Q.23 The primitive streak is derived from
1.anterior epiblast cells
2.hypoblast cells
3.Hensen’s node
4.blastocoel
The primitive streak plays a pivotal role in early avian and mammalian embryogenesis, marking the onset of gastrulation where it establishes bilateral symmetry and gives rise to mesoderm and endoderm. In this MCQ analysis for Q.23—”The primitive streak is derived from”—the correct answer is option 1: anterior epiblast cells, as confirmed by standard embryology texts and exam resources.
Option Analysis
Option 1: Anterior Epiblast Cells
Epiblast cells, specifically from the anterior region of the epiblast layer in the bilaminar disc, converge and migrate to form the primitive streak around day 17 of human development (stage 6 in chick embryos). These columnar epithelial cells undergo convergent extension, thickening along the midline from posterior to anterior.
This is the origin point, driven by signaling from the posterior marginal zone (e.g., Koller’s sickle in chicks), making it the precise source.
Option 2: Hypoblast Cells
Hypoblast cells form the lower layer (visceral endoderm) of the bilaminar disc and contribute to extraembryonic tissues like the yolk sac, but they do not participate in primitive streak formation. The epiblast and hypoblast remain separated by a basal lamina during initial cell movements.
Incorrect, as streak arises solely from epiblast proliferation and ingress.
Option 3: Hensen’s Node
Hensen’s node (primitive node) forms at the anterior end of the primitive streak after it elongates, acting as an organizer for notochord and neural tube induction. It is a derivative of the streak itself, not its source.
Thus, this option reverses the developmental sequence and is wrong.
Option 4: Blastocoel
The blastocoel is the fluid-filled cavity in the blastocyst, present before gastrulation, and collapses during disc formation without contributing cells to the streak. It provides space for early migrations but is not a cellular source.
Incorrect, as the streak derives from epiblast cells post-blastocyst stage.
Developmental Mechanism
During gastrulation, epiblast cells ingress through the primitive streak via epithelial-to-mesenchymal transition (EMT), displacing hypoblast to form definitive endoderm while mesoderm spreads laterally. This process is regulated by BMP, Wnt, and Nodal signaling, with the streak regressing anteriorly to form Hensen’s node.
Key for exams: Primitive streak absence leads to situs inversus or defects like sacrococcygeal teratomas.
Exam Relevance
For competitive exams like CSIR NET, GATE Life Sciences, or CUET PG, this tests gastrulation basics. Focus: Epiblast → primitive streak → mesendoderm fates. Practice similar MCQs on trilaminar disc formation.
