human reproduction biology 269

Assuming:

You paired up with an appropriate partner

You elicited normal sexual responses from each other

A few milliliters of semen, containing 200- to 600-million sperm, were deposited high in the vagina, just below the cervix of the uterus

Scenario I:

The woman is not within a few days before or after ovulation

Her cervix will be plugged with thick mucous, so few if any sperm will enter the uterus and subsequently the Fallopian tubes.

The few sperm which might make it into the Fallopian tubes are not pushed distally (toward the ampulla)

The sperm do not meet an oocyte and die within 48 hours

No fertilization occurs

Scenario 2:

The woman has just ovulated, or she will ovulate within 1 - 2 days.

The mucous plug in her cervix will be thin and watery; sperm can easily penetrate it

Most sperm will die in her vagina, but a few million (1 - 2%) are pulled through the cervix, into the uterus, and into the Fallopian tubes where they are propelled toward the ampulla (+100,000 make it this far)

Sperm meet the oocyte, and fertilization can occur

Fimbria surrounding the opening of the Fallopian tube sweep this structure into the infundibulum

At ovulation, oocyte is surrounded by a thick membrane called the zona pellucida, and by follicular cells called the corona radiata

Oocyte is viable for 12 – 24 hours, so fertilization must occur within this time.

Fertilization occurs within ampulla of Fallopian tube

If fertilization does not occur, oocyte will degenerate and die

A few thousand sperm reach the ampulla of the Fallopian tube within 30 to 60 minutes after ejaculation

But

They are not capable of fertilizing oocyte

They must undergo processes called capacitation

and acrosome reaction

which will take 8 to 10 hours after ejaculation

Prior to ejaculation, membrane around head of sperm is strongSemen contains chemicals to prevent weakeningTail is moderately active

After ejaculation, these chemicals in semen diluted / washed away.

Secretions from uterus & oviducts cause sperm membrane to weaken, and cause changes which will allow the sperm to more easily bind onto and penetrate the membrane around an oocyte.

Movement of tail increases.

Capacitation:

Occurs as sperm make contact with corona radiata and zona pellucida

Requires 15 to 60 minutes

Acrosome swells, becomes leaky, releases digestive enzymes

Acrosome Reaction:

Enzymes digest path through corona radiata and zona pellucida

Hundreds of sperm usually involved in this digestion

(Acrosome swells, becomes leaky, releases digestive enzymes)

Head of one sperm makes contact with outside of oocyte, binds onto it, and is quickly taken into it.

Fast block: Plasma membrane of oocyte changes its electrical charge, which prevents other sperm from binding onto it.

Two systems prevent entry of more than one sperm:

Slow block: Zona pellucida swells, pushing other sperm away from the oocyte.

Once head of sperm enters oocyte, its nucleus is released into the cytoplasm.Nucleus of sperm (23 chromosomes)and nucleus of oocyte (23 chromosomes) meet and fuse together.

Cell now called zygote, with 46 chromosomes

Zygote undergoes repeated divisions (mitosis), called cleavage, as it is transported through Fallopian tube (oviduct) toward uterus

Eventually forms solid mass of 20 – 30 cells called morula, same size as the zygote. This reaches uterus 3 to 4 days after fertilization

As oocyte was developing, follicle in ovary secreted estrogen, which caused endometrium to thicken and prepare for pregnancy.

At time of ovulation and fertilization, follicle develops into corpus luteum which secretes progesterone, keeping endometrium ready for pregnancy.

When morula reaches uterus, endometrium is thick with lots of secretions.

Morula does not embed in uterus immediately.

Stays loosely attached for 4 to 5 days, nourished by secretions of endometrium.

Continues dividing into +100 cells, forming hollow sphere called blastocyst. Cavity in center = blastocyst cavity.

Blastocyst develops mass of cells at one end, called embryoblast or inner cell mass, which will develop into the embryo.

Cells surrounding cavity, called trophoblast, will develop into the placenta.

Cells of trophoblast secrete digestive enzymes, forming finger-like projections into endometrium.

Blastocyst burrows into endometrium, which will heal over. This process is called implantation.

Trophoblast begins to secrete new hormone, human chorionic gonadotropin (HCG) as it matures into placenta.

HCG stimulates corpus luteum of ovary to remain active and to continue secreting progesterone instead of shrinking.

This progesterone keeps endometrium of uterus from dying and being shed. Thus, menstruation does not occur.

HCG = hormone detected by pregnancy tests

HCG produced + 8 weeks, then placenta mature enough to produce progesterone.

Corpus luteum in ovary slowly degenerates, but progesterone from placenta prevents new follicles from developing

As trophoblast is causing implantation and developing into placenta,

Embryoblast is enlarging and developing into embryo

Fluid-filled space called amniotic cavity develops between embryoblast and trophoblast

Embryoblast = flat disk between amniotic cavity and blastocyst cavity. Now called embryonic disk or simply embryo.

(12 – 14 days after fertilization)

Back to the blastocyst

Pre-embryonic period now over, and embryonic period begins.

Fertilization – 2 wks = Pre-embryo

During next six weeks of embryonic period, cells of embryoblast will differentiate into beginnings of all organs.

2 wks – 8 wks = Embryonic period

After 8 weeks organs will grow, develop into their final forms, and begin to function during the fetal period.

8 wks – birth = Fetal period

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