Embryology Flashcards
What is fertilisation?
Process by which the male gamete and female oocyte unite to form a zygote
How many chromosomes are in a diploid cell?
46 chromosomes
How many chromosomes are in a haploid cell?
23 chromosomes
What promotes ovulation?
Oestrogen
What is the process of ovulation?
Oestrogen promotes release of oocyte onto ovary surface.
Oocyte enters into the oviduct and is transported down the fallopian tube by peristaltic contractions.
Takes 4 days for the oocyte to reach the uterus.
What is the process of fertilisation?
- Sperm travels to oviduct to fertilise oocyte.
- Sperm attaches to zona pellucida
- Head bursts open releasing enzymes to loosen cell junctions
- Sperm drills through zona pellucida and merges with oocyte membrane.
- Membranes fuse forming pronuclei where the chromosomes of each gamete mix.
- Zygote is formed and cleavage starts.
What is the process of cleavage?
The zygote divides into smaller blastomeres (no growth of cells).
after 1 day the 2 cell blastomere is formed.
After 2 days the 4 cell blastomere is formed
After 3 days the 8 cell blastomere forms.
What happens at the 8 cell stage of the blastomere?
Compaction of the embryoblast cells begins.
When does the morula start to form?
Day 4.
Compaction of the cells in forms a 16-32 cell blastomere called a morula
How does the morula differentiate into the blastocyst?
Day 5 - 6:
- Outer embryoblast cells differentiate to form the trophoblast layer surrounding the inner cells.
- The epiblast cells compact to one end of the morula and form the inner cell mass (ICM).
- In doing so, a cavity is created called the blastoceol which is a fluid filled space.
- The zona pellucida starts to disintergrate and the blastocyst is formed by a process called hatching.
After how many days does implantation occur?
About day 7.
What is the process of implantation?
Day 7-10:
- Adhesion molecules on the endometrial epithelium and trophoblast cells of the blastocyst combine as the blastocyst attaches to the apical epithelial layer.
- Attachment of the blastocyst occurs above the ICM, this means that the trophoblast cells start proliferating and invade into the endometrial layer.
- The trophoblast cells form 2 layers; the cytotrophoblast, which lines the endometrial surface and the ICM and the syncitiotrophoblast cells which are multinucleated.
- The syncitiotrophoblast cells form overlying the ICM and eventually proliferate to surround the blastocyst entirely embedding it into the uterus lining by day 10.
How is the placenta formed?
Day 10:
- Vacuoles form in the syncitotrophoblast called lacunae which by day 12 form and interconnecting network for blood flow.
- Synctial lacuna fuse with maternal sinuosoids to allow maternal blood to enter the system.
- Cytotrophoblast cells penetrate into the surrounding syncitiotrophoblast and form primary villi which will contain the fetal capillaries.
- 21 days after fertilisation, teritary villi form and a network of capillaries linking maternal blood to the embryonic blood supply is formed. (the placenta)
What are the layers of the uterus lining?
External perimetrium
Medial Myometrium
Internal Endometrium.
What cycle conditions the endometrium?
Ovarian cycle
What are the stages of the ovarian cycle?
28 day menstrual cycle:
1. proliferative stage
2. Secretory stage
3, Menstrual / gravid phase (either no fertilisation or fertilisation)
What hormone is secreted by the syncitiotrophoblast?
hcG - Human chorionic gonadatropin.
What is the purpose of hcG?
Maintains pregnancy by maintaining the production of progesterone critical for the endometrium.
What do most abnormal implanations of the blastocyst occur?
95% in the ampulla
What can an abnormal implantation of an embryo lead to?
An extrauterine ectopic pregnancy.
What happens to the ICM at day 7?
ICM starts to differentiate into 2 layers:
Epiblast cells and hypoblast cells.
These form the bilayer embryoic disk
What cavity is formed with the differentiation of the ICM?
Amniotic cavity.
How does the amniotic cavity develop?
The epiblast cells have a cavity which is the amniotic cavity where the cells lining the cavity are termed the amnioblasts.
What happens to the Hypoblast cells during implantation?
Day 10:
- The hypoblast cells migrate to line the blastoceol
- These cells flatten formin the extracoelomic membrane.
- Cytotrophoblast cells and the extracoelomic cells differential and merge to form the extraembryonic mesoderm
What is the Chorionic cavity?
A cavity which is formed in the extraembryonic mesoderm which eventually surrounds the mebryo but remains attached at the connecting stalk to the chorion.
When can gastrulation occur?
After the corionic cavity surrounds the embryo around 3 weeks after fertilisation?
What is gastrulation?
The process of the formation of the 3 germ layers.
What is the process of gastrulation?
- Primitive streak forms caudially in the epiblast layer of the bilaminary embryonic disk.
- The primitive node develops at the cephalic end.
- Epiblast cells start to migrate towards the primitive node.
- Some cells change shape and detach from the epiblast to begin migrating through the node and into the epiblast and hypoblast layers. (invagination)
- The first cells form the endoderm layer.
- The next cells to invaginate form the mesoderm layer.
- The epiblast cells that do not invaginate form the ectoderm layer.
What does gastrulation do?
Establishes the bodies axis.
What are the specialisations of the germ layers?
Ectoderm - Skin and nervous sytem
Mesoderm - Musculoskeletal, Cardiovascular and urogential systems
Endoderm - GIT, Lungs, pancreas, liver
What forms within the mesoderm germ layer?
The Notochord
What are the derivatives of the ectoderm germ layer?
Surface ectoderm
Neural tube
Neural Crest
What does the surface ectoderm form?
epidermis (skin)
hair, nails
mouth
eyes
What does the Neural tube form?
Brain
Spinal cord
motor neurones
retina
What does the neural crest form?
PNS
Adrenal medulla
Teeth
When does neurulation start?
About 19 days after fertilisation.
What signals the differentiation of the ectoderm?
The notochord.
What is the process of neurulation?
The notochord signals for the differentiation of the ectoderm into the ectoderm and neural plate.
- Neural plate rises up at the edges brining them together at the midline.
- The edges of the plate fuse together and form the neural tube.
- Neural crest cells detatch from the neural plate and migrate to thier final positions.
When does the neural tube close?
At 22 days the neural tube closes by fusion in the middle and then proceeds to fuse cranially and caudially.
By day 23, The tube closes at the anterior neuropore (head) and posterior neuropore (end of spine)
What clinical correlations are linked to failure of the neural tube closing?
Failure of anterior neuropore - Anencephally
Failure of posterior neuropore - Spina bifida
What is apparent at day 26 after fertilisation?
The neural tube forming the brian and spinal cord.
What is the importance of folate (vitamin B9)?
Folic acid should be taken whilst pregnant to prevent neural tube defects and ensure complete closure.
How much folate should be taken whilst pregnant?
400ug daily during the first 12 weeks.
What are the derivatives of the mesoderm layer?
Paraxial mesoderm
intermediate mesoderm
lateral plate mesoderm
How do the mesoderm derivatives form?
- cells close to the midline of the mesoderm proliferate to form the paraxial and lateral plate mesoderms.
What does the paraxial mesoderm form?
Axial skeleton
Axial muscleslimb muscles
What does the intermediate mesoderm form?
Urogenital system
What does the lateral plate mesoderm form?
Limb skeleton
mesentariescirculatory system
Heart
What is lateral flding?
Week 4:
Right edges of the embryonic trilaminar disk move ventrally to fuse with their corresponding layers on the opposite side.
What is Cephalocaudal folding?
growth of the brain vesicles causes the embryo to fold.
Endoderm pinches inwards and the fore and hindgut regions are estabilished.
What does the endoderm divide into?
Froms 2 tubes:
One which will develop into the GI Tract
The other becomes the respiratory tube and will develop into the respiratory system.
What types of twins are formed when are in separate chorionic sacs?
Dizygotic
monozygotic
What are dizygotic twins?
Non identical twins occuring when two oocytes are fertilised and each one implants separately.
What are monozygotic twins?
identical twins occuring when a 2 cell zygote splits fo form 2 blastocysts which are implanted separately.
What type of twins are formed when they share a chorionic sac?
Monozygotic & monoamniotic
Monozygotic and diamniotic
how do Monozygotic & monoamniotic twins arise?
occur when 1 blastocyst ends up with 2 primitive streaks from a single bilaminar disk
How do Monozygotic and diamniotic twins arise?
Occur when 1 morula forms the ICM which the splits and forms 2 embryonic disks
How are conjoined twins formed?
1 morula/blastocyst however there is partial division of the ICM or primitive streak leading to the formation of conjoined twins.
How is polyspermy blocked?
- occurs in two phases
- fast electrical block which causes a change in the membrane potential
- slow black which is when a calcium wave causes cortical granules to fuse with the plasma membrane. Release of lysosomal enzymes alters the structure of the zona pellucid to inhibit sperm binding and penetration
