4.3 – EARLY 4.3 – EARLY DEVELOPMENT OF THE DEVELOPMENT OF THE

HUMAN ZYGOTEHUMAN ZYGOTE

LEARNING LEARNING OUTCOMESOUTCOMES

To describe what fertilisation is,To describe what fertilisation is,

To describe in simple terms the early To describe in simple terms the early development of a zygote,development of a zygote,

To name the two main developmental To name the two main developmental stages of a zygote in preparation for stages of a zygote in preparation for implantation,implantation,

Development of Development of Human ZygoteHuman Zygote 4 millions4 millions sperms are released during sperms are released during

copulation copulation only small number will reach only small number will reach the oviduct & only the oviduct & only one sperm will fuse with one sperm will fuse with the ovumthe ovum fertilisationfertilisation..

ZygoteZygote a a fertilisedfertilised egg egg

After formation of zygote, After formation of zygote, a protective layer a protective layer is formis form around itself to prevent the entry of around itself to prevent the entry of other sperms.other sperms.

4 days after fertilisation 4 days after fertilisation zygote divides zygote divides repeatedly by mitosis to repeatedly by mitosis to form a solid ball of cells form a solid ball of cells morulamorula

Morula develop into a Morula develop into a hollow ball of cells hollow ball of cells blastocyst/ blastula (each cell called blastocyst/ blastula (each cell called blastomere)blastomere)..

Blastocyst has a localised thickening called Blastocyst has a localised thickening called the inner cell massthe inner cell mass. The remainder of the . The remainder of the blastocyst is made up of cells called blastocyst is made up of cells called trophoblasttrophoblast..

The blastocyst will be implanted at The blastocyst will be implanted at the endometrium of the uterus after the endometrium of the uterus after 7 days of fertilisation.7 days of fertilisation.

MorulaMorula & & blastocystblastocyst are the two main are the two main stages in the development of a stages in the development of a zygote in preparation for zygote in preparation for implantation.implantation.

IMPLANTATIONIMPLANTATION After After 7 days7 days, the blastocyst begins to embed , the blastocyst begins to embed

itself in the endometrium of the uterus itself in the endometrium of the uterus implantationimplantation..

The trophoblast The trophoblast forms villiforms villi which are in which are in intimate contact with the endometrial tissue.intimate contact with the endometrial tissue.

Enzymes are secretedEnzymes are secreted by the embryonic cells by the embryonic cells which digest & break down maternal which digest & break down maternal capillaries in the endometrium.capillaries in the endometrium.

The The villi are bathed by a pool of maternal villi are bathed by a pool of maternal bloodblood, enabling exchange of materials , enabling exchange of materials between the mother & the growing embryo.between the mother & the growing embryo.

LEARNING LEARNING OUTCOMESOUTCOMES

To describe the formation of twins,To describe the formation of twins,

To compare identical twins with To compare identical twins with fraternal twins,fraternal twins,

to state the functions of the to state the functions of the placenta in the foetal development,placenta in the foetal development,

The explain the advantages of the The explain the advantages of the foetus having a separate circulatory foetus having a separate circulatory system from that of the mother.system from that of the mother.

IDENTICAL TWINS

FRATERNAL TWINS

SIAMESE TWINS

IDENTICAL TWINS

IDENTICAL TWINSIDENTICAL TWINS Involves only Involves only one spermone sperm & & one ovum.one ovum.

One zygote One zygote is form is form splits into two splits into two halves by mitosis halves by mitosis each half each half develops into a foetus.develops into a foetus.

Share same placentaShare same placenta

IDENTICAL TWINS :SIMILARITIES :

-SAME HAIR TEXTURE

-SAME COLOUR OF EYES

-SAME SEX

DIFFERENCES:

FINGER PRINT

FRATERNAL TWINS

FRATERNAL FRATERNAL TWINSTWINS

Involve Involve two ovatwo ova & & two spermstwo sperms..

Two ova are released at the same Two ova are released at the same time by the ovaries time by the ovaries each fertilised each fertilised by a sperm by a sperm two zygotestwo zygotes will be will be formed formed develops into an develops into an independent foetus.independent foetus.

Have Have two separate placentastwo separate placentas..

FRATERNAL TWINSTHEY MAY HAVE:

1. SAME OR DIFERENT HAIR COLOUR OR HAIR TEXTURE

2. SAME OR DIFFERENT EYE COLOUR

3. SAME OR DIFFERENT SEXES

4. DO NOT SHARE SAME PLACENTA

SIAMESE TWINS

SIAMESE TWINS

SIAMESE TWINSSIAMESE TWINS In the course of formation of identical In the course of formation of identical

twins, if the splitting of the zygote stops twins, if the splitting of the zygote stops halfway, while the embryo continues to halfway, while the embryo continues to develop, then develop, then Siamese twinsSiamese twins will be will be formed.formed.

Identical twins jointed at Identical twins jointed at certain parts of certain parts of their bodiestheir bodies/ certain organs./ certain organs.

Possible to separate by Possible to separate by surgerysurgery if they do if they do not share vital organs.not share vital organs.

SIAMESE

TWINS

1.SAME SEXES

2.SAME GENETIC PARTICULAR

3.SAME APPEARANCE

SIAMESE TWINSUSUALLY THE BODIES WILL JOIN AT THE:

1.HEAD

2.CHEST- SHARING HEART

3.FRONT ABDOMEN – SHARING LIVER AND

DIGESTIVE SYSTEM

4. HANDS

5. CAN SEPARATE DEPEND DEGREE OF

SHARING OF INTERNAL ORGAN.

WORK SHEET

2. DRAW FLOW CHARTS FOR THE FORMATION OF IDENTICAL TWINS,FRATERNAL TWINS AND SIAMESE TWINS.

TWINSCONCLUSION

FUNCTIONS OF THE FUNCTIONS OF THE PLACENTA IN FOETAL PLACENTA IN FOETAL

DEVELOPMENTDEVELOPMENT

LEARNING LEARNING OUTCOMESOUTCOMES

To state the functions of the To state the functions of the placenta in foetal development.placenta in foetal development.

To explain the advantages of To explain the advantages of foetus having a separate foetus having a separate circulatory system from that of circulatory system from that of the mother.the mother.

The organs in a The organs in a foetus are not fully foetus are not fully developed yet developed yet depends on the depends on the mother for mother for foodfood & & oxygenoxygen, & to get rid , & to get rid of its of its waste productswaste products..

Functions in the Functions in the exchange of exchange of substances between substances between the foetus & the the foetus & the mothermother..

PLACENTPLACENTAA

The exchange occurs via The exchange occurs via trophoblastic villi (TV).trophoblastic villi (TV).

TV TV human chorionic human chorionic gonadotrophin (HCG)gonadotrophin (HCG) stimulates stimulates the Corpus Luteumthe Corpus Luteum to increase in to increase in size & continue to produce size & continue to produce oestrogen & progesteron oestrogen & progesteron maintain the structure of maintain the structure of endometriumendometrium & inhibit FSH & inhibit FSH production.production.

The foetus is The foetus is connected to the connected to the placenta by the placenta by the umbilical cord umbilical cord ((umbilical arteries & umbilical arteries & umbilical veinumbilical vein).).

In the placenta, the In the placenta, the umbilical arteries umbilical arteries branch into a branch into a network of blood network of blood capillaries in the capillaries in the trophoblastic villitrophoblastic villi of of the placenta.the placenta.

Surrounding the Surrounding the villi is a space villi is a space filled with filled with maternal blood.maternal blood.

In the placenta, In the placenta, foetal blood & foetal blood & maternal blood do maternal blood do not mix. They are not mix. They are separated by two separated by two layers of cells layers of cells which facilitates which facilitates diffusion.diffusion.

Oxygen, food & Oxygen, food & some some antibodiesantibodies diffuse from the diffuse from the maternal blood into maternal blood into the foetal blood.the foetal blood.

Carbon dioxideCarbon dioxide, & , & nitrogenous waste nitrogenous waste productsproducts diffuse diffuse from the foetal from the foetal blood into the blood into the maternal blood & is maternal blood & is excrete by the excrete by the mother.mother.

Foetal blood containing oxygen & food Foetal blood containing oxygen & food is carried back to the heart of the is carried back to the heart of the foetus by the umbilical vein.foetus by the umbilical vein.

Any molecules which is Any molecules which is small enough small enough will be able to crosswill be able to cross from the maternal from the maternal blood to the foetal blood.blood to the foetal blood.

Harmful substancesHarmful substances alcohol, alcohol, nicotine, other drugs, lead particles, nicotine, other drugs, lead particles, viruses (rubella & HIV) are able to viruses (rubella & HIV) are able to cross the placenta.cross the placenta.

Why it’s so important for the Why it’s so important for the foetus to have a separate foetus to have a separate circulatory system from the circulatory system from the

mother’s?mother’s? If the foetus has a blood group which is If the foetus has a blood group which is

not compatible with the mother’s, the not compatible with the mother’s, the mixing of their blood could lead to mixing of their blood could lead to agglutination & death.agglutination & death.

Blood pressure of maternal circulation Blood pressure of maternal circulation is much higher is much higher than the foetal than the foetal circulation circulation prevents from damaging prevents from damaging the delicate tissues & organs of the the delicate tissues & organs of the developing foetus.developing foetus.