Glandular epithelial tissue cells produce substances called secretions, that can be used and other parts of the body or eliminated from the body. These secretions can be:
- mucous membraneswhen thick and rich in mucus, eg salivary glands
- serouswhen fluid, aqueous, clear and rich and protein. Eg pancreatic secretory glands
- They can also be mixedwhen mucous and serous secretions occur together. Ex. Parotid salivary glands.
The glands can be unicellular, such as the goblet gland (which occurs for example in the tracheal epithelium), or multicellular, like most glands.
In yellow goblet glands of the intestine.
In green the secretory part of exocrine and endocrine multicellular glands, respectively.
At multicellular glands They always originate from the lining epithelia, by proliferation of their cells into the underlying connective tissue and further differentiation.
Three Types of Multicellular Glands
Exocrine glands: present the secretory portion associated with ducts that release their secretions out of the body (such as sweat, lacrimal, mammary and sebaceous glands) or into body cavities (such as salivary glands);
Endocrine Glands: do not have ducts associated with the secretory portion. The secretions are called hormones and are released directly into the blood and lymph vessels. Examples, pituitary gland, thyroid glands, parathyroid glands and adrenal glands;
Mixed Glands: present endocrine and exocrine regions at the same time. This is the case of the pancreas, whose exocrine portion secretes digestive enzymes that are released into the duodenum, while the endocrine portion is responsible for the secretion of the hormones insulin and glucagon. These hormones act, respectively, in reducing and increasing blood glucose levels.