Histological Structure of Large Exocrine Glands

Histological Structure of Large Exocrine Glands

The relationship between the secretory tissue (parenchyma) of glands and the supporting connective tissue is similar in most larger glands. Externally the entire gland is surrounded by a layer of dense connective tissue, the capsule. Connective tissue sheets (septa) extend from the capsule into the secretory tissue and subdivide the gland into a number of lobes. Thinner connective tissue septa subdivide the lobes into a number of lobules. Reticular connective tissue (hardly visible in H&E stained sections) surrounds and supports the secretory units of the glands (alveoli, acini etc.) and the initial parts of the excretory ducts if present.
Blood and lymph vessels as well as nerves penetrate the capsule and form a delicate network between the secretory units and the initial parts of the duct system.
The main excretory duct conveys the secretory product to one of the external surfaces of the body. Other parts of the duct system are named according to their relation to the lobes and lobules of the gland.

• Lobar ducts are are large branches of the main duct which extend to the lobes of the gland. They may be called
• Interlobar ducts if they are found in the connective tissue surrounding the lobes. Interlobar ducts branch and give rise to
• Interlobular ducts, which are found in the connective tissue surrounding the individual lobules of the gland. Branches of the interlobular ducts enter the lobules and are now called
• Intralobular ducts. The terminal branches of the duct system, which connect intralobular ducts with the secretory units of the gland, are called
• Intercalated ducts.

The appearance of the different portions of the duct system is quite variable from gland to gland and may allow the identification of the gland. Quite often, the appearance of parts of the duct system also permits some deductions about their functions.

Note that lobes and lobules are defined by their relationship to each other. Many small lobules may form one large lobe. Neither size nor the spatial relationship between different parts of the tissue can be unequivocally determined in a single, two-dimensional section of the tissue. Lobes and interlobar ducts may therefore be difficult to distinguish from lobules and interlobular ducts.

Suitable Slides

alveolar gland: lactating mammary gland H&E - see lab section on the Female Reproductive System page.
serous and mucous acinar glands: sections of parotid gland, sublingual glandor tongue (lingual salivary glands) - H&E

Parotid Gland, Human - H&E Find an area of secretory tissue at low magnification, and scan over this area at high magnification. Within the lobules and between the acini of the parotid you can find two types of ducts. Since they are both located within the lobules they are both intralobular ducts. Striated ducts are lined by a simple tall columnar epithelium. Intercalated ducts are lined by a simple cuboidal epithelium and connect individual acini to the striated ducts. Try to capture the features of the acini, intercalated and striated ducts in one compound drawing which shows how they connect to each other. Label your drawing.

Parotid Gland, Human and Sublingual Gland, Human - H&E
Many secretory cells and the secretory structures formed by them belong to one of two morphologically distinct forms: serous or mucous. Serous secretions have a low viscosity, i.e. they are rather "watery". Mucous secretions have a high viscosity, i.e. they are rather "slimy". The apical cytoplasm of the cells forming serous acini is usually well-stained. Secretory vesicles are visible in the apical cytoplasm in well-preserve tissue. The nuclei are round or slightly ovoid and located in the basal cytoplasm of the cells. The bluish color of the basal cytoplasm reflects the presence of large amounts of rough endoplasmatic reticulum.
The contents of the secretory vesicles in the apical cytoplasm of cells forming mucous acini are only weakly stained. These empty-looking vesicles give the apical cytoplasm of mucus-producing cells a distinct "foamy" or "frothy" appearance. The nuclei of mucous cells appear darker and smaller than the nuclei of serous cells. They also seem to be "pressed" against the basal limit of the cells and may look flattened with an angular ("edgy") outline. Glands containing mucous acini (e.g. the sublingual glands) are called mucous glands. Glands containing serous acini (e.g. the parotid glands) are called serous glands. If both types of acini are present the gland is muco-serous.
Identify and draw serous and mucous acini at high magnification. Label your drawing. Make sure that the features which characterise serous and mucous acini are visible in your drawing - if necessary use a little artistic freedom.