Topic 5.7: Reproduction

Homepage
Topic Index
Human Physiology Index
5.7.1 Structure of the reproductive system
Male Reproductive System
        
male reproductive system

 

 

 

 

 

 

 

 

 

              
male reproduction system          
           
Female Reproductive System
          
  female reproductive system

     

  • (a) Ovary
  • (b) Oviduct
  • (c) Myometrium
  • (d) Uterus
  • (e) Cervix
  • (f) Vagina
  • (g) Endometrium
female reproductive system  
         
         
5.7.2 Hormones

Puberty

        
  • Puberty is when a sexually immature male or female makes the physiological transition to being sexually mature.
  • Sexual maturity is used here to mean that the individual can produce gametes.
  • Puberty occurs over a number of years and usually begins in early teen years. There is however much natural variation in the exact length of puberty and when it begins and finishes.
  • In both females and males the process is begun by the secretion from the brain of hormones. (FSH and LH).
  • These secretions target the testes, ovary and uterus. These organs then respond by producing their own hormones
  • The testes produce testosterone in boys stimulates the development of secondary sexual characteristics (see 5.7.3)
  • The ovary produces oestrogen's and progesterone's that bring about the development of secondary sexual characteristics in girls. (see 5.7.3)
        

 

Menstrual Cycle:

       

Sexual maturity in women is marked by the beginning of the menstrual cycles. These cycles coordinate the development and release of an egg with the conditions required in the uterus to support a pregnancy.

The cycle is controlled by hormones from both the brain and the ovary.

The natural cycle repeats until there is either a pregnancy or the woman reaches menopause and the end of the reproductive phase of her life.

 

        
menstrual cycle

(a) Follicle Stimulating Hormone (FSH) is secreted by the pituitary gland of the brain and stimulates the development of a primary follicle.

(b) Primary follicle cells secrete oestrogen which in turn increase the secretion of FSH in a positive feedback.

(c) The oestrogen thickens the lining of the uterus in preparation for a fertilised egg.

(d) The peak of oestrogen secretion causes the pituitary to release a surge of LH. This loosens the now mature egg which is released in ovulation

  • LH reduces the secretion of Oestrogen
  • LH stimulates the empty follicle to develop into the corpus luteum
        
         

(e) Progesterone and oestrogen together stop any more LH and FSH being secreted from the pituitary. (negative feedback)

  • This prevents further follicle development or ovulation.

(f) Progesterone maintains the lining of the thickened endometrium in preparation for the implantation of a fertilised egg.

(g) If implantation does not take place then the Corpus luteum degenerates and fails.

  • The progesterone production stops.
  • The endometrium breaks down and the 'menstrual period' begins
  • The inhibition of FSH and LH by ovarian hormones has been removed and so they begin their secretions again of FSH.
  • A new cycle has begun.
        
       
5.7.3 Secondary Sexual Characteristics
        
Male
Female
          
 

The hormones from the brain stimulate the testes to produce testosterone. This male hormone then brings about the development of secondary sexual characteristics

  • Penis and testes grow large
  • Glands including the prostate and the seminal vesicles are able to secrete the fluids that make up semen.
  • There are other changes including an increase in muscle mass and skeletal growth
  • Increase in body hair particularly around the face.
  • The voice deepens as the vocal cords lengthen with larynx growth
  

The hormones from the brain stimulate the ovaries and the uterus. The ovaries in turn produce the hormones oestrogen and progesterone. These hormones bring about the development of secondary sexual characteristics.

  • Increase in the size of reproductive organs including the uterus.
  • Increase in breast tissue and the development of pubic hair.
  • Increase in skeletal development and in particular the pelvis
  • The beginning of menstrual cycles and ovulation

 

          
         
5.7.4 Copulation and Fertilisation
          
  • As mammals humans reproduce via internal fertilisation and gestation
  • Copulation is the technical term for sexual intercourse.
  • Semen is ejaculated from the penis into the vagina.
  • The sperm cells swim within the semen through the narrow cervix and into the uterus.
  • The sperm cells actively swim against a fid current generated by the cilia that line the oviducts
  • Should this coincide with an egg being inside the oviducts then a fertilisation is possible.
   intercourse              
fertilisation  

A single sperm digests its way through the outer cells that surround the egg

Having penetrated the egg the sperm(n) and egg(n) nuclei combine to for the zygote (2n)

This new combination of chromosomes constitutes the genome of a new person.

zygote  
         
5.7.5 Embryonic Development
          
embryonic development  

The image represent the stages in the early development of the embryo. You can click the image for an image of the cell stages of the embryo.

  • The fertilised cell divides by mitosis into 2 then 4 then 8 and so on.
  • By day 7 this has formed a hollow ball of cells called the blastocyst.
  • The blastocyst implant into the endometrium and begins to develop towards the early foetal stages
          
5.7.6 Placental membranes
twin pregnancy    

This is an ultra sound image of a two month twin pregnancy.

  • The image shows the position of the endometrium and two foetus
  • There is amniotic fluid surrounding each foetus to protect and support the foetus
  • The fluid is retained by an amniotic membrane
  • In this example there are two amniotic membranes one for each foetus.
         
5.7.7 Placenta
          
placenta

The placenta grows from the embryonic tissue. It attaches to the endometrium of the uterus. Unlike other mammals the human placenta is invades the endometrium very deeply.

  • a) Umbilical cord attaches the foetus to the placenta
  • b) Umbilical arteries carry deoxygenated blood to the maternal blood to be re-oxygenated. The foetal blood has a much stronger affinity for oxygen than maternal blood
  • c) Umbilical veins carry the oxygenated blood back to the circulatory system of the foetus.
  • d) Placental tissue deep in the endometrial tissue
  • e) Myometrium
  • f) Endometrium which already has the placenta
  • g)Maternal blood supply
  • h) open ended arterioles forming pools of blood(i)
  • j) Chorionic villi embedded deep in the endometrium and in close association with maternal blood
        

Exchange a cross the placenta:

From mother to foetus:

  • Glucose by passive diffusion
  • Amino acids by active transport
  • Lipids by membrane lipid diffusion
  • Oxygen is released by the maternal haemoglobin. The haemoglobin of the foetus has a higher affinity for the oxygen.

From Foetus to Mother:

  • carbon dioxide is taken up by the maternal plasma and transported to the lungs of the mother for excretion
  • urea pass's into the maternal blood and passes to her kidneys for excretion
       
       
5.7.8 Birth and its hormonal control
     
control of birth

After nine months in the uterus the foetus is fully grown and takes up all the space available.

  • These cramped conditions push the baby down stretching the lower walls of the uterus. This sends impulses to the mother brain.

  • The foetus responds to the cramped conditions by producing hormones from the placenta (prostaglandin) which causes myometrial contraction

  • Progesterone is the hormone of pregnancy and at this stage the high levels of this hormone become less active.

All these changes stimulate the secretion of oxytocin from the pituitary and this causes the myometrial contractions of labour

      

Positive feedback:

  • In this system the stimuli to the brain increases the oxytocin production

  • In turn the oxytocin stimulate myometrial contraction

  • Myometrial contraction further stimulates the pituitary of the mother to release more oxytocin

  • The strength and frequency of the myometrial contractions is further increased.

  • In turn this further stimulates more oxytocin production

  • The process builds with stronger and stronger contractions

  • Final the child passes though the cervix and vagina to be born

  • Contractions continue for a further period until the placenta is delivered (after birth).

      
     
5.7.9 Family planning and Contraception
     
contraception  

(a) Contraceptive pills: a chemical method of contraception. One version uses a combination of progesterone and oestrogen that inhibits ovulation. Others are single hormones that require very careful management when taken.

      
       

(b) Interuterine device (IUD) placed inside the uterus an exact understanding how this works is unclear. A possible explanation is that it 'irritates' the endometrium such that rejects implantation of embryo's. The device is made from plastic or copper and inserted by a doctor. Nevertheless this device is very effective.

      
       
 

(c) Condom is another mechanical method of contraception that prevents the sperm from reaching the egg. Composed of a thin barrier of latex this is placed over the erect penis and captures semen on ejaculation. This is also a good barrier to prevent the transmission of sexual diseases.

      
 

(d) The cap is another barrier method again made from latex the cap is placed over the cervix to prevent the entry of sperm in semen. This technique requires that the cap is put in position in advance of sexual intercourse and that it is used in combination with a spermicidal cream. When used correctly this is an effective contraceptive however this is not a barrier against the transmission of sexual diseases.

 

     
 

(e) Sterilisation is a surgical and near permanent solution for contraception. In men this involves cutting the vas deferens (vasectomy) and prevent sperm entering the semen. In this state men still ejaculate normally and release semen however this does not contain sperm. In women the surgery cuts or ties the oviducts thus preventing sperm from reaching the egg in fertilisation. Today it is possible in some cases to reverse both sterilisation's however there are complications and this could never be relied on as a temporary solution to contraception.

      
 

(f) A behavioural form of contraception is abstinence That is the individual makes the choice to delay sexual intercourse until the decision to conceive a child is made. This method finds favour with a number of religious groups and there are currently some interesting statistics related to this an the transmission of HIV. However this assumes that the individual is able to make free choices and to enjoy the freedom of controlling their own sexual behaviour without social or cultural pressures.

      
           
5.7.10 Ethical Issues of Family planning and contraception

The decision on whether to use any method will be determined by an individuals circumstances. Opinions differ on the use of contraceptives and will be affected by a number of different factors including:

  • Health & Medical
  • Age
  • Status
  • Religion
  • legal
           
5.7.11 Amniocentesis
         
 

Amniocentesis is a technique that allows the prenatal diagnosis of conditions including chromosomal abnormalities and biochemical disorders.

(a) The uterus

(b)amniotic fluid.

(c) Hypodermic needle removing amniotic fluid through the abdominal wall
  • Amniotic fluid is removed from the 15th - 18th week
  • 1ml per week of gestation
  • Test include karyotyping for Down's Syndrome (mothers 35 have 1/178 chance, 40 have a 1/63 chance, 48 a 1/8 chance)
  • Test Alpha fetal proteins as a marker for neural tube disorders such as Spina biffida
  • Complication are 1/200 risk
  • Results take up to 7 days
         
           
5.7.12 In Vitro Fertilisation (IVF)
 
 
 
5.7.13 Ethical Issues with in vitro fertilisation

Advantages of IVF: there are as many reasons for this treatment as there are people seeking this treatment. As examples

  • over comes infertility
  • allow families for people who must be sterilised e.g.. radiography/chemo therapy cancer patients

Disadvantages of IVF:

  • what happens to unwanted embryo's
  • what happens to orphaned embryo's
  • should infertility be by-passed