EPITHELIAL GLANDS | CELL BIOLOGY

Types of epithelial glands ( 2-1 )

Most glands develop as an epithelial downgrowth extending into the underlying connective tissue.

Exocrine glands remain connected to the surface of the epithelium by an excretory duct that transports the secretory product to the outside.

Endocrine glands lack an excretory duct , and their product is released into the blood circulation.

Endocrine glands are surrounded by fenestrated capillaries and commonly store the secretions they synthesize and release after stimulation by chemical or electrical signals.

Exocrine and endocrine glands can be found together (for example, in the pancreas), as separate structures in endocrine organs (thyroid and parathyroid glands) or as single cells (enteroendocrine cells). Endocrine glands will be studied later.

Components of exocrine glands ( 2-2 to 2-5 )

An exocrine gland has two components:

• 1.

  A secretory portion .

• 2.

  An excretory duct .

The secretory portion of a gland may be composed of one cell type (unicellular , for example, goblet cells in the respiratory epithelium and intestine) or many cells (multicellular) .

The excretory duct can be simple (see 2-2 ) or branched (see 2-3 ). A gland is called simple when the excretory duct is unbranched. The gland can be branched when the excretory duct subdivides.

According to the shape of the secretory portion , glands with an unbranched excretory duct can be:

• 1.

  Simple tubular gland .

• 2.

  Simple coiled gland .

• 3.

  Simple alveolar gland (Latin alveolus , small hollow sac; plural alveoli) , also called acinar (Latin acinus , grape; plural acini ).

Simple tubular glands are found in the small and large intestine. The sweat glands of the skin are typical coiled glands . The sebaceous gland of the skin is an example of an alveolar gland .

The gastric mucosa and endometrium have branched secretory portions . Note that the secretory portions are branched but not the excretory duct.

Instead of a single excretory duct in simple glands, the excretory duct can be branched .

Tubular and alveolar secretory portions can coexist with branching excretory ducts (see 2-4 ). Then, the gland is called a branched (or compound) tubulo-alveolar (or acinar) gland. An example are the salivary glands (see 2-4 and 2-5 ). The exocrine pancreas is an example of a branched alveolar gland consisting of just alveoli (see 2-4 ).

Composition of secretion of an exocrine gland ( 2-5 )

Based on the composition of the secretion , exocrine glands can be classified as follows:

• 1.

  Mucous glands , when their products are rich in glycoproteins and water. An example is the sublingual glands.

• 2.

  Serous glands , with secretions enriched with proteins and water. Examples are the parotid gland and the exocrine pancreas.

• 3.

  Mixed glands , which contain both mucous and serous cells. An example is the submandibular (also called submaxillary) gland.

Components of a branched (compound) gland ( 2-4 and 2-5 )

A branched, or compound, exocrine gland consists of functional epithelial components (secretory acini and ducts) called parenchyma (Greek parenkhyma , visceral flesh) and supporting connective tissue, including blood and lymphatic vessels and nerves, called stroma (Greek stroma , layer, bed).

A branched exocrine gland is surrounded by a connective tissue capsule .

Septa (Latin saeptum , partition) or trabeculae (Latin diminutive of trabs , beam) extend from the capsule into the glandular tissue. Septa support the major branches of the excretory duct, blood and lymphatic vessels and nerves.

Large interlobar septa divide the gland into a number of lobes . Branches from the interlobar septa, interlobular septa , subdivide the lobes into smaller compartments called lobules .

During development, a main excretory duct gives rise to branches that lie between lobes, inside the interlobar septa. Small branches derived from each of these ducts generate small subdivisions.

These branches can be found first between lobules (in interlobular septa) and within lobules (intercalated ducts and striated ducts) .

Acini are drained by intercalated and striated ducts lined by a simple cuboidal–to–simple columnar epithelium. Intercalated ducts are surrounded by little connective tissue.

The epithelial lining of interlobular ducts is pseudostratified columnar. Lobar ducts are lined by a stratified columnar epithelium.

Mechanisms of secretion of an exocrine gland ( 2-6 )

Exocrine glands can also be classified on the basis of how the secretory product is released .

During merocrine secretion (Greek meros , part; krinein , to separate), the product is released by exocytosis .

Secretory granules are enclosed by a membrane that fuses with the apical plasma membrane during discharge or exocytosis. An example is the secretion of zymogen granules by the exocrine pancreas.

During apocrine secretion (Greek apoknino , to separate), the release of the secretory product involves partial loss of the apical portion of the cell .

An example is the secretion of lipids by epithelial cells of the mammary gland. Proteins secreted by epithelial cells of the mammary gland follow the merocrine pathway (exocytosis).

In holocrine secretion (Greek holos , all), the secretory product constitutes the entire cell and its product . An example is the sebaceous glands of the skin, which produce a secretion called sebum .

Plasma membrane ( 2-7 )

The plasma membrane consists of lipids and proteins . The phospholipid bilayer is the fundamental structure of the membrane and forms a bilayer barrier between two aqueous compartments: the extracellular and intracellular compartments. Proteins are embedded within the phospholipid bilayer and carry out specific functions of the plasma membrane such as cell-cell recognition and selective transport of molecules.

Types of lipids and lipid domains (see 2-7 )

Membrane lipids have three general characteristics and functions:

• 1.

  Cell membranes consist of polar lipids with a hydrophobic portion that self-associates and a hydrophilic portion, that interacts with water-containing molecules. This amphipathic property enables cells and organelles to establish an internal setting separated from the external environment.

• 2.

  Lipid domains enable some intramembranous proteins to aggregate and others to disperse (see below).

  Phospholipids, ceramide and cholesterol are synthesized in the endoplasmic reticulum. Sphingolipids assembly takes place in the Golgi apparatus.

• 3.

  As discussed in Chapter 3 , Cell Signaling | Cell Biology | General Pathology, lipids (for example, phosphatidylinositol and diacylglycerol) can participate in signaling functions.

The four major phospholipids of plasma membranes are phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and sphingomyelin . They represent more than one-half the lipid of most membranes.

A fifth phospholipid, phosphatidylinositol , is localized to the inner leaflet of the plasma membrane.

In addition to phospholipids, the plasma membrane of animal cells contains glycolipids and cholesterol .

Glycolipids, a minor membrane component, are found in the outer leaflet, with the carbohydrate moieties exposed on the cell surface.

Cholesterol , a major membrane constituent, is present in about the same amounts as are phospholipids.

Cholesterol, a rigid ring structure, is inserted into the phospholipid bilayer to modulate membrane fluidity by restricting the movement of phospholipid fatty acid chains at high temperatures. Cholesterol is not present in bacteria.

Two general aspects of the phospholipid bilayer are important to remember:

• 1.

  The structure of phospholipids accounts for the function of membranes as barriers between two aqueous compartments . The hydrophobic fatty acid chains in the interior of the phospholipid bilayer are responsible for the membranes being impermeable to water-soluble molecules.

• 2.

  The phospholipid bilayer is a viscous fluid . The long hydrocarbon chains of the fatty acids of most phospholipids are loosely packed and can move in the interior of the membrane. Therefore, phospholipids and proteins can diffuse laterally within the membrane to perform critical membrane functions.

It is important to emphasize that cellular plasma membranes are heterogeneous; they differ in their biophysical properties and composition. In fact, certain membranes display preferential domains of cholesterol and saturated lipids.

The relatively ordered lipid domains, called lipid rafts , are asymmetrically distributed in the inner and outer leaflet and are transient.

So, which is the function of the lipid rafts? They provide distinct physical properties to the membrane by recruiting distinct lipids and proteins (such as kinases of the Src family) during cell signaling (see Box 2-A ).