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Embryo 7 weeks after conception.jpg
A human embryo, seven weeks old
or nine weeks' gestational age
MeSH D004622
TE E1.
FMA 69068
Anatomical terminology

An embryo is an early stage of development of a multicellular diploid eukaryotic organism. In general, in organisms that reproduce sexually, an embryo develops from a zygote, the single cell resulting from the fertilization of the female egg cell by the male sperm cell. The zygote possesses half the DNA from 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.

Multicellular organism Organism that consists of more than one cell

Multicellular organisms are organisms that consist of more than one cell, in contrast to unicellular organisms.

Ploidy Number of sets of chromosomes in a cell

Ploidy is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes.

Eukaryote taxonomic group whose members have complex structures enclosed within membranes

Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, unlike prokaryotes, which have no membrane-bound organelles. Eukaryotes belong to the domain Eukaryota or Eukarya. Their name comes from the Greek εὖ and κάρυον. Eukaryotic cells also contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, and in addition, some cells of plants and algae contain chloroplasts. Unlike unicellular archaea and bacteria, eukaryotes may also be multicellular and include organisms consisting of many cell types forming different kinds of tissue. Animals and plants are the most familiar eukaryotes.


In human pregnancy, a developing fetus is considered as an embryo until the ninth week, fertilization age, or eleventh-week gestational age. After this time the embryo is referred to as a fetus. [1]

Pregnancy time when children develop inside the mothers body before birth

Pregnancy, also known as gestation, is the time during which one or more offspring develops inside a woman. A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy can occur by sexual intercourse or assisted reproductive technology. Childbirth typically occurs around 40 weeks from the last menstrual period (LMP). This is just over nine months, where each month averages 31 days. When measured from fertilization it is about 38 weeks. An embryo is the developing offspring during the first eight weeks following fertilization, after which, the term fetus is used until birth. Symptoms of early pregnancy may include missed periods, tender breasts, nausea and vomiting, hunger, and frequent urination. Pregnancy may be confirmed with a pregnancy test.

Prenatal development includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal development until birth.

Gestational age is a measure of the age of a pregnancy which is taken from the woman's last menstrual period (LMP), or the corresponding age of the gestation as estimated by a more accurate method if available. Such methods include adding 14 days to a known duration since fertilization, or by obstetric ultrasonography. The popularity of using such a definition of gestational age is that menstrual periods are essentially always noticed, while there is usually a lack of a convenient way to discern when fertilization occurred.


First attested in English in the mid-14c., the word embryon derives from Medieval Latin embryo, itself from Greek ἔμβρυον (embruon), lit. "young one", [2] which is the neuter of ἔμβρυος (embruos), lit. "growing in", [3] from ἐν (en), "in" [4] and βρύω (bruō), "swell, be full"; [5] the proper Latinized form of the Greek term would be embryum.

Medieval Latin form of Latin used in the Middle Ages

Medieval Latin was the form of Latin used in Roman Catholic Western Europe during the Middle Ages. In this region it served as the primary written language, though local languages were also written to varying degrees. Latin functioned as the main medium of scholarly exchange, as the liturgical language of the Church, and as the working language of science, literature, law, and administration.

Greek language language spoken in Greece, Cyprus and Southern Albania

Greek is an independent branch of the Indo-European family of languages, native to Greece, Cyprus and other parts of the Eastern Mediterranean and the Black Sea. It has the longest documented history of any living Indo-European language, spanning more than 3000 years of written records. Its writing system has been the Greek alphabet for the major part of its history; other systems, such as Linear B and the Cypriot syllabary, were used previously. The alphabet arose from the Phoenician script and was in turn the basis of the Latin, Cyrillic, Armenian, Coptic, Gothic, and many other writing systems.

Latinisation of names, also known as onomastic Latinisation, is the practice of rendering a non-Latin name in a Latin style. It is commonly found with historical proper names, including personal names, and toponyms, and in the standard binomial nomenclature of the life sciences. It goes further than romanisation, which is the transliteration of a word to the Latin alphabet from another script.


Embryonic development of salamander, circa the 1920s
Embryos (and one tadpole) of the wrinkled frog (Rana rugosa) Wrinkledfrog embryos.jpg
Embryos (and one tadpole) of the wrinkled frog (Rana rugosa)

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. In a placental mammal, 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 eleventh 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 later stages.

Blastula embryogenesis

The blastula is a hollow sphere of cells, referred to as blastomeres, surrounding an inner fluid-filled cavity called the blastocoele formed during an early stage of embryonic development in animals. Embryo development begins with a sperm fertilizing an egg to become a zygote which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoele is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.

Gastrulation Phase in the early embryonic development of most animals.

Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a multilayered structure known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body, and internalized one or more cell types including the prospective gut.

Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation: the ectoderm, endoderm, and mesoderm form the internal organs of the organism.

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. 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.

Diploblasty is a condition of the blastula in which there are two primary germ layers: the ectoderm and endoderm.

Triploblasty is a condition of the blastula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. A fourth "layer" consists of the germ cells that are set aside in the embryo at the blastula stage, which are incorporated into the gonads during organogenesis. 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.

A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans produce two or three primary germ layers. Some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as chordates produce a third layer, between these two layers. making them triploblastic. Germ layers eventually give rise to all of an animal’s tissues and organs through the process of organogenesis.

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.[ citation needed ] 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.

Neurogenesis is the process by which nervous system cells, known as neurons, are produced by neural stem cells (NSC)s, and it occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs), radial glial cells (RGCs), basal progenitors (BPs), intermediate neuronal precursors (INP)s, subventricular zone astrocytes, and subgranular zone radial astrocytes, among others. Neurogenesis is most active during embryonic development, and is responsible for producing all the various types of neurons of the organism, but continues throughout adult life in a variety of organisms. Once born, neurons do not divide, and many will live the lifetime of the animal.

Brain organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals

The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head, usually close to the sensory organs for senses such as vision. The brain is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains approximately 14–16 billion neurons, and the estimated number of neurons in the cerebellum is 55–70 billion. Each neuron is connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells.

Plant embryos

The inside of a Ginkgo seed, showing the embryo Ginkgo embryo and gametophyte.jpg
The inside of a Ginkgo seed, showing the embryo

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 also produce an embryo, but do not produce seeds. In these plants, the embryo begins its existence attached to the inside of the archegonium on a parental gametophyte from which the egg cell was generated. The inner wall of the archegonium lies in close contact with the "foot" of the developing embryo; this "foot" consists of a bulbous mass of cells at the base of the embryo which may receive nutrition from its parent gametophyte. The structure and development of the rest of the embryo varies by group of plants. As the embryo has expanded beyond the enclosing archegonium, it is no longer termed an embryo.

Research and technology

Embryos are used in various fields of research and in techniques of assisted reproductive technology. An egg may be fertilized in vitro and the resulting embryo may be frozen for later use. The potential of embryonic stem cell research, reproductive cloning, and germline engineering are currently being explored. Prenatal diagnosis or preimplantation diagnosis enables testing embryos for diseases or conditions.

Cryoconservation of animal genetic resources is a practice in which animal germplasms, such as embryos are collected and stored at low temperatures with the intent of conserving the genetic material.

The embryos of Arabidopsis thaliana have been used as a model to understand gene activation, patterning, and organogenesis of seed plants. [6]

In regards to research using human embryos, the ethics and legalities of this application continue to be debated. [7] [8] [9]

Researchers from MERLN Institute and the Hubrecht Institute in the Netherlands managed to grow samples of synthetic rodent embryos, combining certain types of stem cells. This method will help scientists to more deeply study the very first moments of the process of the birth of a new life, which, in turn, can lead to the emergence of new effective methods to combat infertility and other genetic diseases. [10]

Fossilized embryos

Fossilized animal embryos are known from the Precambrian, and are found in great numbers during the Cambrian period. Even fossilized dinosaur embryos have been discovered. [11]

Miscarriage and abortion

A complete spontaneous abortion at about 6 weeks from conception, i.e. 8 weeks from LMP Human Embryo - Approximately 8 weeks estimated gestational age.jpg
A complete spontaneous abortion at about 6 weeks from conception, i.e. 8 weeks from LMP

Some embryos do not survive to the next stage of development. When this happens naturally, it is called spontaneous abortion or miscarriage. [12] There are many reasons why this may occur. The most common natural cause of miscarriage is chromosomal abnormality in animals [13] or genetic load in plants. [14]

In species which produce multiple embryos at the same time, miscarriage or abortion of some embryos can provide the remaining embryos with a greater share of maternal resources. This can also disturb the pregnancy, causing harm to the second embryo. Genetic strains which miscarry their embryos are the source of commercial seedless fruits.

Abortion is the process of artificially (non-naturally) removing the embryo through deliberate pharmaceutical or surgical methods.

See also


  1. "embryo". Mayo. Archived from the original on 20 February 2016. Retrieved 21 February 2016.
  2. ἔμβρυον Archived 2013-05-31 at the Wayback Machine , Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  3. ἔμβρυος Archived 2013-05-31 at the Wayback Machine , Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  4. ἐν Archived 2013-05-31 at the Wayback Machine , Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  5. βρύω Archived 2013-05-31 at the Wayback Machine , Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  6. Boscá, S; Knauer, S; Laux, T (2011). "Embryonic development in Arabidopsis thaliana: from the zygote division to the shoot meristem". Frontiers in Plant Science. 2: 93. doi:10.3389/fpls.2011.00093. PMC   3355580 . PMID   22639618.
  7. Freedman, Jeri. "America Debates Stem Cell Research." New York, NY: The Rosen Publishing Group, 2008.
  8. Sandel, Michael J. "The Case Against Perfection." Michael J. Sandel, 2007.
  9. Zavos, Panayiotis. "Reproductive Cloning is Moral." Ed. James Woodward. The Ethics of Human Cloning: At Issue. Farmington Hills, MI: Greenhaven, 2005. 14–24.
  10. "Scientists grow synthetic mouse embryos from stem cells" Archived 2018-05-08 at the Wayback Machine New Atlas, May 4, 2018
  11. Morelle, Rebecca. "Dinosaur embryo fossils reveal life inside the egg". BBC News. Archived from the original on 24 September 2015. Retrieved 8 August 2015.
  12. miscarriage "Archived copy". Archived from the original on 2016-05-12. Retrieved 2016-05-15.CS1 maint: Archived copy as title (link)
  13. Conrad Stöppler, Melissa. Shiel Jr., William C., ed. "Miscarriage (Spontaneous Abortion)". MedicineNet.com. Archived from the original on 2004-08-29. Retrieved 2009-04-07.
  14. Kärkkäinen, Katri; Savolainen, Outi; Koski, Veikko (1999). "Why do plants abort so many developing seeds: bad offspring or bad maternal genotypes?". Evolutionary Ecology. 13 (3): 305–317. doi:10.1023/A:1006746900736.
Preceded by
Animal development
Succeeded by
Fetus, Hatchling, Larva

Related Research Articles

Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of regeneration, asexual reproduction, metamorphosis, and the growth and differentiation of stem cells in the adult organism.

Gamete haploid cell that fuses with another haploid cell during fertilization (conception) in organisms that sexually reproduce

A gamete is a haploid cell that fuses with another haploid cell during fertilization (conception) in organisms that sexually reproduce. In species that produce two morphologically distinct types of gametes, and in which each individual produces only one type, a female is any individual that produces the larger type of gamete—called an ovum —and a male produces the smaller tadpole-like type—called a sperm. In short a gamete is an egg or a sperm. This is an example of anisogamy or heterogamy, the condition in which females and males produce gametes of different sizes. In contrast, isogamy is the state of gametes from both sexes being the same size and shape, and given arbitrary designators for mating type. The name gamete was introduced by the Austrian biologist Gregor Mendel. Gametes carry half the genetic information of an individual, one ploidy of each type, and are created through meiosis.

Zygote single diploid eukaryotic cell formed by a fertilization event between two gametes

A zygote is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information necessary to form a new individual. In multicellular organisms, the zygote is the earliest developmental stage. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring.

Fertilisation union of gametes of opposite sexes during the process of sexual reproduction to form a zygot

Fertilisation or fertilization, also known as generative fertilisation, insemination, pollination, fecundation, syngamy and impregnation, is the fusion of gametes to initiate the development of a new individual organism or offspring. This cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms the haploid male gamete combines with two haploid polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation.

Human development is the process of growth to maturity. The process begins with fertilisation, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The egg then lodges in the uterus, where an embryo and later fetus develop until birth. Further growth and development continues after birth, and includes both physical and psychological development, influenced by genetic, hormonal, environmental and other factors. This continues throughout life, through childhood, and adolescence into adulthood.

Chimera (genetics) single organism composed of two or more different populations of genetically distinct cells

A genetic chimerism or chimera (/kɪˈmɪərə/ ky-MEER or /kaɪˈmɪərə/ kə-MEER, also chimaera is a single organism composed of cells with distinct genotypes. In animals, this means an individual derived from two or more zygotes, which can include possessing blood cells of different blood types, subtle variations in form and, if the zygotes were of differing sexes, then even the possession of both female and male sex organs. Animal chimeras are produced by the merger of multiple fertilized eggs. In plant chimeras, however, the distinct types of tissue may originate from the same zygote, and the difference is often due to mutation during ordinary cell division. Normally, genetic chimerism is not visible on casual inspection; however, it has been detected in the course of proving parentage.

Embryology branch of biology studying prenatal biology

Embryology is the branch of biology that studies the prenatal development of gametes, fertilization, and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology.

Egg cell haploid female reproductive cell or gamete

The egg cell, or ovum, is the female reproductive cell (gamete) in oogamous organisms. The egg cell is typically not capable of active movement, and it is much larger than the motile sperm cells. When egg and sperm fuse, a diploid cell is formed, which rapidly grows into a new organism.


Ectoderm is one of the three primary germ layers in the very early embryo. The other two layers are the mesoderm and endoderm, with the ectoderm as the most exterior layer. It emerges and originates from the outer layer of germ cells. The word ectoderm comes from the Greek ektos meaning "outside", and derma, meaning "skin."

Sporophyte diploid multicellular stage in the life cycle of a plant or alga

A sporophyte is the diploid multicellular stage in the life cycle of a plant or alga. It develops from the zygote produced when a haploid egg cell is fertilized by a haploid sperm and each sporophyte cell therefore has a double set of chromosomes, one set from each parent. All land plants, and most multicellular algae, have life cycles in which a multicellular diploid sporophyte phase alternates with a multicellular haploid gametophyte phase. In the seed plants, (gymnosperms) and flowering plants (angiosperms), the sporophyte phase is more prominent than the gametophyte, and is the familiar green plant with its roots, stem, leaves and cones or flowers. In flowering plants the gametophytes are very reduced in size, and are represented by the germinated pollen and the embryo sac.


The blastocyst is a structure formed in the early development of mammals. It possesses an inner cell mass (ICM) which subsequently forms the embryo. The outer layer of the blastocyst consists of cells collectively called the trophoblast. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. The trophoblast gives rise to the placenta. The name "blastocyst" arises from the Greek βλαστός blastos and κύστις kystis.


A blastocoel is a fluid-filled cavity that forms in the animal hemisphere of early amphibian and echinoderm embryos, or between the epiblast and hypoblast of avian, reptilian, and mammalian blastoderm-stage embryos.

Embryonic development also embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.

Human Fertilisation and Embryology Act 1990

The Human Fertilisation and Embryology Act 1990 is an Act of the Parliament of the United Kingdom. It created the Human Fertilisation and Embryology Authority which is in charge of human embryo research, along with monitoring and licensing fertility clinics in the United Kingdom.

Human embryonic development process of cell division and cellular differentiation of the embryo that occurs during the early stages of development

Human embryonic development, or human embryogenesis, refers to the development and formation of the human embryo. It is characterised by the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the human body entails growth from a one-celled zygote to an adult human being. Fertilisation occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of development commences. Embryonic development in the human, covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus. Human embryology is the study of this development during the first eight weeks after fertilisation. The normal period of gestation (pregnancy) is nine months or 38 weeks.

Beginning of human personhood opinion as to the precise time when human personhood begins

The beginning of human personhood is the moment when a human is first recognized as a person. There are differences of opinion as to the precise time when human personhood begins and the nature of that status. The issue arises in a number of fields including science, religion, philosophy, and law, and is most acute in debates relating to abortion, stem cell research, reproductive rights, and fetal rights.