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EMBRYO
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An embryo (Greek: έμβρυον) is a multicellular diploid eukaryote in its earliest stage of development.
Description
In organisms that reproduce sexually, once a sperm fertilizes an egg cell, the result is a cell called the zygote that has all the DNA of two parents. In plants, animals, and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism. The term embryo refers to the early stages of this development, after the zygote has divided at least once, but before the process has completed to produce the next stage of development.
In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of morula, blastula, and gastrula. This process involves the creating of a hollow sphere of cells in which three tissue layers develop. In plants. the process is not so standard. The stages of embryo development in plants is more dependent upon the kind of plant being produced.
Information
- Plants: In botany, a seed plant embryo is part of a seed, consisting of precursor tissues for the leaves, stem (see hypocotyl), and root (see radicle), as well as one or more cotyledons. Once the embryo begins to germinate — grow out from the seed — it is called a seedling. Plants that do not produce seeds, but do produce an embryo include the bryophytes and ferns. In these plants, the embryo is a young plant that grows attached to a parental gametophyte.
- Animals: The embryo of a placental mammal is defined as the organism between the first division of the zygote (a fertilized ovum) until it becomes a fetus. In humans, the embryo is defined as the product of conception from implantation in the uterus through the eighth week of development. An embryo is called a fetus at a more advanced stage of development and up until birth or hatching. In humans, this is from the eighth week of gestation. In other vertebrate animals, the embryo develops inside an egg. Invertebrate animals also develop from an embryo.
Growth of the Human Embryo
Week 1-4 After conception, the embryo will begin to "search" for a place to attach to the wall of the uterus (endometrium). When it comes into contact with the endometrium and implants itself there, the connections between the mother and the embryo will begin to form, including the umbilical cord. The embryo's growth centers around an axis, which will become the spine and spinal cord.
Week 5-6 Chemicals produced by the embryo stop the woman's menstrual cycle. The brain begins to develop, showing brain wave activity at about the 6th week. The heart will begin to beat around the same time. Stubs begin to be visible where arms and legs will grow later. All of the main organs begin to grow. The head represents about one half of the embryo's axial length, and more than half of the embryo's mass.
Week 7-8 The embryo's blood type becomes apparent. Embryo is capable of motion, and the eyes begin to form. Most organs have developed or have begun developing. At the end of the 8th week, the embryonic stage is over, and the fetal stage begins.
See also
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