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The Pancreas and Salivary Glands

Objectives

  1. Recognize and distinguish the pancreas and salivary glands.
  2. Distinguish each of the 3 salivary glands.
  3. Understand the structure of the glandular elements and the relationship between structure and function.

Salivary Glands

The glands of the oral cavity may be classified as serous, mucous or mixed according to the nature of their secretory product(s). The basic unit of structure is a saccular or tubular arrangement of mucous and serous secretory epithelial cells enclosing a lumen. One or several of these secretory units may empty into a small intralobular duct which joins others to form progressively larger intralobular, the interlobular ducts and finally the interlobar and excretory ducts. Portions of the intralobular ducts are involved in altering the ionic composition of the secretion. The glands are usually well vascularized, and are under control of the autonomic nervous system.

In H & E stained sections, the cells making up a mucous alveolus are columnar or pyramidal, with colorless or mildly basophilic non-granular cytoplasm. The nucleus is very flattened at the extreme base of the cell. Cell boundaries between adjacent cells are usually clear. In a serous acinus, the cell boundaries are not distinct, and the component pyramidal cells are usually smaller and more basophilic than those of mucous acini. The nucleus lies in the basal cytoplasm, but is oval or round and not flattened against the base of the cell. The apical cytoplasm is less abundant, and ideally contains an accumulation of acidophilic secretory granules although these are often lost in preparation.

Since the precise chemical nature of mucous and serous secretions of various organs can differ, mucous and serous acini from various locations are not necessarily identical in histological appearance, although the above descriptions include overall characteristics generally observed.

Secretory acini are scattered throughout the oral cavity, but three groups of acini form named anatomical entities, the three paired salivary glands. Of these, the parotids and submandibulars (submaxillaries) are more clearly defined as distinct organs, while the sublinguals are embedded within the tongue. The salivary glands produce about 1–2 liters of saliva per day.

Study the parotid gland (slide #38). Under very low magnification note the separation of the gland into lobules by septa of connective tissue containing large, pink interlobular ducts as well as blood vessels and nerves. Often the septa appear to be embedded within the lobules as the architecture is not completely regular. With the 10x lens, note that this gland is composed predominantly of dark-staining serous acini and ducts. The latter are the acidophilic (pink) structures with round nuclei in the center of tall cells. Examine serous acini and note the rounded basally located nuclei and dark granular staining of the cytoplasm. Usually the gland lumen is not easily seen. The acini lie in a fine network of connective tissue. The absence or scarcity of mucous acini and the presence of adipose tissue between acini, i.e., in intralobular locations, aid in the identification of the parotid gland. The lumen of each individual acinus opens into an intercalated duct (first portion of the intralobular duct) which is lined with low cuboidal, pale-staining cells. In cross-section they are smaller than secretory acini, are more acidophilic, and tend to have more centrally located nuclei. Try to find longitudinal sections showing the continuity between secretory acini and intercalated ducts. More often, the continuity between intercalated ducts and the next portion of the duct system, the striated ducts, can be seen. Striated ducts are lined by cuboidal to columnar acidophilic cells and are so-named because of characteristic striations in the basal portions of the cells which are due to the alignment of mitochondria between deep infoldings of the basal cell membrane. The nucleus is in the central part of the cell above the striations. The latter are sometimes difficult to discern because of postmortem changes.

Lobular nature
Intralobular ducts
Striated duct
Striated & Inter- calated ducts

EM of basal region of striated duct

Myoepithelial cell

 

With the high-dry or oil immersion lens, note myoepithelial cells with long, flattened nuclei (small and triangular in cross section) and little cytoplasm, visible in some acini and ducts; they are located between the bases of adjacent secretory cells within the basement membrane. In contrast, the nuclei of the serous epithelial cells are round and lie above the basement membrane more or less in the center of the acinar or duct cell. Myoepithelial cells are not found in the pancreas.

The striated ducts empty into interlobular ducts in the connective tissue septa where they become larger in diameter. Interlobular ducts fuse to form progressively larger structures. Those with walls consisting of multiple layers of cells are called excretory ducts. The cells bordering the lumen in these ducts usually have rounded apical surfaces.

   

Study the submandibular gland (slide #39). The duct system is similar to that of the parotid. Note that this gland is composed mostly of dark-staining (purple) serous acini, but also contains a substantial number of pale (pink) staining mucous acini. In general, the nuclei of mucous cells are flattened and lie close to the basement membrane. Contrast those with the rounded, more central nuclei of serous acini. Groups of dark-staining serous cells form caps called "serous demilunes" often seen in sections as crescents which partially surround some of the mucous acini and empty their secretion into a common lumen. Other features are as described for the parotid gland.

   

The sublingual gland (slide 48), even, is a mixed gland but predominantly mucous. It has very short striated ducts which are extremely difficult to find and lacks intercalated ducts. The intralobular ducts present are called "mucous tubules" and cannot be distinguished from the secretory end-pieces in conventional sections. Excretory ducts of varying sizes may be seen as well as some skeletal muscle.

The Pancreas

The pancreas is a compound tubular-alveolar gland composed of serous acini (exocrine portion), and islets of Langerhans (endocrine portion). The islets of Langerhans are collections of cell cords separated from the exocrine acinar tissue by a thin connective tissue sheath.

The acini and their ducts resemble a bunch of grapes. The ducts lie in dense connective tissue septa which provide the main support of the gland. The vascular and nervous supply of the acini (and islets) are also located in these septa. Branches of these vessels terminate as capillary networks around the acini and form dense networks within the islets. The capillaries, in both cases, are of the fenestrated type.

The alveoli (acini) are groups of cells which vary in shape from spheres to elongated tubes and may appear quite irregular in some sections. The cells are pyramidal in shape and surround a central lumen which is usually difficult to visualize. During secretory activity, acidophilic granules fill the apical portions of the cells, while mitochondria and the endoplasmic reticulum with its associated basophilic RNA are located basally. The acini and ducts are enclosed by a basement membrane.

Examine the human pancreas (slide #41) at low magnification and note its lobular nature. Some regions may be poorly preserved. The connective tissue capsule may or may not be present but note the thin septa penetrating into the gland, dividing it first into lobes and then lobules. Distinguish between basophilic exocrine and the slightly acidophilic endocrine tissue. The Islets of Langerhans vary in size and usually appear as round cords of light pink cells. At 40x magnification note the different appearance of islets and exocrine cells. The islets make up only one percent of the total pancreatic mass and are not uniformly distributed within the organ. It is possible to have sections lacking islets. Try to find a more or less round cluster of exocrine cells forming an acinus. Note the details of the secretory cells such as the dense basal basophilia and the brightly eosinophilic secretory granules in the apical half of the cells. A capillary network surrounds the acini but is difficult to distinguish.

   

Locate centroacinar cells of the exocrine pancreas. These are duct cells which form the junction between the secretory endpiece and the duct. The ease of identification will depend upon the staining, so study both slides #40 and #41. Find well-defined acini in a well preserved region of the slide. The nuclei of centroacinar cells appear to be located in the center of the acini and mark the beginning of the intralobular ducts. Occasionally longitudinal sections of the first, narrow, free portions of the intralobular ducts can be observed emerging from the acini. Find the intralobular ducts among the closely packed acini. The pancreas lacks striated ducts and the intralobular duct is similar to the intercalated duct of salivary glands. It often has a collapsed lumen and the cells are cuboidal with little cytoplasmic staining in contrast to acinar cells. They are responsible for secretion of a bicarbonate-rich fluid. The pancreas also lacks myoepithelial cells. Finally, the interlobular ducts can be seen in the connective tissue septa along with some blood vessels.

The different types of secretory cells in the islets of Langerhans are difficult to differentiate in H & E sections. Examine a section of guinea pig pancreas (slide #40), which has been especially fixed and stained for this purpose. The acinar cells stain a deep purple and the cords of secretory cells in the islets include alpha (A)-cells filled with red-staining granules containing glucagon and beta (B) cells filled with blue-green staining granules that contain insulin.