Pharyngeal Floor Research Articles

Precocious evagination of the embryonic chick thyroid in ATP-containing medium

ABSTRACT The identification of actin and myosin in many cell types other than muscle has given support to the hypothesis that contractile proteins are involved in cell shape changes. However, there is no direct evidence that a contractile process participates in morphogenetic movements during organogenesis. As a first step in testing this possibility, thyroid placodes of chick embryos were treated with an incubation medium containing Triton X-100 and ATP. Pharyngeal regions, isolated in Medium 199, were photographed at timed intervals. At a concentration of 10−3 M ATP, the thyroid region formed a deep pit within minutes after addition of this ‘contraction medium’, whereas evagination requires approximately 7 h in ovo. Treatment with Medium 199 containing either Triton X alone, ATP alone, or Triton X and pyrophosphate did not result in evagination of the thyroid. Substitution of other nucleotides for ATP suggested a specific requirement for ATP. Surgical removal of selected portions of the pharyngeal floor and examination of sectioned material by light microscopy indicated that the cells involved in the shape change were located at the periphery of the thyroid placode before treatment. The sharp bends that were formed in ‘contraction medium’ cannot be explained entirely by pinching of cell apices at the point of folding; rather the effect of several forces is indicated in the evagination.

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

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.

Ciliary feeding mechanisms in anuran larvae.

THE feeding mechanism and mode of feeding of tadpoles of Xenopus lævis have been described, notably by Bles1 and by Weisz2. These workers agree in postulating a ciliary mechanism; but their accounts of the nature and functioning of this mechanism differ widely. Bles recognized a ciliated pharyngeal groove lying along the outer margin of the branchial chamber on each side as the main food-collecting channel. Weisz, on the other hand, holds that food particles never reach the branchial chambers, and makes no mention of the ciliated paths which lie at their outer margins and converge on the œsophagus. He states that the middle region of the pharyngeal floor (called by him the “pharyngobranchial tract” and said to be ciliated) is responsible for creating ciliary currents by which food particles pass backwards into the alimentary canal.

ADENOCARCINOMA OF TONGUE ARISING FROM VESTIGE OF MEDIAN ANLAGE OF THYROID GLAND

The origin of the median anlage of the thyroid gland as a diverticulum from the mesobranchial region of the pharyngeal floor, which lies between the ventral extremities of the first two gill pouches, was conclusively demonstrated by Norris 1 in a study of human embryos. According to Joll, 2 maldevelopment of the median anlage is known to give rise to a variety of abnormalities, which may be located in the midline from the foramen caecum to the level of the suprasternal notch. They include solid tumors, cysts, sinuses and fistulas. A solid tumor of the thyroglossal tract is most common in the region of the foramen caecum, although microscopic amounts of thyroid tissue may occur in any part of the primitive tract. Such a tumor usually consists of well differentiated thyroid tissue and constitutes a lingual thyroid. Ray 3 collected 250 cases of this condition recorded in the literature from

FURTHER OBSERVATIONS AND EXPERIMENTS ON THE SO-CALLED THYROID CARCINOMA OF THE BROOK TROUT (SALVELINUS FONTINALIS) AND ITS RELATION TO ENDEMIC GOITRE

FURTHER OBSERVATIONS AND EXPERIMENTS ON THE SO-CALLED THYROID CARCINOMA OF THE BROOK TROUT (SALVELINUS FONTINALIS) AND ITS RELATION TO ENDEMIC GOITRE