Download WHEN A SPERM MEETS AN EGG…

WHEN A SPERM MEETS AN EGG…
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Conception
The ability to reproduce is necessary for the continuation of the human race. New
life occurs when male and female sex cells unite at conception. The sex cells are
needed to give birth to a new life. These sex cells are given the chance to fuse
and form a new life during sexual intercourse.
In mammals, such as human beings, conception begins with the act of
intercourse. A women is most likely to conceive just after the time she ovulates –
when an ovum has been released from the ovary. During intercourse, sperms
from the man’s penis are deposited into the woman’s vagina. The semen which
contains as many as 400 million sperms are deposited in the vagina. Most of the
sperms leak out of the vagina again, but some begin to swim up in the vagina,
making their way at a very rapid rate to the uterus and the fallopian tubes. As the
time of ovulation, the mucus in the cervix is thinner than usual to allow the
sperms to pass through more easily.
As sperms reach the fallopian tubes where an ovum is likely to be found, one
sperm may then join the ovum. This is commonly known as fertilization.
Fertilization occurs in 5 stages as follows:
− Penetration which occurs when the outer layer of the sperm unites with the
membrane surrounding the ovum.
− Union takes place where the contents of the sperm head have access to the
ovum.
− The sperm as a whole enters the ovum.
− Activation where the ovum develops a reaction which prevents other sperms
from gaining entry to it.
− Fusion of nuclear materials from the two sex cells (ovum and sperm), after
which cell division takes place.
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Development of the fertilized ovum
After fertilization, the fertilized ovum which has become a zygote travels from the
fallopian tube to the uterus. As it travels, a series of events and changes begins
to take place. First, the zygote undergoes cell division. It divides into 2 cells, then
4, then 8, then 16 and so on in a geometric progression. Three to 5 days after
fertilization, the zygote reaches the uterus.
Six to 7 days after fertilization, implantations occurs. The zygote that has become
the blastocyst penetrates and sinks into the uterine lining or endometrium.
The cell divides itself into ectoderm or outer skin which will cover the outer
surface of the body and the endoderm or inner skin which will line the inner
surface. Cells in the position of the ectoderm nearest the inner skin further
multiply to form the skin and nervous system. The rest of the outer skin become
amnion, the membrane which envelopes the embryo. Then the mesoderm is
developed to form the connective tissues, bones, muscles, blood and blood
vessels.
Six to 7 days after fertilization, implantation occurs. The zygote which has
become the blastocyst penetrates and sinks into the thickened uterine lining or
endometrium.
Sometimes, the blastocyst cannot travel from the fallopian tube to the uterus.
Instead, it implants itself in the walls of the fallopian tube. In such rare cases,
fertilization and implantation take place in the ovary. Whether the pregnancy is
tubal, ovarian or abdominal, the general term used is ectopic pregnancy.
At the moment of conception, the genes and chromosomes from the mother and
father unite to form a unique individual with particular traits and characteristics.
Occasionally, two ovum will be released at the same time. If this happens, it is
possible for both to be fertilized and fraternal twins to be conceived. Identical
twins are a result of the division of one fertilized ovum into two completely
separate cells that then continue to develop into 2 babies. Siamese twins are
extremely rare. These are identical twins that started to separate but some parts
of the body remained joined. Surgeons may try to separate the two infants after
birth. Sometimes both survive, sometimes one, sometimes neither.