WBR0884

Author	[[PageAuthor::Serge Korjian M.D. (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Embryology
Sub Category	SubCategory::Musculoskeletal/Rheumatology
Prompt	[[Prompt::A 4-year-old boy is brought to the pediatrics clinic for his regular well baby check-up. The child seems to be developing well and has not missed any of his milestones so far. His physical exam is unremarkable except for a persistent soft spot on the skull suspicious for delayed closure of the fontanelles. Which of the following tissues share the same embryologic origin with the bones responsible for fontanelle closure?]]
Answer A	AnswerA::Arachnoid mater
Answer A Explanation	AnswerAExp::The arachnoid mater is derived from neural crest cells similar to the bones of the skull.
Answer B	AnswerB::Skeletal muscles
Answer B Explanation	AnswerBExp::Skeletal muscles are derived from mesoderm unlike the bones of the skull which are dervied from neural crest cells.
Answer C	AnswerC::Kidneys
Answer C Explanation	AnswerCExp::the kidneys are also derived from mesoderm.
Answer D	AnswerD::Retina
Answer D Explanation	AnswerDExp::The retina is derived from neuroectoderm.
Answer E	AnswerE::Anterior pituitary
Answer E Explanation	AnswerEExp::The anterior pituitary is derived from surface ectoderm.
Right Answer	RightAnswer::A
Explanation	[[Explanation::Fontanelles as membranous separations between bones of the skull. They allow for stretching of the cranial cavity to accommodate the rapidly expanding brain early on. Posterior fontanelles usually ossify very early on, within 2-3 months of birth. Anterior fontanelles on the other hand typically close between 4 months and 2 years, half of which occur in the first year. Craniosynostosis is defined as premature complete ossification of the sutures that leads to increased intracranial pressure and deformation of the skull. The bones of the skull, unlike the rest of the skeletal system, are derived from neural crest cells. Other bones are generally derived from the mesodermal layer. Other derivatives of neural crest cells include: Chromaffin cells of the adrenal medulla Parafollicular C cells of the thyroid Schwann cells Cranial nerves Majority of ANS Odontoblasts Aorticopulmonart septum Pia and arachnoid mater Melanocytes Celiac ganglion Educational Objective: Both the arachnoid mater and the bones of the skull are derived from the neural crest cells. References: Crane JF, Trainor PA. Neural crest stem and progenitor cells. Annu Rev Cell Dev Biol. 2006;22(1):267-86. Aisenson MR. Closing of the anterior fontanelle. Pediatrics. 1950;6(2):223-6. First Aid 2014 page 553 ]]
Approved	Approved::Yes
Keyword	WBRKeyword::Embryologic derviatives, WBRKeyword::Neural crest cells, WBRKeyword::Bones of the skull, WBRKeyword::Arachnoid mater, WBRKeyword::fontanelles
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