|
| |
The thyroid gland is (or was) a part of the gastrointestinal system
In the lowest animals (invertebrates) (Gorbman I958) iodoproteins are formed
in fibrous exoskeletal tissues - to be mentioned - in the pharynx. The
iodoproteins formed in the pharynx - originally in the pharyngeal teeth,
another part of the skeleton - were released into the gastrointestinal
tract, where they were hydrolyzed to provide the "thyroid" hormones. Later
in the evolutionary series (Amphioxus, Ammocetes) the pharyngeal organ
became enlarged and the iodoproteins were products of an exocrine glandular
activity. After metamorphosis in Ammocetes, the iodoprotein-forming tissue
was separated from the rest of the pharynx, the duct was lost, and in the
evolving - now separate - pharyngeal organ active proteinase started to
occur to hydrolyze the iodoproteins. The products of hydrolysis - of course,
the hydrolysis of thyreoglobulin - are released into the blood as thyroid
hormones. Thus the definitive type of the thyroid gland evolved, including
the human thyroid gland. But, even in humans, the iodine-accumulating activity
remains present in the stomach and in the salivary glands, although these
other parts of the gastrointestinal tract do not produce significant amounts
of thyroid hormones. But the question arises: what is the biological advantage
of this iodine-accumulating activity of the human stomach and the salivary
glands? Why was this function preserved?
|
|