TOB Module
Glandular Tissues and How Cells Secrete
(acknowledgements to Dr Colin Ockleford B.Sc., Ph.D., F.R.C.Path.)
Definition of a Gland
An epithelial cell or collection of
cells specialised for secretion.
Classification of Glands
• By destination
• By structure
• By nature of the secretion
• By the method of discharge
Classification by destination
• Exocrine – glands with ducts
• Endocrine – into the bloodstream
‘ductless glands’
Classification by Structure
Secretory part: unicellular / multicellular
acinar (alveolar) / tubular
coiled / branched
Duct system:
Simple gland (single duct)
Complex gland (branched ducts)
Branching ducts:
Main > interlobular > Intralobular > Intercalary
(ducts define structure of complex glands)
Unicellular goblet cells in the upper respiratory epithelium
Unicellular goblet cells in the epithelium of a villus
Classification by Structure
Secretory part: unicellular / multicellular
acinar (alveolar) / tubular
coiled / branched
Duct system:
Simple gland (single duct)
Complex gland (branched ducts)
Branching ducts:
Main > interlobular > Intralobular > Intercalary
(ducts define structure of complex glands)
Goblet cells in tubular colonic crypts
Classification by Structure
Secretory part: unicellular / multicellular
acinar (alveolar) / tubular
coiled / branched
Duct system:
Simple gland (single duct)
Complex gland (branched ducts)
Branching ducts:
Main > interlobular > Intralobular > Intercalary
(ducts define structure of complex glands)
Classification by nature of secretion
• Mucous – poorly stained in H & E sections
• Serous – eosinophilic in H & E sections
Mucous and serous cells of the submandibular salivary gland
S
M
Classification by method of secretion
• Merocrine
• Apocrine
• Holocrine
Merocrine secretion is exocytosis
• Membrane bounded component approaches cell surface
• It fuses with plasma membrane
• Its contents are in continuity with the extracellular space
• Plasma membrane transiently larger
• Membrane retrieved, stabilising cell surface area
Apocrine secretion
• Non-membrane bounded structure (e.g. lipid) approaches
cell surface
• Makes contact and pushes up apical membrane
• Thin layer of apical cytoplasm drapes around droplet
• Membrane surrounding drop[let pinches off from cell
• Plasma membrane transiently smaller
• Membrane added to regain original area
Apocrine secretion
Myoepithelial cells
assist secretion of
milk from acini.
Apocrine sweat glands
occur in the axillae,
areolae of nipples
and genital and
perineal regions.
Cytoplasmic blebbing
is not indicative of true
apocrine secretion.
Despite their name
these cells use
merocrine secretion.
Eccrine sweat gland (also
merocrine) showing secretory
portion and duct.
Myoepithelial cells contract in
order to facilitate the transport
of luminal contents towards
the duct.
Holocrine secretion
• Disintegration of the cell
• Release of contents
• Discharge of whole cell
Sebaceous gland
cells undergoing
holocrine secretion
to fill the hair follicle
with sebum.
(Medium power
micrograph)
Sebaceous
gland cells
undergoing
holocrine
secretion.
(High power
micrograph)
Endocytosis
• Engulfing material initially outside the cell
• Opposite of exocytosis (merocrine secretion)
• Endo- & Exo-cytosis are coupled in transepithelial
transport
Transepithelial Transport
• material endocytosed at one surface
• transport vesicle shuttles across cytoplasm
• material exocytosed at opposite surface
N.B. Molecules too large to penetrate membranes
can be shutted across from one component of
the body to another.
Golgi Apparatus:
Structure
• Stack of disc-shaped cisternae
• One side of discs are flattened; other concave
• Discs have swellings at their edges
• Distal swellings pinch off as migratory Golgi Vacuoles
A Golgi body consisting of curved, stacked cisternae
Golgi Apparatus:
Function
• Packaging through condensation of contents
• Transport
• Adding sugars to proteins and lipids (Glycosylation)
Golgi Product Destinations
• Majority extruded in secretory vesicles
• Some retained for use in the cells (e.g. lysosomes)
• Some enters the plasma membrane (Glycocalyx*)
* Sweet husk
Glycosylation & Specificity
• Branching sugars offer complex shapes for specific
interactions in the glycocalyx
• Destruction of this layer by enzymes alters many
specificity based properties of cells:
- adhesion to substrates & neighbouring cells
- mobility of cells
- communication with neighbouring cells
- contact inhibition of movement and division
Control of Secretion
• Nervous
• Endocrine control
• Neuro-endocrine control
• Negative feedback chemical mechanism
The three major salivary glands: parotid, submandibular and
sublingual.
Parotid gland acini
Striated
duct
lumen
Typical mixed secretory end-pieces of the submandibular gland showing tubular
mucus-secreting glands capped with crescent-shaped cells serous (serous demilunes)
Sublingual gland in which mucous cells predominate. Mucous cells are capped
with a minor proportion of crescent-shaped serous demilunes. The shape of serous
demilunes is artefactual.
The Pancreas, an exocrine
and endocrine gland.
Three islets of Langerhans surrounded by exocrine pancreatic acini.
An islet of Langerhans surrounded by exocrine pancreatic acini (high power).
The thyroid gland
Simple cuboidal
epithelium of the
thyroid gland
surrounding
homogeneous
colloid in each
follicle.
Parathyroid glands
Chief or Principal cells, oxyphil cells and adipose cells of the parathyroid gland
The adrenal or
suprarenal glands
Summary 1
• Epithelial cells form glands: the organs of secretion
• Glands are:
• tubular / acinar / coiled / branched
• simple / complex
• endocrine / exocrine
• serous / mucous
Summary 2
• Secretion is Merocrine, Apocrine or Holocrine
• Nascent proteins & lipids are processed in the Golgi
• Golgi products are retained, exported or added to
membrane
• Glycosylation confers additional specificity
• Control: chemical, neural, endocrine or neuro-endocrine