Creator: Charlotte O’Leary BSc, MBChB•Reviewer: Dimitrios Mytilinaios MD, PhDLast reviewed: Could 31, 2021Reading time: 9 minutes
Epithelia are a bunch of tissues derived from all three embryonic germ layers, that are concerned in absorption, secretion, selective diffusion and bodily safety. Epithelia primarily concerned in secretion are organized into buildings often known as glands. Glands are invaginations of epithelial tissue and may be divided into two foremost sorts:
Exocrine glands Endocrine glands
This text will focus on the construction and performance of exocrine and endocrine glands in addition to examples of those glands. This can be adopted by any related scientific pathology.
Đang xem: Classification of exocrine glands
Exocrine glandsEndocrine glandsClinical notes
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Exocrine glands launch their secretions onto an epithelial floor through a duct. Exocrine glands encompass two foremost components, a secretory unit and a duct. The secretory unit consists of a bunch of epithelial cells, which launch their secretions right into a lumen. A duct is lined with epithelium and is concerned in transporting the secretions from the secretory unit to an epithelium-lined floor.
Classification by form
Exocrine glands may be categorised into a wide range of classes when it comes to their construction. They are often categorized in accordance with the form of their secretory unit. Secretory items formed as a tube are known as tubular, whereas spherical items are known as alveolar or acinar, when the pancreas is concerned. Exocrine glands may also be comprised of each tubular and alveolar secretory items and. On this case, they’re known as tubuloalveolar. They may also be categorized in accordance with whether or not their duct is branched or not. An unbranched duct is known as a easy gland, whereas a branched duct is called a compound gland. An instance of a easy gland is a sweat gland, whereas the pancreas is an instance of a compound gland.
Compound tubuloalveolar blended salivary gland (histological slide)
Classification by perform
Exocrine glands may also be categorised into a wide range of classes when it comes to their perform. They are often categorised into 3 subtypes in accordance with their sort of secretory product:
Serous glands Mucous glands Combined glands
Serous glands produce serous fluid, a watery substance containing enzymes. Mucous glands are concerned within the manufacturing of mucus, a viscid (sticky) glycoprotein. Combined glands are comprised of each serous and mucous glands and secrete a blended substance containing each serous fluid and mucus.
Combined seromucous glands (histological slide)
Classification by secretion mechanism
Exocrine glands may also be categorised into one other 3 subtypes in accordance with their secretion mechanism:
Merocine glands Apocrine glands Holocrine glands
Merocrine glands are the commonest and launch their secretory merchandise through exocytosis. The main secretory merchandise of those glands are normally proteins. Apocrine glands launch their secretory merchandise contained inside membrane-bound vesicles. Such a secretion is uncommon and these glands are discovered within the breast and represent some sweat glands. Holocrine glands launch entire secretory cells, which later disintegrate to launch the secretory merchandise. Such a secretion is seen in sebaceous glands related to hair follicles.
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Apocrine sweat gland (histological slide)
The discharge of secretory merchandise from the secretory unit is aided by some contractile cells, often known as myoepithelial cells. These cells comprise traits of each muscle and epithelial cells and lie between the secretory unit and basement membrane. The cytoplasmic processes of those cells envelop the secretory unit in order that contraction of those cells leads to launch of the secretory merchandise from the secretory items and into the ducts. Launch of secretory merchandise happens in response to stimulation by hormones or autonomic nerve impulses.
Endocrine glands launch their secretory merchandise straight into the bloodstream, slightly than through a duct. These glands are surrounded by a powerful connective tissue capsule, which has fibrous extensions often known as trabeculae. These trabeculae present inner help and provides the gland a lobular look. Endocrine glands launch secretions often known as hormones, which journey through the bloodstream to achieve their goal cells, the place they elicit practical adjustments. The hormones are generally saved intracellularly inside secretory vesicles and are launched intermittently through exocytosis. An exception to that is the thyroid gland, which shops its hormone extracellularly as an inactive precursor molecule. Secretion of hormones is normally regulated by detrimental suggestions, the place an increase within the degree of hormone within the blood decreases its secretion.
Sebaceous glands are easy, branched, acinar, exocrine glands situated inside the pores and skin. They secrete a fatty substance sebum, into the follicular duct, which surrounds the hair shaft. Sebum helps preserve the pores and skin versatile and prevents water loss. These are often known as holocrine glands, as sebum is launched when the secretory cells degenerate.
Sebaceous gland (histological slide)
The pituitary gland is a small endocrine gland inside the mind concerned in hormone synthesis and regulation. It consists of two components, the anterior pituitary or adenohypophysis, and the posterior pituitary, or neurohypophysis. The anterior pituitary secretes:
development hormone (GH)prolactinadenocorticotrophic hormone (ACTH)follicle-stimulating hormone (FSH)luteinising hormone (LH)thyroid-stimulating hormone (TSH)
ACTH and TSH journey to their goal organs, the adrenal gland and the thyroid gland, respectively in an effort to stimulate the discharge of additional hormones. The posterior pituitary secretes antidiuretic hormone (ADH), in any other case often known as vasopressin, and oxytocin.
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Pituitary gland (anterior view)
The pancreas is an organ comprised of each exocrine and endocrine glands. The vast majority of the pancreas has an exocrine perform and secretes an enzyme-rich alkaline fluid into the pancreatic duct, which joins the frequent bile duct earlier than emptying into the duodenum. The exocrine glands secrete the proteolytic enzymes trypsinogen and chymotrypsinogen, that are activated to trypsin and chymotrypsin within the duodenum and support in digestion.
Pancreas (anterior view)
The exocrine pancreas additionally secretes bicarbonate ions, which neutralise the acidic chyme because it reaches the duodenum. There are additionally clusters of endocrine glands situated inside the exocrine tissue and these are known as islets of Langerhans. The 2 foremost hormones launched from the endocrine glands of the pancreas are insulin and glucagon. The pancreas additionally secretes:
somatostatinvasoactive intestinal peptide (VIP)pancreatic polypeptide (PP)motilinserotoninsubstance P in smaller portions
Different examples of glands embrace:
sweat glandssalivary glandsthe thyroid glandparathyroid glandsthe pineal gland
Adrenal glands (anterior view)