we follow Kardong table 13.1 in having some developmental contribution of pouch 4 in mammals, but this isn't reflected in EHDAA2. Consider adding distinct term for mesenchyme (see EHDAA2), to indicate NC constribution.

A small outgrowth of the pharyngeal epithelium that is the site of lymphocyte cell production. Willett et al, 1999.[TAO]

uberon

UBERON:0005562

variability of developmental origin: In fish, thymic primordia are generated by all the pouches except the first. However, in avians the thymus arises from pouches 3 and 4, whereas in humans it is only generated by the third pouch[PMID:16313389] The thymus arises from the second pouch in frogs, 2-6 in cartilaginous fish, 2-3 in reptiles, 3 or 4 in bony fish, birds and mammals. The final number is variable - 5 paired organs in sharks, 4 in caecilians, 3 in urodeles, 1 in many teleost, anurans and many mammals