Re: Question about sperm and egg?

On Tue, 13 Jun 2006 13:18:49 -0400 (EDT), DrBenway <DB@xxxxxxxxxxxxx>
wrote:

On Mon, 12 Jun 2006 13:32:06 -0400 (EDT), r norman
<NotMyRealEmail@xxxxxxxxxxxx> wrote:


The word "egg" has too many meanings. In one sense, it is the ovum,
the single haploid cell that gets fertilized by the sperm, another
single haploid cell. In another sense, it is what a chicken lays (or
the equivalent in many other animals). That other egg contains a
shell along with a lot of other stuff. That egg also usually contains
a developing multicellular embryo, unless it is that non-biological
domestic product, the unfertilized chicken egg.

All amniotes, whether egg-layers like chickens, snakes and lizards,
crocs, turtles and birds or live-bearers like placental mammals
produces an ovum that contains nutrient material, the yolk. Of course
this ovum contains only a haploid genome, one set of chromosomes. At
fertilization, the sperm contributes the second set to produce a
zygote, a single diploid cell with two sets of chromosomes. This set
of chromosomes is the only contribution of the sperm. All of the
other material including all the cellular machinery and, especially,
any cytoplasmic gene regulatory factors that may be present, come from
the mother. The shell, if present, and the fluid/protein "white" is
added usually after fertilization.

The zygote, still containing the nutrient yolk, and inside the
complete "egg" in the larger sense, starts to divide to form the
embryo and the exra-embryonic structures like the chorion, amnion, and
allantois. As the cells divide to form the embryo, the yolk is
usually excluded from the developing embryonic cells but remains
attached as the yolk sac. Developing blood vessels connect the embryo
to the yolk sac so the embryo can continue to grow using the nutrients
from the yolk sac. For egg-layers, the yolk is the only source of
nutrients for the embryo. When the growth is complete, the yolk sac is
completely consumed -- the chick or snake or whatever ready to hatch
contains only the amino acids and carbohydrates and fats present in
the original yolk. The embryo can metabolize, taking in oxygen and
given off carbon dioxide through the chorion and shell, but can't take
in any nutrients -- they all must be present in the yolk. When the
organism hatches, the shell along with the extraembryonic tissues --
the chorion, the amnion, and the allantois -- are discarded (or
eaten).

Placental mammals and a few other non-mammalian organisms work a bit
differently. We still have yolk but very little. The earliest embryo
grows exactly the same as the other amniotes, producing an amnion,
chorion, and allantois and develops a pattern of blood vessels that
initially grows into the yolk mass. However the chorion becomes the
placenta and embeds itself into the uterus of the mother. The blood
vessels from the embryo grow into the placenta so they can exchange
nutrients, respiratory gases, and wastes with the circulation of the
mother. In this way, the embryo can grow far larger for a much longer
time because it is not limited to the nutrients that can fit into the
yolk. The placenta can simply provide as large a quantity of
nutrients as needed. At birth, the extra-embryonic membranes (the
placenta, with amnion, chorion, and allantois, often called the
"afterbirth") is discarded (or eaten).

In any event, the only contribution of the father is the genetic
material in the haploid sperm. Everything else needed to make the
next generation, the other half of the genetic material, all the
cellular machinery, and all of the nutrients that go into the body of
the new generation, come from the mother.



Very nice summation!!
I wonder if you (anyone) would care to add a little about the
duration or the life cycles of sperm and egg cells

I have never been clear on the issue of the "freshness" of
egg and sperm. Since a human (all mammals?) child (both sexes) is
actually BORN with ALL the primary DUPLOID sex cells that many years
later (after puberty) split via Meiosis into the Haploid sperm and egg
cells.

I'd really appreciate a further explanation of the process's that take
place between birth and puberty to these primary sex cells.(oocytes?)
Do they reproduce themselves by standard Mitosis or are they
a finite number determined while the embryo is still in the womb?

The number of haploid cells produced by each sex is an interesting
topic even assuming 40 years x 13 cycles/year x an unusual max of 2
eggs/cycle = 1040 eggs! and much closer to half that, lets say 800
ovum in a woman's life. (If I'm correct, only 1 viable ovum per
female meiosis)

Yet males produce millions of sperm (daily?) And even if there is a
2 to 1 ratio (or is it 4 to 1?) it implies that there would need be
millions of these Duploid source cells to undergo meiosis to yield
millions of the haploid Sperm.

Apparently there is massive mitosis and meiosis in males yet I believe
I have seen it stated that the female is actually born with that fixed
finite number of primaries (no further mitosis) And these
same primaries last her entire life! (thus the incidence of birth
defects increases with a females age, eggs go stale).

What I would really like to understand is that at time of
fertilization both male and female haploid gametes are relatively
"freshly" produced by a recent meiosis, but how old, were their
respective Duploid source cells?. (pre or post puberty)

Thanks so much for any input
my apologies for any blatant mistakes, (please correct them)
Genetics or Biology where never my fields of interest until recently.
Now I find it the most incredibly fascinating of all sciences.


Different organisms work differently -- plants also produce sperm and
eggs although the seed plants modify this so that they are hidden.
But let's stick with animals.

Males produces large numbers of small, motile sperm. Females produce
many fewer because of that yolk business. The growing embryo cannot
feed itself until it hatches or develops sufficiently into a larva
that can. So the more yolk that can be crammed into each egg, the
better chances each embryo has to develop and survive. That limits
the number of eggs produces. Also animals that care for the young
limit the number of offspring by limiting egg production. We humans
produce so few eggs that it turns out that all the potential eggs are
produced before birth, arrested in an early stage of meiosis. Oddly
enough, the second meiotic cell division does not occur until AFTER
fertilization! Males continue to produce large quantities of sperm
during their adult lives, at least during the reproductive season for
most mammals. That is because the diploid germ cells that give rise
to sperm, the spermatogonia, continue to divide by mitosis during
adult life produce large numbers of primary spermatocytes. These are
the cells that divide by meiosis to produce the final sperm. Note:
After one cell divides ten times, it produces about 1000 offspring.
After twenty generations, it produces one million and after thirty it
produces one billion. Yes, it takes a lot of cells dividing to
produce all those sperm! But if each cell is, say, 10 microns in
diameter, then one million will fit in one cubic millimeter of tissue
and one billion will fit in one cubic centimeter.

The "freshness" of the cells is not an issue provided the cells are
specialized to last for long (or for short) times and that the
long-lived ones live in an appropriate environment. Bees, for
example, mate just once. (Actually with several males, but just once
in her life.) The queen bee stores the sperm internally and uses that
storage to fertilize all the eggs she will produce for the rest of her
life -- several years usually. That is, she uses the sperm to
fertilize the eggs that will become workers or other queens. The
males result from unfertilized eggs. So female bees result from
freshly divided eggs but very old sperm. Humans, as I indicated
before, result from freshly divided sperm but eggs that have newly
divided through the first meiotic cell division but before that have
just sat for the lifetime of the mother.


.

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