Glandular tissue is a specialized form of epithelial tissue found throughout the body dedicated to forming and releasing various substances. This tissue originates from surface-covering cells but differentiates to create structures known as glands. Its primary purpose is the synthesis and discharge of products that serve a specific function elsewhere in the body. Glandular tissue performs the body’s secretory work, whether providing lubrication, aiding in digestion, or regulating systemic processes.
The Specialized Structure
Glandular tissue develops from the epithelial layer. These specialized cells typically have shapes ranging from cuboidal to columnar, reflecting their high metabolic activity for production and release. The cells are organized into functional units, often forming small clusters called acini or elongated structures known as tubules, which are the sites of substance creation.
These cell clusters are anchored to a non-cellular layer called the basement membrane. This membrane provides structural support and separates the glandular tissue from the underlying connective tissue, which supplies necessary nutrients and blood flow. The physical arrangement of these secretory cells is adapted to efficiently produce and package specific substances, such as proteins, lipids, or complex carbohydrates.
The Process of Secretion
The main function of glandular tissue is secretion, which involves synthesizing a product and then expelling it from the cell. This release occurs through one of three distinct mechanisms, classified by the amount of cellular material lost.
Merocrine Secretion
The most common method is merocrine secretion, where the product is packaged in vesicles and released via exocytosis without damaging the cell. Examples include the production of saliva and sweat from eccrine sweat glands.
Apocrine Secretion
A less common method is apocrine secretion, which involves the loss of a small piece of the cell’s apical portion along with the secretory product. Mammary glands use this mechanism to release the fatty components of milk.
Holocrine Secretion
Holocrine secretion is the most destructive method, as the entire secretory cell ruptures and disintegrates to release its product. This process is used by sebaceous glands in the skin to produce sebum, an oily substance that lubricates the skin and hair.
Endocrine Versus Exocrine Glands
Glandular tissue is separated into two broad categories based on how secretory products are delivered.
Exocrine Glands
Exocrine glands utilize a duct system, which carries the secretions to an epithelial surface, such as the skin or the lining of an internal organ. These products typically act locally where they are released, performing functions like lubrication or digestion. Examples include salivary glands, which release digestive enzymes into the mouth, and sweat glands, which release perspiration onto the skin for thermoregulation. The substances produced by exocrine glands are diverse, including mucus, digestive juices, tears, and milk.
Endocrine Glands
In contrast, endocrine glands are ductless, releasing their secretory products directly into the surrounding interstitial fluid. These substances, known as hormones, are quickly absorbed into the bloodstream for transport throughout the body. This allows the hormones to travel to distant target cells and organs, enabling a broad, systemic regulation of bodily functions. Endocrine glands such as the thyroid, pituitary, and adrenal glands are examples of this tissue type.
Glandular Tissue and Common Health Contexts
Glandular tissue is important in the context of breast health, as the adult female breast contains 15 to 20 lobes composed of mammary glandular units. These units are surrounded by supportive connective tissue and adipose, or fatty, tissue. The proportion of glandular and fibrous tissue relative to fatty tissue determines breast density, a factor considered in medical imaging.
The mammary gland, a modified sweat gland, is highly sensitive to reproductive hormones, particularly estrogen and progesterone. These hormones stimulate the growth and development of the glandular ducts and lobules. This hormonal sensitivity highlights the tissue’s importance in processes like puberty, the menstrual cycle, and pregnancy.
The endocrine variety of this tissue is important for maintaining homeostasis, or the body’s internal balance. Glands like the pituitary, often called the “master gland,” and the thyroid, which regulates metabolism, produce hormones that coordinate cellular activities across multiple organ systems. The precise output from these glands controls functions ranging from energy use and growth to mood and stress response.