WBR0138

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Alison Leibowitz)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Embryology
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::A 23-year-old man is referred to a neurologist for evaluation of a progressively worsening vertigo and hearing loss in both ears for the past 4 months. The patient has no past medical history. The patient has no significant past medical history. He denies allergies, smoking, alcohol intake, or use of illicit drugs. A brain MRI is performed and shows bilateral cerebellopontine angle tumors. Which of the following is the most likely embryological origin of the cells from which this patient’s masses arose?]]
Answer A	AnswerA::Mesoderm
Answer A Explanation	[[AnswerAExp::The mesoderm gives rise to muscle, bone, connective tissue, spleen, kidneys, adrenal cortex, cardiovascular and lymphatic structures, wall of gut tube, serious linings of body cavities, vagina, ovaries, testes, and dermis.]]
Answer B	AnswerB::Endoderm
Answer B Explanation	[[AnswerBExp::The endoderm gives rise to the entire alimentary canal except for the mouth, pharynx, the terminal part of the rectum (which are lined by involutions of the ectoderm), and the lining cells of all the glands which open into the digestive tube, including those of the liver, gallbladder, and pancreas. In addition, the endoderm gives rise to the respiratory tract (the trachea, bronchi, and alveoli of the lungs), endocrine system (lining of the follicles of the thyroid gland, parathyroid, and thymus), the auditory system (epithelium of the auditory tube, eustachian tube, and tympanic cavity), and the urinary system (urinary bladder and part of the urethra).]]
Answer C	AnswerC::Surface ectoderm
Answer C Explanation	[[AnswerCExp::The surface ectoderm gives rise to the epidermis of the skin (whereas the dermis is derived from mesoderm), glands, hair, sweat glands, nails, epithelium of the mouth and nasal cavity, salivary glands, and glands of mouth and nasal cavity, mammary glands, tooth enamel (although dentin and dental pulp are derived from ectoderm), and epithelium of the anterior pituitary. The anal canal below the pectinate line also arises from the surface ectoderm. Finally, the surface ectroderm also gives rise to opthalmic structures, such as the lens, cornea, lacrimal gland, tarsal glands, and the conjunctiva, and sensory organs of the ear and the the olfactory epithelium.]]
Answer D	AnswerD::Neuroectoderm
Answer D Explanation	[[AnswerDExp::The neuroectoderm gives rise to the CNS (brain and spinal cord), neurohypophysis, oligodendrocytes, astrocytes, ependymal cells, and pineal gland. Additionally, ophthalmic structures, such as the retina and the optic nerve, are also derived from the neuroectoderm.]]
Answer E	AnswerE::Neural crest
Answer E Explanation	[[AnswerEExp::The neural crest gives rise to the PNS (including celiac and dorsal root ganglia, cranial nerves, schwann cells), autonomic nervous system, pia and arachnoid mater, melanocytes, chromaffin cells of the adrenal medulla, parafollicular C cells of the thyroid gland, bones of the skull and facial cartilage, odontoblasts, ciliary body of the eye, and the aorticopulmonary septum of the developing heart.]]
Right Answer	RightAnswer::E
Explanation	[[Explanation::Neurofibromatosis type 2 (NF2) is an autosomal dominant genetic disorder characterized by its predisposition to the development of tumors, namely multiple schwannomas, meningiomas, and ependymomas. In contrast to neurofibromatosis type 1 (NF1), NF2 is not classically associated with the development of neurofibromas. The hallmark of NF2 is the development of bilateral vestibular schwannomas. Patients typically present with hearing loss, tinnitus, and/or vertigo. The presence of bilateral cerebellopontine angle tumors, the age of the patient, and and the clinical involvement of the CN VIII are all characteristic of NF2. The embryological origin of schwann cells, which abnormally proliferate into bilateral schwannomas in NF2, is the neural crest. Schwann cells are highly plastic, neural crest-derived cells that ensheath the peripheral nerves. Neural crest cells migrate between the ectroderm and the neural tube during development and gives rise to the PNS (including celiac and dorsal root ganglia, cranial nerves, schwann cells), autonomic nervous system, pia and arachnoid mater, melanocytes, chromaffin cells of the adrenal medulla, parafollicular C cells of the thyroid gland, bones of the skull and facial cartilage, odontoblasts, ciliary body of the eye, and the aorticopulmonary septum of the developing heart. Educational Objective: The neural crest is the embryological origin of schwann cells. Patients with NF2 may complain of vertigo, hearing loss, and tinnitus, which often suggest the presence of bilateral schwannomas, a characteristic feature of the disease. References: Evans GR. Neurofibromatosis type 2 (NF2): a clinical and molecular review. Orphanet Journal of Rare Diseases. 2009;4:16. First Aid 2014 page 444]]
Approved	Approved::Yes
Keyword	WBRKeyword::Embryology, WBRKeyword::Neural crest, WBRKeyword::Schwannoma, WBRKeyword::Brain tumor, WBRKeyword::Cancer, WBRKeyword::Neurofibromatosis, WBRKeyword::NF2, WBRKeyword::Mesoderm, WBRKeyword::Development
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