Human Reproduction -3 Flashcards
explain the process of reach of sperm to ovum
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 (Figure 2.11b). The ovum released by the ovary is also
transported to the ampullary region
where fertilisation takes place.The process of fusion of a sperm
with an ovum is called fertilisation.
all coitus do not lead to pregnancy
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.
how is polyspermy prevented
During fertilisation, a sperm comes in
contact with the zona pellucida layer
of the ovum (Figure 2.10) and induces
changes in the membrane that block
the entry of additional sperms. Thus,
it ensures that only one sperm can
fertilise an ovum.
when does second meiotic division occur
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.
sex determination
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
what happens to the diploid zygote after fertilisation
The mitotic division starts as the zygote moves through the isthmus
of the oviduct called cleavage towards the uterus (Figure 2.11) and forms
2, 4, 8, 16 daughter cells called blastomeres.
what is morula
The embryo with 8 to 16 blastomeres is called a morula (Figure 2.11e). The morula continues to
divide and transforms into blastocyst (Figure 2.11g) as it moves further
into the uterus.
describe the blastocyst
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 (Figure 2.11 step H). This is called implantation
and it leads to pregnancy.
describe the placenta
After implantation, finger-like projections appear on the trophoblast called
chorionic villi which are surrounded by the uterine tissue and maternal
blood. The chorionic villi and uterine tissue become interdigitated with
each other and jointly form a structural and functional unit between
developing embryo (foetus) and maternal body called placenta
what are the functions of the placenta
i)The placenta facilitate the supply of oxygen and nutrients to the
embryo and also removal of carbon dioxide and excretory/waste materials
produced by the embryo. The placenta is connected to the embryo through
an umbilical cord which helps in the transport of substances to and from
the embryo.
ii)Placenta also acts as an endocrine tissue and produces
several hormones like human chorionic gonadotropin (hCG), human
placental lactogen (hPL), estrogens, progestogens, etc. In the later
phase of pregnancy, a hormone called relaxin is also secreted by
the ovary. Let us remember
that hCG, hPL and relaxin
are produced in women
only during pregnancy.
iii) In
addition, during pregnancy
the levels of other hormones
like estrogens, progestogens,
cortisol, prolactin, thyroxine,
etc., are increased severalfolds in the maternal blood.
Increased production of these
hormones is essential for
supporting the fetal growth,
metabolic changes in the
mother and maintenance of
pregnancy
describe what happens to the embryo after implantation
Immediately after
implantation, the inner cell
mass (embryo) differentiates into an outer layer called ectoderm and an inner layer called endoderm. A
mesoderm soon appears between the ectoderm and the endoderm. These
three layers give rise to all tissues (organs) in adults. It needs to be mentioned
here that the inner cell mass contains certain cells called stem cells which
have the potency to give rise to all the tissues and organs.
what are the developments in the foetus in the first trimester
The human pregnancy lasts 9 months.
In human beings, after one month of pregnancy, the embryo’s
heart is formed. The first sign of growing foetus may be noticed by listening
to the heart sound carefully through the stethoscope.
By the end of the
second month of pregnancy, the foetus develops limbs and digits.
By the
end of 12 weeks (first trimester), most of the major organ systems are
formed, for example, the limbs and external genital organs are welldeveloped.
explain foetal developments in 2nd and 3rd trimester
The first movements of the foetus and appearance of hair on
the head are usually observed during the fifth month. By the end of about
24 weeks (end of second trimester), the body is covered with fine hair,
eye-lids separate, and eyelashes are formed. By the end of nine months
of pregnancy, the foetus is fully developed and is ready for delivery.
define parturition
The average duration of human pregnancy is about 9 months
which is called the gestation period. Vigorous contraction of the uterus at
the end of pregnancy causes expulsion/delivery of the foetus. This process
of delivery of the foetus (childbirth) is called parturition. P
what is the complex process that results in parturition
Parturition is
induced by a complex neuroendocrine mechanism. The signals for
parturition originate from the fully developed foetus and the placenta
which induce mild uterine contractions called foetal ejection reflex. This
triggers release of oxytocin from the maternal pituitary. Oxytocin acts on
the uterine muscle and causes stronger uterine contractions, which in
turn stimulates further secretion of oxytocin. The stimulatory reflex between
the uterine contraction and oxytocin secretion continues resulting in
stronger and stronger contractions. This leads to expulsion of the baby
out of the uterus through the birth canal – parturition. Soon after the
infant is delivered, the placenta is also expelled out of the uterus.
