Embryo

Embryo

Embryo
Embryos (and one tadpole) of the wrinkled frog (Rana rugosa)
Identifiers
Code TE E1.0.2.6.4.0.8
Anatomical terminology

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of fertilization until birth, hatching, or germination.

In humans, an embryo is generally considered to be between the first and the eighth week of development after

Preceded by
Zygote
Stages of human development
Embryo
Succeeded by
Fetus
  • UNSW Embryology - Educational website
  • A Comparative Embryology Gallery
  • 4-H Embryology, University of Nebraska-Lincoln Extension in Lancaster County
  • Video with embryo of a small-spotted catshark inside the egg on YouTube

External links

  1. ^ ""embryo"". OED. Retrieved 7 November 2014. 
  2. ^ Larsen's human embryology (4th ed., Thoroughly rev. and updated. ed.). Philadelphia: Churchill Livingstone/Elsevier. 2009. p. 505-507 .  
  3. ^ ἔμβρυον, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  4. ^ ἔμβρυος, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  5. ^ ἐν, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  6. ^ βρύω, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  7. ^ a b c NIH Medical Encyclopedia http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm
  8. ^ Gazzaniga, Mike S.The Ethical Brain "not until the end of week 5 and into week 6 (usually around forty to forty-three days) does the first electrical brain activity begin to occur. This activity, however, is not coherent activity of the kind that underlies human consciousness, or even the coherent activity seen in a shrimp's nervous system."
  9. ^ 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.  
  10. ^ a b 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.  
  11. ^ Rodeck, Charles; Whittle, Martin. Medicine: Basic Science and Clinical Practice (Elsevier Health Sciences 1999), p. 835.
  12. ^ Stöppler, Melissa Conrad; William C. Shiel, Jr., ed. "Miscarriage (Spontaneous Abortion)". MedicineNet.com. Retrieved 2009-04-07. 
  13. ^ a b Jauniaux, E.; P. Kaminopetros and H. El-Rafaey (1999). "Early pregnancy loss". In Martin J. Whittle and C. H. Rodeck. Fetal medicine: basic science and clinical practice. Edinburgh: Churchill Livingstone. p. 837.  
  14. ^ Department of Health (2007). "Abortion statistics, England and Wales: 2006". Retrieved 2007-10-12. 68% were at under 10 weeks 
  15. ^ Healthwise (2004). "Manual and vacuum aspiration for abortion".  
  16. ^ Bankole, Akinrinola, Singh, Susheela, & Haas, Taylor. (1998).Reasons Why Women Have Induced Abortions: Evidence from 27 Countries. International Family Planning Perspectives, 24 (3), 117–127 & 152. Retrieved 2006-01-18.
  17. ^ Finer, Lawrence B., Frohwirth, Lori F., Dauphinee, Lindsay A., Singh, Shusheela, & Moore, Ann M. (2005).Reasons U.S. women have abortions: quantitative and qualitative perspectives. Perspectives on Sexual and Reproductive Health, 37 (3), 110–8. Retrieved 2006-01-18.
  18. ^ Rumbold, Graham. Ethics in nursing practice, p. 120 (Elsevier Health Sciences 1999).
  19. ^ Freedman, Jeri. "America Debates Stem Cell Research." New York, NY: The Rosen Publishing Group, 2008.
  20. ^ Sandel, Michael J. "The Case Against Perfection." Michael J. Sandel, 2007.
  21. ^ Zavos, Panayiotis. “Reproductive Cloning is Moral.” Ed. James Woodward. The Ethics of Human Cloning: At Issue. Farmington Hills, MI: Greenhaven, 2005. 14–24.
  22. ^ Gould, Stephen. Ontogeny and Phylogeny, p. 206 (1977): "recapitulation was not 'disproved'; it could not be, for too many well-established cases fit its expectations."

Notes

See also

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 (plantlet). Bryophytes and ferns are among plants that produce an embryo, but do not produce seeds. In these plants, the embryo is a young plant that grows attached to a parental gametophyte.

The inside of a Ginkgo seed, showing the embryo

Plant embryos

Fossilised animal embryos are known from the Precambrian, and are found in great numbers during the Cambrian period. Even fossilised dinosaur embryos have been discovered.

Fossilised animals

Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the second in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny".[22] 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 sequencing rule is not absolute, but it is recognized as being partly applicable to development of the human embryo.

During organogenesis, molecular and cellular interactions between germ layers, combined with the cells' developmental potential, or angiogenesis (formation of new blood vessels from pre-existing ones), chondrogenesis (cartilage), myogenesis (muscle), osteogenesis (bone), and many others.

During germ layers are called endoderm, ectoderm, and mesoderm. The position and arrangement of the germ layers are highly species-specific, however, 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.

In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of division of the zygote (a fertilized ovum) until it becomes a fetus. An ovum is fertilized in a fallopian tube through which it travels into the uterus. 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. However, animals which develop in eggs outside the mother's body are usually referred to as embryos throughout development; e.g. one would refer to a chick embryo, not a "chick fetus" even at late stages.

Embryonic development of salamander, circa the 1920s.

Other animals

Human embryos are being researched to determine their use in treating diseases. Their use in stem cell research, reproductive cloning, and germline engineering are currently being explored. The morality of this type of research is debated because an embryo is often used.[19][20][21]

Research

As of 1999, current medical technology does not allow an embryo to survive outside the uterus under any circumstances, or to be transplanted from the uterus of one woman to that of another.[18] A human embryo is therefore not considered viable.

Viability

Prenatal diagnosis or preimplantation diagnosis involves testing embryos for diseases or conditions.

Embryos are used in various techniques of assisted reproductive technology, such as in vitro fertilization and embryo donation. They may be subject to embryo cryopreservation for later use if IVF procedures have resulted in more embryos than is currently needed. Some aspects, e.g. selective reduction, are issues in the beginning of pregnancy controversy.

Use in ART and diagnosis

Common reasons for purposely aborting an embryo include a desire to delay or end childbearing, concern over the interruption of work or education, issues of financial or relationship stability, perceived immaturity and health concerns.[16][17]

Induced (i.e. purposeful) abortion of an embryo may be accomplished by a variety of methods, including both pharmaceutical and surgical techniques. Vacuum aspiration is the most common surgical method of aborting an embryo within the United States.[15]

The majority of induced abortions occur during the embryonic period. For example, in England and Wales during 2006, 68% of them occurred by the end of the embryonic period.[14]

Induced abortion

The most common natural cause of abortion of an embryo is chromosomal abnormality,[12] which accounts for at least 50% of sampled early pregnancy losses.[13] Advancing maternal age and a patient history of previous spontaneous abortions are the two leading risk factors.[13]

Studies using very sensitive early pregnancy tests have found that 25% of embryos are aborted by the sixth week LMP (since the woman's last menstrual period), even if a woman does not realize it.[9][10] Abortions after the sixth week LMP happen in 8% of pregnancies.[10] The risk of them is "virtually complete by the end of the embryonic period," with a rate of only two percent after 8.5 weeks LMP.[11]

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

A complete spontaneous abortion at about 6 weeks from conception, i.e. 8 weeks from LMP

Miscarriage

Week 6–8
fetal stage begins.[7]
Week 4–5
Chemicals produced by the embryo stop the woman'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.[7]
Week 1–3
5–7 days after fertilization, the blastocyst 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.[7]

In this stage of human development, the human is highly undeveloped. Below is the process of how an embryo transforms.

A 10mm embryo from an ectopic pregnancy, not having passed through in the oviduct (fallopian tube) successfully. This embryo is about five weeks old (or from the 7th week of pregnancy).
An embryo 8 weeks after fertilization (i.e. 10 weeks LMP)
At left is an embryo 4 weeks after fertilization (i.e. 6 weeks LMP). At right is a fetus 8 weeks after fertilization (i.e. 10 weeks LMP).

Human

First attested in English in the mid-14c., the word embryon derives from Medieval Latin embryo, itself from Greek ἔμβρυον (embruon), lit. "young one",[3] which is the neuter of ἔμβρυος (embruos), lit. "growing in",[4] from ἐν (en), "in"[5] + βρύω (bruō), "swell, be full";[6] the proper Latinised form of the Greek term would be embryum. imanali

Etymology

Contents

  • Etymology 1
  • Human 2
    • Miscarriage 2.1
    • Induced abortion 2.2
    • Use in ART and diagnosis 2.3
    • Viability 2.4
    • Research 2.5
  • Other animals 3
    • Fossilised animals 3.1
  • Plant embryos 4
  • See also 5
  • Notes 6
  • External links 7

The development of the embryo is called reproduce sexually, once a sperm fertilizes an egg cell, the result is a cell called the zygote, which possesses half the DNA of each of its two parents. 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.

[2]