Acinus: Structure, Function & Diseases

By the acinus, medicine understands the glandular end and at the same time the functional unit of various organs. For example, acini are found in the lungs, liver and pancreas or salivary glands. Especially the tissue of the parotid gland acini can be affected by degeneration or inflammation.

What is acinus?

An acinus is the term used to describe the secretory tip of glands. Literally translated, the Latin term means “grape” and in this case refers to the shape of the anatomical structure. The acinus is the functional component of each body gland in which the production of the glandular secretion takes place. In addition to glandular endings, the functional units of body organs are also referred to as acinus by analogy. Examples of this are the lungs, in whose fine-tissue area the actual gas exchange takes place. The acini of the body differ in their nature depending on the morphology of the associated glandular tissue and the consistency of their secretion. The mode of secretion also affects the anatomical shape of the glandular end. The best known acini are those of the liver, pancreas, and salivary glands.

Anatomy and structure

Regardless of the type of gland and the consistency of the secretion, all acini have a duct around which epithelial cells are arranged. These cells are comparatively large and dense cells with a relatively narrow lumen. Their exact size and actual density depends on the gland. The lumen of the duct can also vary from gland to gland and depends on the consistency of the secretion. Usually, the more viscous the secretion, the larger the duct diameter. In most cases, acini are surrounded by myoepithelial cells that have contractile filaments. The glandular cells are polar jointed. The cell organelles for the formation of secretion are arranged basally. In addition, substances from the blood are absorbed into the acinus basally. Apically, the acinar duct is located. Mixed glands with seromucous secretion have additional serous glandular cells on the secretory acinus. The histologic section through this structure is known as the Ebner’s crescent. The pancreas exhibits an unusual structure on the acinus. Cells of the gland extend into the lumen of the acinus and participate in the production of bicarbonate. These cells are also known as centroacinar cells.

Function and Tasks

All acini in the human body are active as functional units of a particular organ. The acinus of the glands is where the production of the particular secretion takes place. This secretion is usually hormones or other messenger substances, such as growth factors. The production of such substances and the release of the secretion from the respective glands fulfills vital tasks in the body. Growth processes are stimulated or inhibited by the secretions of the glands. Equally relevant are the substances for the immune system and all other body functions, so especially for the metabolism and the maintenance of a stable circulation. The acini of the organs differ from those of the glands in that no secretion is produced in them, or not exclusively. On the other hand, the acini of the lungs are functionally relevant for gas exchange. From the bronchiolus terminalis, the respiratory tract transforms into so-called bronchioli respiratorii. In this area of the lungs, alveoli are already seated in the lung wall. From here, the airways become visibly smaller and transform into the alveolar ducts, which open into the sacculi alveolares. The acini are this tissue including all alveoli and airways of a single bronchiolus terminalis. Gas exchange occurs from the bronchioli respiratorii. In this context, medicine therefore speaks of gas exchange in the acinus. However, because the epithelia of this structure produce the surfactant, the pulmonary acinus is also largely assigned glandular functions.

Diseases

In all acini of the human body, cells may degenerate under certain circumstances, giving rise to a malignant cancer. In this context, the talk is also of an acinar cell carcinoma. This phenomenon particularly affects the parotid gland. The degree of malignancy in these tumors is comparatively low, so that the prognosis is relatively favorable. In the context of acinar cell carcinoma, secretion-producing cells are newly formed.With such a tumor in the parotid gland, taste changes and facial paralysis usually present themselves, since one of the facial nerves runs directly through the glandular ducts of the parotid gland. Pain is also a common symptom of the condition. Metastases are rare in acinar cell carcinomas of all types, but may adversely affect prognosis in some circumstances. Acinar cell carcinoma of the salivary glands, in particular, is readily treatable and has a survival rate of more than 80 percent after ten years. Unfortunately, however, recurrences often occur with this type of carcinoma. Therefore, affected individuals must attend cancer screenings throughout their lives to detect and treat any recurrences early. In addition to degeneration, acinar tissue is often affected by inflammation. Acinar inflammation occurs primarily in the pancreas and can cause lasting damage to the cells. Under certain circumstances, inflammation in other tissues of the body also spreads to the acinus or the opposite phenomenon takes place. To avoid permanent damage, the painful phenomena are treated as soon as possible with anti-inflammatory drugs.