Development of cell-to-cell relationships in the thyroid gland of the chick embryo.

Thyroid glands of 7 to 21 day-old chick embryos were examined by electron microscopy using freeze-fracture and thin-section preparations. The primitive follicle lumen first appears between two adjacent epithelial cells in 8 day-old embryos, and is formed in the region of a focal tight junction (macula occludens). The focal tight junction develops into the zonula occludens when the primitive follicle lumen first forms. The zonula occludens is, at first, composed of 4.6 +/- 2.45 strands, but with increasing embryonic age the number of strands increases to 5.9 +/- 1.41 in 13 day-old, and 8.0 +/- 1.75 in 19 day-old embryos. Thyroglobulin stored within the embryonic gland lumina is isolated from the mesenchymal tissue even at the first appearance of these follicle cavities. Well developed gap junctions already occur in the thyroid gland of the 7 day-old embryo, so that an intimate relationship and communication between these cells already exists at the time of their functional differentiation.