Week 5 - Epithelium & Connective Tissue Series 4: Glandular Epithelium
Glandular Epithelium:
A type of tissue that includes epithelial cells with a secretory function. Involves synthesising and releasing products into extracellular environment
Glands:
Glands are an example of epithelial cell or group of cells that produce + release a secretion which differs in composition from tissue fluid or blood.
Secretions include:
Mucus
Gastric Secretions
Sebum
Milk
Type of Glands:
Exocrine - Release secretions onto a surface
Has ducts which transport secretions to surface via dedicated channels.
Structure:
Secretory portion/secretory units, which contain cells specialised for secretion
Duct cells - Transport secretion out of gland to activity sites
Origin: All glands arise when cells on surface epithelium divide and are inserted into underlying CT during embryonic development.
This forms a epithelium cord/tubule
Cells in cord re-organise to form channels that become ducts
Cells become secretory cells
Endocrine - Release secretions in form of hormones into blood capillaries
Has no ducts
Hormones act as chemical messengers triggering physiological responses in specific target cells:
Hormones are released into extracellular space + blood capillaries, which travel via blood to target sites. These then bind to receptors to initiate a response.
Origin: Similar to Exocrine
Unicellular Gland - Gland composed of a single cell. E.G. Govlet cells in epithelium lining within respiratory and intestinal tracts.
Function - Secrete substances, such as mucus, onto epithelial surface.
If there is a high demand for secretions in various parts of the body, lots of glandular epithelium will be formed below surface epithelium.
Classification of Glands:
Simple vs Compound (How many ducts/branching)
Simple = 1 Duct, Compound = > 1 Duct
Tubular vs Acinar/Alveolar (Secretory cell shapes)
Tubular = Shapes like a tube, Acinar = Like a leaf clover
Secretory units in Glands:
Cells of secretory units show polarity. A secretory unit is called an acinus/alveolus.
Plural = Acini
Acini:
Structure:
Grape/Basket Shaped
Located at ends of duct systems
Usually pyramidal (broad base to CT, narrower apex near lumen)
Features:
Appearance of secretory cell is related to the secretion produced
Other secretions are released on external surface of body such as milk, tears, sweat, sebum
Examples:
In digestive + respiratory system, acini produce one of two secretions:
Mucous secretions from Mucous acini (Main one)
Serous Secretions Serous acini
Mucous Acini:
Different staining characteristics than serous acini
Main component of mucous secretion is carbohydrates/sugars which are stored in apical cytoplasm of mucous cells
Secretory granules are larger and pale in colour
Serous Acini:
Add less viscous and more watery secretions, so that mucous secretions can easily move to locations where it is used.
Not all secretory units are acini though. There are
Secretory Tubules:
Cuboidal or columnar rather than pyramidal
Secretory cells that line a tube, such as sweat glands and crypts in colon
Seromocous Glands:
Glands that have both serous and mucous secretory cells, such as salivary glands.
Acini contains both serous cells and mainly mucous cells in varying proportions between glands. Different types of salivary glands have different proportions of serous:mucous cells.
Serous acini = all serous cells
Mucous acini = mostly mucous cells, but some serous cells
Methods of secretion:
Merocrine (Known as eccrine, exocytosis)
Only secretion is released from the cell.
Product has to be enclosed in membrane.
Mode used by most exocrine + endocrine glands
Aprocrine:
Secretion + some cytoplasm is lost
Examples:
- Lipids in mammary glands; apocrine sweat glands. Lipids are not surrounded by a membrane, hence exocytosis cannot occur. Therefore they move to apical surface, where membrane is broken down causing secretion of some cytoplasm.
Holocrine:
Entire cell becomes secretion
Examples:
- Sebaceous glands in skin
Myoepithelial cells:
Epithelial cells with muscle-like contracting properties. These cells are triggered by hormones or signals from the nervous system
Basket-like arrangement, which function to squeeze the secretory cells to release secretion in duct. Located outside of secretory cells, but has long cytoplasmic processes which can reach the secretory cells.
This is important for controlling the timing of secretion release for saliva, sweat, milk from glands
For example: Role in sweating
Increase in body temperature
Activates thermoregulator centre in brain
Nerve impulses travel from brain to sweat glands
Stimulate myoepithelial cells
Secretory cells are contracted
Release of sweat, and evaporation from skin
Restored body temperature
Compound Glands:
These glands such as salivary glands, are too large for clusters of secretory cells to be together, so they have to be divided up into distinct areas/regions called lobules.
The secretory + acini need:
Duct system for transporting secretion
Blood supply
Nerves and or blood supply for secretion control
CT supporting secretory cells/acini, blood vessels, nerves and ducts
CT septa surrounding lobules and CT capsule surrounding glands.
Learning Objectives:
Identify the characteristics of glandular epithelium and glands
Glandular Epithelium:
A type of tissue that includes epithelial cells with a secretory function. Involves synthesising and releasing products into extracellular environment
Glands:
Glands are an example of epithelial cell or group of cells that produce + release a secretion which differs in composition from tissue fluid or blood.
Secretions include:
Mucus
Gastric Secretions
Sebum
Milk
Distinguish between exocrine & endocrine glands
Exocrine - Release secretions onto a surface
Has ducts which transport secretions to surface via dedicated channels.
Structure:
Secretory portion/secretory units, which contain cells specialised for secretion
Duct cells - Transport secretion out of gland to activity sites
Endocrine - Release secretions in form of hormones into blood capillaries
Has no ducts
Hormones act as chemical messengers triggering physiological responses in specific target cells:
Hormones are released into extracellular space + blood capillaries, which travel via blood to target sites. These then bind to receptors to initiate a response.
Appreciate that the appearance of a gland reflects the organisation of its secretory cells &/or units & ducts, & the composition of the secretions produced
Secretory units in Glands:
Cells of secretory units show polarity. A secretory unit is called an acinus/alveolus.
Plural = Acini
Acini:
Structure:
Grape/Basket Shaped
Located at ends of duct systems
Usually pyramidal (broad base to CT, narrower apex near lumen)
Features:
Appearance of secretory cell is related to the secretion produced
Other secretions are released on external surface of body such as milk, tears, sweat, sebum
Examples:
In digestive + respiratory system, acini produce one of two secretions:
Mucous secretions from Mucous acini (Main one)
Serous Secretions Serous acini
Mucous Acini:
Different staining characteristics than serous acini
Main component of mucous secretion is carbohydrates/sugars which are stored in apical cytoplasm of mucous cells
Secretory granules are larger and pale in colour
Serous Acini:
Add less viscous and more watery secretions, so that mucous secretions can easily move to locations where it is used.
Secretory units in Glands:
Cells of secretory units show polarity. A secretory unit is called an acinus/alveolus.
Plural = Acini
Acini:
Structure:
Grape/Basket Shaped
Located at ends of duct systems
Usually pyramidal (broad base to CT, narrower apex near lumen)
Features:
Appearance of secretory cell is related to the secretion produced
Other secretions are released on external surface of body such as milk, tears, sweat, sebum
Examples:
In digestive + respiratory system, acini produce one of two secretions:
Mucous secretions from Mucous acini (Main one)
Serous Secretions Serous acini
Mucous Acini:
Different staining characteristics than serous acini
Main component of mucous secretion is carbohydrates/sugars which are stored in apical cytoplasm of mucous cells
Secretory granules are larger and pale in colour
Serous Acini:
Add less viscous and more watery secretions, so that mucous secretions can easily move to locations where it is used.
Distinguish between the methods of release of secretions from secretory epithelial cells, & understand the reasons for these differences
Merocrine (Known as eccrine, exocytosis)
Only secretion is released from the cell.
Product has to be enclosed in membrane.
Mode used by most exocrine + endocrine glands
Aprocrine:
Secretion + some cytoplasm is lost
Examples:
- Lipids in mammary glands; apocrine sweat glands. Lipids are not surrounded by a membrane, hence exocytosis cannot occur. Therefore they move to apical surface, where membrane is broken down causing secretion of some cytoplasm.
Holocrine:
Entire cell becomes secretion
Examples:
- Sebaceous glands in skin
The type of secretion and its release method depend on trying to achieve minimal damage to the cell and also is adapted to the gland’s specific function, such as cooling the skin, producing lubrication, or delivering hormones.