Early Embryo Development And Maternal Recognition Of Pregnancy Flashcards
at fertilisation, 2 haploid gametes fuse to form…
New diploid zygotes: spermatozoa perspective
Describe the steps of fertilisation from the spermatozoa perceptive
(Formation of male proncleus)
-Capacitated sperm reach ampulla (need to be motile enough to penetrate)
-interact with Zona Pellicuda via ZP3 protein—> leads to sperm-egg fusion (many sperms will fuse to egg but only one form the final zygote)
-spermatozoa undergo acrosome reaction; outer sperm head membrane dissolves
-dissolving causes release of proteolytic enzymes & uncovering of ZP2 receptors
-these enzymes help with penetration of the ZP
-after penetration, sperm-egg membrane fusion occurs with help of specific proteins on sperm head
-nucleus enters oocyte and decondenses
-this forms the male pronucleus
Capacitated cell meaning
Undergone maturation
Describe the steps of fertilisation from the secondary oocyte (the egg) perspective
(Formation of female pronuleus)
- 1st spermatozoon enters egg; calcium waves are released across the cytoplasm
- Calcium waves lead to;
-cortical granule reaction-> the hardening of the ZP
-polyspermy block-> no more spermatozoa can fuse and make connection with plasma membrane - Egg in metaphase 2 resumes in meiosis II leads to formation of female pronucleus
What happens after both male and female pronucleus have formed?
Genetically new zygote has formed; containing 2 separate pronuclei and separate maternal & paternal chromosomes
-there is an immediate entry into the first cleavage division
In a zygote, can you tell which. Pronucleus is male (paternal) and which is female (maternal)
No, you cannot
Name the structures of the fertilised egg
On anki
Name the steps involved in early embryo development, after fertilisation, which lead to maternal recognition
- Cleavage
- Blastocyst formation
- ‘Pregnancy’ signal
1st step of embryo development:
1. Embryo cleavage
Development of the single fertilized cell (zygote) into a multicellular complex within the Zona Pellucida (Z.P.)
Describe the steps of Embryo cleavage; 1st step of embryo development
-Zygote forms into morula- propelled downwards within uterine tubes
-morula is made up of 16 blastomeres- these are totipotent
-cell becomes progressively smaller
-morula enters uterine horn
-morula divides until its reached 64 divisions; at this stage it has formed the blastocyst- this is the beginning of cell differentiation
2nd step of embryo development;
2. Blastocyst development
Blastomeres in the morula become very compacted and begin to form tight junctions with adjacent cells, & blastocyst development begins
Describe the steps of blastocyst development
-Cell divisions continue in morula—> compression of blastomeres & compaction
-cell differentiation begins= loss of totipotency
-fluid absorption occurs—> central fluid filled cavity is formed, forming the blastocyst
What are the 3 types of blastocyst?
Trophectoderm/trophoblast
Blastocoele (fluid filled cavity)
Inner cell mass (ICM) cells
Which of the 3 types of blastocysts remain pluripotent?
ICM cells
What do the trophoblasts become in embryo
What do the ICM cells become?
-these become outermost layer of placenta
-these become the proper embryo
3rd step of embryo development
3. Hatching & expansion of the blastocyst
-what must the blastocyst do to continue growing?
Needs to break free from the Z.P to continue growing
Describe the steps of hatching step of embryo development
-Blastocyst grows
-zona pellicuda breaks and this is called hatching of the blastocyst
-blastocyst begins to elongate and grow and expand due to trophoblast cells multiplying rapidly
-this leads to elongation of the embryo
What happens after hatching
Either implantation or attachment to maternal endometrium in the uterus
Implantation; invasively implants itself or just attaches itself non invasively
What do the trophectoderm/trophoblast cells provide after hatching?
Provide signals for maternal recognition of pregnancy eg ruminant, humans
Which blastocyst cells form the embryonic shield on surface of expanding embryo
ICM cells
How many days after fertilisation does hatching occur in the bovine?
8-10 days
Degree of embryo expansion reflects…
Different strategies for signalling ‘pregnancy’ to dams
Which embryo expansion image edicts which animal?
On anki
Which animal is the exception to expansion of blastocyst and why
The horse- develops a capsule round the embryo, in addition it doesn’t elongate, but remains spherical- called embryonic vesicle
What do the ‘pregnancy’ signals coming from the trophoblast cells prevent?
Prevent luteolysis in species with a cyclic CL lifespan that is shorter than gestation (pretty short lifespan)- this is to preserve the pregnancy and continue its growth, also means no other ovulation can occur at the same time
What is the importance of maintained function in the corpus luteum
For maintenance of progesterone secretion from the CL
What are the effects of progesterone on the reproductive tract?
- Stimulates endometrial gland secretions essential to support embryo development
- Reduces contractility of the reproductive tract thus leads to embryo quiescence
- Increases uterine growth
- Exerts negative feedback on hypothalamic GnRH secretion: no surges, no pre-ovulatory follicle development or mating
What is maternal recognition of pregnancy?
The prevention of luteolysis initiated by signals from the embryo
In which species are signals needed to prevent luteolysis?
Species with a Short CL lifespan
Which species have a short CL lifespan and require pregnancy signals to prevent luteolysis?
Cow
Ewe
Sow
Mare
Which mechanism is used to stop luteolysis in the:
-cow
-ewe
-sow
-mare
- Suppresses Oxytocin Receptors
2.Suppresses Oxytocin Receptors
3.Redirects PGF2a into uterine lumen
4.Suppresses localised PGF2a synthesis
Name the maternal recognition hormone responsible for preventing luteolysis in each of these animals:
-cow
-ewe
-sow
-mare
-Bovine Trophoblast protein
-Ovine Trophoblast protein
-Oestradiol
-Vets not sure about the mare
The more functional the CL, the more likely…
Is the maternal recognition Of pregnancy
What is the early embryo nourished by?
Endometrial secretions
When equine embryo Hatches it develops into…
free-moving embryonic
vesicle travelling up and down uterine horns until day 16
(Diagram on anki)
What does the movemnt of embryonic vesicle in horse do?
signals maternal recognition of pregnancy = prevents CL regression i.e regulates luteolysis
which species have active luteolysis and which have passive luteolysis?
Active;
Cow
Ewe
Sow
Mare
Passive;
Human
Rodent
Name the maternal recognition hormones found in humans and rodents
Humans- Human Chorionic Gonadotrophin
Rodents- prolactin pulses, Chorionic Gonadotrophin,
PGE2, placental lactogens, prolactin-like pepide
What do species with passive luteolysis need extra of to prevent luteolysis?
extra luteotrophic stimuli from the embryo(s) must prevent CL regression until the placenta can take over progesterone production
In early pregnant human females, enhancement of luteotrophic stimuli is via… what does it do?
hCG which stimulates LH receptors and progesterone synthesis continues
True or false: In species with a long CL lifespan such as the bitch no embryonic signals are needed to prolong CL function
TRUE
During fertilisation, which process do spermatozoa have to undergo specifically ybefore they can penetrate the zona pellucida of the egg
Acrosome and capacitation
What stage of meiosis are oocytes in at the time of fertilization?
Metaphase II
What is a fertilised oocyte called?
Zygote
How quickly does first cell division occur after fertilization?
22-24 hours
