Form 6 Biology – REPRODUCTION

FERTILIZATION

Is the process whereby the nucleus of the male gamete fuses with that of the female gamete to form diploid zygote nucleus.

• This process occurs high up in the fallopian tube. Before fertilization the spermatozoa has to undergo capacitation.
  

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CAPACITATION

Is the mechanism by which the spermatozoa undergo activation before fertilizing the ovum. It takes about 7 hours and involves the following processes.

1. Removal of a layer of glycoproteins and plasma proteins from the outer layer/surface of spermatozoa; glycoprotein are added by the epididymis while the plasma proteins are from the semen. These are removed by the enzymes in the uterus.
   

   Removal of cholesterol which toughens the sperm membrane and prevents premature release of acrosomal enzymes from the sperm head membrane by the enzyme in the uterus.
   

   The advantage of capacitation is that it prevents the wastage of sperms. The membrane becomes more permeable to Ca²⁺ ions.
   

   The calcium ions (ca²⁺) have two factions:-
   

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• Increase/enable the beating of the sperm flagellum.
  
• Promote acrosomal reaction. 
  

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MECHANISM OF FERTILIZATION

Fertilization comprises of two types of chemical reactions:-

1. Acrosomal reactions.
   
2. Cortical reactions.
   

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STEPS:

1. The sperm migrates through the coat of follicle cells and binds to a receptor molecule in the zona pellucida of the egg.
   
2. The binding includes the acrosomal reactions in which the sperm releases digestive enzymes into the zona pellucida. These enzymes are proteases (acrasome) and hyalurionidase. The latter digests the hyaluronic acid which binds granulosa cells together.
   
3. With the help of this hydrolytic enzyme, the sperm reaches the egg and the membrane protein of the sperm binds to the receptor on the egg membrane.
   

   This induces the influx of Ca⁺ ions that depolarize the egg membrane. This is the first block of polyspermy.
   

4. The plasma membrane fuses making it possible for sperm nucleus to enter the egg.
   
5. The sperm egg cell fusion causes Ca²⁺ influx.
   

   This inturn triggers a cortical reaction in which secretions beneath zona pellucida. These secretions swell up with water, push any remaining sperms away from the egg and creating impermeable fertilization membrane. Then the enzymes harden the zona pellucida. This functions as the flow block to polyspermy.
   

6. The nucleus of the secondary oocyte is triggered to undergo meiosis II which produces an octid is transformed into an ovum. The nucleus of the ovum and that of spermatozoa bulge becoming pronida which later fuse in the actual act of fertilization forming zygote.
   

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NB: If not fertilized, the secondary oocyte dies off ovulation and never finishes meiosis.

POST – FERTILIZATION CHANGES IN THE EGG

After fertilization, the following changes occur in the egg.

1. The zygote becomes ready for the cleavage and for the formation of the embryo.
   
2. The oxygen consumption of the zygote increases enormously.
   
3. The metabolic rate of the zygote increases greatly for instance the amount of amino acids and the permeability of the plasmalema of the egg increases the volume of the egg decrease the exchange of phosphate and sodium ions between the zygote and the surrounding media, diffusion of the calcium ions from the egg started and the hydrolysing activities of the
   
4. Protein synthesis in started.
   

   SIGNIFICANCE OF FERTILIZATION
   

5. The fertilization ensures the usual specific diploid of the organisms by the fusion of the male and female pronuclei.
   
6. The fertilization establishes definite polarity in the eggs. This fertilization provides new genetic constitution to the zygote.
   
7. The fertilization activates the egg for the cleavage.
   
8. Fertilization provides a new genetic constitution to the zygote.
   
9. Fertilization combines characters or two parents thus introducing variations and making the resulting individual better equipped for the struggle for existence. This happens only in cross fertilization.
   
10. The fertilization also increases the metabolic activities and the rate of protein synthesis of the cell.
    

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THE CONCEPT OF STERILITY

STERILITY:

Is the failure of the matured mammal to fertilize or to be fertilized. 

CAUSES OF INFERTILITY

FEMALE INFERTILITY                                                        MALE STERILITY/INFERTILITY

IMPOTENCE:

Is the failure of penis to erect, this can be temporary i.e. reversible impotence caused by such factors as;-

1. Depression due to social, economic and ethnic reasons.
   
2. Fear due to inferiority complexity, disease contraction, hesitating to commit sin.
   

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Also impotence can be permanent irreversible due to genetic disorder, diseases, hormonal problems etc.

Copulation – (i.e. seduction, romance and the subsequent intercourse) ensures the transfer of sperms from the male reproductive organs to female reproductive organs for fertilization.

DEVELOPMENT OF THE ZYGOTE AND EMBRYO

This includes 5 stages:
1)Cleavage
2)Blastulation
3)Gastrulating
4)Neurilation
5)Organogenesis

CLEAVAGE

• Two hours after fertilization the zygote divides mitotically to form two cells.
  
• The process by which the zygote divides is called cleavage and the resulting cells are called blastomeres.
  
• After 6 hours the zygote cleaves for the second time forming four blastomeres.
  
• Initially the process is regular but with time it becomes irregular in where it produces a ball of cells called MORULA.
  
• Cleavage does not lead to increase in size of the morula because cells still in the zona pellucida.
  
• The process takes place 72 hours.
  

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           Cleavage increases surface area to volume ratio of each cell which enhances:-

• Rapid nutrient uptake i.e. food and oxygen.
  
• Waste removal.
  
• Cleavage also forms many cells which will form different types of embryonic tissues.
  

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BLASTULATION

Is the process whereby morula is transformed into a blastula or blastula or blastocyst.

• While cleavage is taking place the zygote is in the oviduct moving slowly by the beating action of the cilia in the oviduct (tubules). When it reaches in the uterus the hard zona pellucida gets peeled off by the enzymes in the uterus and leaves an outer layer of cells called trophoblast.
  

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The cells in the centre of the morula migrate and accumulate at one end where they form an inner cell mass; the result of this cellular migration is the formation of the fluid filled cavity called bastocoel.

IMPLANTATION

Is the process whereby the blastocyst embades into the uterine wall.

• As soon as the trophoblast is in contact with the uterine wall it starts secreting enzymes that eat through the endometrium wall thereby pouring a way for blastocyst to embed.
  

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Trophoblast develop finger like processes called trophoblast villi (chorion villi) which are for the absorption of nutrients from the uterine wall. The trophoblast is also endocrine in function as it secretes a human chorionic gonadotrophs hormone (HCG).

The function of the HCG is like that of LH.

• To maintain the corpus luteum       secretion of oestrogen and progesterone also done.
  
• Inhibit menstruation to pregnant woman.
  
• Forms the basis of the pregnancy test (urine – pregnancy test UPT)   dip the litmus paper into the urine if you see two red marks; the person is pregnant; if only one colour is seen then the person is not pregnant.
  
• The process of penetrating in the uterine wall continues until finally the blastocyst becomes completely embeded in the glands and blood vessels of the uterus. This is the actual act of implantation.
  

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GASTRULATION

This is transformation of blastula into germ layer called gastrula.

• It s a stage at which the embryo develops germ layer.
  
• During gastrulation, the cells on one side of the embryo inviginates forming a small pore called blastopore.
  
• The process ends when germ layers are ready formed.
  

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NB: All the five stages are summarized in the diagram below:-

Through this pore, blastopore about half of the cell from outside move to the inside and at this point, the embryo is said to turn on itself.

The result of this cellular migration is the development of two germ layers, the outer (ectoderm) and the inner (endoderm).

The blastocoel becomes an archenteron the future digestive tract. The blastopore is the future anus.

Finally the third layers the mesoderm form between the ectoderm and endoderm forming a three layered embryo.

• Gastrulation is important in placenta development because cell’s location in a particular layer determines its fate e.g.
  

  Ectoderm – Develops nervous system, sense organs, epidermis the skin, hair nails and skin glands, neural egest.
  

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          Mesoderm – Develops into bones blood, muscles, dermis of the skin and reproduction system.

          Endoderm – Develops digestive and respiratory system and many glands.

EXTRA EMBRYONIC MEMBRANES AND THEIR ROLES

After implantation the embryo develops four membranes. They are called extra embryonic membranes because they are found outside the embryo and these include:-
  i)  Chorion
  ii)  Amnion.
  iii) Allantos.
  iv) York sac.

I. CHORION

This is the outer most membrane which is derived from the trophoblastic cells.

It has villi that forms the part of the placenta, therefore the roles of chorion are;-

1. To form parts of the placenta.
   
2. To absorb nutrients from the mother to the foetus means of villi.
   
3. Since it is an outer member, it then protects the foetus.
   

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II. AMNION

This is the innermost membrane which lines the cavity surrounding the embryo.

This cavity (amniotic cavity) is filled with the amniotic fluid secreted by the amniotic cells.

The amniotic fluid acts as shock absorber cushioning the embryo against mechanical and physical shock.

III. ALLANTOIS

This is a sack like outgrowth which develops from embryonic gut; it fuses with the chorion at the point called allantois-chorion where the placenta develops.

As the embryo continues to grow the allantois develops into umbilical cord the tube which carries blood vessels (embryonic) to end from the chorionic villi.

IV. YOLK SAC

This has got no obvious function in humans and other mammal it becomes buried in placenta.

In reptiles and birds the yolk sac is important as it absorbs food from the yolk and transfers it to the midgut of the developing embryo.

PLACENTA

• A placenta begins when extension of chorionic villi penetrates more and more deeply into the endometrium like the roots of a tree in the ‘soil’ uterus. 
  
• As they digest their way through the uterine blood vessels the villi become surrounded by pools of free blood the latter forms placental sinuses.
  
• A placenta is a linking structure between the foetus and the mother. It is the structure that partly develops from the mother and parity develops from the embryo. It thus has the foetal and maternal side.
  
• At the placenta, the materials are exchanged between the foetus and the mother. However, their vascular systems are not in physical contact. The exchange of materials is therefore by simple diffusion.
  

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Why are the material and foetal blood not
allowed to mix?

Maternal blood is under relatively higher pressure compared to foetal blood; this could damage the delicate tissues of the developing foetus

If the two bloods were to mix, the foetal blood could be recognised as a foreign by maternal blood. The maternal blood immune system could respond by killing the foetus.

This is because half of the genetic materials come from the father and so the foetal cells are not identical to those of the mother.   
               i) Progesterone. 

ii) Oestrogen.

iii) Human chorionic gonadotrophic hormone.       

ROLES OF PLACENTA

1. It allows the exchange of materials between the foetus and the mother without mixing up the two blood.
   
2. It is a means of passage of oxygen, water, acids, glucose (i.e. nutrients) to the foetus (acts as intestine).
   
3. Means of passage of carbondioxide, urea and other wastes from the foetus to the mother so as to allow the excretion by the mother and prevent harmful substances to accumulate in the foetus i.e. acts as lungs and kidneys.
   
4. Allows certain antibodies to pass into the foetus providing it with some immunity against diseases. This is called Natural passive immunity.
   
5. It protects the foetus by preventing certain pathogens and their toxins from crossing the placenta. Though, some manage to cross. Eg. Treponema pallidum (for syphilis) and HIV.
   
6. It prevents hormones and some chemical substances like alcohol to pass through the foetus.
   

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Qn:

Placenta serves as a link between foetus and mother. At the same time it acts as a barrier between them. By reference to the functions of placenta explain what those statements mean.  

TWINS PUZZLE AND MULTIPLE BIRTH

MULTIPLE BIRTH AND THEIR CAUSES:

Multiple birth are cases in which more than one baby are born from the same mother and they result from the same pregnancy.

• In mammals like cats, rabbits, dogs and pigs multiple birth are common cases as the ovulation several oocytes are released each of them is fertilized by separate spermatozoan.
  

  Humans are commonly giving birth to only one young individual.
  

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Multiple birth occurs due to;

1. More than one secondary oocyte being released at ovulation and then fertilized by spermatozoa.
   
2. One ovum being fertilized by spermatozoan and zygote cleave into 2,3 …etc blastomeres each of which develops into an embryo after separation. 
   

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