Pharygeal pouches and origin of parafollicular cells of the thyroid???? - USMLE Forums
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Old 09-04-2010
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Thyroid Pharygeal pouches and origin of parafollicular cells of the thyroid????

Dears,
in Kaplan's home study program it is mentioned that there is a 5th phayrngeal pouch that forms the ultimobranchial bodies which give rise to calcitonin secreting parafollicular (C) cells of the thyroid.
but in High Yield embryology... it is mentioned that the 5th pharyngeal pouch completely regresses in human and that the ultimobranchial bodies come from the ventral wing of the 4th pharyngeal pouch (which is against what is written in kaplan)... which one is correct???
second thing... whether the parafollicular cells (which are rised from ultimobranchial bodies of endodermal orgin) are originated from the 4th or 5th pharyngeal pouches that means that they are of endodermal orgin... how come they are always classified as of neural crest cell origin?? is it migration thing or what??
Can any one explain please
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Old 09-04-2010
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Arrow Fourth Pharyngeal Pouch and Ectodermal C cells

The correct one is the Fourth pharyngeal pouch. Remember that there's no fifth arch (lost), the sixth arch immediately follows the fourth. However, some embryologists would like to name the fourth pouch as rudimentary fifth pouch (reference) that's why your confusion.

The ultimobranchial body itself is of endodermal origin (all pharyngeal pouches are endodermal and all pharyngeal grooves are ectodermal) however the parafollicular cells (C Cells) jump into the utlimobranchial body and these cells are derived from the neural crests (ectoderm) that's why you end up having thyroid gland (endoderm) containing C cells (ectoderm).

References:
http://wiki.medpedia.com/Development_of_Head_and_Neck
http://www.amjmed.com/article/S0002-...079-1/abstract
http://www.mercksource.com/pp/us/cns.../000013743.htm
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2435076/
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Old 09-04-2010
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Quote:
Originally Posted by lee-usmle View Post
The correct one is the Fourth pharyngeal pouch. Remember that there's no fifth arch (lost), the sixth arch immediately follows the fourth. However, some embryologists would like to name the fourth pouch as rudimentary fifth pouch (reference) that's why your confusion.

The ultimobranchial body itself is of endodermal origin (all pharyngeal pouches are endodermal and all pharyngeal grooves are ectodermal) however the parafollicular cells (C Cells) jump into the utlimobranchial body and these cells are derived from the neural crests (ectoderm) that's why you end up having thyroid gland (endoderm) containing C cells (ectoderm).

References:
http://wiki.medpedia.com/Development_of_Head_and_Neck
http://www.amjmed.com/article/S0002-...079-1/abstract
http://www.mercksource.com/pp/us/cns.../000013743.htm
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2435076/
Thanks Lee, the most important thing for me is that for the sake of USMLE there is no 5th pharyngeal pouch (even if Kaplan's home study program books mention 4th and 5th and 6th)
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