Learning ObjectivesBy the end of the lesson you should be able to:Label a sperm and ovumDescribe a zygoteExplain what happensafter fertilization
Gametes – e.g. sperm or egg (ovum)Gametes is the name givento sex cellsMen produce spermWomen produce eggs, OvaSperm are produced in greatnumbers in the testes(millions every day)Ova mature one at a time inthe ovaries (takes 28 days)
Sperm cellThe sperm cell has 23unpaired chromosomes fromthe father, these arecontained in the nucleus (3)The head has an enzymefilled acrosome (1), thisbreaks through the zonapellucida of the ovumThe mid-piece (4) containsmitochondria to provideenergy for the tail.The tail (flagellum) (5)thrashes to propel the spermforward
OvumThis large structure hasplenty of nutrient rich yolk inits cytoplasmIt’s nucleus is bigger but stillonly contains 23chromosomes from themotherThe zona pellucida is a jellylayer through which thesperm must break to reachthe nucleus of the eggThere may also be old folliclecells stuck to the egg
The zygoteOnce a sperm has fused withan egg it is known as azygoteIn a human the 23 unpairedchromosomes from thesperm join with the 23unpaired chromosomes inthe ovumThis makes the full humancomplement of 46chromosomes forming aunique zygote.
FertilizationOnce the zygote is formedfertilization is complete, it isstill not guaranteed thatpregnancy will take place.If a zygote is formed thecorpus luteum will notdegenerate (why is thisuseful?)The zygote finds its way intothe uterus and implants(pregnancy has begun)
First cell divisionsAs the zygote travels along itbegins to divide.The yolk makes this possibleThe one becomes two, thetwo become four etc.By the time it reaches theuterus the yolk is used up andthe zygote must implantIf the dividing zygotecompletely splits at this earlystage identical twins areformed.
First 3 monthsThe Progesterone and Oestrogen inhibit FollicleStimulating Hormone (FSH) production (why?)Progesterone and oestrogen also cause the breasts toenlargeThe corpus luteum finally degenerates after 3 monthsBy this time the placenta is fully formed (it takes overthe role of the corpus luteum in producing progesteroneand oestrogen)