Ex 3 - Fertilization and early embryonic development

Question 1

When do oocytes (2n) begin meiosis?

Answer 1

During fetal development

*They then arrest and remain dormant until after puberty

Question 2

Meiosis I –> Meiosis II

Answer 2

Meiosis I resumes when an individual follicle is recruited –> paired chromosomes divide and one polar body is extruded –> Meiosis II begins when a follicle is recruited –> it is then arrested in metaphase II

Question 3

What stage of meiosis is the oocyte in at ovulation? When does meiosis resume?

Answer 3

metaphase II

Meiosis resumes only if the oocyte is fertilized by a sperm

(nb: in the dog, oocyte is ‘immature’ when it ovulates - still in meiosis I)

Question 4

Oocyte release and fertilization locations

Answer 4

Oocyte and its assoc’d cells are released from the follicle at ovulation –> picked up by the infundibulum (oviduct) –> transported to the ampulla (ciliary action & sm m contraction)

Question 5

Where does fertilization occur?

Answer 5

ampulla/isthmus junction

Question 6

Where are the sperm reservoirs?

Answer 6

cervic and/or uterotubular junction

*sperm can last a few days in these areas

Question 7

What is sperm capacitation?

Answer 7

changes in glycoproteins on surface of sperm cell

alteration of lipid structure of sperm plasma membrane

hyperactive motility

Question 8

Describe sperm fertilization of the oocyte

Answer 8

1. sperm penetrates the cumulus cells surrounding oocyte
2. sperm then binds to ZP3
3. Influx of Ca++ trigger the acrosome reaction –> sperm fuse with oocyte membrane
4. hyper motile sperm burrow through the ZP –> enters the perivitelline space –> sperm is internalized into cytoplasm of oocyte

Question 9

What is “Zona Hardening”?

Answer 9

sperm and oocyte binding causes release of phospholipase from the sperm –> this triggers release of Ca++ from internal stores –> release of cortical granules –> these alter the ZP so no other sperm can bind

Question 10

what is the sperm binding site on ZP?

Answer 10

ZP3

Question 11

what is a zygote?

Answer 11

a diploid cell resulting from the fusion of two haploid gametes; a fertilized ovum

Question 12

How is the zygote formed?

Answer 12

meiosis II resumes (stim by Ca++) –> 2nd polar body extruded –> M & F pronuclei are fused –> chromosomes are duplicated (mitosis) –> cell cleaves into blastomeres

*This is what we look for during in vitro fertilization

Question 13

How often do blastomere cells divide?

Answer 13

every 12-24 hours

• each division results in smaller individual cells

Question 14

What do we call the developing embryo when it reaches 16-32 cells?

Answer 14

Morula

• ZP is still intact

Question 15

What is the fluid-filled cavity in the embryo called?

Answer 15

blastocele

Question 16

What is the outer layer of the embryo called?

Answer 16

Trophoblast

• this is what forms the placenta

Question 17

What is the purpose of the inner cell mass?

Answer 17

forms the fetus

Question 18

How long does it take the embryos to exit the oviduct and enter the uterus?

1. pigs?
2. cows?
3. mares?

Answer 18

1. 3.5 days
2. 4-7 days
3. 5.5-6.5 days

Question 19

What must equine embryos produce to allow passage through UTJ?

Answer 19

Embryos must produce PGE2!

This relaxes the sm muscle of the oviduct and allows embryo passage

*Unique to horses

Question 20

Zona hatching in ruminants and mice

Answer 20

development of embryo results in pressure –> ZP cracks –> blastocyst stage escapes and squeezes out

Question 21

Zona hatching in equine

Answer 21

Expand and the zona thins –> ZP is shed

• Equine embryos (in vivo) develop a glycoprotein capsule
• this protects the embryo after zona is shed

Question 22

What hormone is required by ALL FEMALES to maintain pregnancy?

Answer 22

P4!!

Question 23

What is the initial source of P4?

Answer 23

CL

*in some spp, P4 is eventually produced by placenta

Question 24

Can we give P4 to maintain a pregnancy?

Answer 24

little evidence to support routine use of supplemental exogenous P4 in order to maintain pregnancy

*use of exogenous P4 is common in mares, “Regu-mate”

Question 25

Maternal recognition of pregnancy (MRP)

Answer 25

LA embryos must signal the uterus to prevent secretion of PGs and luteolysis

*a decrease in P4 will result in pregnancy failure

Question 26

MRP - bovine/ovine

Answer 26

Embryos secrete IFNtau –> inhibits production of oxytocin receptors in the endometrium
(oxytocin can’t stimulate PFG2a synthesis)

Embryos elongate to till the uterine hormones ipsilateral to the CL
- elongation allows IFNt to contact the entire endometrium to signal MRP –> this is imp. b/c PGFs travel via counter-current transport system in these guys
- IFNt must contact the uterine horn that contains the CL
(PGF is only released into the IPSILATERAL horn)

Question 27

MRP - pigs

Answer 27

produce E2 which re-routes PFG secretion into the uterine lumen –> rather than release into systemic circulation

Question 28

MRP - equine

Answer 28

unknown in horses!

The embryos remain spherical and migrate throughout the entire uterine lumen, multiple times a days

This migration signals MRP to the entire endometrium –> important as equine PGF travel via systemic circulation

Question 29

What are dizygotic twins?

Answer 29

aka Fraternal twins or non-identical

*Arise from two ovulations that were fertilized separately

Question 30

What are monozygotic twins?

Answer 30

aka Identical twins

*Arise from a single oocyte –> developing embryo splits

**horses –> twins often do not survive

**sheep/goats –> twins/triplets are common

Question 31

Is it possible to split embryos to create identical twins via new technology?

Answer 31

yes