Electron microscoic studies on the development of the thyroid gland of chick embryo

Summary

Normal thyroid glands of White-Leghorn chick embryo, ranging in age from 6 to 17 days of incubation were studied with the light as well as electron microscope.

• 1. By light microscopy, the thyroid tissue from 6 to 10 days of incubation consists of the epithelial cell cord and the loose mesenchyme, and the PAS-positive droplets begin to be found distinctly throughout the epithelial cell cord in a 9 day-old embryo.

• 2. By electron microscopy, the thyroid cell of a 7 day-old chick embryo is characterized by numerous abundant polysomes. Though a few mitochondria and small Golgi area are noticed, the rough-surfaced endoplasmic reticulum is very poor. This structure is developed more and more with increase of incubating days.

• 3. In an 8 day- (rarely in a 7 day-) old embryo, the primitive follicle lumen appears between two epithelial cells. Fairly long microvilli are well developed and a terminal bar forms a ring at the edge of the lumen. Around the lumen are numerous small vesicles considered to be derived from the Golgi field. The developmental endoplasmic reticulum, the Golgi apparatus lying near the lumen, numerous vesicles which might be derived from the Golgi region, a central flagellum projecting from the epithelial cell, and an appearances of the terminal bar are considered to be important elements in making up the primitive follicle lumen. In a few cases, the primitive follicle lumen has a shallow intracellular bay or extension which could be interpreted as an intracellular follicle lumen.

• 4. A few small dense granules suggesting lysosomes appear at 8 days (rarely at 7 days) of incubation in or near the Golgi region.

• 5. A few large intracellular less dense droplets are first found near the primitive follicle lumen at 10 days of incubation. It is not yet solved whether this droplet is a secretory substance or a resorbed one.

• 6. By mitosis of the epithelial cells and fusion of the follicle lumens, the primitive follicle lumen enlarges with increase of days. The loose mesenchyme begins to penetrate into the epithelial cell cord to make up each follicle at 10–11 days of incubation, and the typical follicle is mostly completed at 14 days of incubation.

• 7. All the mesenchyme cells including the endothelial cell and connective tissue cell are characterized by abundant polysomes and poorly developed cytomembrane system. The rough-surfaced endoplasmic reticulum is developed more and more with incubating days in the endothelial as well as connective tissue cells. Rerely, a round cell, high in electron density, suggesting an erythroblast is seen in contact with the endothelial cell in 7–9 days old embryos.