Date of Award
Dissertation - Restricted
Doctor of Philosophy (PhD)
Victor De Vlaming
Robert H. Fitts
Brian R. Unsworth
Environmental factors have long been thought to play a role in reproductive processes. The observation that, in the wild, many animals will breed only at selected times of the year, was probably the first indication that some factor in the external milieu could affect reproduction.
Light has been the factor that, traditionally, has been studied the most. Depending on the breeding season of a particular species, either increasing or decreasing amounts of daylight can either stimulate or suppress reproductive function. However, other factors, temperature being the most obvious, also vary seasonally. In general, increasing amounts of light will also be accompanied by increasing mean temperature.
The series of hormonal and structural changes that. transform the non-breeding adult animal into one capable of reproducing bear a striking resemblance to the events associated with puberty. It seems reasonable to assume, therefore, that the process of sexual maturation may also be affected by environmental inputs. Once again, light is the factor most studied. Changing the intensity, duration or spectrum of light exposure is, in fact, capable of either delaying or advancing puberty.
There are currently two major hypotheses concerning the control of sexual maturation in the rat. Although not necessarily mutually exclusive, these two theories focus on two different aspect of the maturational process. The first centers on the- hypothalamus and its control of the hormones of the anterior pituitary. This theory states that maturational changes take place in those hypothalamic centers that are responsible for the control of gonadotropic hormone secretion. These changes are such that the sensitivity to the negative feedback effects of gonadal steroids are altered and the blood levels of the gonadotropins rise. This in turn, leads to an increase in gonadal steroid output which initiates the secondary sexual organ growth and the changes characteristic of puberty.
The second major theory of sexual maturation concentrates on the end organs themselves. It states that either spontaneously, or possibly as the result of exposure to constant levels of gonadotropic hormones the sensitivity of these organs to their appropriate stimulatory hormones increases. In the case of the gonads, this leads to an increased output of sex steroids, which stimulates secondary sexual organ development. Alternatively, the sensitivity of the secondary organs themselves may be altered, thus eliminating the need !or an increase in gonadal steroids,
The series of papers that constitutes this thesis examines the effects of lighting and temperature on the process of puberty in the rat. It also attempts to explain the changes in sexual maturation elicited by these environmental changes in light of the current theories of the control of the onset of puberty.