Histology 1.4. : Glands
Gland: a single epithelial cell, or grouping of cells specialized
for secretion.
Secretion: energy-consuming process by which the cell takes up
small molecules and transforms them via intracellular
biosynthesis into a more complex product then actively
releases it from the cell. The product is utilized by the
organism in several ways.
Excretion: the organism gets rid of harmful or toxic metabolic
end-products or useless waste material.
Classification of glands:
I. Based on morphology: unicellular and multicellular
Unicellular, intraepithelial gland: goblet cell
Intestinal epithelium:
simple columnar epithelium (2)
goblet cells (1)
the arrow shows the nucleus (3)
TEM image of the same
type of unicellular gland,
the goblet cell
CLASSIFICATION OF MULTICELLULAR GLANDS:
With ducts: exocrine glands
Without ducts: endocrine glands
CLASSIFICATION OF EXOCRINE GLANDS
(based on morphology):
Simple tubular
Coiled tubular
Branched
tubular
Simple acinar
Branched
acinar
Compound
tubular
Compound
acinar
Compound
tubuloacinar
SOME EXAMPLES:
Simple tubular gland: intestinal gland of Lieberkühn
Schematic drawing
LM microphoto
Coiled tubular gland: sweat glands of the skin
Schematic drawing
LM microphoto
Branched tubular gland: fundic glands in the stomach
Schematic drawing
LM microphoto
Fundic gland from the stomach - endpiece
Simple acinar gland:
Frog skin: mucous
and toxin-producing glands
Compact form
without lumen:
sebaceous gland of
mammalian skin
Branched acinar gland: larger sebaceous glands of the skin
Compound tubular gland: esophageal gland
duct segment
capillary
secretory acini
capillary
lumen
Compound tubuloacinar glands:
Mandibular gland
Parotid gland
Composition of compound glands:
Parenchyma composed of lobes and lobules, ducts
The duct system of the compound glands:
Acinus
Intercalated duct
Striated (salivary duct)
Interlobular duct
Lobar duct
Main duct
Model of the gland: a bunch of grapes: berry= acinus
stalk of berry: intercalated duct
Intralobular striated (salivary) duct
Interlobular duct
II. Type of secretory product:
1. Serous gland:
Pancreas
(see the micrograph)
and parotid gland
are purely serous
producing thin
watery fluid
rich in proteins
(mainly enzymes)
composed of acini with narrow lumen (1)
secretory cells have round, basally located nuclei (2)
the cytoplasm of the cells is basophilic (3)
3
1
2
2. Mucous glands:
Esophageal glands
(see the micrograph)
are purely mucous
producing thick
viscous fluid, rich in
mucopolysaccharides
for lubrication and
protection of internal
body surfaces
composed of acini with wide lumen
secretory cells have flattened nuclei at the base
their cytoplasm is very weakly stained
3. Seromucous glands: mucous acini surrounded by serous cells
forming a demilune shape
The submandibular gland of some species (monkey, human, cattle) is
seromucous. Red arrows point at mucous cells, blue arrows point at
the demilune –shaped group of serous cells (Demilune of Gianuzzi)
Seromucous gland, haematein-eosin
staining
Seromucous gland, alciane blue
staining
III. Modes of secretion:
1.
•
•
•
Merocrine secretion:
The secretory process is an exocytosis
The secretory cell remains completely intact
Most of the glands secrete in a merocrine manner
Exocrine pancreas
Submandibular gland
2. Apocrine secretion:
• the secretum is gathered at the apical portion of the cell
• the secretum leaves the cell membrane-bounded (pinched off)
• the cell remains alive, but a part of it goes with the secretory droplet
Examples: sweat and mammary glands
Membrane-bound secretory droplets of a sweat gland
3. Holocrine secretion:
• the secretory cell gradually fills up with secretum
• the cell organelles degenerate
• the cell dies, its membrane breaks and the secretum empties
The sebaceous gland is a holocrine gland. Dead cells are replaced
by the mitotic divison of basal cells
Absorptive epithelium:
its main function is to absorb. Morphology of these epithelial cells exhibit
cuticular border (intestine) or brush border (kidney tubules).
Please note: at fine structural level both are microvilli !
Intestinal epithelium: the arrow shows
the cuticular border
EM micrograph of the apical
surface with microvilli
Pigmented epithelium:
Epithelial cells contain melamosomes: brown color
EM
Pigmented epithelium in the eye
at LM level shows brown pigmentation.
LM
At EM level the melanosomes appear as
electron dense bodies in the cytoplasm.
Sensory epithelia:
Main function is sensation
Types: primary, secondary sensory epithelium
true nerve cells
Primary sensory epithelium: olfactory epithelium
Secondary type of sensory epithelia:
Example: sensory cells of the taste buds