Chick Embryo Thyroid 



175 



Fig. 3. Tli>roi(.i from a just-hatched chick. The canalicular 

 structures are well developed. In the apical part of some 

 cells, osmiophilic granules and vacuoles, developed from 

 canaliculi, are scattered. Note a sinusoidal capillary with 

 endothelial cells. Magnification -< 2000. 



the thyroid of the rat, the capillaries which surround 

 the follicles of chick thyroid are of a sinusoidal type. 



During the first period, thyroxine and tetramethyl 

 thiourea have no evident action on the thyroid. On 

 the other hand, they produce specific changes in 

 the second phase of the differentiation. Following 

 treatment with thyroxine, a few cells appear normal, 

 but the other present a washed-out appearance and 

 are practically devoid of canalicular structures. The 

 canaliculi which differentiate are very small and, 

 according to Vidal (22), the formation of follicular 

 colloid is very reduced. After administration of tetra- 

 methylthiourea, the follicles present columnar cells 

 surrounding a reduced follicular cavity, as described 

 by Adams and Bull (I). The cytoplasm is clear and 

 filled by canaliculi which are strongly twistened by an 

 abundant pale substance. 



During the differentiation of the chick thyroid, 

 the apparition of canalicular structures, permit to 

 separate two periods which correspond to those 

 previously observed after the use of radioiodinc (16, 

 18). 



In the first period, before 12 days, the colloid, the 

 canaliculi and the uptake of radioiodine are very 

 poor. In this case, the extracellular colloid droplets 

 seen by electron microscope, correspond to the 

 chromophobic colloid (21) and probably to the 

 P.A.S. positive droplet*; of the classic microscopy 

 (5, 19, 26). The role of the vacuoles and osmiophilic 

 filaments in the colloid remain unknown. 



In the second period, from 12 days, the physio- 

 logical and histological differentiation of the thyroid 

 advances rapidly. Radioiodine storage and hormonal 

 synthesis increase strongly (2, 16, 20, 26). Simul- 



tancousl> follicles ditVcrcnliatc and colloid develops 

 (17, 21). 



Interrelationships exist between these two groups 

 of phenomena (18). Indeed, previous works on the 

 thyroid of adult mammals ha\c shown that the 

 radioiodine is included in the follicular colloid, after 

 a short passage through the glandular cells (II, 12). 

 The radioiodinc is also stored in ihc colloid of the 

 chick embryo thyroid (10). 



The iodine metabolism o\ the th\roid is therefore 

 related to the colloid whose synthesis follows the 

 apparition o\ the canalicular structures. Their sudden 

 extension, between 12 and 13 days, explains the rapid 

 increase of colloid dcsclopment and radioiodine 

 uptake. 



The factors governing the formation of the cana- 

 liculi are an interesting subject for research. The 

 most important point seems to be the embryonic 

 hypophysis as shown by the use of thyroxine and 

 tetramethyl thiourea. 



Indeed, following treatment with thyroxine, the 

 formation of canaliculi and, consequently, the col- 

 loid elaboration are strongly inhibited. This hor- 

 mone is known to act by the way o\ the hypophysis 

 whose thyrotrophic function is reduced by a high 

 level of thyroxine. On the other hand, the antithyroid 

 compound produces an excessive canalicular devel- 

 opment. This is due to the increase of the T.S.H. 

 whose high level is demonstrated by the hypertrophy 

 of the chick embryo thyroid (I, 8). 



In conclusion, the development of the canalicular 

 structures depends on the hypophysis which so 

 controls the histophysiological differentiation of the 

 thyroid, after 12 days. This explains the modifications 

 of the embryonic thyroid when the hypophyso- 

 thyroideal axis is affected by hypophysectomy (6, 7, 

 13). antithyroid drugs (1,8), or in vitro culture (19). 



Therefore, other factors, as iodide, can intluence 

 the canaliculi and colloid (unpublished data). Such 

 a fact could explain the absence of thyroideal modi- 

 fications sometimes observed after embryonic hypo- 

 physectomy (25). 



Rfferences 



1. Adams, A. E. and Bull, A. L., Amii. Rcc. 104. 421^38 



(1949). 



2. Blanqui r. P., Stoll, R.. Maraid, R., and Caput, L., 



Conipt. rend. soc. hiol. 147. 676 678 (1953). 



3. Bkrnhard, v.. Haguenau, F., Gauthier, A., and 



Obprlinc;. C. Z. Zellforsch. 37. 281 (1952). 



4. Braunstlim R. H.. Fkllinger, K.. and Pakfsch, F., 



Endocrinology 53. 123-133 (1953). 



5. Carpenter, E., Beatty, J., and Chamhi ks, R. D., J. 



E.xpll. Zool. 121, 249-269 (1954). 



6. FUGO, N. W., J. E.xptl. Zool. 85. 271 297 (1940). 



7. FuGO, N. W. and Witschi, E., .-Xcta hiologica larvincaH 



(1938). 



8. Grossowicz, N., Proc. Soc. E.xpil. Biol. Med. 32, 51 



(1946). 



9. Haguenau, F. and Bernhard, W., .Arch. Anat. Micr. et 



Morplwl. Exp. 44. 27-55 (1955). 

 10. Hans borough, L. A. and Mustapha Khan, /. Exptl. 

 Zool. 116, 447-452 (1951). 



