4-10-9 Morphological changes may also result from pollutant exposure, but it is necessary to distinguish between primary causes in which the pollutant directly affects the cellular integrity of the gonad, and secondary causes which result from under or over secretion of pituitary gonadotrophin.
4-10-10 Pollutants can also affect a range of developmental processes within the embryo and larvae.
4-10-11 In addition to their toxic effects, some pollutants may act as hormone agonists or antagonists.
4-10-12 To act, a hormone must bind to a receptor.
4-10-13 In the case of large peptide molecules such as gonadotrophin, these are located in the cell membrane, but cause concentration changes within the cell of second messengers such as c-AMP or calcium.
4-10-14 Small molecules such as steroids pass through the membrane and bind to receptors within the cell, which then initiate translation of relevant genes within the DNA of the nucleus.
4-10-15 Such receptors may recognize a pollutant, such as an environmental estrogen, bind it and act in the same way as if they had bound the natural hormone.
4-10-16 Such pollutant will act as hormone mimics.
4-10-17 Alternatively the receptor may bind a pollutant, but fail to elicit the same changes that occur when it binds to the natural hormone, or it may be bound so strongly that it cannot be displaced by the natural hormone (Figure 4.11).
4-10-18 The anti-androgenic action of the DDT metabolite DDE in mammals (Kelce et al., 1994, 1995) may result from such mechanism where the receptor site is blocked and cannot bind the natural hormone.
4-10-19 In both cases, the relative affinity of the pollutant and the natural hormone for the receptor site will determine the activity.
4-10-20 In the case of membrane receptors for peptide hormones such as gonadotrophin, pollutants could affect either binding of the hormone to its receptor or the production and action of the second messenger.
4-10-21 Inhibition of adenyl cyclase or changes in calcium balance could be particularly important in this respect.
4-10-22 In addition to these direct factors affecting the endocrine part of the gonad, pollutants may affect gamete development by disruption of hormonal balance, nutrition and growth.
4-10-23 In the ovary, pollutants may be transferred into the oocytes where they dan affect normal development of the larvae.
4-10-24 Such development may be regulated by thyroid hormones, growth hormones and a range of intracellular messengers.
4-10-25 Sperm motility, and hence fertilization, can be affected by developmental abnormalities resulting from either faults in the hormones from the interstitial (Leydig) cells, or by nutritional abnormalities resulting from disturbance of the Sertoli cells.
4-10-26 The liver plays an important role in metabolizing steroids, and hence in maintaining the plasma levels.