Embryo

An embryo (irregularly from Greek: ', plural ', 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, hatching, or germination. In humans, it is called an embryo until about eight weeks after fertilization (i.e. ten weeks Last Menstrual Period or LMP), and from then it is instead called a fetus.

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 half of the DNA of each of 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.

Animal embryos
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. 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. 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.

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. 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". 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.

Fossilised animal embryos
Fossilised animal embryos are known from the Precambrian, and are found in great number during the Cambrian period. Fossil dinosaur embryos have also been discovered.

The human embryo





 * 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.


 * 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. (This brain activity does not necessarily indicate human consciousness. At this point, brain activity is still less advanced that the nervous system of a shrimp.) 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.


 * 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.

Miscarriage
Some embryos do not survive through to 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. Miscarriages after the sixth week LMP happen in 8% of pregnancies. 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.

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

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

Induced (i.e. purposeful) abortion of an embryo can be accomplished by a variety of methods, including both surgical and non-surgical techniques. Suction-aspiration is the most common surgical method of aborting an embryo.

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.

Use in ART and diagnosis
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.

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

Viability
A human embryo is not considered viable, because it cannot survive outside the uterus. Current medical technology does not allow an embryo to be transplanted from the uterus of one woman to that of another.

Research
Human embryos are being researched to determine their use in treating diseases. Stem cell research, reproductive cloning, and germline engineering are all currently being explored. The morality of this research is also debated because an embryo is often used.

Plant embryos
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.