Embryogenesis

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Embryogenesis

Embryogenesis (compound of the Greek

Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
: eµß???-???es?? "embryo-genesis") is the process by which the embryo

Embryo

An embryo is a multicellular organism ploidy eukaryote in its earliest stage of development, from the time of first cell division until birth, Egg , or germination....
is formed and develops. It starts with 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....
, egg, which, after fertilization, is then called a zygote

Zygote

A zygote is a cell that is the result of fertilization. That is, two ploidy cells—usually an ovum from a female and a sperm cell from a male—merge into a single ploidy cell called the zygote ....
. The zygote undergoes rapid mitotic

Mitosis

Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter nuclei....
division

Cell division

Cell division is a process by which a cell , called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle....
s, the formation of two exact genetic replicates of the original cell, with no significant growth (a process known as cleavage

Cleavage (embryo)

Cellular differentiation

In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a single zygote to a complex system of Tissue and cell types....
, leading to development of an embryo.

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Encyclopedia

Embryogenesis (compound of the Greek

Greek language

Embryo

Ovum

Zygote

Mitosis

Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter nuclei....
division

Cell division

Cleavage (embryo)

Cellular differentiation

Plant embryogenesis

Plant embryogenesis is the process that produces a plant embryo from a fertilised ovule by asymmetric cell division and the differentiation of undifferentiated cells into tissues and organs....
, but this article addresses the common features among different animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s.

The zygote

The egg cell (and hence the fertilized egg) is always asymmetric, having an "animal pole" (future ectoderm

Ectoderm

The ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers.Generally speaking, the ectoderm differentiates to form the nervous system, Epidermis , and the outer part of integumentary system....
and mesoderm

Mesoderm

One of the three germ layers found in the embryos of animals more complex than cnidarians, making them triploblastic. Mesoderm forms in the embryo during gastrulation when some of the cells migrating inward to form the endoderm, produce an additional layer that lies between the endoderm and the ectoderm....
), two of three primitive tissue types, and a "vegetal pole" (future endoderm

Endoderm

Endoderm, is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron from the inner layer of the gastrula, which develops into the endoderm....
), it is also covered with different protective envelopes. The first envelope, the one which is in contact with the membrane

Cell membrane

The cell membrane is the interface between the cellular machinery inside the cell and the fluid outside.It is a semipermeable lipid bilayer found in all cell ....
of the egg, is made of glycoproteins and is called vitelline membrane (zona pellucida
Zona pellucida

The zona pellucida is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the latter, external but not extraneous to it....
in mammals). Different taxa show different cellular and acellular envelopes outside of the vitelline membrane.

Cleavage

The zygote undergoes rapid cell divisions

Cleavage (embryo)

In embryology, cleavage is the division of cell s in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote....
with no significant growth, producing a cluster of cells that is the same size as the original zygote. The different cells derived from cleavage, up to the blastula stage, are called blastomere
Blastomere

A blastomere is a type of Cell produced by division of the ovum after Human fertilization....
s. Depending mostly on the amount of yolk in the egg, the cleavage

Cleavage (embryo)

In embryology, cleavage is the division of cell s in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote....
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

Placenta

The placenta or afterbirth is a highly vascularized ephemeral organ present in Placentalia vertebrates that connects the developing fetal tissues to the uterine wall....
or milk

Milk

Milk is an opaque white liquid produced by the mammary glands of female mammals . It provides the primary source of nutrition for newborn mammals before they are able to digestion other types of food....
. On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk; i.e. birds and reptiles. Because cleavage is impeded in the vegetal pole, there is 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
Radial (sea urchin Sea urchin Sea urchins are small, spiny, globular creatures that compose most of class Echinoidea. They are found in oceans all over the world. Their shell, or "test", is round and spiny, typically from 3 to 10 cm across.... , amphioxus) Bilateral (tunicate Tunicate Tunicate, also known as urochordata, tunicata is the subphylum of a group of underwater saclike filter feeders with incurrent and excurrent Siphon s, that are members of the phylum Chordata.... s, amphibian Amphibian Amphibians , such as frogs, toads, salamanders, newts and caecilians, are cold-blooded animals that metamorphose from a juvenile, water-breathing form to an adult, air-breathing form.... s) Spiral (annelid Annelid The annelids, collectively called Annelida , are a large Scientific classification of animals comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches.... s, mollusks) Rotational (mammal Mammal Mammals are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, with which they feed their young.... s)	Discoidal (fish Fish A fish is any marine biology vertebrate animal that is typically ectothermic , covered with scale , and equipped with two sets of paired fins and several unpaired fins.... , bird Bird Birds are wing, Bipedalismal, endothermic , vertebrate animals that lay egg . There are around 10,000 living species, making them the most numerous tetrapod vertebrates.... s, reptile Reptile Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded vertebrates that have skin covered in scale as opposed to hair or feathers.... s) Superficial (insect Insect Insects are the biggest class of arthropods and the only ones with wings. They are the most diverse group of animals on the planet. They are most diverse at the equator and their diversity declines toward the poles.... s)

Blastula and Gastrula

After the cleavage has produced over 100 cell

Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
s, the embryo is called a blastula
Blastula

The blastula is an early stage of embryonic development in animals. It is also called blastosphere. It is produced by cleavage of a fertilized ovum and consists of a spherical layer of around 128 cells surrounding a central fluid-filled cavity called the blastocoel....
. 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
Blastocyst

The blastocyst is the structure formed in early embryogenesis, after the formation of the blastocoel, but before implantation.It possesses an inner cell mass, or inner cell mass which subsequently forms the embryo proper, and an outer layer of cells, or trophoblast which later forms the placenta....
, characterized by an inner cell mass

Inner cell mass

In early embryogenesis of most eutherian mammals, the inner cell mass is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus....
that is not present in the blastula. The blastocyst must not be confused with the blastula; even though they are similar in structure their cells have different fates.

During gastrulation
Gastrulation

Gastrulation is a phase early in the development of animal embryos, during which the morphology of the embryo is dramatically restructured by cell migration....
cells migrate to the interior of the blastula, consequently forming two (in diploblastic animals) or three (triploblastic) germ layer

Germ layer

A germ layer is a group of cell s, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sea sponge produce two or three primary tissue layers ....
s. The embryo during this process is called a gastrula. The germ layers are referred to as the ectoderm

Ectoderm

Mesoderm

Endoderm

Endoderm, is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron from the inner layer of the gastrula, which develops into the 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
Invagination

Invagination means to fold inward or to sheath. In biology, this can refer to a number of processes.*Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation, the massive reorganization of the embryo from a simple spherical ball of Cell , the blastula, into a multi-layered organism,...
- forming the mouth
Mouth

The mouth, buccal cavity, or oral cavity is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva....
, anus
Anus

The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as coprolite ; food material after all the nutrients have b...
, and archenteron
Archenteron

The primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal....
Delamination - the external cells divide, leaving the daughter cells in the cavity
Polar proliferation

Other major changes during gastrulation:

Heavy RNA transcription using embryonic genes; up to this point the RNA
RNA

Ribonucleic acid is a type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nucleobase, a ribose sugar, and a phosphate....
s used were maternal (stored in the unfertilized egg).
Cells start major differentiation
Cellular differentiation

In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a single zygote to a complex system of Tissue and cell types....
processes, losing their pluripotentiality.

In most animals, a blastopore

Blastopore

A blastopore is an opening into the archenteron during the embryonic stages of an organism. The Embryological origins of the mouth and anus protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the 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 deuterostome

Deuterostome

Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
s the anus forms from the blastopore, while in protostome

Protostome

Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phylum, they make up the Bilateria, mostly comprising animals with symmetry #Bilateral symmetry and triploblastic germ layers....
s it develops into the mouth. See Embryological origins of the mouth and anus

Embryological origins of the mouth and anus

The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two clades: the protostomes and deuterostomes....
for more information.

Organogenesis

At some point after the different germ layers are defined, organogenesis
Organogenesis

In embryogenesis, organogenesis is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. Internal organs initiate development in humans within the 3rd to 8th weeks in utero.The germ layers in organogenesis differ by three processes: folds, splits, and condensation....
begins. The first stage in vertebrate

Vertebrate

Vertebrates are members of the subphylum Vertebrata, chordates with Vertebras or Vertebral columns. The grouping sometimes includes the hagfish, which have no vertebrae, but are genetically quite closely related to lampreys, which do have vertebrae....
s is called neurulation
Neurulation

Neurulation is a part of organogenesis in vertebrate embryos. Steps of neurulation include the formation of the dorsal nerve cord, and the eventual formation of the central nervous system....
, where the neural plate

Neural plate

In human embryology, formation of neural plate is the first step of neurulation. It is created by a flat thickening opposite to the primitive streak of the ectoderm....
folds forming the neural tube

Neural tube

In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord....
. Other common organs or structures which arise at this time include the heart

Heart

The heart is a muscle organ in all vertebrates responsible for pumping blood through the blood vessels by repeated, rhythmic contractions, or a similar structure in annelids, mollusks, and arthropods....
and somite

Somite

A somite is a division of the body of an animal. In vertebrates this is mainly discernible in the embryo stage, in arthropods it is a characteristic of a hypothetical ancestor....
s, but from now on embryogenesis follows no common pattern among the different taxa of the animal kingdom

Animal kingdom

The term Animal kingdom may refer to:* Animal kingdom * Animal, a type of living organism,a multicellular organism* Kingdom referring to animals, as different from Plants ...
.

In most animals organogenesis along with morphogenesis
Morphogenesis

Morphogenesis , is the physical process that gives rise to the shape of an organism. It is one of three fundamental aspects of developmental biology along with the control of cell growth and cellular differentiation....
will result in a larva

Larva

A larva is a young form of animal with indirect developmental biology, going through or undergoing metamorphosis .The larva can look completely different from the adult form, for example, a caterpillar differs from a butterfly....
. The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.

External links

, PMAP The Proteolysis Map
The Proteolysis Map

The Proteolysis MAP is an integrated web resource focused on proteases....
-animation
(retrieved November 20, 2007)
(retrieved November 20, 2007)
of embryogenesis of the frog Xenopus laevis from shortly after fertilization until the hatching of the tadpole; acquired by MRI ()

Embryogenesis

The zygote

Cleavage

Blastula and Gastrula

Organogenesis

See also

External links