Everything about Embryonic Development totally explained
Embryogenesis is the process by which the
embryo is formed and develops. It starts with the
fertilization of the
ovum, egg, which, after fertilization, is then called a
zygote. The zygote undergoes rapid
mitotic divisions, the formation of two exact genetic replicates of the original cell, with no significant growth (a process known as
cleavage) and
cellular differentiation, leading to development of an embryo. It occurs in both animal and
plant development, but this article addresses the common features among different
animals.
The zygote
The egg cell (and hence the fertilized egg) is always asymmetric, having an "
animal pole" (future
ectoderm and
mesoderm), two of three primitive tissue types, and a "
vegetal pole" (future
endoderm), it's also covered with different protective envelopes. The first envelope, the one which is in contact with the
membrane of the egg, is made of
glycoproteins and is called
vitelline membrane (
zona pellucida in
mammals). Different
taxa show different cellular and acellular envelopes outside of the vitelline membrane.
Cleavage
The zygote undergoes rapid
cell divisions with no significant growth, producing a cluster of cells that's the same size as the original zygote. The different cells derived from cleavage, up to the blastula stage, are called
blastomeres. Depending mostly on the amount of
yolk in the egg, the
cleavage can be
holoblastic (total) or
meroblastic (partial).
Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via
placenta or
milk. On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk; for example birds and reptiles. Because cleavage is impeded in the vegetal pole, there's a very uneven distribution and size of cells, being more and bigger at the animal pole of the zygote.
In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms:
Cleavage patterns followed by holoblastic and meroblastic eggs>
| Holoblastic |
Meroblastic |
|
Discoidal (fish, birds, reptiles)
Superficial (insects)
|
Blastula and Gastrula
After the cleavage has produced over 100
cells, the embryo is called a
blastula. The blastula is usually a spherical layer of cells (the
blastoderm) surrounding a fluid-filled or yolk-filled cavity (the
blastocoel).
Mammals at this stage form a structure called the
blastocyst, characterized by an
inner cell mass that isn't present in the blastula. The blastocyst must not be confused with the blastula; even though they're similar in structure their cells have different fates.
During
gastrulation cells migrate to the interior of the blastula, consequently forming two (in
diploblastic animals) or three (
triploblastic)
germ layers. The embryo during this process is called a
gastrula. The germ layers are referred to as the
ectoderm,
mesoderm and
endoderm. In
diploblastic animals only the ectoderm and the endoderm are present .
Among the different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:
- Epiboly - expansion of one cell sheet over other cells
- Ingression - cells move with pseudopods
- Invagination - forming the mouth, anus, and archenteron
- Delamination - the external cells divide, leaving the daughter cells in the cavity
- Polar proliferation
Other major changes during gastrulation:
In most animals, a blastopore is formed at the point where cells are entering the embryo. Two major groups of animals can be distinguished according to the blastopore's fate. In deuterostomes the anus forms from the blastopore, while in protostomes it develops into the mouth.
Organogenesis
At some point after the different germ layers are defined, organogenesis begins. The first stage in vertebrates is called neurulation, where the neural plate folds forming the neural tube. Other common organs or structures which arise at this time include the heart and somites, but from now on embryogenesis follows no common pattern among the different taxa of the animal kingdom.
In most animals organogenesis along with morphogenesis will result in a larva. The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.
Further Information
Get more info on 'Embryonic Development'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://embryogenesis.totallyexplained.com">Embryogenesis Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
