William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA
Glands are composed of secretory epithelial cells and their supporting
connective tissue, nerves and blood vessels.
A VARIETIES OF GLAND
l Epithelial secretory layer, e.g., lining stomach and uterus.
2 Single cells amongst others in an epithelium - goblet cells,
secreting glycoproteins, which with water, make mucus. Mucus is vital for the
protection and lubrication of epithelial surfaces.
3 Intra-epithelial clusters of glandular cells, e.g., in urethra.
Glands as structures distinct from an epithelium can hold more
synthesizing cells, but remain related to the surface epithelium by a duct -
exocrine type of gland.
Other glands originate in an epithelial layer, but lose their duct and send
their secretion instead into blood capillaries - endocrine or ductless glands.
4 Exocrine glands, which may be:
5 Endocrine glands making hormones: details in Chapters 26 and 27.
- (a) Simple, with one duct have secretory units (end-pieces) of a
.. (i) tubular (straight, coiled, branched), or
.. (ii) acinar/alveolar (dilated acini may be termed saccules).
- (b) Compound with a branching duct system and secretory
units of three forms:
.. (i) tubular,
.. (ii) acinar/alveolar,
.. (iii) mixed tubulo-alveolar.
6 Mixed exocrine and endocrine glands, e.g., pancreas.
7 Mixed germinal exocrine/cytogenic (forming reproductive cells) and
endocrine - testis and ovary.
8 Neurosecretory nerve cells and their axons constituting a
neurofibrous gland are an exception to glands' being epithelial.
This classification takes on more meaning when all glands in all the
organs have been studied.
11 B STRUCTURE OF A COMPOUND EXOCRINE GLAND
l Encapsulated in fibrous CT which sends in partitions around lobes.
2 Septa (sheets of CT) divide the glandular tissue further into
lobules. Septa carry ducts, blood and lymphatic vessels, and autonomic nerves
3 Each lobule contains:
Table 5. Secretory passages of a compound exocrine gland.
- Many epithelial, parenchymal cells grouped as tubules or
alveoli, cut at a variety of angles to the plane of section.
- In each tubular or alveolar secretory unit, the cells lie on a BL and face
inwards towards a very small lumen.
- The lumens lead to ducts, also seen in the lobule.
- Outside the BLs, in the spaces between alveoli are the blood capillaries,
CT cells and autonomic nerve fibres of the supporting stroma.
- A duct system runs through and out of the lobule and the gland,
converging and widening as shown in Table 5.
Structure and site Lined by
Intercellular canaliculi (alveolus) Alveolar secretory cells
Alveolar lumen (alveolus) Alveolar secretory cells
Intercalated duct (intralobular) Squamous or cuboidal epithelium
Intralobular duct (intralobular) Cuboidal epithelium
Interlobular duct (interlobular septum) Columnar epithelium
Lobar duct (interlobar septum) Pseudo-stratified columnar epithelium
Final duct (lamina propria of tract) Stratified columnar epithelium
4 Compound exocrine glands were classified by their secretory
product as serous (water+enzymes), mucous (glycoproteins), or
mixed serous and mucous.
l Serous acini have pyramidal darkly basophilic cells, with spheroid
nuclei and apical zymogen (pro-enzyme) granules.
2 Mucous acini are made up of pale cells, with the nuclei flattened towards
their bases, and a cytoplasm crowded with mucus/mucin droplets, which can be
stained to reveal the presence of the sulphated or neuraminic-acid/sialic-acid
moieties that confer viscosity on mucus.
3 Mixed acini/alveoli:
Mixed glands may also form two products by having pure mucous and pure serous
- (a) Mucous cells, in the majority, surround the lumen.
- (b) Serous cells lie at one end as a serous crescent/demilune
between the mucous cells and the BL.
- (c) Serous secretion may pass in fine intercellular canaliculi
between the mucous cells to reach the lumen.
C CYTOLOGY OF SECRETION
l Serous secretion (e.g., in pancreatic exocrine acini)
l Enzymes formed are proteins, and the path of synthesis can be revealed by
following radioautographically the fate of tritium-labelled amino acids,
Other serous products include antimicrobial proteins.
2 In the basal region of the cell, amino acids are chain-linked at the
ribosomes attached to the GER, in sequences determined by mRNA from the
nucleus. The energy needed is released by plentiful mitochondria.
3 The protein passes into the cisternae of the GER and
4 travels in the cisternal space to near the Golgi complex.
5 The protein is shuttled to the cis/forming/proximal face of the supra-nuclear
Golgi complex by transporting vesicles.
6 Condensing vacuoles concentrate the secretion before its dispatch
from the concave trans/secretory/maturing/distal face of the Golgi to become
7 membrane-bound, apical, zymogen storage granules.
8 With appropriate stimulation, the granules pass to the cell's luminal
membrane for release by exocytosis, whereby the granule's enclosing
membrane fuses with the cell's, which then breaks allowing the granular
content to spill out into the acinar lumen.
2 Mucous secretion (e.g., by goblet cell)
l Oligosaccharides are completed by the Golgi complex, sulphated, if
necessary, and linked with a protein to form
2 mucin, stored as droplets dilating the apical cytoplasm.
3 Granular ER - for synthesis of the core protein of the
glycoprotein and of sugar-attaching (glycosylating) enzymes - is well developed
in the narrow basal stem of the cell.
4 After one cycle of activity, the gut goblet cell is normally shed to be
replaced from a pool of undifferentiated cells.
5 Mucous cells of salivary glands are not shed. They have GER and, when they
are immature, or in the early secretory phase with little mucin accumulated,
they are basophilic and may resemble serous cells.
6 The mucin type of glycoprotein has its hundreds of chains of sugar moieties
attached to the peptide core - the apomucin - by hydroxyls of serine or
threonine - the O-linkage. In contrast, serum-type glycoproteins are
N-linked, since their sugars attach via amido groups of asparagine. The
O- and N-linked classes differ in their affinities for lectins, what agents block
sugar-chain biosynthesis, and in whether the first glycosylation is in the
Golgi complex or GER.
7 The mucin molecules are further classified as neutral or acidic, based in
part on the amount of sialic acid present. The molecules join end-to-end, and
then tangle up for bulk and high viscosity.
3 Liberation of secretion
- Merocrine/epicrine/eccrine manner involves exocytosis, or the
discharge of only secretory material without any loss of cytoplasm, as in a
serous gland. The cell then returns to the synthesizing phase of its secretory
- Holocrine secretion requires that the cell fill itself up with
secretion which is liberated by the cell's breaking open and dying, e.g.,
in a sebaceous gland. Precursor cells must multiply to replace those lost, for
the gland to continue secreting.
- Apocrine way was thought to involve a significant loss of apical
cytoplasm along with the secretion, but not cell death. EM suggests
that this occurs rarely, if at all, and the classic apocrine-merocrine
distinction is invalid. However, apocrine is now applied to a release of
secretion where the product, milk fat, departs from the mammary cell
enclosed in a membrane.
4 Myoepithelial cells (basket cells)
These lie between glandular and duct cells and the BL, and clasp those cells
in long branching processes filled with filaments. They closely resemble
smooth muscle cells, and contract to help squeeze the secretion out of
large exocrine glands (breast and salivary) or the long, tortuous sweat gland.
5 Duct-lining cells
Ducts are not usually passive tubes for conveying secretions. Their
lining cells often are cuboidal or columnar, and acidophilic, with many
basal mitochondria serving active transport mechanisms to modify the
secretion's concentration and electrolyte composition, by actions similar
to those of kidney tubules. Such ducts may be called secretory
or striated (from the many parallel mitochondria); they lead to less
active excretory (drain pipe) ducts.
(Secretory ducts are usually intralobular, but not all intralobular ducts are