Embryology

Embryology

Overview

Embryology is a science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

 which is about the development of an embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 from the fertilization of the ovum
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

 to the fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

 stage. After cleavage, the dividing cells, or morula
Morula
A morula is an embryo at an early stage of embryonic development, consisting of cells in a solid ball contained within the zona pellucida....

, becomes a hollow ball, or blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...

, which develops a hole or pore at one end.

In bilateral animal
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...

s, the blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...

 develops in one of two ways that divides the whole animal kingdom into two halves (see: Embryological origins of the mouth and anus
Embryological origins of the mouth and anus
This article needs to be clarified and may contain errors.The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two natural groupings: the protostomes and deuterostomes.In animals at least as complex...

). If in the blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...

 the first pore (blastopore
Blastopore
A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...

) becomes the mouth of the animal, it is a protostome
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...

; if the first pore becomes the anus then it is a deuterostome
Deuterostome
Deuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...

.
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Encyclopedia

Embryology is a science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

 which is about the development of an embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 from the fertilization of the ovum
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

 to the fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

 stage. After cleavage, the dividing cells, or morula
Morula
A morula is an embryo at an early stage of embryonic development, consisting of cells in a solid ball contained within the zona pellucida....

, becomes a hollow ball, or blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...

, which develops a hole or pore at one end.

In bilateral animal
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...

s, the blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...

 develops in one of two ways that divides the whole animal kingdom into two halves (see: Embryological origins of the mouth and anus
Embryological origins of the mouth and anus
This article needs to be clarified and may contain errors.The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two natural groupings: the protostomes and deuterostomes.In animals at least as complex...

). If in the blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...

 the first pore (blastopore
Blastopore
A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...

) becomes the mouth of the animal, it is a protostome
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...

; if the first pore becomes the anus then it is a deuterostome
Deuterostome
Deuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...

. The protostomes include most invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

 animals, such as insects, worms and molluscs, while the deuterostomes include the vertebrates. In due course, the blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...

 changes into a more differentiated structure called the gastrula.

The gastrula with its blastopore
Blastopore
A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...

 soon develops three distinct layers of cells (the germ layers) from which all the bodily organs and tissues then develop:
  • The innermost layer, or endoderm
    Endoderm
    Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...

    , gives rise to the digestive organs, lungs and bladder.
  • The middle layer, or mesoderm
    Mesoderm
    In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...

    , gives rise to the muscles, skeleton and blood system.
  • The outer layer of cells, or ectoderm
    Ectoderm
    The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...

    , gives rise to the nervous system and skin.


In humans, the term embryo refers to the ball of dividing cells from the moment the zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...

 implants itself in the uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...

 wall until the end of the eighth week after conception. Beyond the eighth week, the developing human is then called a fetus. 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.

History





As recently as the 18th century, the prevailing notion in 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
Epigenesis
Epigenesis may refer to:* Epigenesis , describes morphogenesis and development of an organism* By analogy, a philosophical and theological concept, part of the concept of spiritual evolution* The Epigenesis, a 2010 album by Melechesh...

, originally proposed 2,000 years earlier by Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

. Much early embryology came from the work of the great Italian
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

 anatomists: Aldrovandi
Ulisse Aldrovandi
Ulisse Aldrovandi was an Italian naturalist, the moving force behind Bologna's botanical garden, one of the first in Europe. Carolus Linnaeus and the comte de Buffon reckoned him the father of natural history studies...

, Aranzio, Leonardo da Vinci
Leonardo da Vinci
Leonardo di ser Piero da Vinci was an Italian Renaissance polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist and writer whose genius, perhaps more than that of any other figure, epitomized the Renaissance...

, Marcello Malpighi
Marcello Malpighi
Marcello Malpighi was an Italian doctor, who gave his name to several physiological features, like the Malpighian tubule system.-Early years:...

, Gabriele Falloppio
Gabriele Falloppio
Gabriele Falloppio , often known by his Latin name Fallopius, was one of the most important anatomists and physicians of the sixteenth century....

, Girolamo Cardano, Emilio Parisano, Fortunio Liceti
Fortunio Liceti
thumb|Portrait of Fortunio LicetiFortunio Liceti , also known as Fortunius Licetus, was an Italian scientist.He was born in Rapallo, and studied at the University of Bologna, graduating with doctorates in philosophy and medicine. He then took a position of chair at the University of Pisa. He later...

, Stefano Lorenzini, Spallanzani, Enrico Sertoli
Enrico Sertoli
Enrico Sertoli was an Italian physiologist and histologist who was a native of Sondrio. He studied medicine at the University of Pavia, where one of his instructors was physiologist Eusebio Oehl . He continued his studies of physiology in Vienna under Ernst Wilhelm von Brücke , and in Tübingen...

, Mauro Rusconi, etc. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy
Microscopy
Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...

 improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologists.

Finally Karl Ernst von Baer
Karl Ernst von Baer
Karl Ernst Ritter von Baer, Edler von Huthorn also known in Russia as Karl Maksimovich Baer was an Estonian naturalist, biologist, geologist, meteorologist, geographer, a founding father of embryology, explorer of European Russia and Scandinavia, a member of the Russian Academy of Sciences, a...

 discovered the mammalian ovum in 1827. Modern embryological pioneers include Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

, Ernst Haeckel
Ernst Haeckel
The "European War" became known as "The Great War", and it was not until 1920, in the book "The First World War 1914-1918" by Charles à Court Repington, that the term "First World War" was used as the official name for the conflict.-Research:...

, J.B.S. Haldane, and Joseph Needham
Joseph Needham
Noel Joseph Terence Montgomery Needham, CH, FRS, FBA , also known as Li Yuese , was a British scientist, historian and sinologist known for his scientific research and writing on the history of Chinese science. He was elected a fellow of the Royal Society in 1941, and as a fellow of the British...

. Other important contributors include William Harvey
William Harvey
William Harvey was an English physician who was the first person to describe completely and in detail the systemic circulation and properties of blood being pumped to the body by the heart...

, Kaspar Friedrich Wolff, Heinz Christian Pander
Heinz Christian Pander
Heinz Christian Pander, aka Christian Heinrich Pander was a Baltic German biologist and embryologist who was born in Riga. In 1817 he received his doctorate from the University of Würzburg, and spent several years , performing scientific research from his estate in Carnikava on the banks of the...

, August Weismann
August Weismann
Friedrich Leopold August Weismann was a German evolutionary biologist. Ernst Mayr ranked him the second most notable evolutionary theorist of the 19th century, after Charles Darwin...

, Gavin de Beer
Gavin de Beer
Sir Gavin Rylands de Beer FRS was a British evolutionary embryologist. He was Director of the British Museum , President of the Linnean Society, and received the Royal Society's Darwin Medal for his studies on evolution.-Biography:...

, Ernest Everett Just
Ernest Everett Just
Ernest Everett Just was a pioneering African American biologist, academic and science writer. Just's primary legacy is his recognition of the fundamental role of the cell surface in the development of organisms...

, and Edward B. Lewis
Edward B. Lewis
- External links :* *...

.

After the 1950s, with the DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 helical structure being unravelled and the increasing knowledge in the field of molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

, developmental biology
Developmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...

 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.

Vertebrate and invertebrate embryology


Many principles of embryology apply to both invertebrate animals as well as to vertebrates. 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
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

 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

Modern embryology research


Currently, embryology has become an important research area for studying the genetic
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

 control of the development process (e.g. morphogens), its link to cell signalling, its importance for the study of certain diseases and mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

s and in links to stem cell
Stem cell
This article is about the cell type. For the medical therapy, see Stem Cell TreatmentsStem cells are biological cells found in all multicellular organisms, that can divide and differentiate into diverse specialized cell types and can self-renew to produce more stem cells...

 research.

See also

  • Ontogeny
    Ontogeny
    Ontogeny is the origin and the development of an organism – for example: from the fertilized egg to mature form. It covers in essence, the study of an organism's lifespan...

  • Embryogenesis
    Embryogenesis
    Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...

  • Recapitulation theory
    Recapitulation theory
    The theory of recapitulation, also called the biogenetic law or embryological parallelism—and often expressed as "ontogeny recapitulates phylogeny"—is a disproven hypothesis that in developing from embryo to adult, animals go through stages resembling or representing successive stages...

  • Prenatal development
  • Protostomes
  • Deuterostomes
  • Germ layers
  • Epigenesis (biology)
    Epigenesis (biology)
    In biology, epigenesis has at least two distinct meanings:* the unfolding development in an organism, and in particular the development of a plant or animal from an egg or spore through a sequence of steps in which cells differentiate and organs form;...

  • Developmental biology
    Developmental biology
    Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...

  • Cell signalling
  • Hedgehog signaling pathway
    Hedgehog signaling pathway
    In a growing embryo, cells develop differently in the head or tail end of the embryo, the left or right, and other positions. They also form segments which develop into different body parts. The hedgehog signaling pathway gives cells information that they need to make the embryo develop properly....

  • Morphogens
  • French flag model
    French flag model
    The French Flag Model is a conceptual definition of a morphogen, described by Lewis Wolpert in the 1960s. A morphogen is rigorously defined as a signaling molecule that acts directly on cells to produce specific cellular responses dependent on morphogen concentration...

  • Embryo drawing

Further reading

  • Scott F. Gilbert. Developmental Biology. Sinauer, 2003. ISBN 0-87893-258-5.
  • Lewis Wolpert. Principles of Development. Oxford University Press, 2006. ISBN 0-19-927536-X.

External links