Journey of Fertilisation & implantation

Описание: During copulation (coitus) semen is released by the penis into the vagina
(insemination). The motile sperms swim rapidly, pass through the cervix,
enter into the uterus and finally reach the ampullary region of the
fallopian tube. The ovum released by the ovary is also
transported to the ampullary region
where fertilisation takes place.
Fertilisation can only occur if the
ovum and sperms are transported
simultaneously to the ampullary
region. This is the reason why not all
copulations lead to fertilisation and
pregnancy.
The process of fusion of a sperm
with an ovum is called fertilisation.
During fertilisation, a sperm comes in
contact with the zona pellucida layer
of the ovum and induces
changes in the membrane that block
the entry of additional sperms. Thus,
it ensures that only one sperm can
fertilise an ovum. The secretions of the
acrosome help the sperm enter into the
cytoplasm of the ovum through the
zona pellucida and the plasma membrane. This induces the completion of the meiotic division of the
secondary oocyte. The second meiotic division is also unequal and results
in the formation of a second polar body and a haploid ovum (ootid). Soon
the haploid nucleus of the sperms and that of the ovum fuse together to
form a diploid zygote. How many chromosomes will be there in the zygote?
One has to remember that the sex of the baby has been decided at this
stage itself. Let us see how? As you know the chromosome pattern in the
human female is XX and that in the male is XY. Therefore, all the haploid
gametes (ova) produced by the female have the sex chromosome X whereas
in the male gametes (sperms) the sex chromosome could be either X or Y,
hence, 50 per cent of sperms carry the X chromosome while the other 50 per
cent carry the Y. After fusion of the male and female gametes the zygote
would carry either XX or XY depending on whether the sperm carrying X
or Y fertilised the ovum. The zygote carrying XX would develop into a female
baby and XY would form a male The mitotic division starts as the zygote moves through the isthmus
of the oviduct called cleavage towards the uterus and forms
2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8 to 16 blastomeres is called a morula. The morula continues to
divide and transforms into blastocyst as it moves further
into the uterus. The blastomeres in the blastocyst are arranged into an
outer layer called trophoblast and an inner group of cells attached to
trophoblast called the inner cell mass. The trophoblast layer then gets
attached to the endometrium and the inner cell mass gets differentiated
as the embryo. After attachment, the uterine cells divide rapidly and covers
the blastocyst. As a result, the blastocyst becomes embedded in the
endometrium of the uterus. This is called implantation
and it leads to pregnancy.