Chapter 12 Pregnancy

• in human reproduction 


  a sperm and an ovum are brought together at fertilisation
• zygote


  resulting fertilised ovum

  develops into an embryo with many different types of cells
• for fertilisation to occur 


  male sperm need to be brought into contact with an ovum produced by the female

  usual method by which this is brought about is sexual intercourse
• erection 


  for sexual intercourse to take place, and for sperm to be deposited in the vagina, the penis must become enlarged and firm

  results from blood rushing into the spaces of the erectile tissue of the penis

  sexual excitation initiates this blood flow
• ejaculation 


  when sexual stimulation of the penis within the vagina becomes sufficiently intense, rhythmic contractions of the epididymis, vasa deferent, seminal vesicles and prostate gland occur

  the contractions propel the contents of the ducts and glands into urethra and then out of the body
• ejaculated material 


  consist of fluid, or semen, which contain sperm
• accompanying ejaculation


  is a rapid heartbeat, an increase in blood pressure and breathing rate, and intensely pleasurable sensations

  reactions constitute an orgasm
• ejaculation normally expels 


  3mL of semen, containing 250-300 million sperm

  besides the sperm, semen contains the secretions of the seminal vesicles, bulbs-urethral glands and prostate glands

  greatest contribution comes from seminal vesicles, which produces a thick fluid containing nourishment for the sperm
• beside nourishment


  semen provides the sperm with a fluid in which to swim, and neutralises the acid nature of the male urethra and female vagina

  it also contains enzymes that activate the sperm once ejaculation has taken place
• when female is sexually stimulated

  erectile tissue in the region of the vaginal opening fills with blood

  this reduces the size of the vaginal opening and tends to increase the stimulation of the penis during sexual intercourse
• arousal (female)


  results in copious secretions of mucus by glands located around the cervix and in the region of the vaginal opening

  these secretions lubricate the epithelial lining of the vagina, allowing easy entry of the penis
• climax 


  as sexual intercourse progresses, the external genitalia are rhythmically stimulated and, when sexual arousal reaches sufficient intensity, the female undergoes an orgasm (climax)

  female orgasm is somewhat like that of a male, with the exception that there is no ejaculation in the female

  however, there may be an increase in the secretion of cervical mucus
• a female does not need to

  reach an orgasm for fertilisation to occur

  it is still not known whether female orgasm helps fertilisation in any way
• insemination


  when the male ejaculates, the sperm are released in the vagina at the entrance to the uterus
• once within the vagina 


  the sperm travel through the cervix and the body of the uterus into the uterine tubes

  they quickly reach the upper portions of the uterine tubes, often within a few minutes
• rate is too fast due solely to the

  swimming motion of the sperm

  muscular contractions of the uterus and uterine tubes help to transport sperm through the female reproductive tract

  at the same time, muscular contractions of the uterine tubes, together with the beating of cilia, transport the ova towards the uterus after ovulation
• of the hundreds of millions of sperm deposited into vagina 


  only a few thousand reach the uterine tubes

  death rate of sperm, called sperm morality, is high, and is one reason why a large number of sperm are required if fertilisation is to occur
• fertilisation normally occurs in the 


  uterine tubes when the ovum is 1/3 of the way down the tube
• secondary oocyte that is released at

  ovulation is at metaphase II

  surrounded by two layers:

  outer corona radiata

  inner zona pellucida
• corona radiata 


  consist of follicle cells held together by cementing materials that contain acid
• zona pellucida 


  is a glycoprotein matrix surrounding the plasma membrane of the oocyte
• outer surface of acrosome contain

  an enzyme

  capable of breaking down the acid in the cementing material that holds the cells of the corona radiata together

  amount of enzyme contained in a single sperm is extremely small and ineffective
• when the several thousand sperm surround oocyte

  there is enough enzyme to loosen the cells of the corona, allowing one sperm to penetrate the corona radiata

  another reason why a large number of sperm is required if fertilisation is to occur
• once the sperm is through the corona radiata


  it encounters the zone pellucida

  initiates acrosomal reaction, causing digestive enzymes from the acrosome to be released
• acrosomal digestive enzymes

  these enzymes break down the glycoprotein matrix of zone pellucida, giving the sperm access to the plasma membrane of the oocyte

  when the plasma membrane of the oocyte and spermatozoa fuse, the nucleus of the sperm enters the ovum
• entrance of one sperm into secondary oocyte stimulates

  formation of a fertilisation membrane around the oocyte, which prevents the entrance of any more sperm

  this ensures that only one haploid set of chromosomes joins the chromosomes of the oocyte
• once the sperm has entered the oocyte

  tail is absorbed and the head begins to move through the cytoplasm in the form of a male pronucleus- the haploid nucleus of the sperm
• entrance of the sperm stimulates

  the secondary oocyte to complete the second meiotic division

  the nucleus of the oocyte develops into a female pronucleus(haploid nucleus of oocyte) which fuses with the male pronucleus to form a single nucleus that now has the diploid number of chromosomes

  fertilisation is complete, and the fertilised oocyte is called a zygote
• zygote 


  single cell that results from the fertilisation of an ovum by a sperm

  potential to grow into a new individual human

  200 types of cells in human body develop from that one initial cell
• after fertilisation

  zygote travels down the uterine tube and begins to divide by mitosis
• the process of mitosis results in

  formation of two cells exactly the same as the original parent cell

  two cells divide again by mitosis into 4, 8, 16 and so on
• six days after fertilisation 


  original zygote has reached the uterus and has developed into a blastocyst
• blastocyst 


  hollow ball of cells that surround a cavity filled with fluid

  at one side of the cavity is a group of about 30 cells called the inner cell mass (embryoblast)
• inner cell mass composed of

  stem cells that will differentiate into the different body cells to form the embryo
• implantation 


  blastocyst remains free within the cavity of the uterus for two or three days, and then sinks into the soft endometrium to become firmly attached to wall of uterus
• implantation enables 


  the blastocyst to gain nourishment for growth and development by absorbing nutrients from the glands and blood vessels of the uterine lining
• continued development of blastocyst depends in

  endometrium being maintained
• high levels of

  oestrogen and progesterone in the blood stop the endometrium breaking down, and so the menstrual cycle ceases

  during early stages of pregnancy, corpus luteum produces these hormones until developing placenta can take role after approx. 8-12 weeks
• embryonic period

  first two months of pregnancy
• after second month of pregnancy 


  developing individual is called a foetus