Definitions

embryo

embryo

[em-bree-oh]
embryo, name for the developing young of an animal or plant. In its widest definition, the embryo is the young from the moment of fertilization until it has become structurally complete and able to survive as a separate organism. Embryology, the scientific study of embryonic development, deals with the period from fertilization until the hatching or birth of an animal or the germination of a plant. However, since the young animal may undergo metamorphosis or may remain wholly dependent on the mother for some time after birth, and since the seedling derives nourishment from food stored in its fleshy cotyledons even after it has sprouted, the exact limit of the time during which an organism is an embryo has not generally been well defined.

Modern embryology, using the techniques of molecular biology, genetics, and other disciplines, has focused on the question of what makes the embryo differentiate (see differentiation), what genetically directed molecular signals tell a single cell to divide and follow the specific pattern of growth and specialization that results in a complex multicellular organism with species-specific and individual characteristics.

Karl Ernst von Baer, who developed the biogenetic law, is generally regarded as the founder of embryology. E. H. Haeckel's "ontogeny recapitulates phylogeny" gave weight to the theory of evolution (see recapitulation). Other researchers in the field of embryology have included C. F. Wolff, M. J. Schleiden, and T. Schwann, developers of the cell theory; F. M. Balfour; H. Spemann; O. Hertwig; F. R. Lillie; and R. Levi-Montalcini.

Developmental Stages

Among humans, the developing young is known as an embryo until eight weeks following conception, after which time it is described, until birth, as a fetus. In organisms that reproduce sexually, the union of the sperm with the ovum results in a zygote, or fertilized egg, which begins a rapid series of cell divisions called cleavage, or segmentation (see mitosis). Each kind of organism has its own characteristic sequence of development, and related species usually have similar developmental patterns.

In a typical animal, cleavage proceeds in the following pattern. Early divisions produce a hollow ball one cell thick, called a blastula, which encloses the blastocoel, or cleavage cavity. The cells divide more rapidly in the area where the nucleus of the ovum was located; this results in an invagination (inpushing) of these cells to form a ball two cells thick (the gastrula). The new cavity thus formed is the gastrocoel, also known as the primitive gut or archenteron, and its opening is the blastopore. The outer layer of cells is called the ectoderm, the inner layer the endoderm. Among the coelenterates (e.g., sponges and jellyfish), these two layers become the chief functional tissues of the adult.

In higher forms of life, a third layer of cells, the mesoderm, develops from one or both of the first two layers and fills the blastocoel, and invagination forms a digestive tract with only a single opening at this early stage. The flatworms (e.g., the tapeworm and the fluke) stop developing at this time. In most organisms, however, a later invagination of the ectoderm results in a gut that is open at both ends. The mesoderm then divides into two layers, the space between them being called the coelom, or body cavity. The embryo now roughly resembles a tube within a tube.

From the three primary germ layers, the organs and tissues develop. In general the ectoderm gives rise to the skin, or integument, the skin appendages (e.g., scales, feathers, hair, and nails), and the nervous system. The endoderm forms the digestive glands, as well as the lining of the alimentary tract and lungs. From the mesoderm develop the major internal organs: the skeletal, muscular, and connective tissue and the circulatory, excretory, and reproductive systems. Sense organs and endocrine glands arise from combinations of all three layers.

Nourishment of the Embryo

In lower animals, which lay their eggs in water, the developing embryo is nourished by yolk, absorbing oxygen from and discharging wastes directly into the water. In terrestrial oviparous forms, the egg contains the yolk and also a surrounding fluid (e.g., the albumen of bird eggs). In mammals, accessory membranes, comprising both embryonic and uterine tissue, develop around the embryo—the amnion, filled with liquid, and the chorion and allantois, which help to form the placenta, through which nourishment and oxygen in the blood of the mother diffuse into the fetus and wastes diffuse back. In the higher plants, the divisions of the fertilized ovum and the differentiation of the tissues to form the embryonic root (hypocotyl), stem (epicotyl), and leaves (cotyledons) occur inside the ovule within the ovary at the base of the pistil. The matured ovule is the seed; the fruit, when it is produced, is the developed ovary.

Early stage of development of an organism in the egg or the uterus, during which its essential form and its organs and tissues develop. In humans, the organism is called an embryo for the first seven or eight weeks after conception, after which it is called a fetus. In mammals, the fertilized egg or zygote undergoes cleavage (cell division without cell growth) to form a hollow ball or blastocyst. During the second week following fertilization, gastrulation (cell differentiation and migration) results in the formation of three tissue types. These three types of tissue develop into different organ systems: the ectoderm develops into the skin and nervous system; the mesoderm develops into connective tissues, the circulatory system, muscles, and bones; and the endoderm develops into the lining of the digestive system, lungs, and urinary system. In humans, by about the fourth week, the head and trunk can be distinguished and the brain, spinal cord, and internal organs begin to develop. By the fifth week, limbs begin to appear and the embryo is about .33 in. (.8 cm) long. By the end of eight weeks, the embryo has grown to about 1 in. (2.5 cm) long and all subsequent change is limited primarily to growth and specialization of existing structures. Any congenital disorders begin in this stage. Seealso pregnancy.

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An embryo (from Greek: ἔμβρυον, plural ἔμβρυα, lit. "that which grows," from en- "in" + bryein "to swell, be full") is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination. In humans, it is called an embryo from the moment of implantation until the end of the 8th week, whereafter it is instead called a fetus.

Development

The development of the embryo is called embryogenesis. In organisms that reproduce sexually, once a sperm fertilizes an egg cell, the result is a cell called the zygote that has all the DNA of two parents. The resulting embryo derives 50 percent of its genetic makeup from each parent. In plants, animals, and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism. The result of this process is an embryo.

In animals

In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of blastula, gastrula, and organogenesis. The blastula stage typically features a fluid-filled cavity, the blastocoel, surrounded by a sphere or sheet of cells, also called blastomeres.

During gastrulation the cells of the blastula undergo coordinated processes of cell division, invasion, and/or migration to form two (diploblastic) or three (triploblastic) tissue layers. In triploblastic organisms, the three germ layers are called endoderm, ectoderm and mesoderm. However, the position and arrangement of the germ layers are highly species-specific, depending on the type of embryo produced. In vertebrates, a special population of embryonic cells called the neural crest has been proposed as a "fourth germ layer", and is thought to have been an important novelty in the evolution of head structures.

During organogenesis, molecular and cellular interactions between germ layers, combined with the cells' developmental potential or competence to respond, prompt the further differentiation of organ-specific cell types. For example, in neurogenesis, a subpopulation of ectoderm cells is set aside to become the brain, spinal cord and peripheral nerves. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including angiogenesis (formation of new blood vessels from pre-existing ones), chondrogenesis (cartilage), myogenesis (muscle), osteogenesis (bone), and many others.

Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny. For example, the backbone is a common structure among all vertebrates such as fish, reptiles and mammals, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, which is the most sophisticated part of the brain, develops last. This rule is not absolute, but it is recognized as being partly applicable to development of the human embryo.

Embryos of plants and animals

  • Plants: 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. Plants that do not produce seeds, but do produce an embryo, include the bryophytes and ferns. In these plants, the embryo is a young plant that grows attached to a parental gametophyte.
  • Animals: The embryo of a placental mammal is defined as the organism between the first division of the zygote (a fertilized ovum) until it becomes a fetus. In humans, the embryo is defined as the product of conception from implantation in the uterus through the eighth week of development. An embryo is called a fetus at a more advanced stage of development and up until birth or hatching. In humans, this is from the eighth week of gestation.

Fossil record

Fossilised embryos are known from the Precambrian, and are found in great number during the Cambrian period.

The human embryo

Growth

Week 1-3 5-7 days after fertilization, the blastula attaches to the wall of the uterus (endometrium). When it comes into contact with the endometrium it performs implantation. Implantation connections between the mother and the embryo will begin to form, including the umbilical cord. The embryo's growth centers around an axis, which will become the spine and spinal cord. The brain, spinal cord, heart, and gastrointestinal tract begin to form.

Week 4-5 Chemicals produced by the embryo stop the woman's menstrual cycle. Neurogenesis is underway, showing brain activity at about the 6th week. The heart will begin to beat around the same time. Limb buds appear where the arms and legs will grow later. Organogenesis begins. The head represents about one half of the embryo's axial length, and more than half of the embryo's mass. The brain develops into five areas. Tissue formation occurs that develops into the vertebra and some other bones. The heart starts to beat and blood starts to flow.

Week 6-8 Myogenesis and neurogenesis have progressed to where the embryo is capable of motion, and the eyes begin to form. Organogenesis and growth continue. Hair has started to form along with all essential organs. Facial features are beginning to develop. At the end of the 8th week, the embryonic stage is over, and the fetal stage begins.

Status

The status of the human embryo is debated by some bioethicists. Some Ethicists believe that an embryo does, in fact, possess personhood. Gilbert Meilaender, Christian ethics professor at the Valparaiso University for example, identifies conception as the point at which a new individual human being comes into existence, since "when sperm and ovum join to form the zygote, the individual's genotype is established. The NIH defines the embryonic stage as the beginning of differentiation, which "leads to the various cell types that make up a human being."

Footnotes

See also


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