Form 6 Biology – REPRODUCTION

TWINS

Defn: Are two or more babies born from the same mother as a result of the same pregnancy.

TYPES OF TWINS:

1. Identical twins
   

edu.uptymez.com

• Result from the same zygote/one zygote hence called monozygotic twins. For the development of identical twins to occur the zygote cleaves into two or more blastomeres.
  

edu.uptymez.com

These separate from one another and upon implantation each one of them develops into an embryo.

• In rare cases, separation of the blastomere fails at some points. Thus leads to twins remaining linked, such twins are referred to as SIAMESE TWINS.
  
• The identical twins share the same placenta and they are in the same chorion and amnion.
  
• Since they develop from the same zygote such twins are of the same genetic constitution and of the same sex.
  

   2. Flatenal/Non identical twins
  

• They develop from two different zygotes hence they are called dyzyotic twins.
  
• In this case two different ova from different ovaries are fertilized by two different spermatozoa forming two different zygotes, which implant in the uterus.
  
• Each of these twins develops in its own placenta and its own embryonic membranes since they develop from different zygotes. Then the zygotes are genetically different and not necessary of the same sex.
  

  DIFFERENCES
  

edu.uptymez.com

BIRTH(PARTUIRITION)

Birth is a process whereby the fully developed features expelled out of the mother’s womb after the GESTATION period is complete.

THE PROCESS OF BIRTH/LABOUR

The labour occurs in three stages shown by distinct events. These events are longer in primigravide than in multigravide.

THE FIRST STAGE

This is the stage of labour pains. During this stage the fully developed foetus has its own hypothalamus stimulated to release ACTRF which in turn stimulates the release of ACTH from the foetal pituitary gland.

The ACTH stimulates foetal gland (adrenal) to release cortical steroids. The released cortico steroids pass across the placenta and enter the maternal circulatory system where they perform the following:-

I) They cause increase in prostaglandins (secreted by uterus).

II) They cause decrease in progesterone following the decrease in progesterone.

• The pituitary gland is allowed to release oxytocin.
  
• The inhibitory effect on myometrium contraction is removed and prostaglandins power the contraction.
  

  Oxytocin therefore causes the contractions of the myometrium where as the prostaglandins secreted by uterus increase the power of contraction. These contractions of myometrium sum up to labour pains.
  

edu.uptymez.com

• As the uterine walls continue to contract, the cervix dilates under the influence of hormone called relaxin. The amnion and chorion rupture releasing the amniotic fluid through the cervix breaking of water. Contractions continue from top to bottom forcing the baby out of the womb.
  
• The body gets engaged into the pelvis and following further contraction, the foetal head gets into the cervix where it causes irritation and increases powers of contraction.
  

edu.uptymez.com

The first stage of labour is terminated .When the diameter of the head is equal to the diameter of the cervix.

THE SECOND STAGE

Is the stage where by the baby is completely delivered out of the mother’s womb.

• As soon as the baby is out, the umbilical cord is ligatured at two points and a cut is made between the two ligatures so as to make the baby totally separated from the mother’s physiological reliance.
  

edu.uptymez.com

THE THIRD STAGE

This is the stage during which the placenta and the extra embryonic membranes after birth is delivered.

The birth of after birth is due to dramatic contraction of the uterus which causes the placenta to detach from the uterine wall.

It is important that after birth is not allowed to remain inside for a long time as its decomposition leads to blood passing.

PARENTAL CARE

This comprises of all activities the parents do for the better growth and bringing of their offspring or the offspring of the near relative.

ASPECTS OF PARENTAL CARE

1. Nutrition -3 month
   

edu.uptymez.com

The body has to be fed on the nutritious food substances, for proper growth and development. After birth, the baby is fed on breast with from its mother.

Breast feeding is highly recommended because mother’s milk contains all important food substances needed by the baby at every stage of its development.

The first milk that a baby is sucking from the mother’s breasts is a special one as it is called colustrum.

This is a yellowish fluid that contains antibodies to provide immunity to the baby. After the first three months, the child continues to be supplied with extra proteinous food substances such as cow’s milk,

eggs, fish, beef etc. for proper growth and development.

1. Protection
   

edu.uptymez.com

• Most of the mammalian parents protect their young against
  

  a) Disease: by providing health services ensuring hygienic handing of food etc.
  

edu.uptymez.com

          b) Climate changes: by providing warmth to the babies.

          c) Predators/enemies: mothers become agressive to ensure that their young ones are not reached.
    
    2. Social interaction/Education

     The young ones must learn how to interact with others and fit into the social structure around them. The youngs need early experience with their parents in order to depend on themselves and to learn

     to live actively in the social unit.

Parental care involves the:-

1. Learning of language.
   
2. Teaching the language.
   
3. Formal education (For human beings, primary, secondary up to tertiary education) thereafter a person becomes independent.
   

edu.uptymez.com

REPRODUCTIVE CYCLES

In order to syncronise the favourable conditions to sexual reproduction, sexual reproduction is naturally in cycle.

For example; plant flowering is at the same date year after year; bleeding (menstruation) of a mature primate is on the same date month after month etc.

In mature female mammals, there is a sexual reproduction cycle, this is known as oestrus cycle (ovarian cycle).

At the onset of puberty there are approximately 400,000 primordial follicles and single follicles in all stages of maturity in the ovary. Ooctyes contained in the primordial follicles migrate out of the extragenital

structures of the coelomate epithelium into the stroma of the primary bipotent gonalds as oogonia during embryonic development.

These then divide mitotically of the roughly 400,000 follicles that are present in the two ovaries at the beginning of sexual maturity, only around 480 reach the graafian follicle stage and are thus able to

release oocytes (ovulation).

• This number is simply derived by multiplying the number of oocytes of cycles per year (12) and the number of years in which a woman is fertile (40).
  

edu.uptymez.com

            Cyclic ovarian functions entailing follicle formation, ovulation, corpus luteum development and luteolysis is regulated by the hypothalamus pituitary system as well as by intraovarian mechanisms

            hypothalamus, pituitary and ovary are there by in dynamic interaction.

OESTRUS CYCLE

Defn:

Oestrus cycle is the total time taken for the development and degeneration of an ovarian follicle.

• In some mammals, this period occurs once in a year, they are said to be monoestrus eg. fox.
  
• Note that; menstrual cycle in human, oestrus cycle in other mammals.
  

edu.uptymez.com

The discharge of the blood marks the end of oestrus cycle in higher mammals of order primate.

• In most mammals, this period is occurring many times in a year, so they are polyestrus.
  

edu.uptymez.com

PHASES OF OESTRUS CYCLE(VERY MINOR)

1. Anoestrus – Period during which no visible sexual activity in females.
   
2. Proestrus – Period during which graafian follicle develops into ovary and secrete oestrogens. Also called follicular phase.
   
3. Oestrus (heat) – Ovulation normally occurs, the female is ready to mate and becomes sexually attractive to male.
   
4. Met oestrus (luteal phase) – corpus luteum develops from raptured follicle.
   
5. Dioestrus – Progesterone secreted by corpus luteum prepares uterus for implantation.
   

edu.uptymez.com

SIGNIFICANCE OF OESTRUS CYCLE

Since it is characterised by ovulation and hence increase sexual urge of female, then it is important in that copulation is syncronised with fertilisation.

MENSTRUAL CYCLE

• This approximately monthly cycle of events associated with ovulation that replaces the oestrus cycle in most primates i.e. human chimpanzee, gorilla, baboon etc.
  
• The lining of the uterus becomes progressively thicken with more blood vessels in preparation for implantation of a fertilized egg (blastocyst).
  
• Ovulation occurs during the middle of the cycle (fertile period). If fertilization does not occur the uterine lining breaks down and discharged is known as period.
  

edu.uptymez.com

In women the fertile period is 11 – 15 days after the end of the last menstruation.

EVENTS OF MENSTRUAL CYCLE/OESTRUS CYCLE

1. Day 1 and 2 anterior pituitary gland releases. FSH and LH.
   
2. These hormones travel by blood and reach the ovary.
   
3. They stimulate the granulosa cells to secrete oestrogen.
   
4. Oestrogen thickens endometrium and inhibits FSH and LH.
   
5. Day 12 LH level rises.
   
6. It stimulates granulosa cells to stop producing oestrogen and start release progesterone.
   
7. Causes ovulation.
   
8. At day 14, secondary oocyte bursts out of the ovary.
   
9. Granulosa cells filled with yellow substance to form corpus luteum.
   
10. Corpus luteum secretes progesterone.
    
11. Endometrium thickens.
    
12. Inhibits FSH and LH.
    
13. Drop in FSH and LH stops progesterone and oestrogen level going up because granulosa cells are no longer stimulated.
    
14. Endometrium no longer thickens and lack of progesterone and oestrogen stimulate the anterior pituitary gland to release FSH and LH.
    
15. Cycle begins again.
    

     
    

edu.uptymez.com

The figure above shows morphological and endocrinological changes during various phases of the cycle.

PHASES OF MENSTRUAL CYLE

1.FOLLICULAR PHASE

This is characterised by:-

• Increased TSH from pituitary gland.
  
• Production of LH from pituitary gland.
  
• Development of follicle.
  

  2.OVULATION
  

edu.uptymez.com

This involves the release of secondary oocyte after maturation of graafian follicle. This process in controlled by LH. One follicle rapidly out places the others and attains a diameter of up to 2.5 cm. This follicle is called mature vesicular (graafian follicle).

            It is produced from the surface of the ovary like a bluster. As it develops the primary oocyte completes meiosis I producing a secondary oocyte. This begins meiosis II.
       
         3.LUTEAL PHASE

Development of corpus luteum following ovulation; the rapture of graafian follicles develops into a yellow body mass called corpus luteum. The latter is endocrine in function and thus it secretes two hormones. Progesterone (large amount) and oestrogen (small amount).

        4.MENSTRUATION

This is characterised by withdrawal of progesterone following the regression of corpus luteum also discharge of blood from vagina.

EVENTS OF MENSTRUAL CYLCLE

The events of menstrual cycle involves:-

1. Ovarian cycle – ovaries.
   
2. Uterine cycle – uterus.
   

edu.uptymez.com

In women, the rythmic hormonal influence leads to the following cyclic events.

1. The ovarian cycle (follicle maturation) that peaks in the ovulation and the subsequent luteinization of the granulosa cells.
   
2. Cyclic alternation of the endemetrium that prepare the uterine mucosa to fertilized oocyte (as ‘nest’ thene).
   
3. THE OVARIAN CYCLE
   

edu.uptymez.com

• A rule, the ovarian cycle lasts 28 days (in majority).
  

  It is divided into two phases:
  

  i. Follicle phase: Requirement of a so called follicle and within this selection of the mature follicle. This phase ends with ovulation. Oestrogen (estradiol) is the steering hormone normally it last 14 days
  

     but this varies considerably.
  

edu.uptymez.com

          ii. Luteal phase: Progesterone production by corpus luteum.

1. The control circuit of the hormonal cycle has two essential elements:-
   

         i. The pulsative liberation of GnRH as well as FSH and LH.
   

         ii.  The long – loop feedback effect of oestrogen and progesterone on the hypothalamic hypophysical system.
   

edu.uptymez.com