GLANDULAR EPITHELIUM

Glands are special types of cells, which are specialized to produce products to be used elsewhere in the body. Glands are classified according to their mechanism of secretion.

1. Exocrine Glands – Secrete their products via ducts onto the apical (or epithelial) surface. These glands are highly specialized epithelial cells and thus are classified as glandular epithelia. These are the types of glands to be studied today

2. Endocrine Glands – Release their products basally which allows secretion to go through the basal lamina, move into the underlying connective tissue, and enter the vascular system. Secretions can affect cells located far away from the endocrine glands. These glands lack a duct system and secrete into the blood or lymph.

3. Paracrine Glands – Similar to endocrine glands but secretions reach target cells by

diffusion through the extracellular space to affect neighboring cells. We will study endocrine and paracrine glands later this semester.

Glandular Epithelia (exocrine glands) are classified based on the six different morphological criteria.

1. Number of Secretory Cells

a. Unicellular Glands – Mucus-secreting goblet cells are the only example of these

single-celled glands in man. These goblet cells secrete mucus and are easily visualized in slides of the small intestine. In routine (H&E) preparations (Figure 13, left photo), the cytoplasmic mucigen is not preserved (and therefore, not stained) giving the cells an empty appearance. When a PAS stain is used, the mucigen stains magenta (Figure 13, right photo).

Figure 13: Goblet cells. Taken from: Wheater’s Functional Histology, a text and colour atlas, p. 94, Figure 5.21 and Junqueira and Carneiro, Basic Histology, a text and atlas, p. 90, Figure 4-32.

Study goblet cells in slides B51, B53, and B61, small intestine, monkey (H&E).

b. Multicellular Glands - These glands have many cells. In addition to the ways that

multicellular glands are classified below. They may also form a secretory sheet of epithelial cells like the linings of the stomach and the uterus.

Study slide A12, stomach, human (PAS). The glandular epithelium that lines the surface and the gastric pits of the stomach is composed of simple columnar cells called surface mucous cells. These contiguous cells possess an apical cap of mucinogen granules, and they create an epithelial surface that is entirely glandular in nature. These surface mucous cells stain magenta with PAS (periodic acid-Schiff) due to the high carbohydrate content of the glycoprotein of the mucin. Simple, branched tubular glands (seen as circular or ellipsoidal profiles) are also present in the gastric lining (mucosa). The deeper portions of these glands are less intensely stained.

Study slide B33, submandibular gland, human (H&E). A great example of a multicellular gland that is covered later in this lab.

2. Location of the secretory cells in relation to the epithelium

a. Intraepithelial Glands – see Goblet cells; described above

b. Extraepithelial Glands – all large exocrine glands

3. Nature of Secretion

a. Serous: A gland cell that produces a thin watery, protein-rich secretion. The cell is

often called a zymogenic cell because its secretion is rich in enzymes or enzyme-precursors. Serous cells comprise acini. They have round, centrally-located nuclei and large amounts of rough endoplasmic reticulum (rER), free ribosomes, prominent Golgi apparati, and numerous secretory granules. The basal cytoplasm is basophilic (due to rER and ribosomal content) while the apical cytoplasm is acidophilic (due to the secretory granules) in H&E stained preparations. The pancreas and parotid glands are entirely serous in composition.

Study slide B31, parotid gland, human (H&E), slide B84, pancreas, human (H&E), and slide B86, pancreas, monkey (H&E).

b. Mucous: A gland cell that produces a viscous secretion, which has a lubricating or

protective function. Small droplets of mucinogen nearly fill the cytoplasm above the nucleus. Mucinogen matures to become mucin. Mucin takes up water to become mucus. Mucous is the adjective for mucus. Mucous cells often comprise tubules. The nucleus of a mucous cell tends to be flat and located near the cell base. The cytoplasm is pale staining with H&E, but it stains bright pink with PAS due to the high carbohydrate content of the glycoprotein of the mucin. The secretion of a serous cell differs from that of a mucous cell in that it is rich in protein and low in carbohydrate. The sublingual gland contains predominantly mucous acini.

Study slide B35, sublingual gland, human (H&E).

c. Mixed (serous-mucous): These glands produce both serous and mucous

secretions. Within these types of glands, serous demilunes, a collection of serous cells arranged like a half-moon and often capping the end of a mucous tubule, can be found. These serous cells secrete into the highly convoluted space intercellular space between the mucous cells. The relative frequency of the three types of secretions (serous, mucous, or mixed) allows the salivary glands to be distinguished from one another.

Study slide B33, submandibular gland, human (H&E) and slide B35, sublingual gland, human (H&E).

4. Mechanism of Secretion

a. Merocrine (a.k.a. Eccrine). No part of the cell is lost, only the secretory product is

expelled by the process of exocytosis. It is the most common mode of secretion and is seen in serous, mucous, and mixed glands.

b. Apocrine. Part of the apical cytoplasm of the cell is lost. Secretion is discharged

within free, unbroken, membrane-bound vesicles. This is a rare type of secretion that is dependent on sex hormones and occurs primarily in apocrine sweat glands and active mammary gland.

Study slide C67, active mammary gland, monkey (H&E).

c. Holocrine. The entire secretory cell is lost (discharged within the lumen of the duct).

This occurs primarily in sebaceous glands.

Study slide A96, scalp, human (H&E).

5. Shape of Secretory Units

a. Tubular - An elongated group of secretory cells with a lumen (which may be small or

large) shaped like a tube.

b. Acinar or Alveolar - A small grape-like (acinus means “grape”) or sac-like (alveolus

means “sac”) group of secretory cells arranged about a small lumen. These cells, as they are in other glandular units, are attached to a basement membrane. Some authors distinguish between acinus and alveolus, but others use them interchangeably. The two terms will be synonymous in this course.

c. Tubulo-acinar – Lumen of secretory units have both of the above listed shapes

(seen in serous demilunes).

6. Arrangement (branched or not) and occurrence of Duct System

a. Simple glands - Glands of this type have an unbranched duct into which the cells

secrete. Each secretory portion empties separately on an epithelial surface.

b. Branched glands - Several secretory units empty into an unbranched excretory

duct.

c. Compound glands - These glands have a highly branched duct system. Secretory

portions empty into an elaborate branched duct system, which, in turn, drain into larger ducts.

Figure 14. Principal types of Exocrine Glands in Man. Taken from: Junquiera and Carneiro, Basic Histology, a text and atlas. p. 83, Figure 4-22.

Types of exocrine glands in man. See Figure 14.

1. Simple tubular glands - These glands are epithelial-lined tubules, which open on

the apical surface. There are three types.

a. Simple straight tubular glands. slide B62, colon, human (H&E). The long

crypts of Lieberkühn, located within the colon, are a great representation of tubular glands that runs a straight, unbranched course. Also, note the numerous goblet cells.

b. Simple coiled tubular glands. slide A96, scalp, human (H&E). Within the

dermis, eccrine sweat glands are located. The deeper portion of these simple coiled tubular glands is easily seen; however, the long unbranched lumen that goes to the apical surface is rarely seen in cross-section. Within this slide, find some myoepithelial cells. These smooth muscle cells, derived from ectoderm, reside between the basement membrane and the base of a secretory cell. These contractile cells are often elongated, spindle-shaped cells that are found in glands derived from ectoderm, e.g., the major salivary glands and sweat glands. Upon contraction, myoepithelial cells assist in the expulsion of secretion from the gland cells. Also, find the eccrine sweat glands and their apical ducts in slide BB41, thick skin, monkey (H&E).

c. Simple branched tubular glands. slide A12, stomach, human (PAS).

Within the glands of this slide, the deeper portion of the tubule branches. These simple branched tubular glands are found primarily in the stomach.

2. Simple alveolar (acinar) glands - The best representation of simple alveolar glands

is the paraurethral glands located in the penile urethra. There are also simple, branched alveolar glands, which can be seen in slide A96, scalp, human, (H&E). In humans, glands of this type have many acini emptying into a common duct, and this can be seen with the sebaceous glands that are emptying into the hair follicles located within the dermis of the scalp in this section. Sebaceous glands are also the best representation of secretion by holocrine mechanism.

3. Simple tubulo-alveolar glands - Some of the secretory cells are arranged as acini

(alveoli) and others are arranged as tubules. Examples of these include some of the smaller glands of the respiratory tract; minor salivary glands located within the oral cavity are other examples.

4. Compound tubular glands - These glands have a highly branched duct system.

The secretory cells at the ends of the ducts are in the form of tubules. The kidney and testes are examples of a compound tubular gland. A compound branched tubular gland that is mucus secreting can be seen beneath the oral surface of the soft palate in slide BB6, human (H&E). Also see slide C57, uterine cervix, human, (H&E). Find the region with irregular folds and indentations of the surface; this is the endocervix (cervical canal), which is lined by tall, mucus-secreting cells.

5. Compound alveolar glands - The duct system is similar to the compound tubular

and compound tubulo-alveolar glands; however, compound alveolar glands differ from other compound glands in that the ducts end in alveoli with dilated sac-like lumina. The lactating mammary gland is a classic example of this type of gland as well as a gland that utilizes the apocrine mechanism of secretion. This compound alveolar gland can be seen in slide C67, active mammary gland, monkey, (H&E). The pancreas, parotid gland, and lacrimal gland are examples compound alveolar serous glands. Look at the following slides: slide A19, lacrimal gland, human (H&E), slide B31, parotid gland, human (H&E), slide B84, pancreas, human (H&E), slide B86, pancreas, monkey (H&E). In slide B84, the cytoplasmic basophilia typical of serous cells is well demonstrated although the acidophilic secretory granules are not easily visualized. However, the granules are well preserved and intensely stained (pink-red) in slide B86. Individual granules are often seen in the acini near the outer margin of the tissue.

6. Compound tubulo-alveolar glands - These glands also have a highly branched

duct system, but some of the ducts end as tubules and others end as alveoli. Two of the major salivary glands, the submandibular and the sublingual glands, are examples of compound tubulo-alveolar glands. See slide B33, submandibular gland, human, (H&E) and slide B35, sublingual gland, human, (H&E). The submandibular gland is also a mixed (serous-mucous) gland, though it is primarily serous in nature with a low (but variable) number of mucous secretory units. The sublingual gland is also a mixed gland, but has a much greater percentage of mucous cells. Look for both serous and mucous secreting cells in these slides. Other slides with examples of compound tubulo-alveolar glands include slide A58, pharyngeal tonsil, human, (H&E) and slide B9, trachea, human, (H&E). Remember, the parenchyma is the main cell type present in the gland (the functional cells of a gland) and the stroma is the connective tissue of a gland or organ. It supports the parenchyma and contains the nerves, blood vessels, and lymphatics.