Extracellular and Intracellular Correlates of Organ Initiation in the Embryonic Chick Thyroid

Abstract

Apical and basal bundles of microfilaments have been suggested as intracellular structures which might cause cellular contractions leading to bending movements during organogenesis. Microfilament bundles may have a different role in some organ primordia. The embryonic chick thyroid develops as an evagination of the pharyngeal floor at the level of the anterior attachment of the heart. The developing thyroid is confined to a pocket surrounded by the developing aortic arches, ventral aorta, and truncus arteriosus. Initiation of thyroid evagination is accompanied by the formation of grooves as two concentric rings in the basal surface of the placode near its margins. The cells surrounding the grooves contain longitudinally oriented bundles of microfilaments and microtubules. Other microfilament bundles are present at the cell apices and bases. The primordium almost doubles in volume during the period of evagination, but the area of pharyngeal floor that it occupies undergoes virtually no increase. A model is presented which describes the role of microfilament bundles during thyroid evagination as stabilizing cellular dimensions. The bending movements are thought to result from cell elongation, increase in cell numbers, and confinement of the thyroid to a limited space by the developing truncus arteriosus and its branches.