Early organogenesis of the embryonic chick thyroid. I. Morphology and biochemistry

Early organogenesis of the embryonic chick thyroid can be separated into five distinct phases. (1) A placode is first recognized at 48 hr in the midventral pharynx. Its cells are compact, undergo extensive blebbing, and possess apical microfilaments. (2) Vesicle formation coincides with the organization of the microfilaments into pronounced bundles with dense midbodies along their lengths. (3) The vesicle detaches from the pharynx, and as the lumen disappears the band of microfilaments dissipates. The primordium now consists of a loose mass of interior cells with a compact surface cell layer. (4) The primordium elongates laterally and divides to form the bilateral glands. (5) After division, mesenchymal and vascular invasion occur constructing the cords which will give rise to follicles. Thyroxine accumulation is first detected with the recognition of the primordium and increases logarithmically until the vesicle begins to detach from the pharynx. Accumulation of hormone then plateaus until invasion occurs and again increases until after hatching. DNA analyses describe a similar profile for cell numbers with a concomitant plateau. Thus, synthesis of thyroid hormones coincides with the appearance of the primordium, is restricted to the primordium, and describes a concerted profile similar to that found in the pancreas (Rutter et al., 1968).