Gland - gland

Cells with secretory function are called glandular cells, and a collection of glandular cells is called a gland. Secretion occurs when a cell takes in substances from outside the cell, synthesizes secretions from those substances within the cell, and releases them outside the cell. When secretions are transported through ducts, it is called exocrine glands, and when the gland has no ducts and substances are released around the cell, then enter the bloodstream and are transported there, it is called endocrine glands. The former type of gland is called an exocrine gland, and the latter type is called an endocrine gland.

Glands can also be classified according to the type of secretion. Cells that secrete proteins and peptides, such as exocrine cells of the pancreas and anterior pituitary cells, are called protein-secreting cells; cells in the sublingual gland, submandibular gland, and trachea are called mucus-secreting cells; steroid-secreting cells in the adrenal cortex and gonads and sebaceous glands are called lipid-secreting cells; cells in the adrenal medulla and pineal gland are called amine-secreting cells; follicular epithelial cells of the thyroid gland are called amino acid derivative-secreting cells; and sweat gland cells and chloride cells in the gills of bony fish that do not belong to the above are classified as "other" secretory cells. In addition, there are many cases in which peptides and amines coexist and are secreted together among endocrine cells. There are also cells that perform secretory functions by definition, such as neurons that release neurotransmitters from the axon terminals and fibroblasts that release collagen, that are not included in the category of glandular cells.

[Kikuyama Sakae]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

分泌機能を有する細胞を腺細胞といい、腺細胞の集合体を腺とよぶ。細胞外から物質を取り込み、細胞内でそれらの物質から分泌物を合成して細胞の外へ放出するのが分泌である。分泌物が導管を通って輸送される場合は外分泌であり、腺に導管がなく物質が細胞の周りに放出され、それが血管に入って運ばれてゆくのが内分泌である。前者の腺を外分泌腺、後者の腺を内分泌腺という。

　また分泌物の種類によって腺を分類する場合もある。膵臓(すいぞう)の外分泌細胞、下垂体前葉細胞などタンパク質やペプチドを分泌する細胞をタンパク分泌細胞、舌下腺、顎下(がくか)腺、気管などは粘液分泌細胞、副腎(ふくじん)皮質や生殖腺のステロイド分泌細胞や皮脂腺は脂質分泌細胞、副腎髄質や松果体などはアミン分泌細胞とよばれ、甲状腺の濾胞(ろほう)上皮細胞はアミノ酸誘導体分泌細胞、上記のものに属さない汗腺細胞や硬骨魚のえらのクロライド細胞などは「その他」の分泌細胞として分類される。なお、内分泌細胞では、ペプチドとアミンが共存していてともに分泌されるという例がかなり多い。軸索末端から神経伝達物質を放出するニューロン、コラーゲンを放出する繊維芽細胞など、定義からは分泌機能を営んでいるにもかかわらず、腺細胞の範囲には入れられていないものもある。

［菊山　栄］

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