Embryo

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.

Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

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]

Farlex Partner Medical Dictionary © Farlex 2012

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.

The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.

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 8^th wk of pregnancy. See Preembryo. Cf Fetus.

McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.

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]

Medical Dictionary for the Health Professions and Nursing © Farlex 2012

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.

Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005

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).

Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005

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.

Mentioned in: Birth Defects, Childbirth, Echinococcosis, Ectopic Pregnancy, Gene Therapy, Human Chorionic Gonadotropin Pregnancy Test, Hypogonadism, Infertility, Miscarriage, Pregnancy, Puberty, Stem Cell Transplantation

Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.

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]

Medical Dictionary for the Dental Professions © Farlex 2012