98. fertilization 99. blastocyst formation 100. implantation

Question 1

what is fertilization
where does it occur
what is it preceded by

Answer 1

process by which male and female gametes fuse

in the ampulla of the uterine tube

insemination- intro of sperm into female reproductive tract

Question 2

What are the main results of fertilization

Answer 2

restores diploid number of chromosomes

determination of the sex of the new individual

initiation of cleavage

without fertilization, the oocyte degenerates 24 hrs after ovulation

Question 3

spermatozoa are not able to fertilize the oocyte immediately. What has to occur?

Answer 3

CAPACITATION: 7 hr conditioning in female repo tract
glycoprotein coat and seminal plasma proteins
are removed from the plasma membrane of
the acrosomal region.

ACROSOME REACTION: occurs after binding to zona pellucida
induced by zona proteins
releases enzymes :

hyaluronidase= penetration of the corona radiata barrier

trypsin-like substance =digestion of the zona pellucida

acrosin (zonalysin) =helps the spermatozoon cross the zona pellucida

Question 4

What are the 3 phases of fertilization

Answer 4

1. penetration of corona radiata
   2.penetration of zona pellucida
2. fusion of oocyte and sperm cell membranes

Question 5

What does each of the phases of fertilization involve

Answer 5

phase 1: 300-500 reach fertilization site but only 1 fertilized

phase 2: sperm binds to zona pellucida and induces the acrosome reaction. release of enzymes allow sperm to penetrate zona and makes contact with plasma membrane.
Zona reaction prevents other sperm from fertilising- inactivates receptor sites on zona for spermatazoa

phase 3: after spermatozoon enters egg, egg responds by:
1. cortical oocyte granules with lysosomal enzymes released. oocyte membrane impermeable prevents other sperm from penetrating (zona reaction)
2. resume meiosis II, 1 mature daughter cell 22+x (these chromosomes in female pronucleus) and second polar body formed
3. spermatazoons nucleus become a male pronucleus, and its tail detatches and degenerates. M+ F pronuclei (haploid) replicate their DNA so that the zygote has diploid amount

Question 6

Examples of assisted reproduction

Answer 6

IVF
.egg fertilized by sperm outside the womb,
.low (20%) success rate
. four or five ova are collected, fertilized,
and placed in the uterus (multiple births)

gamete intrafallopian transfer (GIFT):
. introduction of oocytes and sperm into
the ampulla of the fallopian tube

zygote intrafallopian transfer (ZIFT):
. fertilized oocytes are placed in the
ampullary region of the fallopian tube

intracytoplasmic sperm injection (ICSI)
for severe male infertility:
. single sperm injected into the
cytoplasm of the egg to cause fertilization

Question 7

what happens to a zygote

Answer 7

zygote undergoes cleavage and mitotically divides
these cells from cleavage divishions are called blastomeres
ball of 16 cells = morula

Question 8

what is structure of blastocyst

Answer 8

inner cells gives rise to embryoblast (approx 128 cells)
outer cells forms trophoblast (cytotrophoblast + synctiotrophoblast)
fluid filled cavity = blastocoel

zona pellicuda disappears
Blastocyst enters the uterine cavity (4-5th day)

Question 9

what are the 3 layers of the uterus

Answer 9

endometrium
myometrium
perimetrium

Question 10

what happens in implantation

Answer 10

blactocysts trophoblast(cytotrophoblast) invade endo tissue (adhesion)
these differnetiate to synctiotrophoplast. they are larger and form projections (villi) that lie close to the uterine blood vessels

Question 11

implantation sites

Answer 11

normal: uterine cavity (posterior + lateral)
abnormal: fallopian tube, cervical canal + ovaries