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

An embryo (irregularly from Greek: ἔμβρυον, plural ἔμβρυα, lit. "that which grows," from en- "in" + bryein "to swell, be full"; the proper Latinate form would be embryum) is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth or hatching. 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.


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 a new individual.

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.

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 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.[How to reference and link to summary or text] 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.== Growth of the Human Embryo == Week 1-4 After conception, the embryo will begin to "search" for a place to attach to the woman's uterus. When it finds one and plants itself there, the connections between the mother and the embryo will begin to form, including the umbilical cord.

Week 5-8 Chemicals produced by the embryo stop the mother's menstruational cycle. The brain begins to develope, and the heart will begin to beat. Stubs begin to be visible where arms and legs will grow later. All the main organs begin to grow. 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 embryoic stage is over, and the fetal stage begins.

The human embryo


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 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.[How to reference and link to summary or text] 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]


See also: Miscarriage
File:Human Embryo - Approximately 6 weeks estimated gestational age.jpg

A complete spontaneous abortion at 6 weeks gestational age

Some embryos do not survive until the fetal stage, which begins about two months after fertilization (10 weeks LMP). Embryos may be miscarried, or aborted purposely.

Studies using very sensitive early pregnancy tests have found that 25% of embryos are miscarried by the sixth week LMP (since the woman's Last Menstrual Period), even if a woman does not realize it.[3][4] Miscarriages after the sixth week LMP happen in 8% of pregnancies.[4] The risk of miscarriage is "virtually complete by the end of the embryonic period," with a miscarriage rate of only two percent after 8.5 weeks LMP.[5]

The most common cause of miscarriage of an embryo is chromosomal abnormality,[6] which accounts for at least 50% of sampled early pregnancy losses.[7] Advancing maternal age and a patient history of previous miscarriage are the two leading risk factors.[7]

See also


  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. 2.0 2.1 2.2 NIH Medical Encyclopedia
  3. Wilcox AJ, Baird DD, Weinberg CR (1999). Time of implantation of the conceptus and loss of pregnancy.. New England Journal of Medicine 340 (23): 1796–1799.
  4. 4.0 4.1 Wang X, Chen C, Wang L, Chen D, Guang W, French J (2003). Conception, early pregnancy loss, and time to clinical pregnancy: a population-based prospective study.. Fertil Steril 79 (3): 577–84.
  5. Rodeck, Charles; Whittle, Martin. Fetal Medicine: Basic Science and Clinical Practice (Elsevier Health Sciences 1999), page 835.
  6. Stöppler, Melissa Conrad, William C. Shiel, Jr., ed. Miscarriage (Spontaneous Abortion). URL accessed on 2009-04-07.
  7. 7.0 7.1 Jauniaux, E.; P. Kaminopetros and H. El-Rafaey (1999). "Early pregnancy loss" Martin J. Whittle and C. H. Rodeck Fetal medicine: basic science and clinical practice, Edinburgh: Churchill Livingstone.

A chick embryo of thirty-three hours' incubation

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