Glandular epithelium definition
The glandular epithelium is a group of tissues, other than the covering and lining epithelium, which is specialized for the production and secretion of various macromolecules. Because of its primary function, the glandular epithelium is also called the secretory epithelium. This epithelium lies beneath the covering and lining epithelium and either consists of cells with specific functions or comprise specialized organs called glands. A gland consists of epithelium that secretes substances into ducts (tubes), onto a surface, or eventually into the blood in the absence of ducts. Some unicellular glands, comprised of specialized cells, might be interspersed between the simple cuboidal, simple columnar, and pseudostratified epithelia.
Structure of the glandular epithelium
- Glands develop from covering epithelia in the fetus by cell proliferation and growth into the underlying connective tissue, followed by further differentiation.
- The shape of the cells of the glandular epithelium can range from cuboidal to columnar as the squamous cells quite rare in the secretory epithelium.
- The glandular epithelium of the endocrine glands is rich in vascularization as well as innervation.
- The secretory cells in the glandular epithelium may synthesize, store, and release proteins (e.g., in the pancreas), lipids (e.g., adrenal, sebaceous glands), or complexes of carbohydrates and proteins.
- The cells of some glands (e.g., sweat glands) have little synthetic activity and secrete mostly water and electrolytes (ions) transferred from the blood.
On the basis of the structure of the glands and the presence or absence of ducts, glands are classified into two types:
- Exocrine glands secrete their products into ducts that release the secretions onto the surface of organs such as the skin surface or the lumen of a hollow organ.
- Exocrine glands remain connected to the covering epithelium via tubular ducts which are lined with lining epithelium and carry the secreted products into the site of action.
- The effects of exocrine gland secretions are limited, and some of them would be harmful if they entered the bloodstream.
- The exocrine glands are either unicellular or multicellular. Unicellular exocrine glands are made up of single-celled glands and mostly secrete mucus directly to the apical surface of the lining epithelium.
- The multicellular exocrine glands are composed of several cells and form a specific macroscopic organ with branched or unbranched ducts.
- Each exocrine gland is made up of a secretory unit and one or multiple ducts.
- The secretory unit is supported by the stroma of connective tissues with each unit connected to the duct system of the gland.
- Structurally, the multicellular exocrine glands are further divided into types on the basis of the shape of the secretory unit and whether the ducts are branched or unbranched.
- If the duct of the gland doesn’t have branches, it is called a simple gland.
- If the duct is branched, the gland is a compound gland.
- The glands with round secretory parts are called acinar or alveolar glands.
- The glands with tubular secretory parts are the tubular glands.
- Some glands have both round and tubular secretory parts and are called tubuloacinar glands.
- The exocrine glands are further divided into three types on the basis of the mechanism of release of secretion.
- The glands that release their secretion from the cells in secretory vesicles via exocytosis are termed merocrine glands.
- The glands that accumulate their secretory product at the apical surface of the secreting cell and, later the apical portion pinces off from the rest of the cell to release the secretion are termed apocrine glands.
- Finally, the glands in which the cells accumulate the secretory product in their cytosol, and as the cell matures, it ruptures to release the product, are termed the holocrine glands.
- The cells of the secretory units of the exocrine glands are provided with the well-developed endoplasmic reticulum, and the Golgi apparatus filled apically with secretory granules in different stages of maturation.
- In addition to secretory cells, epithelia of many exocrine glands (e.g., sweat, lachrymal, salivary, and mammary glands) contain contractile myoepithelial cells at the basal ends of the secretory cells.
- The myoepithelial cells are abundant in actin filaments and myosins, which results in the formation of hemidesmosomes connecting the basement membrane as well as other epithelial cells.
- In one particular group of exocrine glands, called the holocrine glands, as the secretory cell matures, it ruptures and becomes the secretory product.
- The sloughed-off cell is then replaced by a new cell.
- Endocrine glands are the group of glands that do not have a duct system and thus, release their secretions directly into the bloodstream.
- The secretions of endocrine glands, called hormones, enter the interstitial fluid and then diffuse into the bloodstream without flowing through a duct.
- Endocrine secretions have far-reaching effects because they are distributed throughout the body by the bloodstream.
- Endocrine glands lose the connection to their original epithelium and therefore lack ducts.
- Thin-walled blood capillaries adjacent to endocrine cells absorb their secreted hormone products for transport in the blood to target cells throughout the body.
- The glandular epithelium of the endocrine glands lack myoepithelial cells and are specialized for either protein or steroid hormone synthesis.
- The cells in the glandular epithelium are rich in the endoplasmic reticulum with a rather thin cell membrane.
- The thin wall of the cell membrane allows the release of the hormones secreted by the secretory cells by diffusion.
- The diffused hormones are then taken by the binding proteins present outside of the secretory cells.
- The receptors responsible for the signaling the transport of hormones from the glands to the target sites are either present either close to the hormone-secreting cells or on the secreting cells itself.
- Histologically, the endocrine glands are of three types:
- Trabecular type: In this type, the cells are arranged in cords like in the liver and the adrenal glands.
- Follicular type: In this type, the cells form spherical structures like in the thyroid glands.
- Disseminated type: In this type, the cells are placed in groups or separately in other organs like the Leydig cells in the testes.
Functions of the glandular epithelium
The most essential function of the glandular epithelium is secretion.
- Exocrine glands secrete watery mucus that helps to reduce body temperature and maintain homeostasis.
- Similarly, the glands like the sebaceous glands and the ceruminous glands secrete oily substances that help lubricate the surface of the covering epithelium providing protection against water loss and microbial invasion.
- The salivary gland in the mouth and digestive glands (pancreas) in the small intestine produce digestive enzymes that help in the digestion of food and aids the absorption of nutrients.
- The hormones secreted by the endocrine glands regulate many metabolic and physiological activities to maintain homeostasis and the proper functioning of the animals.
- The glands like the pituitary and the thyroid glands are responsible for the overall growth and development of the body.
- Similarly, the adrenal glands regulate the metabolism, immune system, blood pressure, and response to stress among other essential functions.
- The ovary and testes of the reproductive system produce the gametes necessary for sexual reproduction.
- The thymus gland in the thoracic cavity is responsible for the production of T-cells required for the removal of foreign antigens from the body.
Examples of the glandular epithelium
- Examples of glandular epithelium include the epithelium of the endocrine glands like the pituitary gland at the base of the brain, the pineal gland in the brain, thyroid and parathyroid glands near the larynx (voice box), adrenal glands superior to kidneys, pancreas near the stomach, ovaries in the pelvic cavity, testes in the scrotum, thymus in the thoracic cavity.
- The exocrine glands include the sweat, sebaceous, and ceruminous glands of the skin and the digestive glands such as salivary glands and pancreas.
References and Sources
- Mescher AL (2016). Basic Histology. Fourteenth Edition. McGraw-Hill Education.
- Tortora GJ and Derrickson B (2017). Principles of Physiology and Anatomy. Fifteenth Edition. John Wiley & Sons, Inc.
- Waugh A and Grant A. (2004) Anatomy and Physiology. Ninth Edition. Churchill Livingstone.
- Stefan TOTH. Glandular epithelium. Department of Histology and Embryology, P.J. Šafárik University, Medical Faculty, Košice.
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