Embryonic cells differentiate into a variety of different cell types. Image from NCBI.

Embryogenesis is the process by which the embryo is formed and develops. An embryo (from Greek:, plural, lit "that which grows" from en- "in" + bryein "to swell be full" is a multicellular It starts with the fertilization of the ovum, egg, which, after fertilization, is then called a zygote. For soil improvement see Fertilization (soil. An ovum (plural ova) is a Haploid Female reproductive cell or Gamete. For other meanings see Zygote (disambiguation. A zygote (from Greek ζυγωτός zugōtos "joined" or "yoked" 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. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei Cell division is a process by which a cell, called the parent cell divides into two or more cells called daughter cells. In Embryology, cleavage is the division of cells in the early Embryo. In Developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized Cell type. It occurs in both animal and plant development, but this article addresses the common features among different animals. 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

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 is also covered with different protective envelopes. The ectoderm is the start of a tissue that covers the body surfaces One of the three Germ layers found in the Embryos of Animals more complex than Cnidarians making them Triploblastic. Endoderm, (sometimes called Entoderm) is one of the Germ layers formed during animal Embryogenesis. 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). The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Not to be confused with Peptidoglycan. Glycoproteins are proteins that contain Oligosaccharide chains ( Glycans) covalently attached The zona pellucida (or zona striata in older texts is a Glycoprotein membrane surrounding the Plasma membrane of an Oocyte. Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands Different taxa show different cellular and acellular envelopes outside of the vitelline membrane. A taxon (plural taxa) or taxonomic unit, is a name designating an organism or a group of Organisms In Biological nomenclature according to

Cleavage

Main article: Cleavage (embryo)

Cleavage during embryogenesis

The zygote undergoes rapid cell divisions with no significant growth, producing a cluster of cells that is the same size as the original zygote. In Embryology, cleavage is the division of cells in the early Embryo. In Embryology, cleavage is the division of cells in the early Embryo. The different cells derived from cleavage, up to the blastula stage, are called blastomeres. A blastomere is a type of cell produced by division of the egg after fertilization. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total) or meroblastic (partial)^[1]. An egg yolk is the part of an egg which serves as the Food source for the developing Embryo inside In Embryology, cleavage is the division of cells in the early Embryo.

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. The placenta is an Ephemeral organ present in placental Vertebrates, such as Eutherial Mammals and Sharks during Gestation Milk is an opaque white liquid produced by the Mammary glands of female Mammals (including Monotremes. 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^[2].

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:

Blastula and Gastrula

Main article: Gastrulation

After the cleavage has produced over 100 cells, the embryo is called a blastula^[3]. Sea urchins are small globular spiny sea cat animals composing most of class Echinoidea. The lancelets (subphylum Cephalochordata, traditionally known as amphioxus) are a group of primitive Chordates. Tunicate, also known as urochordata, tunicata (and by the common names of urochordates, sea squirts, and sea pork) is the Prehistoric amphibian Amphibians (class Amphibia such as Frogs Toads Salamanders Newts Gymnophiona, Sirens and The annelids, collectively called Annelida (from Latin anellus "little ring" are a large phylum of Animals comprising Molluscs are animals belonging to the phylum Mollusca. There are around 250000 extant Species within the phylum with an estimated 70000 Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands Fish are aquatic Vertebrate animals that are typically ectothermic (previously Cold-blooded) covered with scales, and equipped with two Birds ( class Aves) are bipedal endothermic ( Warm-blooded) Vertebrate animals that lay eggs. Reptiles, or members of the class Reptilia are air-breathing Cold-blooded Vertebrates that have skin covered in scales as opposed to hair or feathers Insects ( Class Insecta) are a major group of Arthropods and the most diverse group of Animals on the Earth with over a million described Gastrulation is a phase early in the development of animal Embryos during which the morphology of the embryo is dramatically restructured by Cell migration. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called The blastula is an early stage of Embryonic development in Animals It is also called blastosphere 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^[4], characterized by an inner cell mass that is not present in the blastula. The blastocyst is the structure formed in early Embryogenesis, after the formation of the Blastocoel, but before Implantation. In early Embryogenesis of most Eutherian mammals the inner cell mass (abbreviated ICM and also known as the embryoblast or Pluriblast The blastocyst must not be confused with the blastula; even though they are 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. Gastrulation is a phase early in the development of animal Embryos during which the morphology of the embryo is dramatically restructured by Cell migration. Diploblasty is a condition of the ovum in which there are two primary Germ layers the Ectoderm and Endoderm. Triploblasty is a condition of the Blastula in which there are three primary Germ layers the Ectoderm, Mesoderm, and Endoderm. A germ layer is a collection of cells formed during animal Embryogenesis. The embryo during this process is called a gastrula. Gastrulation is a phase early in the development of animal Embryos during which the morphology of the embryo is dramatically restructured by Cell migration. The germ layers are referred to as the ectoderm, mesoderm and endoderm. The ectoderm is the start of a tissue that covers the body surfaces One of the three Germ layers found in the Embryos of Animals more complex than Cnidarians making them Triploblastic. Endoderm, (sometimes called Entoderm) is one of the Germ layers formed during animal Embryogenesis. In diploblastic animals only the ectoderm and the endoderm are present ^[5]. Diploblasty is a condition of the ovum in which there are two primary Germ layers the Ectoderm and Endoderm.

• 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^[6]
  • Ingression - cells move with pseudopods^[7]
  • Invagination - forming the mouth, anus, and archenteron^[8]
  • Delamination - the external cells divide, leaving the daughter cells in the cavity^[9]
  • Polar proliferation

• Other major changes during gastrulation:
  • Heavy RNA transcription using embryonic genes; up to this point the RNAs used were maternal (stored in the unfertilized egg). Gastrulation is a phase early in the development of animal Embryos during which the morphology of the embryo is dramatically restructured by Cell migration. Pseudopods or pseudopodia (from the Greek word ψευδοπόδια, ψευδός "fake false" Invagination means to fold inward or to sheath In Biology, this can refer to a number of processes 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 anus is an opening at the opposite end of an Animal 's Digestive tract from the Mouth. The primitive gut that forms during Gastrulation in the developing Blastula is known as the archenteron. Transcription is the synthesis of RNA under the direction of DNA Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units
  • Cells start major differentiation processes, losing their pluripotentiality. In Developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized Cell type. Pluripotency in the broad sense refers to "having more than one potential outcome

In most animals, a blastopore is formed at the point where cells are entering the embryo. A blastopore is an opening into the Archenteron during the Embryonic stages of an organism 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. Deuterostomes (taxonomic term Deuterostomia; from the Greek "second mouth" are a superphylum of Animals They are a subtaxon of the Protostomia (from the Greek: mouth first) are a Taxon of Animals Together with the Deuterostomes and a few smaller phyla

Organogenesis

At some point after the different germ layers are defined, organogenesis begins. In animal development, organogenesis is the process by which the Ectoderm, Endoderm, and Mesoderm develop into the Internal organs The first stage in vertebrates is called neurulation, where the neural plate folds forming the neural tube^[10]. Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes Neurulation is a part of Organogenesis in Vertebrate Embryos Steps of neurulation include the formation of the Dorsal nerve cord, and the eventual In human Embryology, formation of neural plate is the first step of Neurulation. 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 and somites, but from now on embryogenesis follows no common pattern among the different taxa of the animal kingdom. The heart is a muscular organ in all Vertebrates responsible for pumping Blood through the Blood vessels by repeated rhythmic 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 A taxon (plural taxa) or taxonomic unit, is a name designating an organism or a group of Organisms In Biological nomenclature according to

In most animals organogenesis along with morphogenesis will result in a larva. MOrphogenesis is an EP by Industrial Black metal band.And Oceans. A larva ( Latin; plural larvae) is a juvenile form of Animal with indirect development, undergoing Metamorphosis (for example The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development. Metamorphosis is a Biological process by which an Animal physically develops after Birth or hatching involving a conspicuous and relatively

See also

• Mammalian embryogenesis
• Drosophila embryogenesis
• Plant embryogenesis
• Cdx2 gene
• Homeobox genes

References

1. ^  What is a cell? 2004. Mammalian embryogenesis is the process of Cell division and Cellular differentiation during early Prenatal development which leads to the development of Drosophila has long been a favorite model system for geneticists and developmental biologists studying 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 Caudal type homeobox transcription factor 2, also known as CDX2, is a human Gene. A homeobox is a DNA sequence found within Genes that are involved in the regulation of patterns of development ( Morphogenesis) in Animals A Science Primer: A Basic Introduction to the Science Underlying NCBI Resources. NCBI.
2. ^  Campbell, Neil A. ; Reece, Jane B. ; Biology Benjamin Cummings, Pearson Education Inc 2002.

External links

• Development of the embryo (retrieved November 20, 2007)
• Human Embryo (retrieved November 20, 2007)
• Video of embryogenesis of the frog Xenopus laevis from shortly after fertilization until the hatching of the tadpole; acquired by MRI (DOI of paper)