Re: Won't gametes eventually run out?

Hi Heidi,

thanks for your question.  You are absolutely right in thinking that fully 
differentiated gametes (eggs, or ova, and sperm) don’t undergo further cell 
division.  Fortunately,  the process which gives rise to these cells, 
called meiosis, occurs on a very large scale, so that plenty of gametes are 
created.  The process differs a little bit between males and females, so 
here’s the lowdown on each process as it occurs in humans:

Spermatogenesis—the meiotic divisions which give rise to the sperm—begins 
with the cells in the testes called spermatogonia.  Each spermatogonium is 
a type of cell which can divide to give rise to another cell like itself as 
well as producing a new cell type (this is basically the definition of a 
stem cell, something you may have heard of in the news lately).  The new 
type of cell, after some growth and 4 morecell divisions, is referred to as 
the primary spermatocyte.  This is the first cell that will actually 
undergo the reduction of nuclear material characteristic of meiosis..  
First the DNA replicates so that there are 2 copies of each chromosome in 
the nucleus of each primary spermatocyte.  The subsequent division 
separates the replicated homologous chromosomes into the two new cells, 
known as the secondary spermatocytes.  These secondary spermatocytes each 
contain half the number of replicated chromosomes as the primary 
spermatocyte, and when they divide, each of the resulting cells, called 
spermatids, will contain one single copy of each type of chromosome.  These 
spermatids will not divide again, but will differentiate into mature 
spermatozoa (sperm).   
The process of spermatogenesis begins at puberty and continues for the rest 
of the life of the male.  The whole process, from spermatogonium to mature 
sperm, takes about 48 days, and once the production gets going, a human 
male can produce about 2 million mature sperm every day!  Sperm that are 
not released through ejaculation are passed out of the body in the urine or 
are recycled by the body.   

Oogenesis is the corresponding process in females which gives rise to ova, 
and there are some differences compared to spermatogenesis.  In humans, 
oogenesis begins before birth, in the developing ovaries of the unborn baby 
girl.   Cells called oogonia divide for several months during fetal 
development, and produce about 7 million more oogonia.  Most of these will 
die off before the birth, but a fraction of them will become the primary 
oocytes which undergo meiosis.   Now for another major difference between 
spermatogenesis and oogenesis:  These surviving primary oocytes will 
"pause" at the beginning of meiosis, until the hormones that are produced 
during puberty signal them to continue.  When a girl enters puberty, one 
primary oocyte proceeds through the first meiotic cell division, and a 
single secondary oocyte is released from the ovary.  Why only one secondary 
oocyte, and not two?  Because the primary oocyte undergoes an unequal cell 
division at this stage.  The homologous chromosomes are still separated 
equally between the two "daughters", but other components of the cell, like 
cytoplasm and organelles, only go to one of the two daughters.  This cell 
becomes the secondary oocyte, and the other "cell", which is really just a 
little packet of DNA with not much surrounding it, is called a polar body, 
and it will not divide again.   If, and only if, this secondary oocyte is 
fertilized by a sperm, it will proceed to the second stage of meiosis.  
This is another unequal division which forms a polar body and an ovum.   If 
fertilization does not occur,  the secondary oocyte will NEVER complete 
meiosis, and will just pass out of the body with the menstrual fluid.  This 
process continues, one oocyte at a time, every month, until a woman enters 
menopause.  During a woman’s life,  about 500 secondary oocytes will be 
produced, but far fewer than that will actually be fertilized.
 
So, there are the highlights of meiosis in males and females, but the 
bottom line (and the answer to your question) is that our bodies produce 
more gametes than we can actually use so that we’ll have plenty to get us 
through our reproductive lives.  Some of this stuff is pretty detailed, and 
a good diagram would really help you if you’re having a hard time 
visualizing the processes I’ve described. The link listed below has great 
diagrams and good explanations to go along with them.  I hope this helps!  
Please feel free to write again if you have additional questions.  

Warm Regards, 

Jen   http://www.hysa.org/~jkimball/BiologyPages/S/Sexual_Reproduction.html