ch 22 pt 2

• pharynx definition:

  anterior endodermal portion of the digestive and respiratory term
• why is the pharynx considered to be a "complicated domain" of the human body?

  constitutes the neck and throat
• what does the embryonic pharynx consist of?

  4 pairs of endoderm-derived pharyngeal pouches
• what are pharyngeal arches and what do they become?

  paired bars of mesenchymal tissue, covered by endoderm internally and ectoderm externally

  the first pair become the auditory cavities of the middle ear and the associated eustachian tubes. the second pair become the walls of the tonsils. the third pair gives rise to the thymus
• what is the thymus derived from (and what does the thymus do)?

  third pair of pharyngeal pouches; will direct the differentiation of T lymphocytes
• what is the first endocrine gland to develop? what is it important for?

  thyroid gland; important for critical embryonic growth
• what is the evolutionary "innovation" that deals with the anatomical problem presented by the esophagus being so close to the trachea?

  the epiglottis
• what is the role of "sonic hedgehog" in regards to cranial neural crest cells?

  acts as a survival factor
• what does the digestive tube "constrict" form?

  esophagus
• describe the "polarity" of the endoderm?

  hoxb9 and hoxc9 are expressed in what will be the posterior end of the digestive tube; mouth has to be on one end and the anus on the other
• why we never see a mouth directly opening into a stomach?

  the gradient prevents it from happening
• what is the role of retinoic acid in pharynx development?

  pharynx can develop only in areas containing little or no retinoic acid, whereas the retinoic acid gradient patterns in the pharyngeal arch endoderm in a graded manner
• the looping of the intestinal tube is driven by what? how is this experimentally known?

  a combination of growth intrinsic to the endoderm couple with the connection of that tube to the dorsal mesentry; surgically separating the chick endodermal gut tube from the dorsal mesentry on embryonic day 12 causes the mesentry to shrivel and the gut to straighten.
• how many cell types can be found in the developing human instestine?

  over 100 cell types
• what are the effects of both low and high doses of shh on gut specification?


  low: shh activates the myocardin genes of undifferentiated mesenchyme cells which induce the mesenchyme to become smooth muscle cells

  high: shh induces BMP4 in the mesenchymal cells and shh blocks the induction of these transcription factors and the smooth muscle phenotype