Embryology
1 - blastula,
2 - gastrula with blastopore;
orange - ectoderm,
red - endoderm.
Dissection of human embryo, 38 mm - 8 weeks
Embryology (from
Greek ἔμβρυον,
embryon, "the unborn, embryo"; and
-λογία,
-logia) is the
science of the development of an
embryo from the
fertilization of the
ovum to the
fetus stage.
Embryonic development of animals
After cleavage, the dividing cells, or
morula, becomes a hollow ball, or
blastula, which develops a hole or pore at one end.
Bilaterals
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:
- The innermost layer, or endoderm, gives a rise to the digestive organs, the gills, lungs or swim bladder if present, and kidneys or nephrites.
- The middle layer, or mesoderm, gives rise to the muscles, skeleton if any, and blood system.
- The outer layer of cells, or ectoderm, gives rise to the nervous system, including the brain, and skin or carapace and hair, bristles, or scales.
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.
Humans
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.
History of embryology
Human embryo at six weeks gestational age
Histological film 10 day mouse embryo
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, originally proposed 2,000 years earlier by
Aristotle. Much early embryology came from the work of the great
Italian anatomists:
Aldrovandi,
Aranzio,
Leonardo da Vinci,
Marcello Malpighi,
Gabriele Falloppio,
Girolamo Cardano,
Emilio Parisano,
Fortunio Liceti,
Stefano Lorenzini,
Spallanzani,
Enrico Sertoli,
Mauro Rusconi, etc.
[1] 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.
[2]
After 1827
Karl Ernst von Baer and
Heinz Christian Pander proposed the
germ layer theory of development; von Baer discovered the mammalian ovum in 1827.
[3][4][5] 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 1950
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.
Vertebrate and invertebrate embryology
Many principles of embryology apply to invertebrates as well as to vertebrates.
[6]
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.
Modern embryology research
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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