Reproductive Toxicology

The term "reproductive toxicology" has three different meanings from different fields of study, although they overlap and are often sandwiched into one book chapter. A toxic substance might damage:

  1. the genetic material of cells, often resulting in mutations that are passed on to succeeding generations,
  2. the reproductive system of the parental generation,
  3. the conceptus.

In the first use of the term, we need to distinguish somatic cells from germ cells. Germ cells are sperm and egg cells, and their precursors. Somatic cells are everything else. We have already dealt with mutations to somatic cells. Cancer is the chief problem there, but its direct consequences are limited to the the individual. Chemical or radiation damage to the germ cells may have consequences for succeeding generations.

For the last term, we need to define some terms also. A "zygote" is a fertilized egg. The egg is fertilized in the fallopian tubes and there are many technical terms for the conceptus in these first few days, until implantation in the walls of the uterus. From conception to about 8 weeks, the conceptus is termed an "embryo" and thereafter until birth it is termed a "fetus." The term conceptus can be used for any point in the process.

The textbook on pages 117-122 describes meiosis. You remember that mitosis is cell replication in somatic cells, which we can think of as a one-step process that results in the formation of two new, complete cells from one. Meiosis is a two-step process that results in the formation of 4 new cells, from one original cell, but each of those cells only has half the genetic material of a complete cell. In the male these cells are termed sperm, in the female they are termed eggs or oocytes. A key feature of meiosis is the intermediate cell replication, whereat the chromosomes line up, so that the homologous maternal and paternal chromosomes are aligned. At that point genetic material is swapped between the homologous chromosomes, so that the chromosomes that are delivered to the next phase are not the same as from the parental phase. In my somatic cells are two copies of Chromosome 8, one from my father and one from my mother. During the first replication phase these will align and swap, so of the two copies of Chromosome 8 that are sent on two the next phase, one may be 90% paternal genes and 10% maternal genes; the other copy Chromosome 8 will be the opposite. In the next phase, meiosis II, the cell replicates and makes two haploid germ cells. Meiosis I, with the crossing over, is particularly sensitive to chemical insults.

Germ cells may carry mutations, similar to those we've discussed in somatic cells. Also important are chromosomal aberrations. A chromosome might be broken, for example by radiation. Or chemicals might interfere with separation of the chromosomes, resulting in one of the germ cells having too few or too many chromosomes. The term aneuploidy refers to the incorrect number of chromosomes. The term polyploidy means entire extra sets of chromosomes. CHO cells (Chinese Hamster Ovary) are susceptible to chromosome damage and are a common research tool when investigating chemicals for ability to cause chromosome aberrations

NEXT.

Module 6 Index