Introduction

Before discussing the salivary glands specifically, we will review glandular tissue in general. Epithelium that mainly functions in secretion is often arranged into structures known as glands. Invaginations of surface epithelium into the underlying connective tissue form what is known as glands. Glands which maintain contact with the surface via a system of ducts are called exocrine glands. Glands which have lost their duct system and are isolated in connective tissue are called endocrine glands. Endocrine glands secrete their products, called hormones, directly into the blood

EXOCRINE GLANDS

The exocrine glands are divided on the basis of their morphology into simple glands with a single, unbranched duct and compound glands which have a branched duct system. The secretory units of both simple and compound glands may be either tubular, consisting of a cylindrical arrangement of cells, or acinar, consisting of a grape-like cluster of cells. Exocrine glands discharge their products by a merocrine, apocrine or holocrine type of secretion.

· MEROCRINE SECRETION

Also known as eccrine secretion, this is the most common form and involves exocytosis of the product, usually proteins. This type of secretion characterizes the exocrine portion of the pancreas.

· APOCRINE SECRETION

This type of secretion results in the discharge of unbroken, membrane-bound vesicles containing secretory material and is characteristic of some types of sweat glands.

· HOLOCRINE SECRETION

In holocrine secretion, the entire cell is broken down in the release of secretory product. This type of secretion is found in the sebaceous glands.

· MYOEPITHELIAL CELLS

The secretory portions of some exocrine glands are surrounded by contractile cells known as myoepithelial cells which lie between the secretory cells and the basement membrane. The contraction of myoepithelial cells facilitates the discharge of product from the secretory cells.

SALIVARY GLANDS

Salivary glands are characterized as being either serous, mucous, or both. Mucous cells are generally larger than serous cells. The cytoplasm stains poorly, if at all, with hematoxylin and eosin. The nucleus of the mucous cells is sometimes flattened near the basement membrane.

On the other hand, the cytoplasm of serous cells is granular. The nucleus is usually slightly eccentric and spherical. The major enzyme secreted by the salivary glands is amylase, but other protein secreted include secretory IgA, lysozyme and mucus. When observing the various glands, carefully note the cytological features of the duct epithelium.

The salivary glands include:

1. Parotid glands which contain almost entirely serous secreting acini. Interlobular (excretory) ducts are found in the connective tissue between the acini. Within the lobules of the glands are 1) intralobular or striated (secretory) ducts and 2) intercalated ducts.

2. Submandibular (submaxillary) gland which is a mixed gland that generally has about 80% serous and 20% mucus-secreting acini in adult humans. The mucous acini (alveoli) are often capped by serous demilunes. All three types of ducts [interlobular, intralobular (striated) and intercalated] are found in the submandibular gland.

3. Sublingual gland which contains a mixture of serous and mucus-secreting acini with the majority being the mucous type. Intercalated ducts are not usually found in the sublingual gland. Fat cells may be seen in the surrounding connective tissue.

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