Embryology Flashcards
Events of first week of pregnancy
Ovulation, fertilisation, implantation
When and where does fertilisation occur?
12-24hrs after ovulation in the fallopian tube.
Name of a fertilised ovum
Zygote
When does mitotic cell division of a zygote occur to form a morula?
Day 2-3
What is a morula?
A solid mass of 16 cells that forms around day 4-6
How and when does the blastocyst form?
Fluid accumulates in the morula and it rearranges into 2 cell populations to form a blastocyst around day 6-10
What is a blastocyst?
A hollow ball of cells consisting of an inner cell mass (embryoblast) and trophoblast (outer cell mass).
What does the inner cell mass (embryoblast) of the blastocyst form?
The embryo itself
What is the name of the outer cell mass in a blastocyst?
Trophoblast
Function of trophoblast
contacts with the endometrium to facilitate implantation and formation of the placenta.
When does implantation of the blastocyst into the endometrium occur?
day 6
2 layers that the trophoblasts differentiate into around day 8
syncytiotrophoblast and cytotrophoblast
Function of syncytiotrophoblast
layer of trophoblast that assists with embedding of blastocyst into endometrium, secrete hCG to prevent menstruation, and contribute to vascular supply.
Function of cytotrophoblast
become outlining cells of embryo
What is the bilaminar embryo?
A two-layered structure formed when the embryoblast (inner cell mass) divides into the hypoblast and epiblast - around day 10-12.
When does the bilaminar embryo form?
Day 10-12
2 layers of the bilaminar embryo
Hypoblast and epiblast
What structure in the bilaminar embryo does the epiblast form?
Forms the amniotic cavity containing amniotic fluid
Which cell types does the epiblast form?
Embryonic ectoderm, endoderm and mesoderm (mesendoderm)
Function of hypoblast layer in the bilaminar embryo
Forms the yolk sac and is displaced to form the endoderm and mesoderm.
Function of yolk sac
provides nutrients and gas exchange during weeks 2-3 until placenta develops in week 4
Difference between identical and fraternal twins
Identical twins are formed by one single fertilisation and so share the same genetic code whereas fraternal twins are formed by 2 fertilisations so have different genetic codes. Identical twins share one placenta while fraternal twins have 2 separate placenta.
Does the inner cell mass divide to produce identical or fraternal twins?
Identical
What happens during gastrulation (week 3)?
The embryo develops an axis (cranial/caudal) and the 3 germ cell layers
Name of the groove on the epiblast surface that guides gastrulation
primitive streak
Where do epiblast cells migrate towards during gastrulation?
The primitive node (located at the cranial end of the primitive streak) which contains a primitive pit
How does the trilaminar embryo form?
The first set of epiblast cells migrate through the primitive streak and displace the hypoblast cells to become the endoderm. The second set of epiblast cells move to occupy the space between the endoderm and epiblast to form the mesoderm. The remaining epiblast cells differentiate into the ectoderm.
How does the endoderm layer form?
Epiblast cells migrate through the primitive pit and displace the hypoblast cells forming the endoderm.
How does the mesoderm layer form?
Epiblast cells migrate through the primitive pit and lie between the epiblast and newly formed endoblast.
How does the ectoderm layer from?
differentiation of the remaining epiblast cells.
Organogenesis definition
the development of the major organs during weeks 3-8. By the end of the embryonic period (week 8) all organ systems have formed in their primitive state.
Which is one of the first organs to be formed?
central nervous system
What is the forerunner of the CNS?
the neural tube
How does the neural tube form?
Above the notochord, neuroectoderm cells form neural folds which creates a neural groove. The neural folds merge to form the neural tube while the neural crests join and detach from the neuroectoderm and the neural tube.
What does the neural crest eventually form?
The PNS (initially splits to form the dorsal root ganglia)
Name of the defect in which the neural groove persists leading to the incomplete closure of the neural tube
spina bifida
What is the mesenchyme?
Embryonic connective tissue (formed of mesenchymal cells and ECM) that gives rise to all connective tissues in the body. Derived from mesoderm.
What is the neural crest also known as?
Ectomesenchyme
What is the ectomesenchyme?
A 4th layer derived from the ectoderm but has characteristics of mesenchyme.
Which structure leads to the development of most head and neck tissues?
Ectomesenchyme (neural crest)
Examples of mesenchyme in the head produced by the ectomesenchyme layer
dentine, cementum, pulp, periodontal ligament, jaw bones, muscles.
From which structure does the peripheral nervous system arise from?
Ectomesenchyme
How does the embryo take shape from the trilaminar embryo?
The front and back ends of the embryo begin to fold under the middle, around the yolk sac. Some important organs appear (heart tube, foregut, midgut, hindgut, lung bud, liver bud) by day 25.
Which membrane forms the digestive system?
Cloacal membrane
When is the term embryo used?
Up to and including week 8
When is the term foetus used?
From week 9 onwards
What are the 3 germ layers?
Endoderm, mesoderm, ectoderm
What are stem cells?
Unspecialised cells that can differentiate into more specialised cells
Which embryonic stem cells are totipotent?
The inner cell mass (embryoblast) in the blastocyst.
At which stage do the embryonic stem cells become pluripotent?
In the trilaminar embryo - cells are restricted to the cell types of the specific germ layer.
Why are adult stem cells labelled as multipotent?
They have committed cell lines e.g. bone marrow stem cells.
What does stem cell development involve?
Signalling molecules that switch genes on/off.
What does the ectoderm form?
epidermis of skin, mucous membranes of mouth and anus, tooth enamel. Nervous system including brain and spinal cord (from neural tube)
What is the name of the muscle that develops on either side of the neural groove?
Somite (from paraxial mesoderm)
What does the mesoderm form?
most connective tissues (dermis, tendons, cartilage, bone), muscle, circulatory system, kidney and urinary system, reproductive system, serous membranes.
What does the endoderm form?
Alimentary canal (pharynx to rectum), respiratory system, parts of the urogenital system, liver and pancreas.
Result of developmental abnormality in the first 2 weeks
spontaneous abortion (miscarriage)
When do most developmental defects occur?
Week 3-12 (first trimester) - period of greatest sensitivity
When is the period of functional maturity?
Weeks 13-39
Example of a developmental abnormality that results in the malformation of the face
Treacher-Collins syndrome
When does the development of the deciduous and permanent dentition begin?
week 6
