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THE ROLE OF THE THYROID IN ENDOCRINE CONTROL MECHANISMS* J. E. RALLt As more insight has been gained into the remarkable ability ofbacteria such as Escherichia coli to adapt to varying nutritional states, antibiotics, and foreign environments, one might ask what evolutionary advantage can accrue to a multicellular organism. Increasingly, it seems that the advantages for larger many-celled eukaryotes probably lies in environmental niches that bacteria cannot efficiently fill. The problems imposed by multicellularity are numerous; perhaps the most important of the special features required for efficiency and survival is sensitive and precise intraorganismic control systems. I will discuss in broad context the development of these control systems in metazoa. Clearly, control systems must enhance an organism's chances for survival by making responses to fundamental needs accurate and coordinated. These elementary needs may be generally divided into (1) the ability to adapt to changes in nutrition, including periods of starvation; (2) the ability to survive a large variety of other unfavorable or stressful situations; and (3) the ability to reproduce successfully. In many vertebrates this will include provision for care and rearing of the young. The control systems involved in bacteria are largely genetic. A genetic mechanism has considerable logic to it, since information for the synthesis of all the necessary enzymes can be coded into a single DNA molecule. Then the exigencies of the environment can trigger directly the utilization or cessation of utilization of the requisite information by physical covering or uncovering of the specific portions of DNA involved. Since there is only one cell and one DNA molecule,«this is an efficient control device. In multicellular organisms the problem of control evidently becomes more difficult. All cells have the same DNA, but the efficiency of mul- ?Presented in part at the Symposium on Molecular Mechanisms and Regulation of Thyroid Hormone Biosynthesis: An Example of International Collaboration, Rome, October 18-20, 1972. tNational Institute of Arthritis, Metabolism, and Digestive Diseases, National Institutes of Health, Bethesda, Maryland 20014. I thank Dr. Jan Wolff for extensive discussions, several of the ideas, and refinements thereof, embodied in this discussion. 218 J J. E. Rail · Thyroid Control Mechanisms ticellularity derives from cell specialization. Hence, a straightforward modulation ofDNA transcription would turn on (or off) enzyme synthesis in all cells. Clearly more specificity is required. In some instances, this appears to be provided for by specific hormones and/or quite specific receptors on the plasma membrane of the target cells [1, 2]. In other instances, hormones and cytoplasmic binding proteins appear to confer specificity [3-5]. A problem which I shall not consider still remains: the mechanism for synthesis both of the hormones and of the receptors by only certain cells when all cells possess DNA of identical composition. It is debatable whether a nervous system or some type of chemical control mechanism developed first. One might consider the ubiquitous cyclic adenosine monophosphate as the first hormone, since it causes isolated slime mold cells to aggregate into a true multicellular organism [6]. This is secretion of a chemical by one cell which affects the activity ofanother cell. This mechanism or similar ones developed during evolution to generate an endocrine system by which specialized cells in response to certain stimuli produce substances called hormones, which influence tissues remote from the site of origin of the hormone. As one looks at more primitive organisms, such as insects or mollusks, it is apparent that early in evolutionary history the development of a nervous system occurred. This permitted a quick triggered response to an external stimulus so that the organism could flee, put up a defense, recognize a member of the opposite sex for reproduction, or find food. The most primitive nervous system consists of a sensory nerve, a connection with a central processing neuron or neurones, and an effector nerve. Communication between neurons and between an effector neuron and, say, a muscle is made possible by secretion at the nerve ending of a chemical—acetyl choline, epinephrine, dopamine, etc. This factor of synthesis of a highly potent compound which causes the first effect makes the nervous system analogous to the endocrine system. Distribution of the effector chemical is achieved by widespread physical extension of...

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