Sexual reproduction in humans

Passage of sperm from epididymides to oviduct.

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Transport in the male

• Sperm must travel from the seminiferous tubule to the oviduct in the female.

• Seminiferous tubules are grouped into bundles of 100.

• One from each bundle emerges (vas efferens) and is attached to the epididymis.

• The sperm become motile when the chemicals in the fluid changes. Each sperm takes about 6-12 days moving from tubule to epididymis.

Transport in the male con’t

• Muscular activity of the walls of the tubes move the sperm from the epididymis into the vas deferens (they are fully mature at this time).

• More fluid is added from the seminal vesicles, prostate and Cowper’s glands to form semen.

• It is believed that the combined fluids increase fertility.

Transport from male to female (sexual intercourse)

• Also called coitus or copulation. • The penis becomes erect when a male

becomes psychologically or physically stimulated.

• Erection is caused by dilation of the arteries entering the penis and increased blood flow into those arteries. At the same time the veins leaving the arteries constrict preventing blood from leaving the penis.

Transport from male to female (sexual intercourse)

• An orgasm climaxes sexual stimulation. It is an explosive wave of intense pleasure accompanied by contraction of the muscles of the prostate gland, seminal vesicles, vas deferens and urethra.

• This causes ejaculation of semen and lasts a few seconds.

• Approximately 3cm3 of semen containing 500 million sperm is released.

Transport from male to female (sexual intercourse)

• During coitus, the blood supply to the clitoris increases causing it to become erect. The labia also swells as blood supply increases.

• Fluid from the blood seeps through the vaginal epithelium and lubricates the vagina. The vagina also expands.

• At orgasm, the muscles of the vagina and uterus contract.

Transport in the female• Sperm are deposited at the top of the vagina

near the cervix. • The alkaline semen helps protect them from

the acidic pH of the vagina (5.7).• The pH changes to about 6.5 which is ideal for

sperm motility and survival. • Most of the sperm leak from the vagina and

do not penetrate the cervix. • The mucus plug of the cervix is thick and does

not allow the passage of sperm.

Transport in the female con’t

• The plug becomes thin enough to allow sperms to pass during the first part of the menstrual cycle before progesterone becomes too high.

• Action of the cilia lining the uterus and oviducts along with the swimming action of the sperm, propel the sperm towards the oviducts.

• Living sperm take 4-8 hours to reach the oviduct.

Capacitation

• Ejaculated sperms must undergo a process before they can fertilize an ovum.

• A layer of glycoprotein (added by the epididymis) and plasma protein (added by the seminal fluid) must first be removed.

• Enzymes in the uterus remove these layers. • Removal increases the permeability of the

membrane to calcium ions which causes a stronger whiplash of sperm tails and promotes the acrosome reaction.

Acrosome reaction

• After capacitation, fertilisation can take place. • Fertilisation takes place in the oviduct and includes

the acrosome reaction. • During this reaction, the acrosome in the sperm

head swells and its membrane fuses with the cell surface membrane surrounding the head of the sperm.

• The enzymes, such as hyaluronidase and proteases are released by exocytosis.

•

Fertilization

• This is the fusion of the sperm nucleus with the egg nucleus to form a zygote (2n).

• Enzymes (particularly hyaluronidase) released by the acrosomes of many sperm digest a path through the granulosa cells.

• By the lashing of the tails the sperm reach the outer surface of the zona pellucida which has special receptors to which sperm can bind.

Fertilization• Another acrosomal enzyme digests a path through

the zona pellucida and the sperm moves to the surface of the secondary oocyte.

• The sperm fuse with microvilli surrounding the secondary oocyte and penetrate its cytoplasm.

• As soon as sperm has penetrated, the cortical granules (lysosomes) release their enzymes which cause thickening of the zona pellucida forming a ‘fertilization membrane’. The receptor sites are also destroyed so no more sperm can bind to the zona pellucida. This is the cortical reaction and prevents the entry of other sperm.

Fertilization• As soon as a sperm enters the cytoplasm, the

entry acts as a stimulus for the completion of meiosis to form an ovum and a second polar body (which immediately degenerates).

• The nucleus of the sperm swells as its chromatin becomes less coiled. Both nuclei of the ovum and sperm are called pronuclei.

• Both pronuclei fuse and this is fertilization. A zygote is formed (2n)