Types of secretory cells, classification of glands

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Glandular epithelium is composed of cells specialized for producing a secretion, storing it in the form of secretory granules, and subsequently releasing it into the extracellular space. Glands may be unicellular—individual cells scattered within the surface epithelium (e.g., goblet cells)—or multicellular, forming organs (glands in the strict sense).

According to their relationship to the surface epithelium (where they secrete their secretions basically), glands are divided into:

• endocrine.
• exocrine.

Endocrine glands

They have no connection to the surface epithelium in the form of a duct. Secretory granules are released directly into the intercellular space and then diffuse straight into the bloodstream. Their secretion is referred to as a hormone.

Exocrine glands

Simple and compound glands:

1 – tubular, 2 – branched tubular, 3 – coiled tubular, 4 – alveolar, 5 – branched alveolar

Exocrine Glands - David Darling

They are connected to the surface epithelium either directly or by means of a duct. They consist of a secretory portion (the functional part) and a duct. For histological purposes, all exocrine glands can be classified according to:

• the shape of their secretory units,
• the branching of their ducts,
• the nature of their secretion,
• the mechanism of secretion.

According to the structure of the secretory units

A secretory unit is the functional unit of an exocrine gland. It is formed by a single layer of cells resting on the basal lamina and producing secretion into the lumen. According to their shape, we distinguish:

• acinar – round shape with a narrow lumen; e.g., pancreas and parotid gland;
• tubular – tube-shaped with a narrow lumen; e.g., eccrine sweat glands and all mucus-secreting glands;
• alveolar – sac-shaped with a wide lumen;
• tubuloacinar – found in mixed glands; the unit is tubular with a rounded end; e.g., submandibular and sublingual glands;
• tubuloalveolar – found in mixed glands; the unit is tubular with a sac-like dilation; e.g., lactating mammary gland or apocrine scent glands.

According to the branching of ducts

Compound glands:

1 – tubular, 2 – alveolar, 3 – tubuloalveolar

• Simple – have one unbranched duct; the secretion is drained either from one or from multiple secretory units (in which case they are called simple branched glands);
• Compound – their ducts are branched; the glands are composed of lobules separated by connective tissue septa; their ducts are further classified as:
  • intralobular,
  • interlobular.

According to the character of the secretion

• Serous gland – produces a thin, watery secretion rich in proteins. Its cells contain abundant rough endoplasmic reticulum, giving them a basophilic appearance. Secretory granules are located apically, and the nuclei are round. The glandular cells form acini.
• Mucous gland – produces a thick secretion rich in mucin. The secretory unit is tubular and composed of mucous cells.

Mechanisms of secretion:

1 – merocrine, 2 – apocrine, 3 – holocrine, 4 – endocrine, 5 – paracrine action

Merocrine secretion

Molecules and ions are secreted gradually, and water follows them by osmosis. Hydrophilic molecules require energy to cross the cell membrane. Examples include secretion of bile acids in the liver, protons in the stomach, and NaCl in sweat glands. Energy is not required for secretion of hydrophobic steroid hormones, which pass through the membrane passively.

Apocrine secretion

The secretion accumulates in the cytoplasm at the apical pole of the cell. The cell surface then bulges outward until a membrane-bound droplet of secretion is released. Example: apocrine gland cells.

Holocrine secretion

This occurs only in sebaceous glands. The cell produces a secretion that gradually fills the entire cell. Under its pressure, all cellular organelles degrade, the pyknotic nucleus fragments, and the cell undergoes apoptosis. The secretion is released when the cell membrane ruptures, spilling out the entire cell contents—so the cell itself becomes the secretion.

Secretion Mechanisms

Sources and References

• LÜLLMANN-RAUCH, Renate. Histologie. 1. vydání. Praha : Grada Pulishing, a. s., 2012. 576 s. ISBN 978-80-247-3729-4.

• KONRÁDOVÁ, Václava, Jiří UHLÍK a Luděk VAJNER. Funkční histologie. 2. vydání. Jinočany : H & H, 2000. 291 s. ISBN 978-80-86022-80-3.

• JUNQUIERA, L. Carlos, José CARNEIRO a Robert O KELLEY, et al. Základy histologie. 1. vydání. Jinočany : H & H, 1997. 502 s. ISBN 80-85787-37-7.