ferilisation Flashcards
week 6
What are the two phases pf embryology?
prenatal and postnatal
What are the two periods within prenatal development?
Embryonic period
Fertilisation –> end of 8th week
Fetal period
9th week –> birth
How long is pregnancy?
38 weeks
(three trimesters)
How is the due date calculated?
40wks + 7 days FROM the First day of last normal menstrual period (LNMP)
What occurs in the embryonic period?
Placenta develops
All major adult organs formed (gastrulation and organogenesis)
What occurs in the fetal period?
growth and development
What occurs in week 1 of embryonic period?
Day 0 = fertilization
Pre-implantation events: cleavage and blastocyst formation
Day 6= implantation: adherence
Day 7 = implantation: invasion
What occurs in week 2 of embryonic period?
Development of placenta begins
Day 12: implantation complete
What occurs in week 3 of embryonic period?
Days 13-14 = primitive streak
Days 14-16 = Gastrulation
Day 17 = start of organogenesis
What happens in week 12 of pregnancy?
placenta maturation
What happens during fetal development?
differentiation and refinement of body structure
Sperm capacitation ( what and how)
Sperm Capacitation
-Sperm can’t immediately fertilize as must spend a few hours in female repro tract to attain capacity to fertilize.
Process
-Osterogen and Vaginal mucus destabilize sperm plasma and trigger motility
-Removal of protein coating
-Reorganization of plasma to expose binding sites
What is the acrosome reaction, what is it stimulated by and what are its actions?
What:
Ca-dependent event in which the acrosome swells and outer acromsal membrabe fuses with overlaying membrane
stimulated by
progesterone and ZP3 (protein)
Actions:
releases digestive enzymes (hyaluronidase and acrosin) from acrosome
What does Hyalurnidase do?
penetration of corona radiata
(digestive enzyme released in acrosome reaction).
What does acrosin do?
zona pellucida digestion
(digestive enzyme released in acrosome reaction).
Summarise the four events of fertilisation
Capacitation: estrogen/mucus destabilize sperm PM and hyperactive motility
Acrosome reaction: progesterone and binding of ZP3 cause increase Ca and release of digestive enzymes from sperm.
Fertilization: sperm digests zona, binds to sperm-binding receptor and fuses with oocyte PM.
Polyspermy block: fusion causes increase Ca and electrical block of oocyte OM, cortical granule release hardens zona and removes sperm binding receptors.
what are the three stages of implantation development?
Cleavage stage
Morula stage
Blastocysts stage
What occurs in the cleavage stage?
2-8 cells
Increase number of cells (but no increase in cell size)
Totipotent blastomeres (generate complete individual)
What happens in the morula stage? How does it appear?
16-32 cells
Inner and outer cell populations formed
= solid ball of blastomeres
What happens in the early blastocysts phase?
morula hollows out, fills with fluid and hatches from the ona pellucide
4 days
What occurs (overall) in the blastocysts stage?
64+ cells
First cell differentation event
Pluripotent inner cell mass = generate complete individual excluding individual
For successful fertilisation, Coitus (sexual intercourse) should occur….
2 days before and 0.5 days after ovulation
How long does sperm and occytes last for?
Sperm = 28-48hrs
oocyte= 6-24hrs
For IVF, how can it be microscopically shown that fertilisation has occurred?
presence of 2 polar bodies
(trapped by zona pellucida)
can’t use cell division as an indicator as high Ca exposure will cause them to divide even if bot fertilised
Pluripotent vs totipotent
P= generate complete individual excluding placenta
T= generate complete individualincluding placenta
Morula - structure and actions
outer and inner cell populations
gap junctions between inner cells
tight junctions between outer cells
Na pumped in = influx of water
What are the three stages of implantation?
Adherence
Invasion
Completion
When does adherence occur and what occurs in it?
6 days after
Protein mediated binding of the trophoblast –> endomer\trium
Digestive enzymes from the trophoblast erode the endometrial layer
Why do syncytiotrophoblasts secrete hCG? When does it do this?
hCG maintains CL and enables it to produce progesterone to support endometrium and pause menstrual cycle
6-8 after fertilsation, for 8-12 wks
What occurs in the invasion stage of implantation and when does it occur?
Trophoblasts differentiate = cytotrophoblasts and syncytiotrophoblasts
Syncytiotrophoblast digest and invade endometrium to secrete hCG = maintenance of CL
7 days after F
What occurs in the completion stage of implantation and when does it occur?
12 days after feralization
Blastocysts enveloped by endometrium
Outline the 4 steps in the formation of the bilaminar embryonic disc
- Inner cell mass differentiates = epiblasts and hypoblasts
- Fluid filled cavities form
- Hypoblast invaginates = allantois with extraembryonic mesoderm.
- extraembryonic mesoderm, cytotrophoblasts and syncytiotrophoblasts = chorion and chorionic villi
What are the 2 fluid-filled cavities form in stage 2 of Bilaminat embryonic disc formation?
Amniotic cavity: between epiblasts and amnion
Yolk-sac: lined by hypoblasts
what are the 4 extraembryonic membranes?
Amnion
Yolk Sac
Allantois
Chorion and Chorionic villi
Functions of the Amnion
Prevents physical trauma
Maintains temperature
Fluid avoids embryonic structures fusing
Permits movement
Functions of Yolk Sac
Brief nutrient supply
Become spat of gut
Source of early blood cells and blood vessels
Function of Allantois
Forms base of umbilical cord
Function of Chorion and Chorionic villi
Forms true placenta with maternal decidua
During week 3 of pregnancy, the primitive streak forms. What is this and on what does it form?
Days 13-14 = primitive streak = midline groove forms along the caudal end of bilaminar disc = longitudinal axis of the embryo
How is the endoderm germ layer formed?
from the epiblast cell that displaces hypoblasts
How is the mesoderm germ layer formed?
from the Epiblast cell that fills middle layer
In what weeks do the germ layers differentiate?
4-8
What do the ectoderm, endoderm and mesoderm differentiate into respectively?
Ectoderm
Nervous system and epidermis
Mesoderm
Most blood vessels, connective tissues, bone and muscles
Endoderm
Epithelial linings and associated glands of digetsive, respiratory and urogenital systems.
What are the 3 steps to sperm penetrating an egg?
- hyperactive motility → buries itself by physically penetrating cumulus cells
- hyaluronidase → penetrates corona radiata (innermost cumulus cells)
- acrosin → digests zona pellucida
What does the zona pellucida do during the cleavage stage?
holds together the cells of the cleaving embryo and protects it from attack by macrophages.
Why must the zona pellucida be removed before the expanded blastocyst can attach itself to endometrium?
How is it removed?
If not removed, it would form a physical barrier preventing contact between trophoblast and endometrial epithelium
How does the blastocyst remove the zona pelluida?
Blastocyst Expansion:
fluid accumulation increases the internal pressure within the blastocyst.
Enzyme Production:
The blastocyst produces and secretes enzymes, (proteases such as trophoblast proteases) degrade the zona pellucida.
Mechanical Pressure:
The accumulation of fluid + enzymatic breakdown= mechanical pressure = zona pellucida to thin and eventually rupture or crack.
Zona Pellucida Rupture:
What is the trophoblas?
The outer cell layer of a blastocyst
secretes enzymes that help digest the zona pellucida
Why is the uterus the best spot for implantation?
- endometrium (secretory phase so highly vascularised)
- space for growth and development
- myometrium for muscular contractions in parturition
- structural support
When the embryo attaches to the uterine wall, what does the trophoblast proliferate and differentiate into?
(what are the two layers of a trophoblast)
Two cell populaions:
Syncytiotrophoblast
Cytotrophoblast
cytotrophoblast
cell population in which cell structure is retained
surround the blastocyst cavity and overlie inner mass = responsible for the proliferation and further differentiation
Syncytiotrophoblast
a cell population in which individual cells have lost cell membrane
forms the ‘leading edge’ of cytotrophoblasts = produce enzymes that allow embryo to invade uterine wall.
Why do trophoblasts invade uterine wall?
The trophoblast needs to invade the uterine lining to establish a connection with the maternal blood supply,
