HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter l6 GLANDS

Glands are composed of secretory epithelial cells and their supporting connective tissue, nerves and blood vessels. Powerpoint
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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.
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 and neurons.
3 Each lobule contains:
  1. Many epithelial, parenchymal cells grouped as tubules or alveoli, cut at a variety of angles to the plane of section.
  2. In each tubular or alveolar secretory unit, the cells lie on a BL and face inwards towards a very small lumen.
  3. The lumens lead to ducts, also seen in the lobule.
  4. Outside the BLs, in the spaces between alveoli are the blood capillaries, CT cells and autonomic nerve fibres of the supporting stroma.
  5. A duct system runs through and out of the lobule and the gland, converging and widening as shown in Table 5.
Table 5. Secretory passages of a compound exocrine gland. 
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 alveoli.

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, e.g., leucine.
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

  1. 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 cycle.
  2. 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.
  3. 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 secretory.)

William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159