Embryo - Yes

Japanese: 胚 - はい

Although it refers to the early stage of individual development of multicellular organisms, it is used in a variety of ways, such as to refer to the developmental stage of an individual after the cleavage stage, or to refer to the stage after germ layer differentiation.

[Taku Shimada]

Embryos in animals

In invertebrates, the morula, blastula, and gastrula are called early embryos, and the subsequent developmental stages are called early embryos. In vertebrates, there is a period when the neural tube is formed, and this period is specifically called the neurula. The length of the embryonic period varies depending on the animal species. The embryonic period is the period from when development progresses until the animal begins to take in food from the outside world. In viviparous animals, an individual in the embryonic period is called a fetus. Cells in the cleavage stage are undifferentiated, and as the germ layers form, there is a tendency for differentiation, but cells in the embryonic period are less differentiated. In addition to being undifferentiated, embryonic cells in the cleavage stage are characterized by a short cell cycle, and during this stage the embryo rapidly divides to increase the number of cells. In Drosophila, cells in the cleavage stage are known to grow at a rate more than 100 times faster than adult cells. During the cleavage stage, there is no cell growth period like after cell division, so the cells become smaller with each division. The rate of cell proliferation during the cleavage stage is particularly high in non-viviparous animals, which is in line with the goal of shortening the period of vulnerability to external enemies as much as possible. As cleavage progresses, the gaps between the cells of each blastomere gradually increase in size, forming a cleavage cavity. The structure in which this cleavage cavity is surrounded by a single layer of cells is called the blastula. Some of the cells then begin to invaginate into the cleavage cavity. As a result of this plastic movement, a sac (gastrucore) is formed, and the single-layered pouch formed by the invaginated cells is called the archenteron, and the opening of the invagination is called the blastopore. In addition to forming the archenteron, this plastic movement has another important meaning. Cells that were far apart during the blastula stage come into contact with each other as a result of invagination, allowing the cells to influence each other. Furthermore, as a result of gastrulation, new embryonic axes, the dorsal-ventral axis and the head-tail axis, are formed in the embryo, and the cells become two-layered, resulting in the differentiation of germ layers. The outer layer is called the ectoderm, the inner dorsal layer the mesoderm, and the inner lateral and ventral layers the endoderm. After the sacculula stage, the notochordal mesoderm separates from the mesoderm to become the notochord. The ectoderm lined with the notochordal mesoderm thickens to become the neural plate, and eventually forms the neural tube. This phenomenon is called induction of neural tissue by the notochordal mesoderm, in which the notochordal mesoderm cells influence the ectodermal cells in contact with them, causing them to differentiate into neural tissue. The induction phenomenon occurs at various points during individual development in the embryonic stage, and plays an extremely important role in morphogenesis. However, the induction phenomenon has yet to be elucidated at the molecular level.

[Taku Shimada]

Embryos in Plants

In plants, when an egg cell in the gametophyte is fertilized, it begins to develop into a sporophyte, but the immature sporophyte that is embedded in the gametophyte or endosperm and receives nutrients from its surroundings is called an embryo (or germ). Mosses, ferns, gymnosperms, and angiosperms have embryos.

(1) Mosses The moss embryo develops inside the archegonia of the gametophyte and differentiates into a foot, a stalk, and a capsule primordium. The foot attaches to the bottom of the archegonia and absorbs nutrients from the gametophyte. As the embryo grows, the stalk extends and the capsule protrudes from the opening of the archegonia.

(2) Ferns In fern embryos, a foot also differentiates near the bottom of the archegonia. A suspensor may also form in the direction of the opening of the archegonia. A suspensor is a part of the embryo that does not participate in the future formation of the sporophyte. In ferns, the axis of the sporophyte often forms at a right angle to the archegonia. A taproot (radicle) differentiates on one side of the axis, and a shoot apex and leaf primordia differentiate on the other side, both of which grow by dissolving the tissue of the gametophyte (prothallus) and eventually extend outward.

(3) Gymnosperms In gymnosperms, the fertilized egg first undergoes free nuclear division, then the embryo stalk is formed, which continues to grow and twist while dissolving the tissues of the female gametophyte. The embryo stalk branches into many branches, and eventually meristematic tissue is formed at the end of each branch, which forms the sporophyte plant. Thus, many young plants are produced from one fertilized egg, but usually only the one that develops the fastest remains until the end, and the others are absorbed by it. Only the part that will form the body of such a young plant is sometimes called an "embryo." To distinguish it from this, the part at the stage where the embryo stalk is growing is called a proembryo. The part that will become the young plant differentiates into a taproot (radicle) near the embryo stalk, followed by a hypocotyl, and then a shoot apex and cotyledon primordium at the tip.

(4) Angiosperms In angiosperms, after double fertilization, the endosperm develops first, and then the fertilized egg begins to divide a little later, growing and developing into the endosperm. The part close to the micropyle often becomes the suspensor, but it is much smaller than that of gymnosperms. The part that enters the endosperm first becomes a spherical mass of tissue, and from this point onwards the shape of a single plant begins to form. The primordium of the taproot (radicle) differentiates in the part close to the micropyle and suspensor, followed by the hypocotyl, and the shoot apex and cotyledon primordia at the tip. The number of cotyledons allows the distinction between dicotyledons and monocotyledons. The embryo of the Poaceae family has special organs such as the scutellum and coleoptile, and there are various interpretations of these. The most common idea is that the scutellum is a modified leaf blade of the cotyledon, and the coleoptile is an independent part of the cotyledon leaf sheath. The degree of development of the embryo within the seed varies depending on the species.

[Takashi Yamashita]

[References] | Bryophytes | Ferns | Seeds | Growth

Morphology of gastrula (sea urchin, frog)
© Satoshi Shimazoe

Morphology of gastrula (sea urchin, frog)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

多細胞生物の個体発生初期の段階をさすが、卵割期以後の発生期個体を意味する場合や、胚葉分化以降を意味する場合などまちまちな使われ方をしている。

［嶋田　拓］

動物における胚

無脊椎(むせきつい)動物では桑実胚、胞胚、原腸胚を早期胚、それ以降の発生段階を初期胚とよぶ。脊椎動物では神経管の形成時期があり、この時期をとくに神経胚という。胚期の長さは動物種により多様である。発生が進んで外界から食餌(しょくじ)をとり始めるまでが胚期とよばれる。胎生動物では胚期の個体を胎児という。卵割期の細胞は未分化であり、胚葉の形成とともに分化の傾向が生ずるが、胚期の細胞は分化の程度が低い。卵割期の胚細胞の特徴は、それが未分化であることのほか、細胞周期が短いことで、この時期の胚は急速に細胞分裂を繰り返して細胞数を増やしていく。ショウジョウバエでは、卵割期の細胞は成体細胞の100倍以上の速度で増殖することが知られている。卵割期には細胞分裂後のような細胞成長期を欠くので、細胞は分裂ごとに小さくなる。卵割期における細胞増殖速度は、非胎生動物ではとくに大きく、外敵に弱い時期をなるべく短縮するという目的に合致している。卵割が進むと各割球細胞間のすきまはしだいに大きくなって卵割腔(こう)となる。この卵割腔を細胞が一重に囲んだ構造となったものが胞胚である。続いて細胞の一部は卵割腔内へ向かって陥入を始める。この造形運動の結果、嚢胚(のうはい)（原腸胞）となるが、陥入した細胞の形成する一重の嚢を原腸、陥入口を原口とよぶ。この造形運動は原腸の形成のほかもう一つ重要な意味をもつ。胞胚期には遠く離れていた細胞どうしが陥入の結果互いに接し合い、細胞間で相互に影響しあえるようになることである。また、原腸形成によって胚に背腹軸および頭尾軸という新しい胚軸が生ずるとともに、細胞も二層となり、胚葉の区別が生ずる。外層を外胚葉、内層背部を中胚葉、内層側部と腹部を内胚葉とよぶ。嚢胚期を過ぎると、中胚葉から脊索中胚葉が分離して脊索となる。脊索中胚葉で裏打ちされた外胚葉は厚みを増して神経板となり、やがて神経管を形成する。この現象を脊索中胚葉による神経組織の誘導といい、脊索中胚葉細胞がそれに接する外胚葉細胞に影響を与えて神経組織へ分化させたのである。誘導現象は胚期の個体発生で各所におこり、形態形成にきわめて重要な働きをしている。しかし誘導現象の分子レベルの解明はいまだなされていない。

［嶋田　拓］

植物における胚

植物では、配偶体の中にある卵細胞が受精すると発達を始めて胞子体となるが、まだ幼い胞子体で、配偶体や内胚乳の中に埋まって、周囲から栄養を供給されている段階にあるものを胚（または胚芽）という。胚があるのはコケ植物、シダ植物、裸子植物、被子植物である。

(1)コケ植物　コケ植物の胚は配偶体の造卵器の中で発達し、足(あし)footと柄と蒴(さく)の原基とに分化する。足は造卵器の底面に張り付いて、配偶体から栄養を吸収する。胚が成長すると柄が伸び、蒴は造卵器の口から外に出る。

(2)シダ植物　シダ植物の胚においても、造卵器の底の近くに足が分化する。また、造卵器の口の方向には胚柄(はいへい)suspensorができることがある。胚柄とは、胚の一部ではあるが、将来の胞子体の形成には参加しない部分のことである。シダ植物の場合、胞子体の軸は造卵器と直角の方向にできることが多い。軸の一方には主根（幼根）、反対側には茎頂と葉原基が分化し、いずれも配偶体（前葉体）の組織を溶かしながら成長し、やがて外へ伸び出す。

(3)裸子植物　裸子植物では、受精卵がまず自由核分裂をし、次に胚柄ができて、雌性配偶体の組織を溶かしながら、曲がりくねって伸び続ける。胚柄は多数の枝分れをするが、やがてそれぞれの枝の先に分裂組織ができ、これが胞子体の幼植物の形をつくる。したがって、1個の受精卵から多数の幼植物ができることになるが、普通は、もっとも早く発達した一つだけが最後まで残り、ほかのものはこれによって吸収される。このような幼植物の体をつくる部分だけを「胚」とよぶことがある。これと区別するため、胚柄が伸びている段階のものは前胚（または初期胚proembryo）とよばれる。幼植物となる部分は、胚柄寄りの位置に主根（幼根）、続いて胚軸、先端部に茎頂と子葉原基とを分化させる。

(4)被子植物　被子植物では、重複受精のあと、まず内胚乳が発達し、やや遅れて受精卵が細胞分裂を始め、内胚乳の中に向かって成長、発達する。珠孔寄りの部分は胚柄となることが多いが、裸子植物と比べるとはるかに小さい。内胚乳の中に入った部分はまず球形の組織塊となり、これからただちに1個の幼植物の形をつくり始める。珠孔や胚柄に近い部分に主根（幼根）の原基が分化し、続く位置に胚軸、先端部に茎頂と子葉の原基ができる。この子葉の数によって、双子葉植物と単子葉植物が区別できるようになる。イネ科の胚には胚盤、子葉鞘(しょう)などの特殊な器官があり、さまざまな解釈が行われている。もっとも普通なのは、胚盤は子葉の葉身が変化したもので、子葉鞘は子葉の葉鞘部分が独立したものとする考えである。種子内での胚の発達の程度は種類によってさまざまである。

［山下貴司］

[参照項目] | コケ植物 | シダ植物 | 種子 | 発生

原腸胚の形態（ウニ、カエル）

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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