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A-covering (or lining) epithelia
B- Secretory (glandular) epithelia.
TYPES OF EPITHELIA
Epithelia can be divided into two main groups
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Glands
Glandular epithelial cells may synthesize, store, and secrete:
• Proteins (e.g; pancreas)
• Lipids (e.g; adrenal, sebaceous glands
• Complexes of carbohydrates and protein (e.g; salivary glands).
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1-Formation of glands from covering epithelia
2-Epithelial cells
proliferate
3- Penetrate
connective tissue
followed by
4-Further differentiation
5-If they maintain
contact with the
surface, exocrine
glands are formed
2-Epithelial cells
proliferate
3- Penetrate
connective tissue
followed by
4-Further differentiation
5-If they lose
contact with
the surface,
endocrine
glands are
formed
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Classification of glands:
• Glands are generally classified into two major groups:
• Exocrine glands
• Release their products onto an epithelial surface, either directly
or through a duct e.g; the salivary glands.
• Endocrine glands release their products ( hormones) into
the blood stream,
e.g; thyroid gland.
• Mixed variety: some glands possess both exocrine and
endocrine function .e.g; pancreas, liver cells.
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Glands can be also classified according to
the number of cells :
• Goblet cells which are present in the lining epithelia of large intestine and the respiratory tract.
• DNES (Diffuse Neuro-Endocrine Systems), or APUD (Amine Precursor Uptake and Decarboxylation) in small intestine.
A. Unicellular glands
• They form most of the glands of the body
• Salivary glands, Lacrimal gland, sweat glands…
B. Multicellular glands
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Mucus cells or goblet cells
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Goblet cell
The goblet cell is
highly polarized
its nucleus and
other organelles
are concentrated
at the base of the
cell.
The apical part of the cell's
cytoplasm is occupied by
membrane-bound secretory
granules containing mucin
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Goblet cell Functions of goblet cells
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Goblet cells in the respiratory tract
You should appreciate polarity What do think about their function?
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Unicellular gland
DNES (Diffuse Neuro-Endocrine Systems), or APUD
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For example, Stomach glands
Multicellular glands
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❖Morphology of ducts and secreting portions.
❖Nature of secretory product.
❖Mode of secretion.
These can also be classified on the basis of:
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Classified according to the duct system and the secretory part
Duct Secretory part
Compoundsimple Acinar (alveolar) tubular Tubulo-acinar
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Morphology of ducts and secreting portions
Classified by structure of duct:
Simple : “if a gland consists of a single secretory passage”.
Compound: “ if a gland containing a branched duct system”.
Categorized by secretory unit:
Tubular
Acinar
Tubulo- acinar
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SIMPLE Glands (Ducts Do Not Branch)
This means simple
Secretory portion
Tube like
+
Simple Tubular
Mucous glands of colon
Example
Simple Tubular
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Simple tubular
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This means simple
Several long secretory
parts joining to drain into
1 duct
+
Simple Branched Tubular
Glands in the uterus
and stomach
Example
Simple Branched Tubular
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Simple branched tubular
Several long secretory parts joining to drain into 1 duct
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Simple Coiled Tubular
This means simple
Secretory portion
is very long and
coiled
+
Sweat glands
Example
Simple Coiled Tubular
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Simple coiled tubular
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A single tube, tightly coiled in 3 dimensions eg. Sweat glands
Simple coiled tubular
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Simple coiled tubular
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Rounded, saclike
secretory portion
This means simple
+
Simple Acinar (or Alveolar)
Small mucous glands
along the urethra
Example
Simple Acinar (or Alveolar)
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Simple acinar
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Simple branched acinar
This means simple
Secretory portion
is very long and
coiled
+
Simple branched acinar
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Simple branched acinar
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Compound
exocrine glands
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Compound Tubular
This means Compound
Several elongated coiled
secretory units and their
ducts converge to form
larger duct
+
COMPOUND Glands (Ducts from Several Secretory Units Converge into Larger Ducts)
Example
Compound Tubular
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Compound tubular
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Compound acinar
This means Compound
Several saclike secretory
units with small
ducts converge at a larger
duct
+
Example
Compound acinar
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Compound acinar
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Compound tubulo - acinar
This means Compound
Ducts of both tubular
and acinar secretory units
converge at larger ducts
+
example
Compound tubulo - acinar
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Compound tubulo - acinar
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Exocrine Glands
Classified according to nature of secretion:
• Serous:
• Mucous:
• Mixed:
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Classification on the basis of nature of
secretory product
1. Mucous glands: produce carbohydrate-rich secretion
which is called mucus,
e.g; Pyloric glands of stomach
2. Serous glands: these glands produce watery, protein-rich
secretions, often high in enzymatic activity e.g;
Exocrine pancreas, the parotid salivary gland.
3. Mixed glands: these glands produce both mucous and
serous secretions e.g; the sublingual and submandibular
salivary glands.
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Parotid gland
Serous gland
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Esophagous gland
Mucous gland
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Mixed gland : Seromucous gland
Submandibular gland
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Classification
on the basis of the
mode of secretion
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Classification on the basis of the mode of
secretion:
• Depending on their mode of secretion i.e; the manner
in which the secretory product is elaborated.
• The exocrine glands are classified into the following:
1. Merocrine (eccrine) glands
2. Apocrine glands
3. Holocrine glands
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Merocrine glands
Merocrine glands :
The secretory product is
delivered in membrane-bounded
vesicles to the apical surface of
the cell. Here, vesicles fuse with
the plasma membrane and
extrude their contents by
exocytosis.
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Example: Sweat gland, Pancreas and salivary glands.
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Apocrine glands
Apocrine glands :
In these glands part of the apical
cytoplasm is lost along with the
secretory material.
1. Lactating mammary glands,
2. Special sweat glands located
in axilla , perianal area &
areola of nipples.
3. Ceruminous glands of the
external auditory meatus (ear)
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Mammary glands
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Holocrine glands
Holocrine glands:
In these glands entire cells laden
with secretory material
disintegrate and all of the cellular
contents are discharged from the
gland as secretions.
e.g; the sebaceous glands of skin