An embryo (irregularly from Greek Greek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical ancient Greek literature and the New Testament of: 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 Multicellular organisms are organisms that consist of more than one cell, and have differentiated cells that perform specialized functions in the organism. Most life that can be seen with the naked eye is multicellular, as are all members of the kingdoms Planimalia diploid Human germ cells have one complete set of chromosomes from the male or female parent. Germ cells, also called gametes, combine to produce somatic cells. Somatic cells therefore have twice as many chromosomes. The haploid number (n) is the number of chromosomes in a gamete. A somatic cell has twice that many chromosomes (2n) eukaryote A eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the in its earliest stage of development Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy, from the time of first cell division Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort of cell division in prokaryotes is known as binary until birth Birth is the act or process of bearing or bringing forth offspring. The offspring is brought forth from the mother. Different forms of birth are oviparity, vivipary and ovovivipary, hatching In most birds, reptiles, insects, fish, and two types of mammals an egg (Latin, ovum) is the zygote, resulting from fertilization of the ovum. To enable incubation the egg is usually kept within a favourable temperature range as it nourishes and protects the growing embryo. When the embryo is adequately developed it breaks out of the egg in the, or germination Germination is the process in which a plant or fungus emerges from a seed or spore and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the growth of hyphae from fungal spores, is also germination. In a more. In humans, it is called an embryo until about eight weeks after fertilization Fertilisation , is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external (i.e. ten weeks LMP Gestational age is the age of an embryo or fetus . In humans, a common method of calculating gestational age starts counting either from the first day of the woman's last menstrual period (LMP) or from 14 days before conception (fertilization). Counting from the first day of the LMP involves the assumption that conception occurred 14 days later), and from then it is instead called a fetus A fetus Pronounced Fee-tus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.

The development of the embryo is called embryogenesis. In organisms that reproduce sexually, once a sperm The spermatid is the haploid male gametid that results from division of secondary spermatocytes. As a result of meiosis, each spermatid contains only half of the genetic material present in the original primary spermatocyte fertilizes an egg cell An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization. In lower plants and algae, the ovum is also often called oosphere, the result is a cell The cell is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular . Other organisms, such as humans, called the zygote A zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development. Zygotes are usually produced by a fertilization event between two haploid cells — an ovum from a female and that has half of the DNA Deoxyribonucleic acid ( /diːˌɒksɨˌraɪbɵ.nuːˈkleɪ.ɪk ˈæsɪd/ (help·info)) (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of of each of two parents. In plants Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004,, animals Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also, and some protists Protists are a diverse group of eukaryotic microorganisms. which include mostly unicellular organisms that do not fit into the other kingdoms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy. Instead, it, the zygote will begin to divide by mitosis Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and 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 The blastula (from Greek βλαστός , meaning "sprout") is an early stage of embryonic development in animals. It is also called blastosphere. It is produced by cleavage of a fertilized ovum and consists of a spherical layer of around 128 cells with a large fluid filled space called the blastocoel in the animal pole of the embryo, gastrula Gastrulation is a phase early in the development of most animal embryos, during which the morphology of the embryo is reorganized to form the three germ layers: ectoderm, mesoderm, and endoderm. The molecular mechanism and timing of gastrulation is different in different organisms. Gastrulation is followed by organogenesis, when individual organs, and organogenesis In animal development, organogenesis is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. Internal organs initiate development in humans within the 3rd to 8th weeks in utero. The germ layers in organogenesis differ by three processes: folds, splits, and condensation. Developing early during. The blastula stage typically features a fluid-filled cavity, the blastocoel A blastocoel or blastocele (also called blastocyst cavity, cleavage cavity or segmentation cavity) is the fluid-filled central region of a blastocyst. A blastocoele forms during embryogenesis when a zygote (a fertilized ovum) divides into many cells through mitosis, surrounded by a sphere or sheet of cells, also called blastomeres A blastomere is a type of cell produced by division of the egg after fertilization. The embryo of a placental mammal Eutheria are a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos). They are distinguished from non-eutherians by various features of the feet, ankles, jaws and teeth. One of the major differences between is defined as the organism between the first division Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort of cell division in prokaryotes is known as binary of the zygote A zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development. Zygotes are usually produced by a fertilization event between two haploid cells — an ovum from a female and (a fertilized ovum An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization. In lower plants and algae, the ovum is also often called oosphere) until it becomes a fetus A fetus Pronounced Fee-tus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth. In humans, the embryo is defined as the product of conception from implantation in the uterus The uterus (from Latin "uterus" , plural uteruses or uteri) or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species. It is within the uterus that the fetus through the eighth week of development. An embryo is called a fetus at a more advanced stage of development and up until birth Childbirth is the culmination of a human pregnancy or gestation period with the birth of one or more newborn infants from a woman's uterus. The process of normal human childbirth is categorized in three stages of labour: the shortening and dilation of the cervix, descent and birth of the infant, and birth of the placenta. In many cases, with or hatching, although by convention some animals are called embryos all the way through hatching, e.g. chick embryos. In humans, this is from the eighth week of gestation Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time . The time interval of a gestation plus 2 weeks is called gestation period, and the length of time plus 2 weeks that the offspring have spent developing in the uterus is called gestational age.

During gastrulation Gastrulation is a phase early in the development of most animal embryos, during which the morphology of the embryo is reorganized to form the three germ layers: ectoderm, mesoderm, and endoderm. The molecular mechanism and timing of gastrulation is different in different organisms. Gastrulation is followed by organogenesis, when individual organs the cells of the blastula undergo coordinated processes of cell division, invasion, and/or migration to form two (diploblastic Diploblasty is a condition of the ovum in which there are two primary germ layers: the ectoderm and endoderm) or three (triploblastic Triploblasty is a condition of the blastula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. The germ layers form during gastrulation of the blastula. Additionally, the term may refer to any ovum in which the blastoderm splits into three layers. All higher and intermediate animals , arise from a triploblastic) tissue layers. In triploblastic organisms, the three germ layers A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers (sometimes called primary germ layers). Animals with radial symmetry, like are called endoderm Endoderm, is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm, ectoderm Generally speaking, the ectoderm differentiates to form the nervous system, tooth enamel and the epidermis and mesoderm In humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to give rise to a number of tissues and structures including bone, muscle, connective tissue, and the middle layer of the skin. Some cells in mesodermal. However, the position and arrangement of the germ layers are highly species-specific, depending on the type of embryo produced. In vertebrates Vertebrates are members of the subphylum Vertebrata, chordates with backbones and spinal columns. About 58,000 species of vertebrates have been currently described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates are comprised of animals from the groups jawless fishs, bony fish,, a special population of embryonic cells called the neural crest The neural crest, a transient component of the ectoderm, is located in between the neural tube and the epidermis of an embryo during neural tube formation. Neural crest cells quickly migrate during or shortly after neurulation, an embryological event marked by neural tube closure 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 In microbiology, genetics, cell biology and molecular biology, competence is the ability of a cell to take up extracellular DNA from its environment. Competence is distinguished into natural competence, a genetically specified ability of bacteria that is thought to occur under natural conditions as well as in the laboratory, and induced or to respond, prompt the further differentiation of organ-specific cell types.^{[citation needed]} For example, in neurogenesis Neurogenesis is the process by which neurons are generated. Most active during pre-natal development, neurogenesis is responsible for populating the growing brain, a subpopulation of ectoderm cells is set aside to become the brain The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all. In vertebrates, the brain is located in the head, protected by the skull and close to the primary, spinal cord The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system. The spinal cord extends down to the space between the first and second lumbar vertebrae; it does not extend the entire length of the vertebral column. It is around 4 and peripheral nerves The Peripheral Nervous System resides or extends outside the central nervous system (CNS), which consists of the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs. Unlike the central nervous system, the PNS is not protected by bone or by the blood-brain barrier, leaving it exposed to toxins and. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including angiogenesis Angiogenesis is the physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over terminology, vasculogenesis is the term used for spontaneous blood-vessel formation, and intussusception is the term for new blood vessel formation by splitting off existing ones (formation of new blood vessels from pre-existing ones), chondrogenesis In embryogenesis, the skeletal system is derived from the mesoderm germ layer. Chondrification is the process by which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondrocytes and begins secreting the molecules that form the extracellular matrix (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.

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.^[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.^[3] "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]

Miscarriage

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.^[4][5] Miscarriages after the sixth week LMP happen in 8% of pregnancies.^[5] 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.^[6]

The most common cause of miscarriage of an embryo is chromosomal abnormality,^[7] which accounts for at least 50% of sampled early pregnancy losses.^[8] Advancing maternal age and a patient history of previous miscarriage are the two leading risk factors.^[8]

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.^[9]

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.^[10]

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.^[11][12] Abortions may also be performed in an instance where rape or incest results in pregnancy.

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 allow an embryo to be transplanted from the uterus of one woman to that of another.^[13]

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 commonly sacrificed.^[14][15][16]

Plant embryos

• 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.

Fossilised embryos

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

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