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Monitoring Stem Cell Research 
then populate the new gonads of whichever type. In humans, the 
primordial germ cells first appear by the end of the fourth week of 
development, and begin their migration to the gonads. The 
primordial germ cells share certain characteristics with embryonic 
stem cells, including self-renewal and pluripotency. Primordial germ 
cells have been recovered from fetuses that were aborted (for 
reasons unrelated to research) and cell lines have been established 
from them, the progeny of which showed characteristics of multiple 
different types of cells. ^ 
After the primordial germ cells populate the gonads, some 
continue to divide by mitosis, producing more like themselves. The 
primordial germ cells are diploid, meaning that they have all the 
normal chromosomes of the organism in pairs. In humans, this means 
that they have 22 pairs of autosomes, and one pair of sex 
chromosomes, or 46 total. Mitosis is the name of the process whereby 
the cell replicates its DNA and then divides equally to result in two 
cells, each cell including an entire complement of DNA just like the 
first cell before the division (in humans, that is the 46 total 
chromosomes mentioned) (Figure 2). 
But if a cell is to become an ovum or sperm ready to combine with 
a gamete of the complementary type to produce a new organism (at 
first a zygote) containing the normal number of chromosomes, it must 
imdergo a special type of cell division whereby each gamete 
acquires only half the diploid number. Each mature ovum or sperm 
must include only 23 single (not paired) chromosomes. Mature ova or 
sperm cells are haploid, indicating that their 23 chromosomes in their 
nuclei are unpaired (and after they combine, then the resulting single 
cell the zygote is again diploid). The process whereby the diploid 
primordial germ cells develop into haploid gametes is called meiosis 
(Figure 3). Mitosis is part of the life cycle of any cell, but meiosis or 
meiotic division occurs only in the development of haploid ova and 
sperm from diploid primordial germ cells. The process itself appears 
as though the cell nucleus is undergoing two rounds of mitosis, but 
omits the step of replicating DNA on the second cycle. In the “first 
round," the differentiating primordial germ cell replicates its DNA, 
and then in the “second round" it divides again (without another 
replication). In the second division, the pairs of chromosomes 
separate, leaving each of the new cells with just one copy of each of 
the 22 (in humans) autosomes and just one sex chromosome. 
PRE-PUBLICATION VERSION 
