WEEK 7 - EPITHELIAL TISSUE Flashcards
EPITHELIAL TISSUE AND GLANDS
what is a tissue?
group of cells similar in structure and function
four types of tissue : epithelial (covering), connective (support), muscle (movement) and nerve (control)
what is epithelial tissue?
- Epi= “on” or “around”
- Thelial= “surface/sheet”
- A continuous layer of closely packed cells that covers the external body surface and lines body cavities, and generally marks off our insides from our outsides
where is epithelial tissue
covers all internal and external body surface, and is the most abundant tissue
skin, mouth, blood vessels, stomach, lung…
functions of epithelial tissue
1.Provide physical protection (from abrasion, dehydration, destruction) e.g. skin
2.Control permeability (selectively filter biochemical substances; eg. capillaries and lung alveoli)
3.Provide sensation (sensory epithelium which contains sensory cells –nose, ear, eyes)
4.Absorb nutrients (eg. in digestive tract) 5.Secretion (glandular epithelium –hormones, mucus, etc.)
6.Transport (ciliatedepithelia sweep mucus, cells and other particles along the surface of the epithelium)
characteristics of epithelial tissue
1.Polarity: apical and basal surfaces
2.Specialised contacts: cell junctions, closely packed cells
3.Attachment: via basement membrane to underlying connective tissue
4.Avascularity: no blood vessels (but richly innervated)
5.Regeneration: can replace lost cells by cell division
polarity
- polarity refers to the presence of an apical surface, it is always free and can be inside lining body cavity or outside on the skin.
The basal surface is not free. - apical surface can be:
-smooth
-microvilli, 20 x surface area, increase absorptive capacity (lining intestine & kidney tubules), job of the microvilli is to increase the surface area increase absorption
-cilia, help to move materials (mucus, etc…, lining trachea) - polarity doesn’t only refer to presence of an apical and basal surface but also to the uneven distribution of intracellular organelles
specialised Contacts
- cell junctions :
tight junctions (pull the membrane over the cell towards the membrane of the other cell, cancelling off any space between the cell, forced to pass through the cells)
desmosomes
gap junctions - cell junctions give the cell strength and permeability by mechanically linking neighbouring cells
attachement
- epithelial tissue is attached to the underlying connective tissue via the basement membrane
- basal lamina : glycoproteins secreted by epithelial cells; selective filter for molecules between connective tissue and epithelium, important to protect underlying tissue
- Reticular lamina : layer of extracellular material (collagen) produced by connective tissue, gives the basement membrane its strength.
avascularity
No Blood vessels: nutrients arrive by diffusion from blood vessels in connective tissue. no space for blood vessels as cells are packed together
regenertaion
- Epithelial tissue has high regenerative capacity: can reproduce rapidly.
- Epithelial cells have a hard life: exposed to mechanical friction, disruptive enzymes, toxic chemicals & bacteria
- Regeneration is essential to maintain tissue integrity and is achieved via cell division from stem cells
- Skin - Renewed once every 35 days (200 times by your age), Stem cells in the basal layer divide & migrate to outer layers
stem cells
- Undifferentiated cells (not committed to a specific cell type/function)
- Self-renewable (able to go through numerous cycles of cell division while still maintaining their undifferentiated state)
- After division, each daughter cell has a choice: it can either remain a stem cell, or it can embark on a course that commits it to terminal differentiation
- Different tissues have different regenerative capacity: high in epithelial tissue; very low in cardiac and nervous tissue
classification of epithelial cells
- number of cell layers :
simple - one layer (for absorption/filtration)
stratifies - more than one layer (for protection) - cell shape :
squamous - flat (flattened nuclei)
cuboidal - cube (nucleus is spherical and is positioned in centre of cell and is large)
columnar - tall/column
simple squamous epithelium
- Description: Single layer of flat cells with disc-shaped central nuclei; the simplest of the epithelia.
- Function: Allows material to pass by diffusion and filtration in sites where protection is not important.
- Location: Lining many body cavities: lung alveoli (air sacs -gas exchange) and capillaries (permeability)
- Lung tissue - one layer of epithelial cells, very thin
- blood vessels - Endothelium: Epithelium lining the entire vascular system. Function: exchange of nutrients and wastes between blood and tissue
simple cuboidal epithelium
- Description : Single layer of cube-like cells with large, spherical central nuclei.
- Function : Secretion and absorption
- Location : Lining kidney tubules; ducts and secretory portions of small glands
- Kidney tubules - Function: absorption of water and small molecules from kidney filtrate into the blood & secretion of wastes from blood into urine
simple columnar epithelium
- Description : Single layer of tall cells with oval nuclei located near the basement membrane; some cells have cilia or microvilli; may contain mucus-secreting unicellular glands (goblet cells).
- Function : Absorption of nutrients; secretion of mucus, enzymes & digestive fluids; ciliated type moves materials (mucus or eggs) by ciliary action.
- Location : Non-ciliated lines most of the digestive tract (stomach to rectum) & ducts of some glands; ciliated lines respiratory tract and uterus
- Digestive tract - Function: absorbing and transporting nutrient
stratified squamous epithelium
- Description : Thick: several layers of flat cells. Basal cells are cuboidal or columnar, metabolically active, divide and produce the cells of the more superficial layers. In the keratinised type, the surface cells are full of keratin and dead
- Function : Protects underlying tissues against abrasion, pathogens, & chemical attack.
- Location : Lining oesophagus, mouth & vagina; (non keratinised usually found in on organs that don’t come in direct contact with external environments). keratinised type in the epidermis of the skin & outer surface of the cervix (keratinised found in organs that need highest level of protection)
*keratin is a very strong protein that will be found on the apical side of the epithelium, after cells die on surface they are filled with keratin - oesophagus - Function: protection from hot food and bacteria
stratified cuboidal epithelium
- Description : Generally, two layers of cuboidal cells.
- Location (rare) : lumen of mammary glands and sweat glands.
- Function : Secretion
- Sweat gland ducts - large spherical nuclei
stratified columnar epithelium
- Description : Several layers of cells in which basal cells are cuboidal and superficial (top) cells are columnar.
- Locations (rare) : Large ducts of mammary glands, salivary gland ducts and urethra.
- Functions : Secretion and protection
pseudostratified columnar epithelium (is actually simple)
- Description : Single layer of cells (some tall, some short); all in contact with basement membrane; some not reaching the free surface; nuclei seen at different levels; may be ciliated.
- Function : Secrete substances, particularly mucus; movement of mucus by ciliary action (dust-trapping mucus in trachea).
- Location : Lines part of the male reproductive tract & the respiratory system (eg. Trachea)
transitional epithelium (can transition from one state to another)
- Description : Several cell layers. Basal cells cuboidal or columnar; surface cells dome shaped or flat (depending on degree of organ stretch).
- Function : Stretches to permit distension of urinary bladder
- Location : Lines urinary bladder
- Transitional epithelium is designed to distend and return to its normal size.
- In empty bladder: epithelium is thick and multilayered, with rounded, domelike cells on the luminal surface.
- In full bladder: epithelium stretches and thin out (squamous cells).
what are glands?
- specialised epithelial tissue structures that secret substances
- secretion :
mucin (goblet cell),
hormones (thyroid gland),
enzymes (pancreas),
electrolytes and waste products (sweat gland) - Secretion is an active process: glandular cells obtain substances from blood and transform them chemically into secretion product.
- Glandular epithelial cells have prominent ER, Golgi apparatus and secretory granules.
glands - classification
Classified by:
* Site of product release–endocrine (internally secreting in tissue and taken by blood) or exocrine (externally secreting outside tissue e.g. body cavity like the stomach or on surface of skin)
* Relative number of cells forming the gland –unicellular (e.g. goblet cell) or multicellular
glands : endocrine
- Secrete their products hormones directly into extracellular space (by exocytosis)
- Hormones enter blood or lymphatic fluid and travel to target organs
- Target organs respond in some characteristic way
- Ductless glands: secretions not released into a duct
- Example: thyroid gland
glands : exocrine
- Secrete their products onto body surfaces (skin) or into body cavities
- More numerous than endocrine glands
- Secrete products into ducts (except for goblet cells – unicellular glands)
- Unicellular or multicellular
- Examples: sweat and salivary glands, liver (secretes bile), pancreas (secretes digestive enzymes
Unicellular Exocrine Gland: Goblet cell
- Found in epithelial linings of digestive and respiratory tracts
- Produce mucin: complex glycoprotein that dissolves in water to form mucus
- Mucus: slimy coating that protects and lubricates surfaces
- In digestive tract: mucus blocks microorganism, toxins, bacteria, digestive enzymes
- In respiratory tract: mucus traps dust particles, bacteria and other inhaled debris; cilia move the mucus towards the pharynx to be swallowed –protects lungs from infections
Multicellular Exocrine Glands
- Multicellular exocrine glands are composed of a duct and a secretory unit (secretory cells)
- Formed by invagination of epithelium into underlying connective tissue
- Connective tissue provides support and nutrients to glands (blood vessels)
- Classified by mode of secretion
Exocrine Glands: Mode of secretion
- Merocrine glands - most –secrete products by exocytosis (sweat glands, salivary glands & pancreas)
- Apocrine glands : a portion of plasma membrane buds off the cell, containing the secretion (mammary glands)
- Holocrine glands : the entire cell disintegrates to secrete its product (sebaceous glands, microscopic glands found in your hair follicles that secrete sebum)
Polarity is one of the five characteristics of epithelial tissue. What is polarity?
Presence of an apical surface (free) and a basal surface. Uneven distribution of organelles & membrane proteins
What does avascularity mean?How do epithelial cells receive nutrients and O2?
There are no blood vessels in the epithelium as there is no space for them between the closely packed cells. Nutrients are delivered by diffusion from blood vessels running in connective tissue which is below the epithelium
What specialised structures are present on the apical surface of epithelial cells and their function?
microvilli and cilia
Microvilli: increase the surface area by 20x increasing absorptive capacity. Cilia: help to move materials (mucus, eggs)
Can you describe the mode of secretion of merocrine, apocrine and holocrine glands?Can you give an example of each
Merocrine glands: secretion by exocytosis (sweat glands)
Apocrine glands: a portion of the plasma membrane buds off the cell, containing the secretion (mammary glands)
Holocrine glands: the entire cell disintegrates to secrete its product (sebaceous glands)
What is the very thin, specialised epithelium of the blood vessels and vascular system called?
endothelium
Small (~ 1 micrometre) finger like projections which increase the surface area for absorption?
microvilli
Motile processes (~ 10 micrometre) that move mucus and other matter along the length of the epithelium?
cilia
what cells have cilia, microvilli or neither?
cilia - lining of the trachea, vas deferent, oviduct
microvilli - lining of the gastrointestinal tract, nephrons of the kidney
neither - blood vessels, skin, lung alveoli, small endocrine ducts, oral cavity/ oesophagus
Think of the function of these epithelia - in the gut and the kidney materials need to be absorbed efficiently so these cells have microvilli. The airways need to be cleared of particles - the cilia here sweep away mucus containing inhaled debris. In the reproductive tracts the oocytes and sperm are swept along the ciliated epithelium
single layer of epithelial cells is known as
simple
A common feature of epithelial cells is
absence of blood vessels
In a pseudostratified epithelium…
All cells are in contact with the basement membrane
what functions are carried out by epithelia?
-absorption
-protection
-contraction
Name the principle cells of the liver and one function carried out by these cells
hepatocyte and functions as…
storage, secretion, degradation, metabolism, produce urea, synthesis, detoxification
Store glycogen and break it down to regulate blood sugar (regulate metabolism)
Synthesise, absorb and degrade cholesterol Metabolise amino acids to produce urea
Degrade toxins (detoxification)
Secrete plasma proteins (into blood) and bile (into bile ducts)
What protein is deposited at the surface of the skin epithelium to form a protective barrier?
keratin
Steroid hormone secreting cells have the following characteristics
Well developed smooth endoplasmic reticulum and Golgi apparatus, an abundance of mitochondria and free lipid
Match the type of epithelium to its locations
- Simple squamous - blood vessels, lung alveoli
- Simple cuboidal - small endocrine ducts, nephrons of kidney
- Simple columnar - oviduct, lining of gastrointestinal tract
- Pseudostratified columnar - lining of the bronchus, vas deferens
- Stratified squamous - oral cavity/ oesophagus, skin
