Mia Patel
The chick embryo is a popular model for embryological studies due to its year-round accessibility, ease of raising, and predictable developmental stages. Cleavage in the chick embryo occurs exclusively in the blastodisc, a small disc of cytoplasm measuring 2-3 mm in diameter, located at the animal pole of the egg cell. This restriction of cleavage to the blastodisc results in the formation of a disc-like structure, with the embryo's cells arranged in a characteristic pattern. The blastodisc, also known as the blastula, is a critical structure in the early development of the chick embryo, serving as the foundation for the formation of the epiblast and hypoblast layers, which contribute to the development of the embryo and its external membranes, respectively. The complex process of gastrulation, which involves the formation of body axes and the development of the primitive streak, further shapes the embryo's structure and function. Despite the wealth of knowledge about the chick embryo, there is still much to uncover regarding the cleavage-stage chick embryos due to their poor accessibility.
| Characteristics | Values |
|---|---|
| Cleavage | Occurs as the zygote is passing along the oviduct before the egg is laid |
| Blastodisc | A small disc of cytoplasm 2-3 mm in diameter at the animal pole of the egg cell |
| Blastodisc composition | Concentrated cytoplasm with yolk spheres graded in size (dorsal ones smaller than ventral ones) |
| Blastodisc layers | Superficial (epiblast) and deep (hypoblast) layers |
| Subgerminal cavity | Gives the central part of the blastodisc a semitransparent appearance; called the area pellucida |
| Area opaca | Surrounding area where the edges of the blastodisc are in contact with the yolk |
| Cell fate | Cells from the epiblast and hypoblast layers of the area pellucida form the future chick embryo |
| Gastrulation | Cellular intrusion occurs along a slit called the primitive groove |
| Primitive streak | Indicates the location of the future vertebral axis; develops after the egg is laid |
| Embryo development | Lifted off the yolk by the development of infoldings below the future head and tail regions |
| Torsion | Chick's head rests sideways on the yolk instead of face-down |
| Cleavage pattern | Meroblastic; segmentation restricted to the blastodisc |
| First cleavage | Meridional furrow near the centre of the blastodisc, about 4.5 hours after fertilization |
| Second cleavage | Meridional, at a right angle to the first |
| Third cleavage | Vertical |
| Fourth cleavage | Vertical, but division is not synchronous |
| Cleavage progression | Becomes irregular, with a disc of smaller cells appearing and remaining connected to the underlying yolk |
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What You'll Learn
The blastodisc is a small disc of cytoplasm at the animal pole of the egg cell
The blastodisc is formed when the animal pole becomes the blastodisc during oocyte activation. This process takes advantage of the pre-existing biased distribution of cytoplasmic and yolk components of the oocyte. After egg activation, this bias is enhanced by the animal pole-oriented streaming of ooplasm, which results in the separation of ooplasm from the yolk and the lifting of the animal cortex to form a one-cell embryo.
The yolky eggs of birds undergo discoidal meroblastic cleavage, which occurs only in the blastodisc. The first cleavage furrow appears centrally in the blastodisc, and other cleavages follow to create a single-layered blastoderm. As in the fish embryo, these cleavages do not extend into the yolky cytoplasm, so the early-cleavage cells are continuous with each other and with the yolk at their bases.
The dorsal-ventral (back-belly) axis is critical to the formation of the hypoblast and to the further development of the embryo. This axis is established when the cleaving cells of the blastoderm create a barrier between the basic albumin above the blastodisc and the acidic subgerminal space below it. This process distinguishes two sides of the epiblast: a side facing the negative and basic albumin, which becomes the dorsal side, and a side facing the positive and acidic subgerminal space fluid, which becomes the ventral side.
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The blastodisc's central part is called the area pellucida
The blastodisc is a small disc of cytoplasm 2–3 mm in diameter at the animal pole of the egg cell. It is a pale area several millimetres across. Cleavage occurs only in the blastodisc, and a flat cavity called the blastocoele or the cleft space develops within its depth, producing superficial (epiblast) and deep (hypoblast) layers of cells. The presence of a subgerminal cavity gives the central part of the blastodisc a semitransparent appearance, and this area is called the area pellucida.
The area pellucida is where the embryo is lifted off the yolk above the subgerminal cavity. The deep cells in the centre of the blastoderm are shed and die, leaving behind a one-cell-thick area pellucida. This part of the blastoderm forms most of the actual embryo. All the cells for the future chick embryo are recruited from the two layers of cells (epiblast and hypoblast) of the area pellucida. The hypoblast produces endoderm, with a small contribution to the notochord. The epidermis will become the outer layer of the skin. Neural tissue will give rise to the brain, spinal cord, and nerves.
The peripheral ring of blastoderm cells that have not shed their deep cells constitutes the area opaca. Between the area pellucida and the area opaca is a thin layer of cells called the marginal zone (or marginal belt). Some of the marginal zone cells become very important in determining cell fate during early chick development. The two-layered blastoderm (epiblast and hypoblast) is joined together at the margin of the area opaca, and the space between the layers forms a blastocoel.
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The area pellucida is semitransparent and has two layers of cells
The blastodisc is a small disc of cytoplasm, a few millimetres in diameter, located at the animal pole of the egg cell. Cleavage occurs in the blastodisc as the zygote passes along the oviduct before the egg is laid.
The blastodisc contains a subgerminal cavity, which gives the central part of the blastodisc a semitransparent appearance. This area is called the area pellucida. The area pellucida is a central clear zone of blastoderm that does not have direct contact with the yolk. The surrounding area where the edges of the blastodisc are in contact with the yolk is called the area opaca. The area pellucida is formed by the migration of cells from Koller's sickle.
The area pellucida has two layers of cells: the epiblast and the hypoblast. The epiblast is the upper layer of the area pellucida, and the hypoblast is the lower layer. The hypoblast spreads from the posterior end and forms a continuous sheet. The hypoblast cells are about 15-20 μm in diameter and yolky, and are attached to one another loosely. The hypoblast does not contribute any cells to the developing embryo. Instead, the hypoblast cells form portions of the external membranes, especially the yolk sac and the stalk that links the yolk mass to the endodermal digestive tube.
The embryo is lifted off the yolk by the development of infoldings below the future head and tail regions of the embryo. These infoldings unite along the sides of the embryo, and the connection to the yolk is progressively restricted. All the cells for the future chick embryo are recruited from the two layers of cells of the area pellucida.
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The blastodisc's outer area is the area opaca
The blastodisc is a small disc of cytoplasm, a few millimetres in diameter, located at the animal pole of the egg cell. Cleavage occurs only in the blastodisc, and the outer area of the blastodisc is called the area opaca.
The area opaca is a peripheral ring of blastoderm cells that have not shed their deep cells. It is a general mottled area all around the developing embryo where it is resting on the yolk. The area opaca is one of three concentric regions of the chick embryo, the other two being the innermost area pellucida and the intermediate narrow ring called the marginal zone. The marginal zone is a ring of extra-embryonic epiblast that immediately surrounds the embryonic epiblast of the area pellucida.
The two-layered blastoderm (epiblast and hypoblast) is joined together at the margin of the area opaca, and the space between the layers forms a blastocoel. The epiblast forms all three germ layers of the embryo proper, while the hypoblast forms portions of the external membranes, such as the yolk sac and the stalk that links the yolk mass to the endodermal digestive tube.
The area opaca plays a role in positioning the primitive streak of the early chick embryo, which is the site of gastrulation. The juxtaposition of the area opaca directly onto the area pellucida induces a new marginal zone from the latter. Additionally, the area opaca can influence the polarity of the adjacent marginal zone.
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The area opaca is where the embryo rests on the yolk
The blastodisc is a small disc of cytoplasm, a few millimetres in diameter, located at the animal pole of the egg cell. Cleavage occurs as the zygote passes along the oviduct before the egg is laid, and is restricted to the blastodisc. The blastodisc is where the cytoplasm is concentrated, at the opposite pole to the yolk mass.
The area opaca is the outermost ring of the blastoderm, a general mottled area all around the developing embryo where it rests on the yolk. The blastoderm is a two-layered structure (epiblast and hypoblast) that is joined together at the margin of the area opaca. The space between the layers forms a blastocoel. The epiblast provides all the cells for the developing embryo, while the hypoblast forms portions of the external membranes, including the yolk sac.
The area opaca is formed during the intrauterine period and sets up its initial tissue structure. It becomes vascularized and is subdivided into an inner area vasculosa and an outer area vitellina, which contribute to the development of extraembryonic membranes, including the yolk sac. The area opaca is responsible for supplying nutrients to the embryo and driving its expansion through a specialized population of edge cells. It also plays a role in establishing embryonic polarity and orchestrating the formation of the marginal zone, an extraembryonic tissue essential for positioning the primitive streak, the first midline structure, at the beginning of gastrulation.
The primitive streak indicates the location of the future vertebral axis. The embryo is lifted off the yolk by the development of infoldings below the future head and tail regions. These infoldings unite along the sides of the embryo, progressively restricting its connection to the yolk.
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Frequently asked questions
Cleavage is restricted to the blastodisc in chick embryos because the blastodisc is a small disc of cytoplasm sitting atop a large yolk, and cleavage occurs only in this area.
The blastodisc is a pale area several millimeters across, located at the animal pole of the egg cell.
Cleavage consists of the division of the zygote into a large number of cellular entities called blastomeres.
The blastomeres produced during cleavage form a solid cellular ball called a morula. The morula then develops into a hollow sphere called a blastula, which contains a cavity called the blastocoel.
The blastodisc plays a crucial role in the early stages of chick embryo development. It is from the blastodisc that the cells for the future chick embryo are recruited. The blastodisc also helps establish the dorsal-ventral (back-belly) axis, which is critical to the formation of the hypoblast and the further development of the embryo.
