Biology - Human Reproduction

gonad

an organ that produces sex cells (gametes)

testis

produces sperm (exocrine function) and testosterone (endocrine function)

epididymis

matures and stores sperm

sperm duct/vas deferens

carries sperm from the epididymis to the urethra

seminal vesicles, cowper’s gland, prostate gland

• produces seminal fluid which nourishes the sperm (contains fructose) and allows them to swim

• sperm and seminal fluid are called semen

urethra

allows the passage of either urine or sperm to outside of the body

penis

places sperm inside the body of a female

scrotum

keeps testes at a lower temperature (35 degrees) as this is the optimum temperature for meiosis to occur

human sperm cell

• head: contains nucleus, which contains 23 haploid cells, and the acrosome, which produces digestive enzymes that breaks down the egg cell’s outer membrane during fertilisation

• collar: contains many mitochondria as the sperm cells need energy (ATP) to swim long distances

• tail

remember that sperm mitochondria (i.e. the collar and the tail) do not enter the egg cell as only the head of the sperm cell enters the egg. remember that non-nuclear inheritance is maternal

non-nuclear inheritance

when genes are passed onto offspring from a source other than the nucleus (mitochondria and chloroplasts contain DNA)

survival days of sperm

0-7 days in the female reproductive system

role of meiosis in sperm and egg production

1. it halves the chromosome number to produce haploid cells

2. it introduces variation

primary sexual characteristics

the presence of the male and female reproductive parts

secondary sexual characteristics

refer to characteristics that distinguish males from females apart from the reproductive organs

this appears during puberty

male secondary characteristics

1. growth of pubic, facial and body hair

2. breaking of the voice/deepening

3. broadening of the shoulders

male sex hormones

testosterone:

• produced in the testes

• its function is the development of secondary sexual characteristics and sex organs

FSH:

• produced in the pituitary gland

• its function is to cause sperm development in testes

LH:

• produced in the pituitary gland

• its function is to cause production of testosterone by the testes

infertility

the inability to achieve conception

reasons for male infertility

reason: low sperm count

caused by:

• the persistent use of drugs such as alcohol, smoking, cigarettes

• using anabolic steroids

• low levels of male hormones

treatment:

• a change in lifestyle, e.g. stopping alcohol consumption/smoking cigarettes/ use of anabolic steroids

• administration of the relevant hormone

ovary - function

• production of egg cells (meiosis)

• production of oestrogen and progesterone

fallopian tube - function

• transports egg from ovary

• fertilisation takes place here

endometrium (uterus lining) - function

• contains blood vessels which provide nutrients to the embryo

• implantation occurs here

uterus

holds the foetus during pregnancy

vagina

• the birth canal

• allows entry of sperm during intercourse

female infertility

reason: failure to ovulate due to endocrine gland function

caused by: hormone imbalance/stress

treatment: administration of relevant hormone/stress reduction

menstrual disorder

disorder: fibroids - benign uterus tumours

symptoms: heavy and prolonged menstrual bleeding

cause: hormonal (abnormal response to oestrogen)

treatment: surgical removal or a hysterectomy (removal of the uterus)

fertilisation

when the egg and sperm fuse to form a zygote

implantation

the embedding/attaching of the fertilised egg (zygote) to the uterus lining

female hormones

oestrogen & progesterone. these cause development of the female secondary sexual characteristics

female secondary characteristics

• the maturing and enlargement of the breqasts

• the widening of the pelvis to allow for birth

• the growth of pubic and underarm hair

the menstrual cycle

a 28-day sequence of events that include menstruation, egg production and preparation of the body for pregnancy

occurs when no fertilisation takes place at the end of the cycle, hence progesterone levels fall and menstruation begins

this cycle begins at puberty and continues until the menopause (end of the woman’s reproductive life)

survival times for egg

12-48 days after ovulation

menstruation

when the lining of the uterus (endometrium) breaks down and is shed from the body

ovulation

the release of an egg from the ovary and is caused by luteinising hormone

fertile period

the time during the menstrual cycle when fertilisation can occur

role of the menstrual cycle

to prepare the female body for pregnancy

development of the graafian follicle

1. the graafian follicle in the ovary contains an immature diploid egg cell (oocyte)

2. this diploid cell divides by meiosis to produce a haploid egg cell

3. the graafian follicle produces oestrogen

4. the graafian follicle bursts at ovulation to release the egg cell

5. after ovulation, the follicle becomes the corpus luteum

6. the corpus luteum secretes the hormone progesterone

the menstrual cycle

Day 1-5

• menstruation

• a new egg is produced in the graafian follicle by meiosis

Day 6-14

• the hormone oestrogen is produced by the graffian follicle

• this hormone causes the endometrium to build up in preparation for implantation

• oestrogen also prevents the development of more eggs

• day 12-16 is our fertile period

Day 14

• ovulation

Day 14-28

• the graafian follicle develops into the corpus luteum (yellow body), and this secretes progesterone

• progesterone maintains the lining of the uterus and also prevents new eggs from forming

*if fertilisation does not take place, the corpus luteum degenerates/dies. progesterone levels fall and as a result the lining of the uterus breaks down on day 28 and the cycle repeated

*remember to know how to analyse a graph regarding the hormones as well

hormonal control of the menstrual cycle

FOLP (must be in order)

1. FSH (follicle stimulating hormone)

2. Oestrogen 

3. LH (luteinising hormone)

4. Progesterone

FSH

• produced by pituitary gland (day 1-5 of the cycle)

• stimulates the development of the graafian follicle

Oestrogen

• produced by graafian follicle in the ovary

• produced from days 5-13

• causes development/buildup of the endometrium

• inhibits FSH production (neg. feedback mechanism)

• stimulates the release of LH (pos. feedback mechanism)

Luteinising hormone

• produced by pituitary gland

• produced on day 14

• causes ovulation

• causes the formation of the corpus luteum

Progesterone

• produced by the corpus luteum in the ovary

• produced from days 14-28

• maintains the endometrium

• inhibits FSH and LH production (neg. feedback mechanism)

what happens if fertilisation takes place?

• corpus luteum does not break down and continues to produce progesterone

• progesterone maintains the endometrium and prepares it for implantation

• high progesterone levels inhibit FSH production

feedback mechanism

when levels of one hormone control the production of itself or another hormone. it can be negative or positive

a negative feedback mechanism is when high levels of progesterone inhibit FSH and LH production

relative numbers, frequency of production and relative size of gametes

egg = low numbers, monthly, large

sperm = high numbers, continuously, small

fertilisation

the fusion of the egg and sperm nuclei to form a diploid zygote. takes place in the fallopian tube

events leading up to fertilisation

1. insemination is the release of semen into the vagina at the cervix

2. the egg releases chemicals to attract the sperm (chemotaxis)

3. many reach the egg but only one sperm cell will fertilise it

4. the acrosome releases enzymes to digest the egg membrane

5. the sperm loses its tail and the head enters the egg

6. the sperm and egg nuclei fuse to form a zygote

7. a fertilisation membrane prevents other sperm cells entering

process of implantation

1. during implantation, a membrane called the amnion develops around the embryo

2. the amnion secretes amniotic fluid

3. amniotic fluid acts as a shock absorber

placenta formation

• the embryo forms an outer membrane called a chorion

• this chorion develops projections (chorionic villi)

• so, the embryonic tissue and the endometrium form the placenta

functions of the placenta

• allows the exchange of nutrients, wastes, gases, antibodies and hormones between the blood of the mother and the embryo

• produces the hormone progesterone (it’s an endocrine gland) to prevent uterus from contracting

• a barrier that prevents the blood of the mother and foetus from mixing

why do the blood of the mother and embryo not mix?

• blood groups of mother and baby may be incompatible

• mother’s blood pressure may cause damage to the embryo

umbilical cord

connects the placenta with the embryo. it contains blood vessels which circulate blood between the embryo and the placenta

stages of development after fertilisation

1. zygote divides by mitosis to form a solid ball of cells called the morula

2. morula forms a hollow ball of cells called the blastocyst

3. the blastocyst is made up of the trophoblast and the inner cells (inner cell mass)

4. blastocyst passes along the fallopian tube and enters the uterus and implants into the endometrium

so…

zygote → morula → blastocyst

all formed via mitosis

further development of the inner cell mass

1. forms the embryonic disc

2. consists of three layers called germ layers

3. in humans, the mesoderm is split by a layer called the coelom. this allows space for more complex organs such as the heart, lungs and kidneys to develop

germ layer

a group of embryonic cells that can develop into different tissues and organs

germ layers + their organs/systems

ectoderm (outside):

• skin, hair, nails, nervous system

mesoderm (middle):

• muscles, skeleton, excretory system, respiratory system, circulatory system

endoderm (inside)

• inner lining of digestive, respiratory and excretory systems

• liver

• pancreas

gestation

the development of the embryo in the uterus until birth. in humans, gestation lasts 40 weeks

development of the embryo

4-5 weeks

• heart forms and starts to beat

• brain develops

• limbs form

6th week

• eyes visible

• mouth, nose, ears form

• skeleton develops

8th week

• body organs formed

• testes or ovaries form

• bone replaces cartilage

• referred to now as a foetus and it continues to grow

end of 3rd month

• nerves and muscles become co-ordinated

• eyes low in face and widely spaced

birth hormones

1. placenta stops making progesterone. progesterone levels fall. walls of uterus contract as a result

2. pituitary gland releases oxytocin, in which it causes contractions of the uterus

labour

stage 1:

• cervix dilates

• mucus plug falls out

• amnion breaks, releasing amniotic fluid

stage 2:

• uterine contractions push foetus through cervix and vagina

• umbilical cord is tied and cut

stage 3: 

• contractions expel the afterbirth (umbilical cord and placenta)

lactation

the secretion of milk from the mammary glands & is caused by a hormone called prolactin, which is secreted by the pituitary gland

advantages of breastfeeding

• colostrum and breastmilk contains antibodies

• ideal balance of nutrients for baby

• has little fat, making it easier to digest than milk

IVF - in vitro fertilisation

• when an egg and sperm cell fuse outside the body

• products of IVF are implanted in the uterus

• number of fertilised eggs are usually implanted to increase the chances of a successful pregnancy

contraception + examples

using methods to prevent fertilisation or pregnancy

e.g.

natural: intercourse avoided during fertile period

mechanical: condom/diaphragm are physical barriers

chemical: contraceptive pill

surgical: sterilisation/vasectomy