174 



R. STOLL, P. BLANQUET, A. P. LACHAPELE, R. MARAUD AND A. MAGIMEL 



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Fig. I. Thyroid from a 9 days old embryo. The gland is of a 

 cordonal type and cells are practically devoid of canalicu- 

 lar structures and colloid. Mitochondria and small vacuoles 

 are visible. Magnification 5000. 



and chicks from 8 days of incubation to 13 days after 

 hatching (Light Sussex). Thyroids were also removed 

 from 18 embryos of 9 and 16 days previously treated at 

 3 days with thyroxine (0.001 mg) or tetramethylthiourea 

 (1.5 mg). Choice and dosage of the drugs were established 

 from our previous investigations. The glands were fixed 

 in osmic acid, buffered according to Palade (1952) and 

 embedded in n-butyl methacrylate. Sections 0.05 /< in 

 thickness were cut with a Porter-Blum microtome and 

 examined in a Philips electron microscope (Mod. 1951). 



In the diflferentiation of the thyroid, electron 

 microscope shows two periods identical to those 

 previously described. In the first one, before 12 days 

 of incubation, the canalicular structures and the col- 

 loid are very poor (fig. 1). 



At 8 days, the gland consists of cellular cords 

 which form spherules progressively later. Cellular 

 nuclei contain a nucleolus and are ovoid, sometimes 

 bean-shaped. The cytoplasm possesses a clearly dis- 

 cernible chondriome and numerous vacuoles variable 

 in size and containing a pale substance. The cana- 

 licular structures, very scarce and small, lie generally 

 at the periphery of the cell and show a clear lengthened 

 center surrounded by a thii osmophilic wall. Be- 

 tween 2 or 3 epithelial cells, some small colloid 

 droplets appear, containing filamentous protrusions, 

 or microvilli inserted on the cellular membrane. 



The second period is characterized by the differ- 

 entiation of follicular colloid associated with sudden 

 and general distribution of the canalicular structure. 

 Its beginning is already discernible in the thyroid of 

 12 days old embryo. Sometimes the cell, very poor 

 in canaliculi, presents the aspect of the preceding 

 period; sometimes the cell, apparently devoid of 

 vacuoles, appears clear and possesses numerous 

 canaliculi which are short and fine (fig. 2); sometimes 

 intermediary aspects are present. Generally speaking. 



the small follicles are poor in canaliculi while the 

 greatest are rich in such formations. The epithelial 

 cords still persisting at this time, keep their initial 

 structures. In the 13 days old embryo, the follicles 

 increase in number and size while short canaliculi 

 exist now in all the cells. Some osmiophilic projec- 

 tions, issued from granules of the apical part of the 

 surrounding cells, are seen in the colloid. 



Afterwards, the follicular structure is generalized 

 in the thyroids removed from the 16, 17, 19 days old 

 embryos. The long and twistened canaliculi fill then 

 almost all the cytoplasm where they lie, following 

 its larger amount. According to the disposition of 

 the canaliculi, three states can be distinguished in 

 the cells of the same follicle. Few cells are narrow 

 and dark, because their canaliculi have almost joint 

 walls and poor, lightly osmiophilic contents. The 

 other cells are cuboidal in shape with a relatively 

 clear appearance. This is caused by the canaliculi 

 which are well developed and moderately filled with 

 a pale ground substance. Among these elements, 

 some cells show apical vacuoles which seem to be 

 related to sectioning of highly dilated canaliculi, in 

 the apical part of the cells. Between these vacuoles, 

 osmiophilic granules, variable in size, are scattered. 

 In all the cells, mitochondria are always covered 

 with a canalicular sheet. 



After hatching, the thyroid shows its well known 

 follicular structure. Around the colloid, with its 

 various osmophilic filaments, the three varieties of 

 previously described cells form the follicular wall 

 where a last cellular state is well recognizable (fig. 3). 

 In this case, the canaliculi are so much filled with 

 the pale ground substance that the cell shows an 

 aspect of colloid degeneration. On the other hand, 

 according to the observations of Monroe (14) on 





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Fig. 2. Thyroid from a 12 days old embryo. The cells sur- 

 rounding the follicular colloid present numerous fine cana- 

 licular structures. Magnification 5000. 



