Early Embryonic Development Flashcards
Briefly describe the formation of an embryo
Sperm fuses with the ovum
This forms a zygote
Blastocyst
Bi laminar disc
Tri laminar disc
Embryo
What forms the zygote ?
Spermatozoon and secondary oocyte (ovum)
What is the function of the acrosome of the sperm ?
Contains enzymes required for the penetration of the zone pellucida of the oocyte.
Where are 2ndary oocytes found ?
In the ovary, arrested in metaphase 2 of meiosis 2
Describe what the cytoplasm of the oocyte is surrounded by.
(inner to outer)
Plasma membrane
Zona pellucida
Corona radiata
What is ovulation ?
The discharge of usually a single secondary oocyte from the ovary as a result of increase in luteinising hormone (LH)
What type of cells are found in the corona radiata ?
Cumulus oophorus cells
When is Meiosis 2 completed ?
When the secondary oocyte is fertilised
The oocyte matures to become ovum
Describe the steps in ovulation
The ends of the uterine tubes come into close contact with the ovary during ovulation.
Fimbriae of uterine tubes ‘sweep’ the ovulated ovum into the uterine tube.
Peristaltic waves of uterine tube musculature bring the 2ndary oocyte into the ampulla of the uterine tube.
What is fertilisation ?
Fusion of the haploid sperm and haploid oocyte to form a diploid zygote.
Where does fertilisation usually take place ?
Ampulla region of the uterine tube
What must spermatozoa undergo to acquire fertilisation capability ?
Capacitation
Acrosome reaction
Describe spermatozoon penetration
Phase 1 : Capacitation (passing through the corona radiata)
Phase 2 : Acrosome reaction (penetration of the zone pellucida)
Phase 3 : Penetration of the oocyte membrane by one spermatozoon, which loses its own plasma membrane.
What enzyme penetrates the corona radiata ?
Hyaluronidase
Interaction with ZP3 receptor
Describe fertilisation
Penetration of corona radiata - by hyaluronidase
Interaction with ZP3 receptor
Acrosome reaction - digests zone pellucida
Membrane fusion
Completion of oocyte meiosis
Release of the contents of cortical granules :
- Degradation of ZP3 to prevent second penetration
Cleavage
Zygote to Morula
Chromosomes become arranged on the spindle, split longitudinally and move to opposite poles.
What happens once the zygote has reached the 2 cell stage ?
It undergoes a series of mitotic divisions, increasing in the number of cells.
These cells, which become smaller with each cleavage division, are known as BLASTOMERES.
How many cells are in the morula ?
16-18 cells
takes 72 hours
Describe the structure of the morula
Inner cell mass - EMBRYOBLAST
which is surrounded by
Outer cell mass - TROPHOBLAST
What does the embryoblast of the morula give rise to ?
Embryo
What does the trophoblast of the morula give rise to ?
Contributes to the placenta
Where do the cavities of the morula form ?
Cavities form in inner cell mass, filled with fluid.
What does a single blastocyst cavity give rise to ?
The single blastocyst cavity will become the primitive yolk sac.
Around what day does the blastocyst reach the uterine cavity ?
On day 6-7, the blastocyst reaches the uterine cavity.
What happens on day 6-7 ?
The blastocyst reaches the uterine cavity.
Trophoblast cells over the embryo blast begin to implant into the epithelial cells.
Host/graft and graft/host reactions are normally suppressed.
How do the cells in the embryoblast differentiate ?
Under the influence of fibroblast growth factors (FGFs) at the early blastocyst stage, cells in the embryoblast differentiate into epiblast and hypoblast cells.
Scattered cells segregate —-> Bilaminar disc
What happens on day 8 ?
On day 8, the blastocyst is partially embedded in the endometrium of the uterus.
What does the trophoblast differentiate into ?
In the area over the embryoblast, the trophoblast differentiates into the inner layer of
- CYTOtrophoblast,
and an outer zone of
- SYNCYTIOtrophoblast (secretes beta-HCG)
What does the syncytiotrophoblast secrete and what is the importance of this ?
Secretes - Beta-HCG
This is used to detect pregnancy
What and where is the amniotic cavity ?
A small cavity appears within the epiblast called amniotic cavity.
What happens at day 15 ?
Around day 15, Primitive streak appears as a thick structure on the epiblast :
- In the midline
- Near the caudal region of the bilaminar embryonic disc
- Defines the body axes
Where is the primitive streak ?
IN the midline.
Near the caudal region of the bilaminar embryonic disc
What does the primitive streak define ?
The body axes
Name the parts of the primitive streak
Primitive :
- pit
- node
- groove
What does the cranial end of the primitive streak contain ?
Primitive node which contains the primitive pit
What is the groove in the primitive streak called ?
Primitive groove
What happens at day 16 ?
Gastrulation
Bilaminar disc —-> Trilaminar disc
Name the 3 layers of the trilaminar disc
Ectoderm
Mesoderm
Endoderm
What is the ectoderm formed from ?
Epiblast
What is the endoderm formed from ?
Hypoblast
Describe invagination
Cells of the epiblast migrate through the primitive streak, detach from the epiblast, displace hypoblast and create a third layer.
Describe what happens on day 17
Cranial and caudal folding
Notochordal plate forms a notochord
What is a notochord ?
A cylindrical structure which gives instructions to other cells/tissues.
The notochord acts as a temporary axial skeleton for the embryo.
What does the paraxial mesoderm give rise to ?
Somites
- Sclerotome
- Myotome
- Dermatome
Head mesenchyme
What does the lateral plate mesoderm give rise to ?
Walls of the body cavities and serous membranes
What does the intermediate mesoderm give rise to ?
The urogenital system
Ectopic pregnancy
Implantation of blastocyst in an abnormal location
IN the ovary or anywhere along the path of migration into the uterine cavity.
What is the most common ectopic site ?
Tubal pregnancies (95%)
What do ectopic pregnancies lead to ?
Can lead to spontaneous tubal abortion or …
Can lead to maternal mortality if undetected after intraperitoneal rupture of the uterine tube and its vessels.
When does the likelihood of tubal rupture and haemorrhage increase ?
From the 2nd through the 5th months
Risk of maternal death
