embryo

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embryo

 [em´bre-o]
a new organism in the earliest stage of development. In humans this is defined as the developing organism from the fourth day after fertilization to the end of the eighth week. After that the unborn baby is usually referred to as the fetus. adj., adj em´bryonal, embryon´ic.

Immediately after fertilization takes place, cell division begins and progresses at a rapid rate. At approximately 4 weeks the cell mass becomes a recognizable embryo from 7 to 10 mm long with rudimentary organs. The beginnings of the eyes, ears, and extremities can be seen. By the end of the second month the embryo has grown to a length of 2 to 2.5 cm, and the head is the most prominent part because of the rapid development of the brain; the sex can be distinguished at this stage.

At the time of fertilization the ovum contains the potential beginnings of a human being. As cell division takes place the cells of the blastoderm (embryonic disk) gradually form three layers from which all the body structures develop. The ectoderm (outer layer) gives rise to the epidermis of the skin and its appendages, and to the nervous system. The mesoderm (middle layer) develops into muscle, connective tissue, the circulatory organs, circulating lymph and blood cells, endothelial tissues within the closed vessels and cavities, and the epithelium portion of the urogenital system. From the endoderm (internal layer) are derived those portions not arising from the ectoderm, the liver, the pancreas, and the lungs.
Embryonic development from 3 weeks through the eighth week after fertilization. CRL is crown-to-rump length. From McKinney et al., 2000.

em·bry·o

(em'brē-ō),
1. An organism in the early stages of development.
2. In humans, the developing organism from conception until the end of the eighth month; developmental stages from this time to birth are commonly designated as fetal.
3. A primordial plant within a seed.
[G. embryon, fr. en, in, + bryō, to be full, swell]

embryo

/em·bryo/ (em´bre-o)
1. in animals, those derivatives of the zygote that eventually become the offspring, during their period of most rapid growth, i.e., from the time the long axis appears until all major structures are represented.
2. in humans, the developing organism from fertilization to the end of the eighth week. Cf. fetus.
3. in plants, the element of the seed that develops into a new individual.em´bryonalembryon´ic

presomite embryo  the embryo at any stage before the appearance of the first somite.
previllous embryo  the embryo before the placental chorionic villi develop.
somite embryo  the embryo between the appearance of the first and the last somites.

embryo

(ĕm′brē-ō′)
n. pl. embry·os
1.
a. The collection of cells that has developed from the fertilized egg of a vertebrate animal, before all the major organs have developed.
b. A collection of such cells of a human, especially from implantation in the uterine wall through the eighth week of development.
2. Botany The young sporophytic plant contained within a seed or an archegonium.
3. An organism at any time before full development, birth, or hatching.
4. A rudimentary or beginning stage: an idea that was the embryo of a short story.

embryo

[em′brē·ō]
Etymology: Gk, en, in, bryein, to grow
1 any organism in the earliest stages of development.
2 in humans the stage of prenatal development from the time of fertilization of the ovum (conception) until the end of the eighth week. The period is characterized by rapid growth, differentiation of the major organ systems, and development of the main external features. Compare fetus, zygote. embryonal, embryonic, embryonoid, adj.
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Embryo: end of fourth week

embryo

An early stages of a developing organism, which follows fertilization of an egg including implantation and very early pregnancy–ie, from conception to the 8th wk of pregnancy. See Preembryo. Cf Fetus.

em·bry·o

(em'brē-ō)
1. An organism in the early stages of development.
2. In humans, the developing organism from conception until the end of the eighth week; developmental stages from this time to birth are commonly designated as fetal.
3. A primordial plant within a seed.
[G. embryon, fr. en, in, + bryō, to be full, swell]

embryo

(em′brē-ō″) [Gr. embryon, growing inside]
1. The young of any organism in an early stage of development.
Enlarge picture
STAGES OF DEVELOPMENT OF HUMAN EMBRYO INCLUDING MATURE FETUS
Enlarge picture
STAGES OF DEVELOPMENT OF HUMAN EMBRYO INCLUDING MATURE FETUS
Enlarge picture
STAGES OF DEVELOPMENT OF HUMAN EMBRYO INCLUDING MATURE FETUS
2. In mammals, the stage of prenatal development between fertilized ovum and fetus. See: tableillustration

Development

First week after fertilization: The zygote begins a series of mitotic divisions called cleavage and forms a morula, a solid sphere of cells. The morula develops into a blastocyst, which has an outer trophoblast and an inner cell mass. The trophoblast gives rise to the chorion, and after implantation in the uterus, becomes the fetal placenta. Second week: The amniotic cavity and yolk sac form within the inner cell mass; they are separated by the embryonic disk, which at this time consists of ectoderm and endoderm. Third week: Mesoderm develops between ectoderm and endoderm; all three germ layers are established.

The epithelium of the alimentary canal, liver, pancreas, and lungs develops from endoderm. Muscle, all connective tissues, blood, lymphatic tissue, and the epithelium of blood vessels, body cavities, kidneys, gonads, and suprarenal cortex develop from mesoderm. The epidermis, nervous tissue, hypophysis, and the epithelium of the nasal cavity, mouth, salivary glands, bladder, and urethra develop from ectoderm.

Embryo (Third through eighth weeks): The embryo increases in length from about 1.5 mm to 23 mm. The organ systems develop and the embryo begins to show human form. During this period of organogenesis, the embryo is particularly sensitive to the effects of viral infections of the mother, e.g., rubella, and toxic chemicals, including alcohol and tobacco smoke, and is sensitive to hypoxemia.

EctodermMesodermEndoderm
Nervous tissueBone, cartilage, and other connective tissuesEpithelium of respiratory tract except nose; digestive tract except mouth and anal canal; bladder except trigone
Sense organsMale and female reproductive tractsProximal portion of male urethra
Epidermis, nails, and hair folliclesHeart, blood vessels, and lymphaticsFemale urethra
Epithelium of external and internal ear, nasal cavity and sinuses, mouth, anal canalKidneys, ureters, trigone of bladderLiver
Distal portion of male urethraPleura, peritoneum, and pericardiumPancreas
Skeletal muscle

embryo

An organism in its earliest stages of development, especially before it has reached a stage at which it can be distinguished from other species. The human embryo is so called up to the eighth week after fertilization. After that it is called a fetus.

embryo

  1. (in animals), the stage immediately after the beginning of CLEAVAGE up to the time when the developing animal hatches, or breaks out of egg membranes, or in higher animals, is born. In humans the developing embryo is called a FOETUS after eight weeks of gestation.
  2. (in plants), the partly developed SPOROPHYTE, which in ANGIOSPERMS is protected within a seed. At one end of the embryo axis is the RADICLE or ROOT, and at the other the apical MERISTEM, or PLUMULE in some forms, and one or two young leaves (COTYLEDONS).

Embryo

In humans, the developing individual from the time of implantation to about the end of the second month after conception. From the third month to the point of delivery, the individual is called a fetus.

embryo

the developing child from conception until 8 weeks in utero, after which it is considered as a fetus

embryo,

n the fertilized ovum, while in its primary developmental stage, two to eight weeks after implantation in the mother's womb.

em·bry·o

(em'brē-ō)
In humans, developing organism from conception until end of the eighth week; developmental stages from this time to birth are commonly designated as fetal.
[G. embryon, fr. en, in, + bryō, to be full, swell]

embryo

(em´brēō),
n an organism in the earliest stages of development; the stage between the time of implantation of the fertilized ovum until the end of the seventh or eighth week of gestation.

embryo

a new organism in the earliest stage of development, i.e. from the time that the fertilized embryo begins to develop a long axis up to the time that the major structures have begun to develop, when it becomes a fetus.

embryo collection
collection of an embryo from the genital tract for the purposes of embryo transfer; surgical and nonsurgical techniques available.
embryo cryopreservation
preservation of embryos by freezing.
hexacanth embryo
the larva with six hooks present in the cestode egg when it escapes from the uterus of the adult tapeworm. Called also oncosphere.
embryo micromanipulation
handling of an embryo under a microscope, for examination, dissection.
embryo transfer
collection of fertilized ova from one female before they become implanted and transfer to another female to complete the gestation. The donor is usually superovulated and then inseminated. Collection may be surgical via a laparotomy or nonsurgical by flushing through the cervix. Collected embryos must be stored carefully. They are evaluated in terms of fertilization, possibly cleaved artificially to create clones, and washed to eliminate the possibility of transferring infection with the embryo. Long-term storage by freezing is a practicable procedure. The recipient needs to be in appropriate stage of uterine receptivity, effected by synchronizing the estrus cycle with that of the donor.
embryo transplant
see embryo transfer (above).