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Embryos (and one tadpole) of the wrinkled frog (Rana rugosa)

Development

6 week old human embryo

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 organogenesis, 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. 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

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 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.

Fossil record

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

The human embryo

Growth

A 10mm embryo from an ectopic pregnancy, still in the oviduct. This embryo is about five weeks old (or from the 7th week of pregnancy).

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. 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 Christian Ethicists believe that an embryo does, in fact, possess personhood. Gilbert Meileander, Christian ethics professor at the private Lutheran university 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]

Footnotes

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."
2. ↑ ^{a b c d} 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.

See also

• Embryogenesis
• Embryology
• In vitro fertilization
• Plant embryogenesis
• Embryo space colonization
• Pregnancy
• Embryo adoption
• Sexual
• Prenate

External links

• Chart of human fetal development
• A Comparative Embryology Gallery
• Development of the human embryo
• The Visible Embryo from fertilization to birth
• The Human Fertilisation and Embryology Authority (HFEA)

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