By Dr Ananya Mandal, MD

Embryology is the study of development of an embryo from the stage of ovum fertilization through to the fetal stage.

The ball of dividing cells that results after fertilization is termed an embryo for eight weeks and from nine weeks after fertilization, the term used is fetus.

Once an egg is released from the ovary during ovulation, it meets with a sperm cell that was carried to it via the semen. These two gametes combine to form a zygote and this process is called fertilization. The zygote then begins to divide and becomes a blastula.

The blastula develops in one of two ways, which actually divides the whole animal kingdom in half. The blastula develops a pore at one end, called a blastopore. If that blastopore becomes the mouth of the animal, the animal is a protostome, and if it forms an anus, the animal is a deuterostome.

Protosomes are invertebrate animals such as worms, insects and molluscs while deuterostomes are vertebrates such as birds, reptiles, and humans.

The blastula continues to develop, eventually forming a structure called the gastrula. The gastrula then forms three germ cell layers, from which all of the bodys organs and tissues are eventually derived. From the innermost layer or endoderm, the digestive organs, lungs and bladder develop; the skeleton, blood vessels and muscles are derived from the middle layer or mesoderm and the outer layer or ectoderm gives rise to the nervous system, skin and hair.

Reviewed by Sally Robertson, BSc

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What is Embryology?

British Dictionary definitions for embryology Expand

the branch of science concerned with the study of embryos

the structure and development of the embryo of a particular organism

Derived Forms

embryological (mbrldkl), embryologic, adjectiveembryologically, adverbembryologist, noun

Word Origin and History for embryology Expand

1859, from embryon (see embryo) + -logy. Related: Embryologist (c.1850).

embryology in Medicine Expand

embryology embryology (m'br-l'-j) n.

The branch of biology that deals with the formation, early growth, and development of living organisms.

The embryonic structure or development of an organism.

embryology in Science Expand

embryology in Culture Expand

The study of the embryo; a major field of research in modern biology.

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Embryology | Define Embryology at Dictionary.com

This article is about the development of embryos in animals. For the development of plant embryos, see Sporophyte.

Embryology (from Greek , embryon, "the unborn, embryo"; and -, -logia) is the branch of biology that studies the development of gametes (sex cells), fertilization, and development of embryos and fetuses. Additionally, embryology is the study of congenital disorders that occur before birth.[1]

After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end.

In bilateral animals, the blastula develops in one of two ways that divides the whole animal kingdom into two halves (see: Embryological origins of the mouth and anus). If in the blastula the first pore (blastopore) becomes the mouth of the animal, it is a protostome; if the first pore becomes the anus then it is a deuterostome. The protostomes include most invertebrate animals, such as insects, worms and molluscs, while the deuterostomes include the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula.

The gastrula with its blastopore soon develops three distinct layers of cells (the germ layers) from which all the bodily organs and tissues then develop:

Embryos in many species often appear similar to one another in early developmental stages. The reason for this similarity is because species have a shared evolutionary history. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism, having evolved from a common ancestor.

Click here to read the main article on Drosophila embryogenesis

Drosophila melanogaster, a fruit fly, is a model organism in biology on which much research into embryology has been done (see figure 1.1.1A and figure 1.1.1B).[2] Before fertilization, the female gamete produces an abundance of mRNA - transcribed from the genes that encode bicoid protein and nanos protein.[3][4] These mRNA molecules are stored to be used later in what will become a developing embryo. The male and female Drosophila gametes exhibit anisogamy (differences in morphology and sub-cellular biochemistry). The female gamete is larger than the male gamete because it harbors more cytoplasm and, within the cytoplasm, the female gamete contains an abundance of the mRNA previously mentioned.[5][6] At fertilization, the male and female gametes fuse (plasmogamy) and then the nucleus of the male gamete fuses with the nucleus of the female gamete (karyogamy). Note that before the gametes' nuclei fuse, they are known as pronuclei.[7] A series of nuclear divisions will occur without cytokinesis (division of the cell) in the zygote to form a multi-nucleated cell (a cell containing multiple nuclei) known as a syncytium.[8][9] All the nuclei in the syncytium are identical, just as all the nuclei in every somatic cell of any multicellular organism are identical in terms of the DNA sequence of the genome.[10] Before the nuclei can differentiate in transcriptional activity, the embryo (syncytium) must be divided into segments. In each segment, a unique set of regulatory proteins will cause specific genes in the nuclei to be transcribed. The resulting combination of proteins will transform clusters of cells into early embryo tissues that will each develop into multiple fetal and adult tissues later in development (note: this happens after each nucleus becomes wrapped with its own cell membrane).

Outlined below is the process that leads to cell and tissue differentiation.

Maternal-effect genes - subject to Maternal (cytoplasmic) inheritance.

Zygotic-effect genes - subject to Mendelian (classical) inheritance.

Humans are bilaterals and deuterostomes.

In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception (tenth week of pregnancy), the developing human is then called a fetus.

As recently as the 18th century, the prevailing notion in western human embryology was preformation: the idea that semen contains an embryo a preformed, miniature infant, or homunculus that simply becomes larger during development. The competing explanation of embryonic development was epigenesis, originally proposed 2,000 years earlier by Aristotle. Much early embryology came from the work of the Italian anatomists Aldrovandi, Aranzio, Leonardo da Vinci, Marcello Malpighi, Gabriele Falloppio, Girolamo Cardano, Emilio Parisano, Fortunio Liceti, Stefano Lorenzini, Spallanzani, Enrico Sertoli, and Mauro Rusconi.[22] According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favoured explanation among embryologists.[23]

Karl Ernst von Baer and Heinz Christian Pander proposed the germ layer theory of development; von Baer discovered the mammalian ovum in 1827.[24][25][26] Modern embryological pioneers include Charles Darwin, Ernst Haeckel, J.B.S. Haldane, and Joseph Needham. Other important contributors include William Harvey, Kaspar Friedrich Wolff, Heinz Christian Pander, August Weismann, Gavin de Beer, Ernest Everett Just, and Edward B. Lewis.

After the 1950s, with the DNA helical structure being unravelled and the increasing knowledge in the field of molecular biology, developmental biology emerged as a field of study which attempts to correlate the genes with morphological change, and so tries to determine which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulated.

Many principles of embryology apply to invertebrates as well as to vertebrates.[27] Therefore, the study of invertebrate embryology has advanced the study of vertebrate embryology. However, there are many differences as well. For example, numerous invertebrate species release a larva before development is complete; at the end of the larval period, an animal for the first time comes to resemble an adult similar to its parent or parents. Although invertebrate embryology is similar in some ways for different invertebrate animals, there are also countless variations. For instance, while spiders proceed directly from egg to adult form, many insects develop through at least one larval stage.

Currently, embryology has become an important research area for studying the genetic control of the development process (e.g. morphogens), its link to cell signalling, its importance for the study of certain diseases and mutations, and in links to stem cell research.

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Embryology - Wikipedia, the free encyclopedia

embryology [embre-olo-je]

the science of the development of the individual during the embryonic stage and, by extension, in several or even all preceding and subsequent stages of the life cycle. adj., adj embryologic.

Science of the origin and development of the organism from fertilization of the oocyte to the end of the eighth week. Usually used to include all stages of prenatal life.

[embryo- + G. logos, study]

1. The branch of biology that deals with the formation, early growth, and development of living organisms.

2. The embryonic structure or development of an organism.

Etymology: Gk, en, bryein + logos, science

Science of the origin and development of the organism from fertilization of the oocyte to the end of the eighth week and, by extension, all subsequent stages up to birth.

[embryo- + G. logos, study]

Science of the origin and development of the organism from fertilization of the oocyte to the end of the eighth week.

[embryo- + G. logos, study]

(embrolj), n the study of the origin, growth, development, and function of an organism from fertilization to birth.

the science of the development of the individual animal during the embryonic stage and, by extension, in several or even all preceding and subsequent stages of the life cycle.

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Embryology | definition of embryology by Medical dictionary

Embryology is the branch of developmental biology that studies embryos and their development. The field of developmental biology encompasses the overall study of the process by which organisms grow and develop, including cell growth, cellular differentiation, and, "morphogenesis," which is the process that gives rise to tissues, organs, and anatomy. Embryology, a subfield of developmental biology, is the study of organisms between the one-cell stage (generally, the zygote) and the end of the embryonic stage, which is not necessarily the beginning of free living.

Embryology was originally a more descriptive science until the twentieth century. Embryology and developmental biology today deal with the various steps necessary for the correct and complete formation of the body of a living organism.

The wonder by which a single, fertilized egg differentiates into diverse cells, tissues, organs, and systems of a fully formed organismthe heart, lungs, brain, arms, endocrine system, muscles, and bones of a human, for instanceremains a mystery that embryologists attempt to unravel.

Embryology is the classic study of morphological changes within the embryo. Aristotle is said to be the first person to undertake a study in embryology. Aristotle observed the ontogeny of chicken embryos by breaking open eggs at various time intervals during incubation.

In the 1950s, with the discovery of the structure of DNA by Watson and Crick, and the rapidly increasing knowledge in molecular biology, developmental biology emerged as a field of study interested in the role that genes play in development. In other words, developmental biologists wanted to know which genes are responsible for each morphological change that occurs in development. Perhaps even more importantly, developmental biologists sought to explain how the various cell types of a multicellular organism arise from a single fertilized cell, the egg.

Development of an embryo can be divided into several stages. The first stage is fertilization, in which the sperm penetrates the egg. The nuclei of the sperm and egg then fuse to form a diploid zygote (with paired chromosomes). Cleavage follows, in which the single cell composing the embryo undergoes mitosis (cell division), resulting in many cells called blastomeres. Each blastomere has the exact same genome (set of DNA) as the zygote. These blastomeres come to compose a solid ball of cells called a morula. The final event of cleavage involves the formation of a blastula, or a hollow ball of blastomeres containing a blastocoel, or fluid-filled cavity.

Gastrulation is the stage in which the blastomeres partition themselves into three distinct germ layers: the ectoderm, mesoderm, and endoderm. The ectoderm is the outermost layer and will eventually develop to form the skin and nervous system. The endoderm is the innermost layer and will eventually develop to form the lining of the gut and internal organs. The mesoderm is the middle layer, which eventually forms the muscles, bones, and heart.

After the forming of the gastrula (the multi-layered structure formed during gastrulation), the cells begin to differentiate, or undergo physical and chemical changes that will determine their individual identities (as muscle cells, kidney cells, etc.). Growth is the last stage, in which cells divide and proliferate, eventually composing all the major organs of the body.

One of the significant questions that early developmental biologists sought to answer was how cell individuation occurs. Almost every cell in the body contains the exact same DNA as every other cell, as they all are derived from the initial zygotic cell. So how is it that some cells become cardiac cells and others become skin cells?

One explanation offered for this question is termed induction, the process whereby the development of a cell, or the fate of a group of cells, is influenced by neighboring cells.

The early development of an egg is influenced by the mother. When an egg is first fertilized, its cytoplasm contains lots of the mothers RNA and proteins. In fact, the fertilized egg does not actually start to transcribe its own DNA until the blastula contains about 4,000 cells. The mothers RNA and proteins are not dispersed homogenously throughout the eggs cytoplasm. Instead, they form gradients, so that each section of the egg has a particular selection and quantity of the mothers RNA and proteins. This is called the maternal effect.

When cleavage events occur, different groups of cells in the blastula are exposed to different environments from one another. The different environments consist of different selections and quantities of the mothers RNA and proteins. The mothers RNA and proteins act as signals for the cells, telling the cells which genes to turn on or off. Thus, because different cells will receive different signals, they will develop differently via cell-intrinsic signals and will produce individual signals of their own.

Induction is held to occur when a cell produces a certain signal, for example, by emitting a protein. The protein may diffuse around the cell source. Cells that are closely neighboring the source will receive lots of the signal, while more distant cells will receive less or none of the signal. Therefore, cells will develop different characteristics and functions depending on their relative location to other cells, and thus their individual cell-cell interactions.

Although the phenomena of induction provides insights into how cells individually differentiate into diverse structures, a comprehensive understanding of this process, from an individual egg cell to particular organs, lacks consensus. Notably, some developmental biologists question an underlying assumption of embryonic development that genes ultimately direct the changes, maintaining that the genetic matter only determines which proteins can be produced, but not the form of the organisms (Wells 1997).

Scientists often use model organisms (a species that is extensively studied to understand particular biological phenomena) to learn about how development occurs in animals generally. Although all species develop somewhat differently, there are also many similarities that occur in development in species. For example, certain groups of genes are conserved between humans and flies and worms. Some common examples of model organisms are the fruit fly Drosophila melanogaster, Caenorhabditis elegans (nematode worm), E. coli, the mouse, the zebrafish, and many others.

Ontogeny (also ontogenesis or morphogenesis) is a term that describes the origin and the development of an organism from the fertilized egg to its mature form. Ontogeny is studied in developmental biology.

In 1866, German zoologist Ernst Haeckel theorized that "ontogeny recapitulates phylogeny" (the evolutionary history of a species), and this theory (which was also independently established by others) became known as the biogenetic law or the theory of recapitulation. The idea that ontogeny recapitulates phylogeny, that is, that the development of an organism exactly mirrors the evolutionary development of the species, repeating the adult forms of the organisms, is discredited today. Likewise discredited is the ancillary principle that the there is "terminal addition"evolution proceeding by adding stages to the end of the ancestral organisms (Gould 1977).

However the phenomenon by which a developing organism will for a time show a similar trait or attribute to that of an ancestral species, only to have it disappear at a later stage, is well documented. That is, embryos seem to repeat the embryonic stages (not adult stages) of their ancestors. For example, embryos of the baleen whale exhibit teeth at certain embryonic stages, only to later disappear. A more commonly given example is the emergence of pharyngeal gill pouches of lower vertebrates in almost all mammalian embryos at early stages of development (April, 2001). Note, however, in terms of this later example, some embryologists state the resemblance of pharyngeal pouches in mammals to the gill clefts of fish is illusory, and there is no embryological reason to make such a claim (Wells 2000).

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Embryology - New World Encyclopedia