16. In chick embryo, the ectoderm generates
(A) alveolar cells
(B) germ cells
(C) neurons
(D) red blood cells
Ectoderm Gives Rise to Neurons in Chick Embryo
Introduction
Embryonic development begins with the formation of three primary germ layers during the process of gastrulation. These germ layers—ectoderm, mesoderm, and endoderm—serve as the foundation for the development of every tissue and organ in the body. Each germ layer follows a specific developmental pathway and differentiates into distinct cell types under the influence of signaling molecules and transcription factors.
Among the three germ layers, the ectoderm forms the outermost layer of the embryo and is primarily responsible for generating the nervous system and epidermis. During neurulation, a specialized region of the ectoderm called the neural plate folds to form the neural tube, which later develops into the brain and spinal cord. Because of this developmental process, neurons originate from the ectoderm.
Correct Answer
Correct Option: (C) Neurons
Detailed Explanation
The ectoderm is the outermost germ layer formed during gastrulation and gives rise primarily to the nervous system and epidermis. During embryonic development, the dorsal ectoderm thickens to form the neural plate. The neural plate folds inward and forms the neural tube, which differentiates into the central nervous system, including the brain and spinal cord. Neural crest cells, which arise from the edges of the neural tube, give rise to many additional structures such as peripheral neurons, Schwann cells, melanocytes, adrenal medulla, and craniofacial cartilage.
Since neurons develop directly from the neural tube and neural crest, both of which originate from the ectoderm, neurons are considered ectodermal derivatives. Therefore, the correct answer is Option (C).
Explanation of Each Option
Option (A): Alveolar Cells
This statement is incorrect. The epithelial lining of the lung alveoli develops from the endoderm of the foregut. Although connective tissues, blood vessels, and smooth muscles of the lungs originate from mesoderm, the alveolar epithelial cells themselves are endodermal derivatives.
Option (B): Germ Cells
This statement is incorrect. Primordial germ cells are specified early in embryonic development and later migrate to the developing gonads. They are not considered derivatives of the ectoderm.
Option (C): Neurons
This statement is correct. Neurons develop from the neuroectoderm through the formation of the neural plate and neural tube during neurulation. The brain, spinal cord, and much of the peripheral nervous system are derived from the ectoderm.
Option (D): Red Blood Cells
This statement is incorrect. Red blood cells originate from hematopoietic stem cells that develop from the mesoderm. Blood cell formation initially occurs in the yolk sac and later in fetal liver, spleen, and bone marrow.
Why Option (C) is Correct
The nervous system develops from the neuroectoderm. During neurulation, the ectoderm forms the neural tube, which differentiates into neurons and other neural cells. Therefore, neurons are classical derivatives of the ectoderm.
Why the Other Options are Incorrect
Why Option (A) is Incorrect
Alveolar epithelial cells originate from the endoderm rather than the ectoderm.
Why Option (B) is Incorrect
Primordial germ cells are specialized embryonic cells and are not ectodermal derivatives.
Why Option (D) is Incorrect
Red blood cells arise from mesoderm-derived hematopoietic stem cells during embryonic development.
Comparison of All Options
| Option | Structure | Germ Layer of Origin | Correct or Incorrect |
|---|---|---|---|
| A | Alveolar Cells | Endoderm | Incorrect |
| B | Germ Cells | Specialized primordial cells | Incorrect |
| C | Neurons | Ectoderm | Correct |
| D | Red Blood Cells | Mesoderm | Incorrect |
Major Derivatives of the Three Germ Layers
| Germ Layer | Major Derivatives |
|---|---|
| Ectoderm | Brain, spinal cord, neurons, epidermis, hair, nails, lens of eye, neural crest derivatives |
| Mesoderm | Muscles, bones, connective tissue, blood cells, heart, kidneys, gonads |
| Endoderm | Digestive tract lining, respiratory epithelium, liver, pancreas, thyroid follicular cells |
Important Ectoderm Derivatives
| Structure | Developmental Origin |
|---|---|
| Brain | Neural tube |
| Spinal Cord | Neural tube |
| Peripheral Neurons | Neural crest |
| Epidermis | Surface ectoderm |
| Lens of Eye | Surface ectoderm |
| Melanocytes | Neural crest |
Biological Significance of the Ectoderm
The ectoderm plays a fundamental role in embryonic development by forming the nervous system and external protective structures of the body. Proper differentiation of the ectoderm is essential for brain development, sensory organ formation, peripheral nervous system development, and skin formation. Abnormalities in ectodermal development may lead to severe congenital disorders affecting the nervous system, skin, or neural crest-derived tissues.
Final Answer
Correct Option: (C) Neurons
In the chick embryo, the ectoderm gives rise to the nervous system through the formation of the neural plate and neural tube. Consequently, neurons are ectodermal derivatives, whereas alveolar cells originate from the endoderm and red blood cells arise from the mesoderm.
