Human Reproduction

Question 1

Fertilization and Development to Blastocyst Stage

Answer 1

During sexual intercourse or copulation the penis is placed inside the vagina and the spermatozoa are released
The spermatozoa swim up the uterus and into the fallopian tube
If an ovum is present in the fallopian tube at this time, one spermatozoan may penetrate and fertilize the ovum, resulting in a diploid zygote
Since the sperm cell come from the male parent and the ovum from the female parent, the zygote contains genetic material from both parents
As the zygote passes down the fallopian tube into the uterus it divides by mitosis to form a hollow ball of cells called a blastocyst

Question 2

Development into an Embryo

Answer 2

By the time it reaches the uterus, the blastocyst develops into an embryo
The embryo becomes attached to the endometrium wall of the uterus. This is referred to as Implantation
The endometrium was already thickened with a rich supply of blood vessels by the hormone Oestrogen secreted by the Graafian Follicle
Now that fertilization has taken place, the corpus luteum secretes the hormone progesterone which makes certain that the embryo remains attached to the uterine wall
It ensures that pregnancy is maintained

Question 3

Gestation

Answer 3

The period during which the embryo develops within the uterus of the mother up to the time the baby is born is referred to as pregnancy. It is also called the gestation period
In humans, pregnancy last for about 280 days
During this period, the embryo increases in shape and size as it grows:
14 days after fertilization the embryo still looks like a large dot about 1 mm in diameter
At about 4 weeks old, it becomes comma-shaped and is about 5 mm in length. we can now see the head, and the eyes and ears are just developing. Inside the head a brain is just beginning to develop
By 12 weeks, i has grown quite a lot. It looks clearly like a human baby. It is about 75 mm in length and weighs about 30 grams. The 12 week (three month) embryo is called a foetus

Question 4

Implantation and Development: Role of the Placenta

Answer 4

Soon after the blastocyst attaches itself to the endometrium wall of the uterus, it develops two membranes around itself
These are called extra-embryonic membranes because they occur outside the embryo (‘extra’ means outside)
These membranes are:
The Chorion on the outside, which forms the chronic villi
An Amnion on the inside, with its cavity, filled with amniotic fluid
The Amniotic Fluid has the following functions:
Acts as a shock absorber, protecting the foetus against mechanical injury
Prevents dehydration of the embryo
Keeps foetus within a small range of temperature
Allows relatively free foetal movement for growth and development
The chorionic villi together with the uterine tissue which the villi are embedded makes up the placenta
A hollow, rope-like tube called the umbilical cord, attached the embryo to the placenta

Question 5

The functions of the Placenta

Answer 5

Serves for attachment of the embryo to the mother
Allows for diffusion of dissolved food from the mother to the foetus
Allows for diffusion of oxygen from the mother to the foetus, and for the diffusion of carbon dioxide from the foetus to the mother
Allows for diffusion of nitrogenous excretory wastes from the foetus to the mother
After about twelve weeks of pregnancy it secrets its own progesterone which maintains pregnancy