The glands are organs made up of specialized cells of the epithelium that produce a specific substance: a secretory product. Glands that release their secretion through a separate secretory duct or directly to an external or internal surface are known as exocrine glands (exo = outside; krinein = secrete). These are distinguished from endocrine glands, which release their internal secretion (hormone) directly to the bloodstream. Glands can be classified according to the nature of their secretion:
- Serous glands produce a thin secretion containing protein, such as the lacrimal glands.
- Mucous glands produce a viscous, slimy secretion as a transporting mucus.
- Mixed glands produce both a serous and a mucous secretion (eg, saliva) that also contains digestive enzymes.
The structure of a gland can be traced back to invaginations (turning inward) of the surface epithelium. These invaginations can be shaped like tubes (tubular form) or small hollows (alveolar form). Branched glandular ducts with a main excretory duct, secondary excretory duct, connecting pieces, and endpieces develop from these basic forms (Fig 6-39).
The glandular cells release their secretory product in a wide variety of ways, which means a further classification of glands can be made:
- Apocrine glands discharge their secretion — once it has formed and accumulated in the apex of the cell—together with a piece of cytoplasm (eg, mammary glands).
- Holocrine glands discharge whole cells as their secretory product (eg, sebaceous glands in the skin).
- Merocrine glands release a fine-grained, droplike secretion through the surface of the cell.
Saliva is a secretion of either a watery (serous) or slimy (mucous) consistency that is formed by salivary glands and is released into the oral cavity. The following distinctions are made:
- Serous saliva is the watery diluting or rinsing saliva; it is rich in salt and protein and contains digestive enzymes.
- Mucous saliva is the slimy lubricating saliva; it is viscous, stringy, and poor in salt and protein.
- Seromucous saliva is a mixed saliva made up of varying proportions of serous and mucous saliva.
The saliva from the parotid gland, mandibular gland, sublingual gland, and the glands of the oral mucosa is a mixed saliva. Its chemical composition and quantity (normally 1.0 to 1.5 liters a day in humans) depend on the diet and on psychologic and nervous factors: Dry foods lead to the release of a mucous lubricating saliva; acids and alkalis cause a watery diluting or rinsing saliva to be produced.
Functions of saliva: The food is moistened, made slippery, and diluted by the saliva to make it easier to swallow; the constant production of saliva helps with self-cleaning of the mouth and leads to constant empty swallowing. Salivary amylase (ptyalin) brings about predigestion in which starch and glycogen are split. The saliva provides a good nutrient medium for microorganisms in the mouth.
The composition of saliva differs considerably from that of blood plasma. Saliva contains the following:
- 99% water to thoroughly wet and dissolve foods
- 0.6% solids, which shed epithelial cells and bacteria
- Mucins as lubricants in the oral area; they make food particles slippery and ease tongue and cheek movements
- Enzymes (ptyalin) for predigestion of carbohydrates
- Salts in the form of sodium, potassium, chloride, and bicarbonate ions
Saliva has a pH of 7 to 8 and has a bactericidal action due to leukocytes. It is usually weakly acidic, clear, odorless, colorless, and viscous.
Mineral components of saliva have the effect of healing (remineralizing) the hydroxyapatites attacked by lactic acid on the tooth surface. They also form deposits of tartar, particularly on the lingual surfaces of the mandibular incisors and the buccal walls of the maxillary molars, while salivary stones (known as sialoliths) may block the excretory ducts of the salivary glands. The electrolytes contained in saliva have corrosive effects on metals in the mouth.