embryo

(noun)

An organism in the earlier stages of development; in humans, usually the cell growth up to the end of the seventh week in utero.

Related Terms

Examples of embryo in the following topics:

  • Characteristics of Amniotes

    • The albumin, or egg white, provides the embryo with water and protein, whereas the fattier egg yolk is the energy supply for the embryo, as is the case with the eggs of many other animals, such as amphibians.
    • While the inner amniotic membrane surrounds the embryo itself, the chorion surrounds the embryo and yolk sac.
    • The chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg's external environment.
    • The amnion protects the embryo from mechanical shock and supports hydration.
    • The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration.

  • Fifth Through Eighth Weeks of Development

    • During this time, the embryo is extremely susceptible to the effects of teratogens.
    • By week eight of gestation, the embryo measures 13 millimeters in length.
    • An embryo from an ectopic pregnancy, still in the oviduct.
    • This embryo is about five weeks old (or from the seventh week of menstrual age).
    • The heart is the dark spot at the center of the image, bulging out of the embryo.

  • Double Fertilization in Plants

    • Ultimately, the embryo and cotyledons fill the seed, at which point, the seed is ready for dispersal .
    • Shown are the stages of embryo development in the ovule of a shepherd's purse (Capsella bursa).
    • (a) In the first stage of development, the terminal cell divides, forming a globular pro-embryo.
    • (b) In the second stage, the developing embryo has a heart shape due to the presence of cotyledons.
    • (c) In the third stage, the growing embryo is crowded and begins to bend.

  • Development of Metabolism

    • At approximately the sixteenth day of human development, the embryo begins to fold ventrally (with the embryo's ventral surface becoming concave) in two directions: the sides of the embryo fold in on each other and the head and tail fold toward one another.
    • As a result, a piece of the yolk sac (the endoderm-lined structure in contact with the ventral aspect of the embryo) is then "pinched off" to become the primitive gut.
    • The midgut is the portion of the embryo from which most of the intestines develop.

  • Yolk Sac Development

    • The yolk sac is vascularized and contributes nutrients to the embryo.
    • The yolk sac, situated on the ventral aspect of the embryo, is lined by extra-embryonic endoderm, outside of which is a layer of extra-embryonic mesenchyme derived from the mesoderm.
    • After circulating through a wide-meshed capillary plexus, it is returned by the vitelline veins to the tubular heart of the embryo.
    • This vitelline circulation absorbs nutritive material from the yolk sac which is conveyed to the embryo.
    • The yolk sac is a membranous sac attached to the embryo that provides nourishment in the form of yolk.

  • Gastrulation

    • During gastrulation, the embryo develops three germ layers (endoderm, mesoderm, and ectoderm), which differentiate into distinct tissues.
    • Each layer gives rise to specific tissues and organs in the developing embryo.
    • In amniotes such as humans, gastrulation occurs in the following sequence: (1) the embryo becomes asymmetric, (2) the primitive streak forms, and (3) cells from the epiblast at the primitive streak undergo an epithelial to mesenchymal transition and ingress at the primitive streak to form the germ layers.The ectoderm gives rise to epidermis, and also to the neural crest and other tissues that will later form the nervous system.
    • However, some common features of gastrulation across triploblastic organisms include: (1) A change in the topological structure of the embryo, from a simply connected surface (sphere-like), to a non-simply connected surface (torus-like); (2) the differentiation of cells into one of three types (endodermal, mesodermal, or ectodermal); and (3) the digestive function of a large number of endodermal cells.
    • The embryo at this stage is only a few millimeters in length.

  • Digestive System Development

    • At approximately the 16th day of human development, the embryo begins to fold ventrally (with the embryo's ventral surface becoming concave) in two directions: the sides of the embryo fold in on each other and the head and tail fold toward one another.
    • The result is that a piece of the yolk sac, an endoderm-lined structure in contact with the ventral aspect of the embryo, begins to be pinched off to become the primitive gut .
    • Sagittal section of embryo at about four weeks showing the primitive gut.

  • Blastocyst Formation

    • The blastocyst, forming early in embryonic development, has two layers that form the embryo and placenta.
    • The blastocyst possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta.
    • As already stated, the cells of the trophoblast do not contribute to the formation of the embryo proper; they form the ectoderm of the chorion and play an important part in the development of the placenta.
    • The blastocyst possesses an inner cell mass from which the embryo will develop, and an outer layer of cells, called the trophoblast, which will eventually form the placenta.

  • The Life Cycle of an Angiosperm

    • Only the large megaspore survives; it produces the female gametophyte referred to as the embryo sac.
    • The two sperm cells are deposited in the embryo sac.
    • One sperm and the egg combine, forming a diploid zygote, the future embryo.
    • Seed food reserves are stored outside the embryo in the form of complex carbohydrates, lipids, or proteins.
    • Anthers and carpels are structures that shelter the actual gametophytes: the pollen grain and embryo sac.

  • Cleavage of the Zygote

    • These species, such as humans and other mammals, receive nourishment as embryos from the mother via the placenta or milk after birth.
    • The trophoblast does not contribute to the formation of the embryo proper; it is from the inner mass cells that the embryo develop.
    • This is named the embryonic pole, since it indicates the location where the future embryo will develop.
    • In the case of monozygotic twins (derived from one zygote), a zygote divides into two separate cells (embryos) at the first cleavage division.
    • Dizygotic twins is the development of two embryos from two different zygotes.