Acinus – Definition, Structure and Clinical Significance
The acinus is the smallest functional unit of a gland or the lung. It plays a key role in secretion and gas exchange within the body.
Things worth knowing about "Acinus"
The acinus is the smallest functional unit of a gland or the lung. It plays a key role in secretion and gas exchange within the body.
What Is an Acinus?
The term acinus (plural: acini) is derived from Latin and originally means berry or grape. In medicine, it refers to the smallest functional unit of certain organs, particularly exocrine glands and the lungs. The grape-like appearance of these tiny structures inspired the name.
Types and Locations
Acini are found in several organs, where they serve distinct physiological roles:
- Pancreatic acini: These are the most well-known acini. They form the exocrine portion of the pancreas and produce digestive enzymes such as amylase, lipase, and proteases, which are secreted into the small intestine.
- Salivary gland acini: In the salivary glands (e.g., parotid, submandibular), acini produce saliva. They are classified as serous (watery secretion), mucous (mucus-rich secretion), or mixed.
- Hepatic acinus: Described by Rappaport, the hepatic acinus is a functional unit of the liver arranged around small portal venules and hepatic arterioles. It is divided into three zones with varying degrees of oxygenation and nutrient supply.
- Pulmonary acinus: In the lungs, the pulmonary acinus refers to the lung tissue distal to the terminal bronchiole. It includes the respiratory bronchioles, alveolar ducts, and alveoli (air sacs), where gas exchange takes place.
Structure of the Acinus
Despite occurring in different organs, acini share common structural features:
- They consist of specialized epithelial cells (acinar cells) that are secretory in nature.
- They are surrounded by a dense capillary network that supplies nutrients and facilitates the transport of secretory products.
- They drain into larger ductal systems via small excretory ducts.
- Myoepithelial cells surround the acini and support secretion through contractile activity.
Clinical Relevance
Diseases affecting acini can have serious consequences:
- Pancreatitis: Inflammation of the pancreatic acini causes premature activation of digestive enzymes within the gland itself, potentially leading to self-digestion of the organ.
- Acinar cell carcinoma: A rare malignant tumor arising from the acinar cells of the pancreas.
- Pulmonary emphysema: In emphysema, the walls of the pulmonary acini -- especially the alveoli -- are destroyed, reducing the surface area available for gas exchange and causing breathlessness.
- Liver cirrhosis: Chronic liver damage disrupts the architecture of the hepatic acinus, severely impairing liver function.
- Sjogren syndrome: This autoimmune disease destroys the acini of the salivary and lacrimal glands, leading to dry mouth and dry eyes.
Diagnostic Methods
Assessment of acinar structures typically involves:
- Histology: Microscopic examination of biopsy samples to evaluate the structure and integrity of acinar cells.
- Imaging: Ultrasound, CT (computed tomography), and MRI (magnetic resonance imaging) can reveal changes in organs containing acini.
- Laboratory tests: Elevated serum levels of amylase and lipase indicate damage to pancreatic acini.
References
- Schiebler, T.H. & Korf, H.W. - Anatomie: Histologie, Entwicklungsgeschichte, makroskopische und mikroskopische Anatomie. 10th edition, Steinkopff Verlag, 2007.
- Kloppel, G. et al. - Pathology of the Pancreas. Springer Verlag, 2nd edition, 2005.
- Weibel, E.R. - The Pathway for Oxygen: Structure and Function in the Mammalian Respiratory System. Harvard University Press, 1984.
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