Early Early Development of Development of

Zygote in Zygote in HumansHumans

FertilisationFertilisation

FertilisationFertilisation1. During sexual intercouse,about 100 to 500 millions sperms will be 1. During sexual intercouse,about 100 to 500 millions sperms will be

jaculatedinto the vagina in the form of semenjaculatedinto the vagina in the form of semen2. Trough the vagina, the sperms move into the uterus and towards 2. Trough the vagina, the sperms move into the uterus and towards

the Fallopian tube.the Fallopian tube.3. In the uterus, the number of sperm is greatly reduced. When the 3. In the uterus, the number of sperm is greatly reduced. When the

sperms reach the Fallopian tube, there are only a few hundreds sperms reach the Fallopian tube, there are only a few hundreds sperm left.sperm left.

4.Of all the sperms that managed to move towards the secondary 4.Of all the sperms that managed to move towards the secondary oocyte, only one sperm will fertilise the secondary oocyte.oocyte, only one sperm will fertilise the secondary oocyte.

5. In the Fallopian tube,the first sperm that reaches the secondary 5. In the Fallopian tube,the first sperm that reaches the secondary oocyte will penetrate into the egg membrane with its head. oocyte will penetrate into the egg membrane with its head.

6.Upon entry of the sperm, the secondary oocyte is stimulated to 6.Upon entry of the sperm, the secondary oocyte is stimulated to undergo meiosis (2). An ovum and a polar body is formed.undergo meiosis (2). An ovum and a polar body is formed.

7. The nucleus of the sperm the fuses with the nucleus of the ovum 7. The nucleus of the sperm the fuses with the nucleus of the ovum to form to form a zygote.a zygote.The process is called The process is called fertilisation.fertilisation.

8. As soon as fertilisation occurs, a wall is built around the fertilised 8. As soon as fertilisation occurs, a wall is built around the fertilised ovum to prevent any further entry of sperms.ovum to prevent any further entry of sperms.

The Nuclei Fuse The Nuclei Fuse TogetherTogether

Development of ZygoteDevelopment of Zygote1. As the the zygote moves down the 1. As the the zygote moves down the

Fallopian tube, it divides repeatedly Fallopian tube, it divides repeatedly trough mitosis to form a blastocyst.trough mitosis to form a blastocyst.

(a) The zygote undergoes mitosis (a) The zygote undergoes mitosis division to division to

form a zygote with two cells.form a zygote with two cells.

Cleavage (divide via Cleavage (divide via mitosis) forms the 2 cell mitosis) forms the 2 cell

stagestage

((b) Both cells the divide to form a b) Both cells the divide to form a zygote with fourzygote with four

cells.cells.

They split again to form the They split again to form the 4 cell stage4 cell stage

(c) Subsequently, mitosis occurs to form a (c) Subsequently, mitosis occurs to form a zygote with eight cells, sixteen cells and zygote with eight cells, sixteen cells and finally a zygote with a few hundred cells.finally a zygote with a few hundred cells.

(d) When the zygote reaches the uterus, it (d) When the zygote reaches the uterus, it has has

become a solid ball with hundreds of become a solid ball with hundreds of cells,cells,

called called morulla.

And eventually form a And eventually form a MorulaMorula

(e) Eventually, the mass of hundreds (e) Eventually, the mass of hundreds of cells changes into a fluid-filled of cells changes into a fluid-filled sphere called sphere called blastocyst. The blastocyst consists of an outer layer of cells and an inner cell mass.

BLASTOCYSTMorulla

((f) The outer layer of cells will later f) The outer layer of cells will later develop into placenta and the inner develop into placenta and the inner cells mass will develop to form an cells mass will develop to form an embryo.

Implantation of Implantation of BlastocystBlastocyst

1.1. During the implantation, the outer layer of the During the implantation, the outer layer of the blastocystblastocyst attaches to the attaches to the trobhoblast villi.trobhoblast villi.

2.2. The villi secrete enzymes to dissolve the cells in the The villi secrete enzymes to dissolve the cells in the uterine wall, forming a cavity that allows the blastocyst uterine wall, forming a cavity that allows the blastocyst to be embedded into the wall.to be embedded into the wall.

3.3. The villi with rich supply of blood capillaries extend The villi with rich supply of blood capillaries extend into the endometrium to implant the blastocyst.into the endometrium to implant the blastocyst.

4.4. In the endometrium,, the inner cell mass in the In the endometrium,, the inner cell mass in the blastocyst will develop to form the blastocyst will develop to form the embryo.

5.5. The villi from the embryonic tissue and the cells of the The villi from the embryonic tissue and the cells of the uterine wall (maternal tissue) will form the uterine wall (maternal tissue) will form the placenta.

6.6. Dissolved gases and nutrients from the mother’s blood Dissolved gases and nutrients from the mother’s blood in the uterine wall could diffuse into the trophoblast in the uterine wall could diffuse into the trophoblast villi of the placenta that contains embryo’s blood.villi of the placenta that contains embryo’s blood.

7. The embryo will develop to form a 7. The embryo will develop to form a foetus within two months.

8. The foetus will then develop throughout the whole term and is then born as a baby.

Formation of Twins.Formation of Twins.(a) (a) Identical twinsIdentical twins1. Identical twins develop 1. Identical twins develop from a a single single zygote. . Only one sperm and

one ovum are involved in fertilisation to produce a single zygote.

2. During the early development stage, the zygote divide completely into two identical zygotes.

3. Each zygote develops to form an embryo within the same blastocyst.

4. Both embryo develop to form two separate foetus but but share theshare the same placenta. same placenta.

5. The two embryos are genetically identical because they developed from a single zygote.

6. The two identical individuals (twins) will have inherited traits and are identical in every aspect; same sex, blood group and physical characteristics.

(b) Siamese twins (conjoined twins)1. There are times during the formation of identical

twins when the zygote does not divide completely.2. As a result, two zygotes are formed but they are

conjoined at certain parts of the body or organ. The conjoined zygotes develop into conjoined embryos and finally become a pair of conjoined twins or commonly known as Siamese twins.

3. The siamese twins may be joined physically or may share certain organs such as stomach, heart or brain.

4. Siamese twins are genetically because they develop from the same single zygote.

5. Hence, Siamese twins are also identically with regards to their sex, blood group and all inherited traits.

Siamese twinsSiamese twins

(c) (c) Fraternal twins.1. Formed when two ova are released from the ovaries

simultaneously and are fertilised by two different sperms respectively.

2. Two separate zygote are formed and each develop individually to form two separate embryos. The embryos develop to form two foetus that have their own placenta.

3. Characteristics if the fraternal twins. - They are genetically different because they are formed

from two different ova and two different sperms. - They are physically different and may or may not be of

the same sex. However, they may have certain similar traits, as other siblings in the family would.

4. Fertility treatment that involves high dosage of FSH can stimulate the ovary to release more than one ovum during ovulation. This may result in multiple births and the formation of fraternal twins.

Structure and Function of Structure and Function of Placenta on Foetus Placenta on Foetus

Development.Development.1. Placenta is an organ by which the embryo is attached onto the uterus wall. It is made of embryonic tissues and maternal tissue from the uterine wall.

2. During implantation, trophoblast villi from the other layer of blastocyst are extended into the uterine wall to form the placenta.

3. Each villus has a network of blood capillaries that are separated from the mother’s blood.

4. The placenta forms a selective barrier betwee the motehr’s blood and the foetal blood.

5. Exchange the substancex occurs through diffusion between the mother’s blood and the foetal blood.

6. The placenta is joined to the foetus by the umbilical cord.7. In the umbilical cord, these are two blood vessels to transport

substances between the placenta and the foetus : umbilical vein and umbilical artery.

8. The umbilical vein transport from the placenta to the foetus while the umbilical artery transport substances from the foetus to the placenta.

9. The mother’s blood that flows into the placenta via the mother’s artery is rich on oxygen and dissolved food materials (nutrients).

10. In the placenta, oxygen and nutrients (glucose, amino acids, mineral salts, lipids and vitamins) from the mother’s blood diffuses across the villi membranes of the placenta into the foetal blood. Then, they are transported to the foetus via the umbilical vein.

11. Carbon dioxide and nitrogenous waste (urea) from the foetus are transported out of the foetus via the umbilical artery of the placenta. These waste product are diffused out of the placenta in to the mother’s blood. The waste products will be removed from the mother’s blood by the mother’s excretory system.

12. Functions of the placenta.(a) Allows exchange of substances

between the mother’s blood and the foetal blood.

(b) Allows the embryo or foetus to attach to the uterine wall.

(c) Transport antibodies from the mother’s blood to the foetus.

(d) Secretes hormones progesterone and oestrogen.

13. Adaption of placenta for its function.(a) Numerous villi in the placenta, to

increase the surface area and to facilitate diffusion.

(b) Thin membrane which separates the foetal blood capillaries from the mother’s blood capillaries.

(c) Both the uterine wall and the placenta are supplied with rich networks of capillaries.

14. The blood of the mother and the foetus do not mix. This is important to;

(a) Protect the foetus from the higher pressure of the mother’s blood, which could damage the delicate foetal blood vessels.

(b) Prevent certain toxic substances in the mother’ blood from entering the foetal blood.