Epithelial tissues form the covering and lining of the body’s surfaces and cavities. A specialized form, glandular epithelium, constitutes the body’s glands and is responsible for producing and releasing substances. The cells within this tissue synthesize specific products from molecules taken from the bloodstream. This secretory function supports numerous bodily processes, from digestion to temperature regulation.
Exocrine and Endocrine Glands
Glandular epithelium is classified into two main types, exocrine and endocrine, based on the presence or absence of a duct system. Exocrine glands secrete their substances through ducts that lead to an external surface, like the skin, or an internal space, known as a lumen. This group includes sweat glands that release sweat to cool the body and salivary glands that produce saliva to aid in digestion.
Endocrine glands lack ducts and release their chemical messengers, called hormones, directly into the surrounding interstitial fluid. From there, hormones diffuse into the bloodstream for transport to target cells and organs. Examples include the thyroid gland, which regulates metabolism, the pituitary gland, and the adrenal glands. The pancreas is unique, with both exocrine functions (releasing digestive enzymes) and endocrine functions (secreting insulin).
Structural Organization
The structure of exocrine glands varies from single-celled to complex multicellular forms. The simplest is the unicellular gland, a single secretory cell interspersed among other epithelial cells. A primary example is the goblet cell, found in the linings of the digestive and respiratory tracts, which secretes mucus for protection and lubrication.
Multicellular glands are more complex and are classified based on the structure of their ducts and the shape of their secretory units. Glands with an unbranched duct are categorized as simple, while those with a branched duct are called compound. A tube-shaped secretory part is called tubular, a flask-like or berry-shaped one is known as acinar or alveolar, and glands that feature both tube and flask shapes are termed tubuloacinar.
Cellular Secretion Processes
The release of products from glandular epithelial cells occurs through three distinct mechanisms: merocrine, apocrine, and holocrine secretion. The most common method is merocrine secretion, where substances are enclosed in vesicles that move to the cell’s surface and release their contents via exocytosis. The cell remains intact during this process. Salivary glands and the acinar cells of the pancreas use this method.
A different method is apocrine secretion, which involves the loss of a small part of the cell. In this process, the secretory product accumulates at the apical, or top, portion of the cell. This part of the cell then pinches off and is released along with the substance. This type of secretion is characteristic of the mammary glands when they release milk fats.
The third mode, holocrine secretion, is the most destructive to the cell. The entire secretory cell disintegrates to release its contents. The cell fills with its product and then ruptures, becoming part of the secretion itself. This process requires continuous cell replacement. Sebaceous glands in the skin, which produce an oily substance called sebum, are a prime example of holocrine glands.
Relevance to Human Health
The function and integrity of glandular epithelium are directly linked to human health, and disruptions can lead to various medical conditions. Tumors that originate in glandular epithelium are relatively common. A benign, or non-cancerous, tumor of this tissue is called an adenoma. When the tumor becomes malignant, or cancerous, it is known as an adenocarcinoma. Adenocarcinomas are one of the most widespread types of cancer, frequently developing in organs such as the colon, breast, and prostate.
Beyond cancer, other diseases can arise from malfunctioning glandular epithelium. Cystic Fibrosis is a genetic disorder that affects exocrine gland function. A defect in a specific protein impairs the transport of ions across cell membranes, leading to the production of abnormally thick and sticky mucus. This thick mucus can clog ducts and passageways, particularly in the respiratory and digestive systems, causing a wide range of serious health complications.