avian Avian may refer to: *Birds or Aves, winged animals *Avian (given name) (russian: Авиа́н, link=no), a male forename Aviation *Avro Avian, a series of light aircraft made by Avro in the 1920s and 1930s *Avian Limited, a hang glider manufacture ...

gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. Be ...

, Koller's sickle is a local thickening of cells at the posterior edge of the upper layer of the area pellucida called the

epiblast In amniote embryonic development, the epiblast (also known as the primitive ectoderm) is one of two distinct cell layers arising from the inner cell mass in the mammalian blastocyst, or from the blastula in reptiles and birds, the other layer is t ...

. Koller's sickle is crucial for avian development, due to its critical role in inducing the differentiation of various avian body parts. Koller's sickle induces

primitive streak The primitive streak is a structure that forms in the early embryo in amniotes. In amphibians the equivalent structure is the blastopore. During early embryonic development, the embryonic disc becomes oval shaped, and then pear-shaped with the ...

and

Hensen's node The primitive node (or primitive knot) is the organizer for gastrulation in most amniote embryos. In birds it is known as Hensen's node, and in amphibians it is known as the Spemann-Mangold organizer. It is induced by the Nieuwkoop center i ...

, which are major components of avian gastrulation. Avian gastrulation is a process by which developing cells in an avian embryo move relative to one another in order to form the three germ layers (

endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...

, and

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...

In-depth definition

The thickening of the epiblast in Koller's sickle acts as a margin separating sheets of cells from posterior side of avian

blastoderm A blastoderm (germinal disc, blastodisc) is a single layer of embryonic epithelial tissue that makes up the blastula. It encloses the fluid filled blastocoel. Gastrulation follows blastoderm formation, where the tips of the blastoderm begins the for ...

s from

hypoblast In amniote embryology, the hypoblast, is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac, which in turn gives rise to ...

s and area opaca endoderm. The blastoderm is a single layer of cells, and the hypoblast and area opaca endoderm cells lie directly below the blastoderm. Koller's sickle arises from the midpoint, between the hypoblast cells and the area opaca endoderm. As blastoderm cells migrate anteriorly they push primary hypoblast cells and form a secondary hypoblast known as the endoblast. Also during this migration, Koller's sickle prevents the hypoblast cells and the area opaca cells from making contact with the blastoderm, which allows the

to form.

Formation of the primitive streak

The primitive streak is induced by the posterior marginal zone (PMZ) of Koller's sickle, which can also induce

. If cell movement in the PMZ is blocked, the primitive streak does not form. Thus, the PMZ acts as an organizer. Cells in marginal zones of the embryo, like the PMZ, are key to development and cell fate determination in chick embryos. Avian gastrulation occurs as cells move though the primitive streak. Hence, primitive streak is analogous to the blastopore lip in

amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

gastrulation. The primitive streak develops from Koller's sickle and the epiblast of the avian embryo. As the cells of Koller's sickle migrate during gastrulation, they form different portions of the primitive streak. The anterior cells of Koller's sickle become the anterior region of the primitive streak, known as Hensen's node. Similarly, the posterior cells of Koller's sickle form the posterior region of the primitive streak. This differential movement is due to expression of different mesodermal marker genes among the cells located in different areas of Koller's sickle. Chordin is expressed in cells of the anterior streak, while Wnt8c is expressed in cells of the posterior streak. The movement is coordinated by a

Wnt signaling pathway The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt is a portmanteau created from the names Wingless and Int-1. Wnt signaling p ...

which is activated by

fibroblast growth factors Fibroblast growth factors (FGF) are a family of cell signalling proteins produced by macrophages; they are involved in a wide variety of processes, most notably as crucial elements for normal development in animal cells. Any irregularities in their ...

from the hypoblast.

Role of the primitive streak

The primitive streak is key in the development of the major body axes. The

primitive groove The primitive streak is a structure that forms in the early embryo in amniotes. In amphibians the equivalent structure is the blastopore. During early embryonic development, the embryonic disc becomes oval shaped, and then pear-shaped with ...

forms as a depression in the primitive streak as it is developing, and allows a space for migrating cells to move into the deeper layers of the embryo. Cells migrate by entering through the dorsal side and moving toward the ventral side of the avian embryo, separating the left and right sections of the embryo. The primitive pit in Hensen's node, at the anterior end of the primitive streak, allows cells to enter which will form the

notochord In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consis ...

and

prechordal plate In the development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material is ...

. Cells that move through the center of the streak will become the heart and kidneys. The lateral plate and the extraembryonic mesoderm arise from the cells that enter at the posterior end of the primitive streak. Epiblast cells near the primitive streak form the neural plate and other dorsal structures, while the epiblast cells far from the streak become epidermis.

Gene influence

Koller's sickle is one of two regions (the other being the caudal boundary region of the area opaca) where expression patterns for genes important for gastrulation are localized. For example, the gene ''Nodal'' is only expressed in Koller's sickle. While a single gene has not been isolated for the creation of Koller's sickle, there is evidence that the Homeobox gene ''Hex'' influences Koller's sickle development. The transcript ''cHex'', which is a product of ''Hex'', has been detected in Koller's sickle during chick embryogenesis. ''cHex'' is also involved with the formation of the hypoblast, the endoderm in an anterior arc that overlaps the cardiogenic region, pharyngeal endoderm immediately adjacent to the forming myocardium, in the endocardium, and in the liver and thyroid gland primordia. It is also possible that the Homeobox gene ''goosecoid'' ( GSC) is involved in the formation of Koller's sickle, as Koller's sickle cells are the first to express the ''goosecoid'' transcript. In general, the ''goosecoid'' gene is thought to be involved in the development of the chicken organizer during gastrulation.

History

Discovery

Koller's sickle was originally described by

August Rauber August Rauber (March 9, 1841 – February 16, 1917) was a German anatomist and embryologist born in Obermoschel in the Rhineland-Palatinate. Rauber was born the fourth of five children to Stephan Rauber and Rosalie née Oberlé. He studied ...

in 1876. Because of this Koller's sickle is sometimes referred to as Rauber's sickle. In 1926, Ludwig Graper first studied the involvement of Koller's sickle in the formation of the primitive streak. The cell movements reminded him of a dance called the Polonaise, in which dancers moved in parallel lines and in which they move from the back of the group to the center. It was not until 2007 that the mechanism for these movements was discovered, by Voiculescu and his associates. They determined that cells move to the center of the epiblast following the activation of the Wnt planar cell polarity pathway by fibroblast growth factors made by the hypoblast.

Current research

There is still a lot that is unknown regarding Koller's sickle, but research is ongoing. By implanting a fragment of quail Koller's sickle into a chicken blastoderm, Drs. Callebaut and Van Nueten observed the formation of a normal secondary primitive streak, mesoderm, and definitive endoderm. This led them to the conclusion that Koller's sickle is the early avian representation of the organizer, and that there is homology between Koller's sickle in avians and the blastoporus in amphibians. Drs. Callebaut and Van Nueten also optimized a method for preparation of unincubated avian eggs, and from this they demonstrated the fact that embryonic regulation is a result of the spatial distribution of Koller's sickle tissue. Additionally, Drs. Callebaut and Van Nueten were able to determine that the differentiation of Koller' sickle cells to sickle endoblast is irreversible, and that the sickle endoblast induces early neurulation; they did this by implanting Koller's sickle tissue into different parts of unincubated chicken blastoderms and observing the effects.

References

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