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An embryo (from Greek: Template:Polytonic, plural Template:Polytonic, lit. "that which grows," from en- "in" + bryein "to swell, be full") is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination. In humans, it is called an embryo from the moment of implantation until the end of the 8th week, whereafter it is instead called a fetus.

Development

The development of the embryo is called embryogenesis. 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. The resulting embryo derives 50 percent of its genetic makeup from each parent. In plants, animals, and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism. The result of this process is an embryo.

In animals

In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of blastula, gastrula, and organogenesis. The blastula stage typically features a fluid-filled cavity, the blastocoel, surrounded by a sphere or sheet of cells, also called blastomeres.

Human embryo at six weeks gestational age, i.e. four weeks after fertilization

During gastrulation the cells of the blastula undergo coordinated processes of cell division, invasion, and/or migration to form two (diploblastic) or three (triploblastic) tissue layers. In triploblastic organisms, the three germ layers are called endoderm, ectoderm and mesoderm. However, the position and arrangement of the germ layers are highly species-specific, depending on the type of embryo produced. In vertebrates, a special population of embryonic cells called the neural crest has been proposed as a "fourth germ layer", and is thought to have been an important novelty in the evolution of head structures.

During orgasmic, molecular and cellular interactions between germ layers, combined with the cells' developmental potential or competence to respond, prompt the further differentiation of organ-specific cell types.^{[citation needed]} For example, in neurogenesis, a subpopulation of ectoderm cells is set aside to become the brain, spinal cord and peripheral nerves. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including angiogenesis (formation of new blood vessels from pre-existing ones), chondrogenesis (cartilage), myogenesis (muscle), osteogenesis (bone), and many others.

Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny."^[1] For example, the backbone is a common structure among all vertebrates such as fish, reptiles and mammals, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, which is the most sophisticated part of the brain, develops last. This rule is not absolute, but it is recognized as being partly applicable to development of the human embryo.

Embryos of plants and animals

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 sex

Fossil record

Fossilised embryos are known from the Precambrian, and are found in great number during the Cambrian period.

The human embryo

Growth

Week 1-3 5-7 days after fertilization, the blastula attaches to the wall of the uterus (endometrium). When it comes into contact with the endometrium it performs implantation. Implantation 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. The brain, spinal cord, heart, and gastrointestinal tract begin to form.^[2]

Week 4-5 Chemicals produced by the embryo stop the woman's menstrual cycle. Neurogenesis is underway, showing brain activity at about the 6th week.^{[citation needed]} The heart will begin to beat around the same time. Limb buds appear where the arms and legs will grow later. Organogenesis begins. The head represents about one half of the embryo's axial length, and more than half of the embryo's mass. The brain develops into five areas. Tissue formation occurs that develops into the vertebra and some other bones. The heart starts to beat and blood starts to flow.^[2]

Week 6-8 Myogenesis and neurogenesis have progressed to where the embryo is capable of motion, and the eyes begin to form. Organogenesis and growth continue. Hair has started to form along with all essential organs. Facial features are beginning to develop. At the end of the 8th week, the embryonic stage is over, and the fetal stage begins.^[2]

Status

The status of the human embryo is debated by some bioethicists. Some Ethicists believe that an embryo does, in fact, possess personhood. Gilbert Meileander, Christian ethics professor at the Valparaiso University for example, identifies conception as the point at which a new individual human being comes into existence, since "when sperm and ovum join to form the zygote, the individual's genotype is established."^[3] The NIH defines the embryonic stage as the beginning of developed human form ^[2] unnecessary

Footnotes

^ Gould, Stephen. Ontogeny and Philogeny, page 206 (1977): "recapitulation was not 'disproved'; it could not be, for too many well-established cases fit its expectations."
^ ^a ^b ^c ^d NIH Medical Encyclopedia http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm
^ Gilbert Meilander, Bioethics: A Primer for Christians (2nd ed.; Grand Rapids: Eerdmans, 2005), p. 29.

External links

[1] Gould, Stephen. Ontogeny and Philogeny, page 206 (1977): "recapitulation was not 'disproved'; it could not be, for too many well-established cases fit its expectations."

[facts-2] NIH Medical Encyclopedia http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm

[3] Gilbert Meilander, Bioethics: A Primer for Christians (2nd ed.; Grand Rapids: Eerdmans, 2005), p. 29.

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 ==Embryos of plants and animals==
 [[Image:Ginkgo embryo and gametophyte.jpg|thumb|260px|The inside of a ''[[Ginkgo]]'' seed, showing the embryo.]]
-*'''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 '''[[cotyledon]]s'''. Once the embryo begins to [[germination|germinate]] &mdash; grow out from the seed &mdash; it is called a '''seedling'''. Plants that do not produce seeds, but do produce an embryo, include the [[bryophyte]]s and [[fern]]s. In these plants, the embryo is a young plant that grows attached to a parental '''[[gametophyte]]'''.
+*'''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 sex
-*'''Animals''': The embryo of a [[placental mammal]] is defined as the organism between the first [[cell division|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 [[childbirth|birth]] or hatching. In humans, this is from the eighth week of [[gestation]].
 ==Fossil record==
 {{Main|Fossil embryos}}