Serous membrane

This refers to the embryonic membrane that covers the outermost part of the developing embryo in some invertebrates such as insects, and in vertebrate reptiles, birds, and mammals. The chorion has a common origin with the amnion, which is one of the embryonic membranes. In vertebrates, early in embryonic development, the ectoderm that is lined with the extraembryonic mesoderm creates folds that extend from the front, back, left, and right and fuse in the center to encase the embryo in a double layer. The outer part of these folds becomes the chorion, which in birds and reptiles spreads in contact with the eggshell membrane. The embryonic membrane that forms inside the chorion is the amnion, and the gap between the amnion and the chorion is called the extraembryonic coelom, into which the allantois fits. The allantois on the outside of the allantois combines with the chorion to form the chorioallantoic membrane. In reptiles and birds, many blood vessels enter the chorioallantoic membrane, which serves as a site for gas exchange with the outside world through the eggshell. In mammals, the chorioallantoic membrane becomes the fetal placenta, which combines with the maternal placenta in the uterus to form the placenta, which provides nutrients and oxygen and expels waste products. In insects, the extraembryonic cell layer in contact with the eggshell becomes the chorioallantoic membrane. The role of the chorioallantoic membrane beyond protecting the embryo is unclear.

In addition, the thin membranes that cover the free surfaces facing the body cavities, such as the pleura, peritoneum, and pericardium in vertebrates, are also called serous membranes. The serous membrane has a smooth surface and is moistened with a small amount of transparent fluid, so organs covered by it can change position without causing friction with other organs.

[Shigeo Takeuchi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

昆虫など無脊椎(むせきつい)動物の一部、脊椎動物の爬虫(はちゅう)類、鳥類、哺乳(ほにゅう)類の胚(はい)発生の際、最外側を包む胚膜をいう。漿膜は胚膜の一つである羊膜と共通の起源をもつ。脊椎動物では胚発生の早い時期に、胚体外の中胚葉に裏打ちされている外胚葉がひだをつくり、それが前後左右から伸びて中央で癒合し、胚を二重に包む。このひだの外側の部分が漿膜となり、鳥類、爬虫類では、卵殻膜に接して広がる。漿膜の内側にできた胚膜が羊膜で、羊膜と漿膜の間隙(かんげき)を胚体外体腔(たいこう)といい、ここに尿嚢(にょうのう)が入り込んでくる。この尿嚢の外側の尿膜と漿膜が合して漿尿膜となる。爬虫類、鳥類では、この漿尿膜に多くの血管が入り込み、卵殻を通して外界とガス交換をする場所となる。哺乳類では、漿尿膜が胎児性胎盤となり、子宮の母性胎盤と合して胎盤を形成し、栄養や酸素の摂取、老廃物の排出などにあたる。昆虫では、卵殻に接している胚体外の細胞層が漿膜となる。胚の保護以上の漿膜の役割については明らかでない。

　なお、脊椎動物における胸膜、腹膜、心膜など、体腔に面した遊離面を覆った薄膜も漿膜とよぶ。漿膜は表面が滑らかで、少量の透明な液によって潤っているため、漿膜に覆われた器官は他と摩擦をおこすことなく、位置を変えることができる。

［竹内重夫］

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