84. What is the role of the notochord during organogenesis in a vertebrate embryo?
(A) Signaling the development of placenta
(B) Induction of neural plate formation
(C) Stimulation of the umbilical chord formation
(D) Suppression of the development of extra-embryonic membranes
The notochord plays a pivotal role in early vertebrate development, acting as a defining feature of chordates. During organogenesis—the phase where major organs form—this structure emerges from the mesoderm and influences key embryonic patterning. If you’re prepping for exams in developmental biology, genetics, or molecular biology, understanding its functions is essential.
This article breaks down a common multiple-choice question (MCQ) on the topic, highlights the correct answer, and explains every option with scientific backing. Let’s dive in.
Correct Answer: (B) Induction of neural plate formation
Why is (B) Correct? Detailed Explanation
The notochord induces neural plate formation through a process called neural induction. Here’s how it works:
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In vertebrate embryos (e.g., frogs, chicks, mice, humans), the notochord forms midline beneath the ectoderm around gastrulation.
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It secretes signaling molecules like Sonic hedgehog (Shh), noggin, chordin, and follistatin from the Spemann-Mangold organizer (or equivalent axial mesoderm).
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These signals inhibit BMP (bone morphogenetic protein) signaling in the overlying ectoderm, transforming it into neuroectoderm—the neural plate.
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The neural plate folds into the neural tube, precursor to the central nervous system (brain and spinal cord).
Without the notochord, neural tube defects like anencephaly or spina bifida can occur. Classic experiments, such as Spemann’s organizer transplants in amphibians, confirmed this inductive role. In humans, notochord signals pattern the ventral neural tube and floor plate.
This makes (B) the precise function during organogenesis.
Why Are the Other Options Incorrect?
Let’s evaluate each distractor with evidence:
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(A) Signaling the development of placenta
Incorrect. The placenta forms from trophoblast cells (extra-embryonic) and uterine tissue, driven by hormones like hCG and signals from cytotrophoblast/syncytiotrophoblast. The notochord is intra-embryonic and absent in non-mammalian vertebrates (e.g., fish, birds) that lack placentas. No direct notochord-placenta link exists. -
(C) Stimulation of the umbilical chord formation
Incorrect (note: likely a typo for “cord”). The umbilical cord arises from the connecting stalk (allantois and yolk sac remnants) and vitelline duct, with vasculature from extra-embryonic mesoderm. Notochord doesn’t influence this; it’s unrelated to cord mesenchyme or Wharton’s jelly formation. -
(D) Suppression of the development of extra-embryonic membranes
Incorrect. Extra-embryonic membranes (amnion, chorion, allantois, yolk sac) develop from trophoblast and hypoblast before organogenesis. Notochord signaling patterns the embryo proper, not suppressing these structures. In fact, amniotes rely on them, and notochord ablation doesn’t affect membrane formation.
Visual Summary: Notochord Functions in Organogenesis
| Function | Description | Key Signals | Outcome |
|---|---|---|---|
| Neural Induction | Inhibits BMP in ectoderm | Shh, Noggin, Chordin | Neural plate → Neural tube |
| Axial Patterning | Defines body axis | Shh gradients | Ventral neural tube, somites |
| Scaffolding | Temporary skeletal role | N/A | Later replaced by vertebral column |
Broader Context for Students and Researchers
In population genetics or plant biology crossovers (your interests), note how notochord homologs influence conserved pathways like Hedgehog signaling, seen in Arabidopsis root patterning. For lab work, CRISPR knockout of Shh in zebrafish models disrupts notochord function, mimicking human neural defects.
Mastering this strengthens your grasp of molecular embryology. Sources: Gilbert’s Developmental Biology (11th ed.), Wolpert’s Principles of Development.
