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Human Reproduction

Reproduction is the formation of new individuals of the same species.
Sexual reproduction is the only method of reproduction in our species.
Sexual reproduction involves the fusion of specialised haploid sex cells forming a diploid zygote from which a new individual develops.
In our species the sexes are separate, each gender producing a different type of gamete.


Reproductive role of the male

  1. Production of sperm (male gametes)
  2. Their transfer into the female

Reproductive role of the female

  1. Production of egg cells (female gametes)
  2. Reception of sperm providing favourable conditions for fertilisation, implantation and development of the baby
  3. To give birth and feed the baby by lactation

Male Urinogenital System (see diagram in textbook)

Testes:

  1. The primary sex organs where the sperm (male haploid sex cells) are produced by meiosis in the seminiferous tubules.
  2. The male hormone, testosterone is produced by the interstitial cells of the testes.

Epididymis
A long coiled tube that receives and stores the sperm from the seminiferous tubules. The sperm mature here and if not ejaculated will be reabsorbed after four weeks.

Sperm Duct (Vas Deferens)
A muscular tube that transfers the sperm from the epididymis to the urethra by peristalsis.

Seminal Vesicle
Secretes a sugar-rich fluid that is also alkaline protecting the sperm in the acidic vagina.

Prostate Gland
Secretes a protective alkaline fluid which also stimulates sperm motility.

Cowper's Gland
Secretes a protective alkaline fluid and mucus for lubrication.

Semen
A liquid mixture of sperm and the fluids of seminal vesicles, prostate gland and Cowper's gland.
The sperm, 50 to 300 million make up only about one thousandth of the volume of the semen.

Penis
Its function is to transfer sperm into the vagina of the female.
Three masses of erectile tissue, when engorged with blood, cause the penis to stiffen and become erect.

Urethra
Conveys urine and semen to the outside.
The sphincter muscle at the base of the bladder is contracted during sexual arousal preventing the release of urine from the bladder - acidic urine would immobilise the sperm in the semen.

Seminiferous Tubules of the Testes (diagram of Testis T.S. - see textbook)
Long coiled tiny tubes, about 1,000 per testis, where the sperm are produced by meiosis.
Sertoli cells protect and nourish the developing sperm which are produced at a rate of 1,000 per second.
FSH from the pituitary stimulates sperm production.
LH (ISCH) from the pituitary stimulates production of testosterone by interstitial cells between the seminiferous tubules.

Male Gamete (Sperm) See Diagram in Textbook
Basically just a haploid nucleus with a flagellum that propels it towards the egg cell.
The middle piece or neck contains mitochondria producing the ATP to supply energy for propulsion.
The head contains the nucleus with the genetic material. It also has a large lysosomes, the acrosome, which carries digestive enzymes to erode a channel through the layers surrounding the egg cell to make contact with it.


Female Reproductive System (see diagram in textbook)

Ovaries
The primary sexual organs.
Produces the female gametes, the egg cell, on average one matures every 28 days.
At ovulation the egg cell is released from the ovary into the coelom (secondary body cavity).

Fallopian Tube (Uterine Tube)
The open funnel end close to the ovary captures the egg cell at ovulation.
Fertilisation of the egg cell usually occurs in the Fallopian tube - if not fertilised it disintegrates.
The 'fertilised egg' is carried to the uterus by the action of cilia on the cells lining the tube and by peristalsis.
The 'fertilised egg' reaches the uterus in about six days.
Sperm have been known to survive in the Fallopian tube for at least six days - nourishment is supplied to them here.

Uterus (Womb)
The site of menstruation, implantation, development of the embryo and labour.
A new uterine lining (endometrium) develops each cycle to receive and nourish the early embryo.
The bulk of the uterine wall is composed of muscle, its strong contractions expel the baby at birth.

Cervix
The narrow opening between the uterus and vagina. A solid mucus plug protects the uterus from pathogens. This mucus softens at the time of ovulation to allow the entry of sperm to continue on their way to meet the egg cell.

Vagina
It receives the penis during sexual intercourse and the sperm are deposited here (insemination).
It is the passageway for menstrual flow and childbirth.
The vagina opens to the exterior at the vulva (external genitalia of the female) between the labia minora.
The opening of the urethra is also present here but closer to the pubic bone.

Structure of Ovary (see diagram in textbook)
Stroma: the central connective tissue.
Germinal Epithelium - the outside layer: produced the reproductive cells in the foetus.
Graafian Follicle: a liquid-filled ball of cells in which the egg cell develops.
Corpus Luteum: a solid ball of cells made from the Graafian follicle after ovulation - secretes progesterone and oestrogen.

At puberty each ovary of the female has about 100,000 primordial follicles. A primordial follicle is a primary oocyte surrounded by a single layer of supporting cells. A primary oocyte is a cell that has begun to divide by meiosis but is arrested at the later Prophase I stage. On maturing into a Graafian follicle the primary oocyte completes Meiosis I forming one functional cell, the secondary oocyte, and a non-functioning reduced 'cell' called a polar body. The secondary oocyte is the reproductive cell released at ovulation. Meiosis II of the secondary oocyte is completed when the sperm penetrates it.

Sperm Journey: seminiferous tubule » epididymis » spermduct » urethra » vagina » cervix » uterus » Fallopian tube

Egg Cell Journey: ovary » coelom » Fallopian tube


The Menstrual Cycle

The menstrual cycle is the repeating series of changes in the uterine lining of a fertile female provided fertilisation and implantation does not happen. This cycle ranges from 24 to 35 days, commonly 28 days.

Events of the Female Reproductive Cycle

  1. Day 1 to day 5
    Day 1 is the first day of menstruation.
    The corpus luteum of the previous cycle has disintegrated so the level of oestrogen and progesterone declined greatly.
    Low progesterone level leads to the menstruation - breakdown and discharge of the outer layer of the uterine lining.
    Low oestrogen and progesterone levels permit the secretion of FSH by the pituitary gland.
  2. Day 6 to day 13
    FSH stimulated the formation and maturation of a primordial follicle into a Graafian follicle.
    The maturing follicle secretes oestrogen which inhibits FSH secretion preventing other follicles maturing.
    Oestrogen also stimulates repair of the uterine lining and a surge of LH just before ovulation.
  3. Day 14
    The surge of LH stimulates ovulation - release of the egg cell from the mature Graafian follicle at the ovary's surface.
    The egg cell is drawn into the Fallopian tube.
  4. Day 15 to day 26
    A corpus luteum develops from the remains of the Graafian follicle.
    The corpus luteum secretes progesterone and oestrogen stimulating the final maturation of the uterine lining.
    High levels of theses hormones also inhibit FSH and LH secretion from the pituitary.
    If implantation does not take place by day 26 the corpus luteum disintegrates.
  5. Day 26 to day 28
    Rapid decline in the levels of progesterone and oestrogen due to the degeneration of the corpus luteum.
    The degenerated corpus luteum becomes a small scar, the corpus albicans.

The first sign that pregnancy has occurred is the failure of menstruation. Implantation occurs about six days after fertilisation. The developing embryo releases a hormone into the mother's blood which maintains the corpus luteum. The surviving corpus luteum continues to secrete progesterone and oestrogen and so the endometrium does not breakdown.


Development up to Implantation
(see diagram in textbook)

  1. Cleavage - increase in the number of cells by division but no overall increase in size.
    Immediately after fertilisation development commences by mitotic cell divisions forming a solid ball of about 100 cells.
    This small solid ball of cells is called the morula.
  2. Cell division continues but water is absorbed forming a hollow ball of cells with a special group of cells at one end.
    The hollow ball is called the blastocyst: outer layer of cells is the trophoblast; the group of cells is called the inner cell mass and the liquid cavity the blastocoel.
  3. The blastocyst is the structure that implants into the endometrium.

Embryonic Period: up to the end week 8

The inner cell mass forms the three primary cell layers (ectoderm, mesoderm, endoderm) which give rise to all the new tissues and organs during this time.

The trophoblast contributes to the formation of the placenta - fully operational by the third month.


Foetal Period: week 8 to birth

The tissues and organs of the embryo are brought to maturation.
A foetus is an embryo that resembles a developed individual.
Pregnancy ends with the birth of the foetus - on average 38 weeks after fertilization or 40 weeks after the last menstruation.


The Functions of the Placenta (see diagram of Foetus in Uterus in the textbook)

  1. Protection of the Embryo: impedes the entry of pathogens from the mother, allows the entry of antibodies from the mother and keeps the embryo separated from the mother's higher blood pressure, prevents exchange of red blood cells avoiding the deadly possibility of agglutination.
  2. Gas Exchange: supply of O2 the mother and excretion of CO2 from the embryo to the mother's blood.
  3. Nutrient Supply: glucose, amino acids, lipids, vitamins and minerals pass to the embryo from the mother's blood.
  4. Endocrine: secretes a variety of hormones including oestrogen and progesterone to maintain the pregnancy and prepare
    the mother's body for birth and lactation.
  5. Excretion: metabolic wastes, CO2 and urea, pass from embryo into the mother's blood.

Transfer of materials by diffusion and active transport is very efficient because the blood of the embryo and mother are very close and there is a very large surface area, 14 m2. (Your skin is 2 m2 )
The placenta is connected to the embryo by the umbilical cord along which run two arteries and a large vein.
The placenta is unique - the only organ made from tissue contributed by two different individuals.


Birth or Parturition - three major stages

  1. Labour: gradual dilation of the cervix to a diameter allowing safe passage of the head into the vagina.
    1. Oxytocin hormone is secreted by the pituitary, stimulating the contraction of the uterine muscles.
    2. The amniotic sac ruptures and the amniotic fluid escapes by way of the vagina.

  2. Birth: passage of the baby from the uterus through the cervix and along the vagina to the outside.
    The umbilical cord is clamped closed near to the baby and cut on the far side of the clamp.

  3. Afterbirth: the expulsion of the placenta from the mother.


Lactation:
Breastfeeding with milk

The placenta produced very high levels of oestrogen and progesterone. After the birth of the baby these hormone levels fall rapidly allowing the pituitary to secrete prolactin hormone.
Prolactin stimulates the glands in the breasts to produce milk.
The suckling of a baby at the breast stimulates the mother's pituitary to release prolactin maintaining milk production and oxytocin which causes the milk duct to contract ejecting the milk from the breast.

Human milk has a lot of advantages for the baby's growth and development. Besides being nutritionally balanced it also contains a wide variety of beneficial chemicals which include mother's antibodies that will protect the child against common pathogens.


Links

A guide to the female reproductive system.
http://www.epigee.org/guide/reproduction.html

This site poses some interesting questions about reproduction - use it to test yourself!
http://www.howe.k12.ok.us/~jimaskew/hbio.htm

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