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An embryo is an early stage of of a multicellular organism. In general, in that reproduce sexually, embryonic development refers to the portion of the life cycle that begins just after and continues through the formation of body structures, such as tissues and organs. Each embryo starts development as a , a single cell resulting from the fusion of (i.e. fertilization of a female by a male cell). In the first stages of embryonic development, a single-celled zygote undergoes many rapid cell divisions, called cleavage, to form a , 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 . These layers will each give rise to different parts of the developing multicellular organism, such as the nervous system, connective tissue, and .

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 . In other multicellular organisms, the word “embryo” can be used more broadly to any early developmental or life cycle stage prior to or .


Etymology
First attested in English in the mid-14c., the word embryon derives from embryo, itself from ἔμβρυον ( embruon), lit. "young one", ἔμβρυον , Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus which is the neuter of ἔμβρυος ( embruos), lit. "growing in", ἔμβρυος , Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus from ἐν ( en), "in" ἐν , Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus and βρύω ( bruō), "swell, be full"; βρύω , Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus the proper Latinized form of the Greek term would be embryum.


Development

Animal embryos
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.

(1975). 072161518X, W.B. Saunders Company. 072161518X
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 ) are called diploblastic, and those that form three (most other animals, from to humans) are called triploblastic. During gastrulation of triploblastic animals, the three germinal layers that form are called the , , and . 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 , embryonic development is considered finished when a bone called the 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 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 broadly as the study of the development of animals.


Plant embryos
Flowering plants () create embryos after the fertilization of a haploid by . The DNA from the ovule and pollen combine to form a diploid, single-cell zygote that will develop into an embryo. The zygote, which will divide multiple times as it progresses throughout embryonic development, is one part of a . Other seed components include the , which is tissue rich in nutrients that will help support the growing plant embryo, and the seed coat, which is a protective outer covering. The first cell division of a zygote is asymmetric, resulting in an embryo with one small cell (the apical cell) and one large cell (the basal cell). The small, apical cell will eventually give rise to most of the structures of the mature plant, such as the stem, leaves, and roots. The larger basal cell will give rise to the suspensor, which connects the embryo to the endosperm so that nutrients can pass between them. The plant embryo cells continue to divide and progress through developmental stages named for their general appearance: globular, heart, and torpedo. In the globular stage, three basic tissue types (dermal, ground, and vascular) can be recognized. The dermal tissue will give rise to the epidermis or outer covering of a plant, ground tissue will give rise to inner plant material that functions in , resource storage, and physical support, and vascular tissue will give rise to connective tissue like the and that transport fluid, nutrients, and minerals throughout the plant. In heart stage, one or two (embryonic leaves) will form. (centers of activity) develop during the torpedo stage, and will eventually produce many of the mature tissues of the adult plant throughout its life. At the end of embryonic growth, the seed will usually go dormant until germination. Once the embryo begins to (grow out from the seed) and forms its first true leaf, it is called a or plantlet.

Plants that produce instead of seeds, like and , also produce embryos. In these plants, the embryo begins its existence attached to the inside of the on a parental 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.

Since all land plants create embryos, they are collectively referred to as (or by their scientific name, Embryophyta). This, along with other characteristics, distinguishes land plants from other types of plants, such as , which do not produce embryos.


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 research, reproductive , and germline engineering are currently being explored. 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.

In regards to research using human embryos, the ethics and legalities of this application continue to be debated.Freedman, Jeri. "America Debates Stem Cell Research." New York, NY: The Rosen Publishing Group, 2008.Sandel, Michael J. "The Case Against Perfection." Michael J. Sandel, 2007.Zavos, Panayiotis. "Reproductive Cloning is Moral." Ed. James Woodward. The Ethics of Human Cloning: At Issue. Farmington Hills, MI: Greenhaven, 2005. 14–24.

Researchers from MERLN Institute and the Hubrecht Institute in the Netherlands managed to grow samples of synthetic rodent embryoids, combining certain types of . 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 and other . "Scientists grow synthetic mouse embryos from stem cells" New Atlas, May 4, 2018


Fossilized embryos
Fossilized animal embryos are known from the , and are found in great numbers during the period. Even fossilized embryos have been discovered.


Miscarriage and abortion
Some embryos do not survive to the next stage of development. When this happens naturally, it is called spontaneous abortion or miscarriage.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.


See also
  • In vitro fertilisation
  • Human embryonic development
  • Plant embryogenesis
  • Prenatal development


Notes

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