- Following statements were made about the events occurring during chick development.
A. The fertilized chick egg undergoes discoidal meroblastic cleavage; however the cleavage does not extend into the yolk cytoplasm.
B. Development of primary hypoblast is mediated by localized migration of a group of highly specified and connected cluster of 30-40 cells.
C. By the stage XIII of chick embryogenesis and little prior to primitive streak formation, the formation of the hypoblast is just complete.
D. Hansen’s node of the chick embryo signifies a region at the anterior end of the primitive streak with regional thickening of cells.
E. Inhibition of Wnt planar cell polarity pathway in the epiblast causes the mesoderm and endoderm to form centrally instead of peripherally.
Which one of the following combinations represents all correct statements?
(1) A, B and D (2) A, C and E
(3) A, B and C (4) A, C and D
The correct combination of statements about chick development events is (4) A, C and D.
-
Statement A is correct: Fertilized chick eggs undergo discoidal meroblastic cleavage restricted to the blastodisc; cleavage does not extend into the large yolky cytoplasm.
-
Statement C is correct: By stage XIII of chick embryogenesis, just before primitive streak formation, the hypoblast is fully formed.
-
Statement D is correct: Hensen’s node is located at the anterior end of the primitive streak and exhibits regional thickening of cells.
-
Statement B is incorrect: Hypoblast formation is not mediated by the localized migration of a small cluster of 30-40 cells but rather by delamination and widespread cellular migration.
-
Statement E is not accurately described in the simplified form; inhibition of the Wnt planar cell polarity pathway disrupts convergent extension movements, but the description of mesoderm/endoderm forming centrally instead of peripherally is an oversimplification.
Introduction
Chick embryogenesis is a model of vertebrate development marked by distinctive early events such as discoidal meroblastic cleavage, formation of the hypoblast, and gastrulation beginning at the primitive streak. Hensen’s node at the anterior primitive streak is a key organizer signaling center. These events are tightly regulated spatially and temporally to shape the embryo’s body plan.
Discoidal Meroblastic Cleavage in Chick Eggs
-
The chick egg has a large amount of yolk, preventing cleavage furrows from passing through the entire cytoplasm.
-
Cleavage is restricted to a small blastodisc (the germinal disc), a flat disc of cytoplasm at the animal pole.
-
This cleavage pattern is termed discoidal meroblastic cleavage and results in the formation of the blastoderm.
Hypoblast Formation Completion by Stage XIII
-
The hypoblast forms early in chick development beneath the epiblast through cell delamination and migration.
-
By stage XIII (Hamburger-Hamilton staging), just prior to primitive streak formation, the hypoblast layer is complete across the blastoderm.
-
The hypoblast plays a role in positioning the primitive streak but does not contribute to the embryo proper.
Hensen’s Node Characteristics
-
Hensen’s node is a thickened region of cells at the anterior tip of the primitive streak.
-
It functions as the avian organizer, inducing the formation of the neural tube and body axes.
-
This thickening also exhibits a central depression called the primitive pit or Hensen’s pit, from which chordamesoderm migrates.
Incorrect Statements and Clarifications
-
Hypoblast development involves widespread cell movements and delamination, not simply migration of 30–40 connected cells, making Statement B incorrect.
-
Inhibition of Wnt planar cell polarity impacts convergent extension and gastrulation movements, but the resulting phenotypes are more complex than central versus peripheral mesoderm and endoderm localization as in Statement E.
Summary Table: Review of Statements on Chick Development
| Statement | Description | Correctness | Rationale |
|---|---|---|---|
| A | Discoidal meroblastic cleavage without yolk penetration | Correct | Cleavage restricted to blastodisc; yolk remains uncleaved |
| B | Hypoblast formed by migration of small cluster of cells | Incorrect | Hypoblast formation involving delamination and wider cell migration |
| C | Hypoblast complete by stage XIII before primitive streak | Correct | Supported by Hamburger-Hamilton staging observations |
| D | Hensen’s node as thickening at anterior primitive streak | Correct | Recognized avian organizer with characteristic morphology |
| E | Wnt PCP inhibition leads to central mesoderm/endoderm formation | Incorrect/Inaccurate | Wnt PCP regulates cell movements; phenotypes complex beyond centralization |
Conclusion
Statements A, C, and D accurately describe chick embryogenesis events related to cleavage type, hypoblast formation timing, and Hensen’s node as an organizer. Statements B and E contain inaccuracies or oversimplifications concerning hypoblast formation and consequences of Wnt pathway inhibition.
FAQ
Q: What type of cleavage does the fertilized chick egg undergo?
Discoidal meroblastic cleavage restricted to the blastodisc, not penetrating the yolk.
Q: When is hypoblast formation complete in the chick embryo?
By stage XIII, shortly before primitive streak formation.
Q: What is Hensen’s node?
A thickened region at the anterior primitive streak acting as the avian organizer.
Q: What are the effects of inhibiting Wnt planar cell polarity in chick embryogenesis?
Disruption of convergent extension movements, leading to complex gastrulation defects.
