Emily Carter
The primitive streak is a structure that forms in the early embryo of amniotes, including chicks. It is the first obvious morphological sign of bilateral symmetry, establishing the second embryonic axis and the site of gastrulation. The primitive streak also initiates germ layer formation, determining the embryonic midline axis. The formation of the primitive streak involves the coordinated movement and rearrangement of cells in the epiblast, with large-scale cell flows and chemotactic cell movement. The marginal zone of the chick embryo contains cells that contribute to the streak, which is induced by signals from the surrounding marginal zone and the area opaca. The primitive streak is essential for understanding the early development of higher animals and has been studied extensively in chick embryos, providing valuable insights into the formation of this crucial structure.
| Characteristics | Values |
|---|---|
| Embryo stage | Initially circularly symmetric |
| Cell movement | Large-scale cell movements, including chemotactic movement and differential adhesion |
| Cell types | Mesenchymal stem cells, mesoderm, endoderm, epiblast, hypoblast |
| Symmetry | Establishes bilateral symmetry |
| Site of formation | Gastrulation, the midline of the embryo |
| Germ layer formation | Initiates germ layer formation |
| Cell arrangement | Cells are arranged along the prospective midline |
| Axis | Determines the second embryonic axis, left-right and cranial-caudal body axes |
| Koller's Sickle | A sickle-shaped assembly of loosely associated cells underneath the epiblast |
| Nodal expression | Induced by inductive signals from the marginal zone |
| Formation time | Approximately 24 hours |
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What You'll Learn
The primitive streak establishes bilateral symmetry
The primitive streak is a structure that forms in the early embryo of amniotes, such as birds and mammals. In amphibians, the equivalent structure is the blastopore. The primitive streak is the first obvious morphological sign of bilateral symmetry in the chick embryo.
The embryonic disc becomes oval-shaped, then pear-shaped, with the broader end towards the anterior and the narrower region projected to the posterior. The primitive streak forms a longitudinal midline structure in the narrower posterior (caudal) region of the developing embryo on its dorsal side. At first formation, the primitive streak extends for half the length of the embryo. In the human embryo, this appears by stage 6, about 17 days after fertilisation.
The primitive streak is formed by the coordinated movement and rearrangement of cells in the epiblast. Two counter-rotating flows of cells meet at the posterior end, where the streak forms. There is little movement in the centre of these flows, while the greatest movement is observed at the periphery of the vortices. The vortex movements are likened to polonaise movement, which is key to the formation of the primitive streak. Cells overlaying Koller's sickle in the posterior end of the chick embryo move towards the midline, meet and change direction towards the centre of the epiblast.
The primitive streak is an essential structure in the development of the chick embryo. It establishes bilateral symmetry, determines the site of gastrulation, and initiates germ layer formation. Mesenchymal stem cells are arranged along the prospective midline, establishing the second embryonic axis, and the site where cells will ingress and migrate during gastrulation and germ layer formation. The primitive streak extends through this midline and creates the left–right and cranial–caudal body axes.
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The marginal zone induces primitive streak formation
Inductive signals from the marginal zone, specifically an anterior-posterior gradient of Vg1 and graded Wnt8c expression in the AO, induce nodal expression in the epiblast overlying Koller's Sickle. These nodal-expressing cells then differentiate to form mesendoderm, initiating gastrulation and the formation of the primitive streak. The primitive streak is the first obvious morphological sign of bilateral symmetry in the chick embryo.
The marginal zone's ability to induce primitive streak formation is attributed to Wnt signalling, with Vg1 and Wnt cooperating to initiate the process. Misexpression of Vg1 along with Wnt antagonists prevents the formation of ectopic streaks, highlighting the importance of Wnt activity in this process. Furthermore, implantation of COS cell aggregates overexpressing BMP4 in posterior regions of the prestreak stage epiblast abolishes the formation of the authentic primitive streak, indicating the necessity of BMP/Cerberus-antagonizing signals in primitive streak induction.
In addition to Vg1 and Wnt, other candidate proteins that mediate the inducing activity of the posterior marginal zone include Activin, Chordin, and Wnt8c. These factors are expressed in the marginal zone of the blastodisc, and implantation of COS cell aggregates overexpressing these factors results in ectopic primitive streak formation. The marginal zone's inductive capacity is not limited to a specific region, as evidenced by the ability of the posterior marginal zone to induce an ectopic primitive streak even in the presence of the hypoblast, which typically has an inhibitory effect on primitive streak formation.
The formation of the primitive streak in the chick embryo is a highly regulated process involving a complex network of signalling pathways and interactions between various cell types. The marginal zone plays a crucial role in initiating this process by providing the necessary inductive signals to the epiblast cells, which respond by differentiating and forming the primitive streak.
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Cell movements during primitive streak formation
The primitive streak is a structure that forms in the early embryo of amniotes, such as the chick. It is the first visible sign of gastrulation, initiating the formation of the three germ layers (endoderm, mesoderm, and ectoderm) and establishing bilateral symmetry. The primitive streak also determines the site of gastrulation, which involves the ingression and migration of mesoderm progenitors to their ultimate position.
During primitive streak formation, there is extensive rearrangement of cells in the epiblast, which is the source of all embryonic material in amniotes. Cells in the posterior two-thirds of the embryo move in two counter-rotating flows that meet at the site of streak formation, with the greatest movement observed at the periphery of the vortices. This movement is likened to the polonaise movement. The cells overlaying Koller's sickle in the posterior end of the embryo move towards the midline, meet, and change direction towards the center of the epiblast. Cells from the lateral posterior marginal zone replace the cells that left Koller's sickle by meeting at the center, changing direction, and extending anteriorly. This movement results in the formation of a single- to multi-layered epithelial sheet that becomes macroscopically visible.
Several mechanisms have been proposed to explain the cellular movements required to form the primitive streak, including active proliferation, oriented cell division, cell-cell intercalation, and chemotactic cell movement. Chemotactic movement, or the response of cells to chemical signals, has been investigated as a potential mechanism for streak formation. Computer simulations have been used to test this hypothesis, suggesting that chemotaxis and differential adhesion between cell types may be sufficient to explain the formation of the primitive streak.
Additionally, the role of specific molecular signals in controlling cell movement during primitive streak initiation is still under investigation. For example, the FGF-family of signalling molecules has been implicated in movement regulation during later stages of gastrulation in the chick embryo, but their role during streak initiation is not yet fully understood. Furthermore, the expression of FGFR1 in the early epiblast and the presence of FGFs during early development indicate a potential role for these molecules in primitive streak formation.
The formation of the primitive streak is a complex process involving the coordination and integration of various cellular movements and signals. While some aspects have been elucidated through computational methods and simulations, further research is needed to fully understand the molecular signals controlling cell movement during this critical stage of embryonic development.
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The role of the epiblast in primitive streak formation
The epiblast is a single epithelial layer of the bilaminar embryonic disc, which is the source of all embryonic material in amniotes. Some of its cells will give rise to the primitive streak. In the chick embryo, the formation of the primitive streak involves the coordinated movement and rearrangement of cells in the epiblast. This movement is known as the Polonaise movement.
During the early stages of primitive streak formation, the embryo appears circularly symmetric. Then, a group of deep mesenchymal cells at the boundary between the AO (area opaca) and AP (area pellucida) in the posterior half of the embryo thicken to form Koller's Sickle, a darker sickle-shaped or lunate region. Inductive signals from the marginal zone, an anterior-posterior gradient of Vg1 and graded Wnt8c expression in the AO, induce nodal expression in the epiblast overlying Koller's Sickle. The nodal-expressing cells then differentiate to form mesendoderm, initiating gastrulation.
Two counter-rotating flows of cells meet at the posterior end, where the streak forms. Cells overlaying Koller's Sickle in the posterior end of the chick embryo move towards the midline, meet and change direction towards the centre of the epiblast. Cells from the lateral posterior marginal zone replace those cells that left Koller's Sickle by meeting at the centre of this region, changing direction and extending anteriorly. As these cells move and concentrate at the posterior end of the embryo, the streak undergoes a single- to multi-layered epithelial sheet transition that makes it a macroscopically visible structure.
Epithelial cells in the lateral edge of the epiblast layer undergo an epithelial-to-mesenchymal cellular transition to delaminate (detach) and migrate down or into the primitive streak. The movement of epiblastic mesenchymal cells down the primitive streak is known as ingression. The first set of cells to move down the primitive streak integrate into the hypoblast layer and form endoderm, the first of the three germ layers. The second set of cells to detach and ingress will fill the space between the endoderm and epiblast layer to form the second germ layer, mesoderm. The remaining epiblast cells will transform into the final germ layer, ectoderm.
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Primitive streak formation via mechanism M4
The primitive streak is a structure that forms in the early embryo in amniotes, such as the chick. It is the first embryonic axial element and defines the future midline of the embryo, serving as a conduit for cell migration during germ layer formation. The primitive streak also establishes bilateral symmetry and determines the site of gastrulation.
The formation of the primitive streak involves the coordinated movement and rearrangement of cells in the epiblast. Initially, the embryo appears circularly symmetric. Then, a group of deep mesenchymal cells at the boundary between the AO and AP in the posterior half of the embryo thicken to form Koller's Sickle, a darker sickle-shaped region. As cells move and concentrate at the posterior end of the embryo, the streak undergoes a single- to multi-layered epithelial sheet transition that makes it a macroscopically visible structure.
One of the mechanisms that contribute to the formation of the primitive streak is mechanism M4. In this mechanism, ST cells attract S cells. This interaction results in the formation of the primitive streak through the movement and arrangement of cells. When the extending streaks formed by mechanism M4 collide and fuse, they do not affect each other until this point of contact. This finding suggests that posterior mesoderm cells produce a diffusible chemical that repels streak tip cells.
The primitive streak is an important structure in the early embryo, and its formation involves various mechanisms that contribute to the development of the embryo and the establishment of its body axes. The interaction between ST and S cells in mechanism M4 is one of the key processes that contribute to the formation of the primitive streak in the chick embryo.
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Frequently asked questions
The primitive streak is a structure that forms in the early embryo in amniotes. It establishes bilateral symmetry, determines the site of gastrulation, and initiates germ layer formation.
The primitive streak is a critical stage in the development of the chick embryo. It is a longitudinal midline structure that forms in the narrower posterior (caudal) region of the developing embryo on its dorsal side.
The formation of the primitive streak involves the coordinated movement and rearrangement of cells in the epiblast. Two counter-rotating flows of cells meet at the posterior end, where the streak forms.
The primitive streak is an important structure during gastrulation in the chick embryo. It is a site where mesoderm and endoderm formation occurs, and it also establishes the left-right and cranial-caudal body axes.
