Embryo

(redirected from embryoly)
Also found in: Dictionary, Thesaurus, Encyclopedia.
Related to embryoly: embryologist

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

(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

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.

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]