Allan L. Goldstein

Is the Thymus an Endocrine Gland?: Old Problem, New Data

It seems appropriate in a volume honoring Dr. Gregory Pincus, a distinguished endocrinologist, to have a contribution that will consider briefly the history of a gland studied by pioneers in experimental physiology and medicine and to indicate more recent developments which not only have attracted renewed attention to this anatomical structure but suggest it should be added to the established list of endocrine glands. It is the purpose of this essay to examine some of the evidence for a possible role ofthe thymus as an endocrine organ. This will be approached by assessing the extent to which studies of the thymus have provided data confirming the postulates generally agreed necessary to establish that a given anatomical structure is an endocrine organ. These data are generally sought in the following experimental stages: (i) extirpation of the tissue or organ with subsequent evidence ofphysiological or biochemical alterations and deficiencies; (2) replacement therapy, that is, efforts to replace the extirpated structure by transplantation of the organ and by administration of suitable extracts of the removed tissue; (3) administration of excessive amounts of an extract of the gland to normal animals in order to study alterations which may result from an exaggerated supply of the

The thymus as an endocrine gland: properties of thymosin, a new thymus hormone.

Publisher Summary This chapter discusses recently assembled evidence that unequivocally establishes a role of major significance for the thymus in both the development and the maturation of the lymphoid system. It presents evidence in support of the thesis that this influence of thymus on the lymphoid system is endocrine in nature and that the thymus gland should be added to the established list of endocrine organs. The chapter focuses primarily on modern concepts of the thymus as an endocrine gland, including effects of extirpation of the thymus, and studies of replacement therapy, the chemistry of thymic fractions, and the biological effects of thymic fractions, both in normal and thymectomized animals. The initial assessment of lymphocytopoietic activity has been based upon an assay developed utilizing adult CBA/W mice. The mechanism by which thymus exerts its endocrine role and the number of hormones secreted by the thymus are in early stages of exploration.

The Thymus Gland: Experimental and Clinical Studies of Its Role in the Development and Expression of Immune Functions

Publisher Summary The mechanism by which the thymus gland exerts its vital influence over the lymphoid system is, at present, largely formulated by hypotheses that are based on rapidly accumulating experimental data. The most primitive vertebrate to possess a thymus-like structure is the lamprey. In most mammals, the thymus is similar to the gland of lower vertebrates. The thymus gland exerts a cardinal role in the regulation of the development and expression of host immunological competence, notably phenomena reflecting cell-mediated immunity. The thymus exerts its regulatory influences by at least two mechanisms: (1) secretion of thymosin, a thymic humoral principle, and (2) functioning as a primary site for the production of classes of lymphoid cells which contribute to host immunity. The increased interest in experimental and clinical studies of the thymus provide a clearer understanding of the chemical nature of thymic secretions and of the processes basic to the contributions of the thymus to normal and pathological states.

Role of Thymosin and other Thymic Factors in the Development, Maturation, and Functions of Lymphoid Tissue*

Publisher Summary This chapter discusses the role of thymosin and other thymic factors in the development, maturation, and functions of lymphoid tissue. The thymus is the central organ in regulating the development and expression of cell-mediated immunity and general host resistance. The present evidence supports the concept that the thymus functions as both a donor of cell to the peripheral lymphoid system and by the production and secretion of one or more humoral factors. The mechanism by which the thymus exerts this apparent endocrine role and the number of hormones secreted by the thymus is in the early stages of exploration. The responses to partially purified thymic fractions of those functions of lymphoid tissue which are fundamental to host immunological competence suggests that several of the diverse crude thymic fractions may contain a single biologically active substance similar to or identical with thymosin. It is also established that the thymus is a major source of lymphocytes for the peripheral lymphoid tissues and circulation. Thus, the thymus exerts a dual function, namely, as an endocrine gland and as a progenitor of lymphoid cells.