A human embryo, seven weeks old
or nine weeks' gestational age
An embryo is an early stage of development of a multicellular organism. In general, in organisms that reproduce sexually, embryonic development refers to the portion of the life cycle that begins just after fertilization and continues through the formation of body structures, such as tissues and organs. Each embryo starts development as a zygote, a single cell resulting from the fusion of gametes (i.e. fertilization of a female egg cell by a male sperm cell). In the first stages of embryonic development, a single-celled zygote undergoes many rapid cell divisions, called cleavage, to form a blastula, which looks similar to a ball of cells. Next, the cells in a blastula-stage embryo start rearranging themselves into layers in a process called gastrulation. These layers will each give rise to different parts of the developing multicellular organism, such as the nervous system, connective tissue, and organs.
A newly developing human is typically referred to as an embryo until the ninth week after conception (see human embryogenesis), when it is then referred to as a fetus. In other multicellular organisms, the word "embryo" can be used more broadly to any early developmental or life cycle stage prior to birth or hatching.
First attested in English in the mid-14c., the word embryon derives from Medieval Latin embryo, itself from Greek ? (embruon), lit. "young one", which is the neuter of ? (embruos), lit. "growing in", from (en), "in" and ? (bru?), "swell, be full"; the proper Latinized form of the Greek term would be embryum.
In animals, fertilization begins the process of embryonic development with the creation of a zygote, a single cell resulting from the fusion of gametes (e.g. egg and sperm). The development of a zygote into a multicellular embryo proceeds through a series of recognizable stages, often divided into cleavage, blastula, gastrulation, and organogenesis.
Cleavage is the period of rapid mitotic cell divisions that occur after fertilization. During cleavage, the overall size of the embryo does not change, but the size of individual cells decrease rapidly as they divide to increase the total number of cells. Cleavage results in a blastula.
Depending on the species, a blastula stage embryo can appear as a ball of cells on top of yolk, or as a hollow sphere of cells surrounding a middle cavity. The embryo's cells continue to divide and increase in number, while molecules within the cells such as RNAs and proteins actively promote key developmental processes such as gene expression, cell fate specification, and polarity.
Gastrulation is the next phase of embryonic development, and involves the development of two or more layers of cells (germinal layers). Animals that form two layers (such as Cnidaria) are called diploblastic, and those that form three (most other animals, from flatworms to humans) are called triploblastic. During gastrulation of triploblastic animals, the three germinal layers that form are called the ectoderm, mesoderm, and endoderm. All tissues and organs of a mature animal can trace their origin back to one of these layers. For example, the ectoderm will give rise to the skin epidermis and the nervous system, the mesoderm will give rise to the vascular system, muscles, bone, and connective tissues, and the endoderm will give rise to organs of the digestive system and epithelium of the digestive system and respiratory system. Many visible changes in embryonic structure happen throughout gastrulation as the cells that make up the different germ layers migrate and cause the previously round embryo to fold or invaginate into a cup-like appearance.
Past gastrulation, an embryo continues to develop into a mature multicellular organism by forming structures necessary for life outside of the womb or egg. As the name suggests, organogenesis is the stage of embryonic development when organs form. During organogenesis, molecular and cellular interactions prompt certain populations of cells from the different germ layers to differentiate into organ-specific cell types. For example, in neurogenesis, a subpopulation of cells from the ectoderm segregate from other cells and further specialize to become the brain, spinal cord, or peripheral nerves.
The embryonic period varies from species to species. In human development, the term fetus is used instead of embryo after the ninth week after conception, whereas in zebrafish, embryonic development is considered finished when a bone called the cleithrum becomes visible. In animals that hatch from an egg, such as birds, a young animal is typically no longer referred to as an embryo once it has hatched. In vivaparous animals (animals whose offspring spend at least some time developing within a parent's body), the offspring is typically referred to as an embryo while inside of the parent, and is no longer considered an embryo after birth or exit from the parent. However, the extent of development and growth accomplished while inside of an egg or parent varies significantly from species to species, so much so that the processes that take place after hatching or birth in one species may take place well before those events in another. Therefore, according to one textbook, it is common for scientists interpret the scope of embryology broadly as the study of the development of animals.
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.
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.
Researchers from MERLN Institute and the Hubrecht Institute in the Netherlands managed to grow samples of synthetic rodent embryoids, combining certain types of stem cells. This method may assist scientists to understand 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.
Some embryos do not survive to the next stage of development. When this happens naturally, it is called spontaneous abortion or miscarriage. There are many reasons why this may occur. The most common natural cause of miscarriage is chromosomal abnormality in animals or genetic load in plants.
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.