FertilizationFertilizationand Pregnancyand Pregnancy

FertilizationFertilizationPregnancy is the presence of

developing offspring in the uterus, an event resulting from fertilization – the fusion of male and female gametes.

For this fusion to occur in humans, the sperm needs to penetrate the egg cell, which is surrounded by a zona pellucida and a corona radiata.

FertilizationFertilizationThe acrosome reaction

A protective covering (the acrosome) surrounds the head of the sperm; it contains enzymes that break down the cells that protect the egg.As the sperm touches the cells surrounding the egg (the corona radiate), the acrosome releases these enzymes, and they digest a path into the egg.

FertilizationFertilizationRole of the zona pellucida

When the sperm cell reaches the zona pellucida, a thick, jelly-like layer surrounding the secondary oocyte, re-

ceptors bind the sperm cell so it can pass through.

FertilizationFertilizationThe cortical reaction

The head of the sperm cell fuses with the plasma

membrane of the oocyte, triggering the cort-

ical granules to release enzymes to thicken the zona pellucida so that it becomes a fertilization envelope

Then no other sperm cells can fertilize the egg.

FertilizationFertilizationThe journey of the sperm cell:

Sperm cells must reach the upper one-third of the uterine tubes for fertilization to occur.

Under the influence of estrogen during the first half of the menstrual cycle, uterine se-

cretions are thin, allow ing sperm cells to swim easily to

their destination.

Fertilization occurs in the fallopian tube when one

sperm penetrates the egg. A diploid zygote results.

Process of Process of in vitro in vitro fertilizationfertilizationFertilization that is done outside the

body (in glass).

Early embryo developmentEarly embryo developmentAfter fertilization: the haploid sets of

chromosomes from the male and female gametes line up at the equator and prepare for the first mitotic division.

This division, within 24 hours after fertilization, is not followed by cell growth, and is, therefore, called a cleavage division.During the first 5 days several cleavage divisions occur within the fallopian tube.

Even though cells are dividing, embryo doesn’t growAt the 8 cell stage compaction occurs – the cells squish together into a small ball called the morula.

Early embryo developmentEarly embryo development

The morula stage, up to 32 cells

Early embryo developmentEarly embryo developmentMorula: A compact mass of 16 to 32

cells.The morula then leaves the

fallopian tube and enters the uterus.

The inside of the morula hollows out, and the cavity

becomes filled with fluid, becoming the blastocyst.

Early embryo developmentEarly embryo developmentThe blastocyst begins to differentiate.

Trophoblast: tissue around the outside of the blastocyst will become the extraembryonic membranes and placenta, after implantation in the uterus.Inner cell mass: a ball of cells inside the trophoblast will become the embryo proper.Blastocoel: the fluid-filled

cavity.

Early Embryonic DevelopmentEarly Embryonic DevelopmentThe blastocyst begins to differentiate.

The rate of cell division increases.By time blastocyst

reaches the uterine cavity there are

~100 cells, and it is ready to plant

itself in the endometrium.

(lining of uterus)

Early embryo developmentEarly embryo developmentPassage of the fertilized egg through

the fallopian tube.

Early embryonic developmentEarly embryonic developmenthttp://www.youtube.com/watch?v=vslaD-mWaNA

http://www.youtube.com/watch?v=28GTvrNvRRE

The placentaThe placenta

Uterus

Day 6

Day 7

Day 18

Day 30

Development of the placenta from cells of the chorion (in green).

The placentaThe placentaStructure:

Disk-shaped organ, contains embryonic and maternal blood vesselsAbout the size of a dinner plate, and weighing about 1 kg allowing exchange of materials by diffusion.Two embryonic blood vessels

contained with-in the umbilical

cord.

PlacentaPlacentaStructure of the placental villus

Maternal blood flows in the inter-villous space around the villi.Cells that separate maternal and fetal blood from the placental barrier must be selectively permeable for exchange of materials.ace area for processes.

Microvilli increases Surface Area Fetal blood that flows through the placental villi is oxygenated and recharged with nutrientsWaste products removed

Distance between mater-nal and fetal blood is as

small as 5 micrometers.

The placentaThe placentaFunction:

produces progesterone/estrogenEstrogen maintains endometrium and inhibits menstrual cycle from 2nd trimester onwardsMaterials are exchanged between the maternal and fetal blood in the placenta.

Nutrients: oxygen, glucose, amino acids, vitamins, antibodies.Waste products: carbon dioxide, urea transports glucose, amino acids, lipids, oxygen, carbon dioxide, antibodies, urea

HCG in early pregnancyHCG in early pregnancyHuman Chorionic Gonadotropin

Secreted by embryonic cells that form the placenta after implantation. Enters the bloodstream, binds to LHCGR (Luteinizing Hormone /Choriogonadotropin Receptor in Uterus and Ovary.

This preserves the corpus luteum beyond 14 days so it can con-tinue producing progesterone to maintain the thickening & vascularization of the endometrial layer in the uterus.

Triggers the hypothalamus to mobilize the mothers stored fat.HCG levels will double every 72 hours

Level will reach its peak in the first 8 - 11 weeks of pregnancy and then will decline and level off for the remainder of the pregnancy.

Pregnancy TestsPregnancy TestsHuman Chorionic Gonadotropin

Can be detected 11 days after conception in the blood.

More accurate.Can be detected 12-14 days after conception in the urine.

A pregnancy test binds the HCG hormone to an indicator and antibody; the indicator is a pigment molecule (ex. a line on the home pregnancy test).

The amnionThe amnionThe fetus is supported and protected by the amniotic sac and amniotic fluid.

The amnionThe amnionAmniotic sac is an extra-embrionic

membrane that extends all the way around the fetus.

Provides a cushioning effectProvides thermal stabilityProvides an environment for free movement of fetusDevelopment of muscles and skeleton of fetus

Hormones and birthHormones and birthAs birth approaches, progesterone

levels (which were elevated throughout pregnancy) begin to decline.

At this time, oxytocin levels begin to rise.

Made by hypothalamus & excreted by pituitary.During parturition, oxytocin

is involved in contractions, dilation of the cervix, and minimizing bleeding.

Hormones and birthHormones and birthWhen a baby is ready to be delivered, it

drops lower in the uterus and puts pressure on the cervix, causing the cervix to stretch.

Hormones and birthHormones and birthThe stretching of the cervix further

stimulates the release of the hormone oxytocin, which induces the uterus to contract more and push the baby further against the cervix.

In turn, this causes the cervix to further stretch,

which releases more oxy-tocin, and thus the cycle

continues =

positive feedback.

ChildbirthChildbirthStretching of the cervix further

stimulates the release of the hormone oxytocin, which induces the uterus to contract.