparatus and secretory granules
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How are glands classified?
-site of product release (endocrine or exocrine (externally secreting))
-relative number of cells forming gland-unicellular or multicellular
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What is an example of a unicellular gland?
goblet cell
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what is an example of a multicellular gland?
duct
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How do endocrine glands secrete their product?
internally into the extracellular space and between epithelial cells by exocytosis. Hormones then enter blood/lymphatic fluid and travel to target organs-these then respond
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What are endocrine glands also known as?
ductless gland- has secretions which are not released into a duct eg. thyroid gland
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What are exocrine glands?
externally secreting glands such as the outside of the skin. Secretes products onto body surfaces or into body cavities.
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What do exocrine glands do?
secrete products into ducts and can be uni or multicellular eg sweat,salivary glands live, pancreas
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Where are unicellular exocrine glands (goblet cells) found?
in epithelial linings of digestive and respiratory tracts.
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What do goblet cells produce?
mucin which is a complex glycoprotein that dissolves in water to form mucus- lubricates and in digestive tract, toxins are blocked to protect epithelial cells
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What does mucus do in the respiratory tract?
traps dust particles, bacteria and other inhaled debris and the cilia moves the mucus towards the pharynx to be swallowed to protect the lungs from infection
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What are multicellular glands composed of?
a duct and a secretory unit (secretory cells)
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How are multicellular exocrine glands formed?
by invagination of epithelium into underlying connective tissue
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What does connective tissue provide to glands?
nutrients from blood vessels and provides support
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How are multicellular exocrine glands classified?
by modes of secretion
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How are nutrients passed from blood to epithelial cells?
from blood to connective tissues to basement membrane to epithelial cells
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What are the 3 types of classification of exocrine glands?
merocrine glands
apocrine glands
holocrine glands
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what are merocrine glands?
secrete products by exocytosis (sweat, salivary gland and pancreas)
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What are apocrine glands?
a portion of plasma membrane buds off the cell containing the secretion
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What are holocrine glands?
the entire cell disintegrates to secrete its products (sebaceous glands)
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