Changes with aging

Abstract

The thymus gland is fully developed at birth and weighs about 10 g. It increases in weight until the age of two, when it plateaus, only to start increasing in weight again between the ages of seven and twelve. At this age its weight has doubled and it has become narrower and longer [1] (Fig. 2.1). From then onwards throughout adulthood, there is a progressive replacement of the majority of the perivascular spaces with adipose and fibrous tissue. This process commences around middle age, and by the age of 50 fat accounts for more than 80% of the total thymic volume [2]. As a result, the thymus changes from a pink-grey gland to a yellowish mass that is difficult to distinguish from the surrounding mediastinal fat, and whose margins can only be determined by its capsule. This loss of working glandular tissue is described as "involution" and results in only a minimum of cortical or medullary tissue remaining in the adult gland. Others, however, have suggested that involution begins as early as birth and proceeds at a rate of approximately of 3% per year until middle age, when it slows to a rate of 1% per year. At this rate the thymus is projected to disappear at the age of 120 years [3-5] (Fig. 2.2). This involution usually does not result in a change in the overall size of the gland. It is a controlled process regulated by a number of gonadal and thymic hormones, and can be accelerated by adrenal corticosteroids and sex hormones [6-8]. Cytokines are also known to contribute to this process [9]. Experimental evidence suggests that thymic involution is neither intrinsic nor irreversible, but probably the result of age-related disruption of neuroendocrine- thymic interactions [10]. Thus, this process can theoretically be reversed by the neuroendocrine replacement of GH, TSH, T3, T4 and LH-RH analogues [11]. © Springer-Verlag Berlin Heidelberg 2007.