fertilisation and 1st week of development

Question 1

3 things that are necessary for the sperm to succeed in fertilisation

Answer 1

1. Sperm climbing (into female tract) meaning a flagellum is needed
2. Sperm capacitation: removal of GPs from PM allowing it to bind to the ZP
3. Acrosome: release of lytic enzymes to break down ZP

Question 2

where does fertilisation occur

Answer 2

the ampulla of the uterine tract (external portion of fallopian tube)

Question 3

overview of fertilisation (4 steps)

Answer 3

1. penetration of corona radiata
2. binding of sperm to oocyte + acrosomal reaction
3. cortical reaction
4. fusion of pronuclei

Question 4

penetration of corona radiata process

Answer 4

sperms reaching the fertilisation site need to destroy the ECM of the coronaradiata cells within the cumulus oophorus to form a path to the 2ary oocyte

early lysosomal reaction: release of lytic enzymes (diff than phase 2 lytic enzymes) to do this

Question 5

binding of sperm to oocyte process

Answer 5

-sperm reaches ZP and binds to the ZP3 receptor (which is species specific)

-triggers acrosomal reaction
-fusion of acrosomal anterior membrane with plasma membrane of sperm
-creates discontinuities of the membrane so release of contents is possible
-ACROSIN: enzyme released

Question 6

what needs to have occured in order for acrosomal reaction to occur

Answer 6

sperm capacitation

Question 7

specialisation that happen within sperm after acrosomal reaction during docking

Answer 7

-actin bundle that is kept between nuc and acrosome in the sperm head is polymerised to form protrusion
-helps sperm enter into the cytoplasm of oocyte

Question 8

cortical reaction process

Answer 8

CORTICAL REACTION: release of cortical granules from periphery of oocyte cytoplasm that digest certain proteins in ZP to make it impermeable
!!! hyaluronic acid is also released into ZP which binds to H20 and pushes sperm further away

THIS PREVENTS POLYSPERMY

Question 9

fusion of pronuclei process

Answer 9

-oocyte is triggered to finish meiosis 2 (after being paused at metaphase 2)

-male and female pronuclei are formed and duplication of DNA occurs to prepare for 1st zygotic division

-fusion of pronuclei membranes to form arrangement into the metaphase plate and undergo the first division

!! this stage is where sex determination occurs

Question 10

what changes does the uterus undergo in the case that fertilisation does occur

Answer 10

-corpus luteum is maintained and becomes gravidic (which releases progesterone to maintain thickness of endometrium for implantation)

!! the trophoblastic tissue release hCG (human chorionic gonadotropin) hormone which stimulates the maintenance of the endometrium thickness

Question 11

Cleavage def

Answer 11

mitotic divisions within embryo to increase the cell number (occurs after 30 hours of fertilisation)

Question 12

overview of events that occur in the first week of embryo development

Answer 12

1. cleavage of embryo
2. 1st embryo layer formed (hypoblast)
3. START of implantation

Question 13

stages of cleavage

Answer 13

1. 2 cell stage -> totipotent blastomeres, with the 2 polar bodies. Surrounded by ZP
2. 4 cell stage -> totipotent blastomeres, polar bodies have been degraded Surrounded by ZP
3. 8 cell stage -> last totipotent stage, surrounded by ZP (early morula is formed)
4. advanced morula -> 16+ cells (blastomeres) until the blastocyst stage. Tight junctions formed to maximise contact, pluripotent cells

HATCHING

5. Blastocyst -> trophoblast (outer), embryoblast (inner cell mass) and blastoceole (fluid filled).

Question 14

Compaction definition

Answer 14

Occurs at the 8cell stage to form the morulla

flattening of cells in the 8 cell stage to form increased tight junctions and mazimise contact –> forms the template for the flat layer of the blastocyst and changes embryo shape

Question 15

4 characteristics that change/remain the same during the cleavage divisions

Answer 15

1. number of cells increases
2. size of cells decreases
3. size of entire embryo stays the same
4. mass of entire embryo stays the same

Question 16

overview of what the layers of the blastocyst will then differentiate into

Answer 16

1. TROPHOBLAST: cytotrophoblast and the syncytiumtrophoblast
2. EMBRYOBLAST: bilaminar disc (epi and hypoblasts)

Question 17

Blastocyt hatching def

Answer 17

the shedding of the ZP from the blastocyst which allows implantation to occur

Question 18

When do the mitotic divisions of the blastomers stop

Answer 18

when the cell has reached its specific correct ratio of nucleus:cytoplasm

Question 19

can can cause infertility in women

Answer 19

the maintenance of the zona pellucida past blastocyst hatching bcos implantation cannot happen with ZP present

Question 20

embryo cavitation definition

Answer 20

Na+ channels form on the trophoblast membrane which causes the movement of Na+ and water into the cell (forming the fluid filled cavity of the blastocyst

Question 21

what tissue does the trophoblast eventually turn into

Answer 21

placenta (extra embryonic)

Question 22

2 steps of implantation and process

Answer 22

STARTED in 1st week and FINISHED in 2nd week

1. Microvilli form on TB membrane and attach to endometrium
2. differentiation of TB into the STB and lytic enzyme release to break down the endometrium

Question 23

What location does implantation usually occur in (healthy)

Answer 23

posterior and superior region of the uterus

Question 24

Ectopic pregnancy locations of implantation

Answer 24

within ampulla
ovary
cervix
retctouterine pouch
abdominal cavity

Question 25

Process of implantation until the end of the 1st week of development

Answer 25

1. Differentiation of TB into CTB and STB.
2. STB: fusion of cells that invade wall of uterus, multinucleated and release lytic enzymes to degrade the EM

Question 26

what is a way we can visualise blastomeres during cleavage

Answer 26

phase contrast microscopy