Germ layer - Germ

In animal embryos, this refers to a group of cells that spread out in layers and share a common fate in the future body structure of the embryo after the sac stage. Animal embryos become multicellular through repeated cell division (cleavage) and become a blastula or an equivalent embryo. During this period, the cells form the embryo as a single layer of epithelial structure. Through sac formation (gastrulation), the embryo becomes a two- or three-layer structure. The cell groups of each layer that are stratified and share a common fate are called germ layers. Animals with simple body structures consist of two germ layers: the archenteron that sinks into a pocket and the outer cell layer that surrounds it. The cell group that forms this archenteron will form the digestive system and the glands associated with it, and is called the endoderm. The outer cell group will form the organs and tissues (sensory organs, nervous system, cortex, etc.) that will interact with the outside world, sense information from the outside world, and protect the inside of the body from the outside world. This is called the ectoderm. The second layer of cells that develops between the inner and outer layers is called the mesoderm. In animals with a relatively simple body structure, this cell group is called mesodermal cells because it does not have a clearly layered structure. The origin of the mesoderm or mesodermal cells varies depending on the animal, but it helps the inner and outer germ layers function and creates various organs and tissues that support them from the inside (bones, muscles, dermis, connective tissue, vascular system, excretory organs, reproductive organs, etc.).

[Shigeo Takeuchi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

動物の胚において、嚢(のう)胚期以後、胚の将来の体制に関して共通の運命を担う、層状に広がる細胞の集団をいう。動物の胚は細胞分裂（卵割）を繰り返して多細胞化し、胞胚あるいはそれに相当する胚となる。この時期に細胞は一層の上皮様の構造として胚をつくる。嚢胚形成（原腸陥入）により、胚は二層あるいは三層の構造になる。このように複層化し、共通の運命を担った各層の細胞群を胚葉とよぶ。体制の簡単な動物は二層の胚葉よりなる。すなわちポケット様に落ち込んだ原腸と、これを取り囲む外側の細胞層である。この原腸をつくる細胞集団は、将来消化器およびそれに付属する腺(せん)を形づくるもので、内胚葉とよぶ。外側の細胞集団は、将来外界と交渉し、外界の情報を感取し、外界から体の内部を保護する器官や組織（感覚器、神経系、皮層など）を形づくる。これを外胚葉とよぶ。この内外二層の間に生ずるもう一層の細胞集団を中胚葉とよぶ。体制の比較的簡単な動物ではこの細胞集団は、はっきりした層状構造をとらないので中胚葉細胞といわれる。中胚葉あるいは中胚葉細胞の起源は動物によって異なるが、内外二胚葉の働きを助け、内から支える諸器官や組織（骨、筋肉、真皮、結合組織、血管系、排出器官、生殖器官など）をつくる。

［竹内重夫］

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