Embryology Flashcards
To cover most aspects of Early Embryonic Development for ESA 1 Note: lateral and cephalocaudal folding are not covered within this deck
What 3 changes allow for one cell to become a multicellular body?
- Growth
- Morphogenesis (form and structure development)
- Differentiation (specialisation for function)
How long does the pre-embryonic last?
2 weeks
Which period goes from the 3rd to 8th week?
Embryonic
What is the final stage of embryonic development called? (after the 9th week)
Fetal
Briefly describe the process of fertilisation (3 points)
- Oocyte released from ovary
- Travels down fallopian tube
- Fertilised by sperm in ampulla forming zygote
What is the ideal site for implantation of the zygote?
Posterior uterine wall
How long is an oocyte viable for?
1 day
How long is sperm viable for?
3 days
How long after fertilisation does it take for cleavage to occur?
30 hours
What does the process of cleavage result in?
A morula made of 2 blastomeres
What is the purpose of the zona pellucida?
This glycoprotein shells prevents polyspermy
How is the blastocyst formed?
Compaction of the blastomeres causes the hatching from the zona pellucida
What are the three main components of the blastocyst?
- Embryoblast (inner cell mass)
- Trophoblast (outer cell mass)
- Blastocyst cavity
What will the trophoblast later form?
Support structures for the embryo (like the placenta)
What will the embryoblast later form?
Bilaminar disc
What is the cytotrophoblast?
A derivative of the trophoblast forming the placental membrane around the yolk sac
What is the syncytiotrophoblast?
A derivative of the trophoblast which are cells that invade maternal sinusoids giving circulation to the foetus
At the stage of implantation how many cells does the conceptus have? What will they form?
107 cells - 8 will make the embryo and 99 will begin development of the foetal membrane
What does the embryoblast differentiate into?
Epiblast and hypoblast
What does the trophoblast differentiate into?
Syncytiotrophoblast and cytotrophoblast
What does the bilaminar disc consist of?
Epiblast and hypoblast
Describe the process of implantation
- The uterine epithelium is breached
- The conceptus implants within the uterine stroma
At which stage of pre-embryonic development are the cells totipotent?
Morula
What does PGD stand for and what is it for?
Pre-implantation Genetic Diagnosis - removal of an embryo to test for serious inheritable diseases
How is the embryonic pole formed?
Rapid development of the syncytiotrophoblast
How is the primitive yolk sac formed?
Heuser’s membrane spreading across blastocyst cavity
Why is the blastocyst cavity formed?
To provide nutrition to the inner cells of the morula
What is the inner cell mass called?
Embryoblast
What is the outer cell mass called?
Trophoblast
In the bilaminar disc which of the layers is closest to the trophoblast (outer cell mass)?
Epiblast
Which of the cells of the bilaminar disc will contribute to the embryo?
Epiblast
Which cavity does the epiblast make with the trophoblast?
Amniotic cavity
Which cavity does the hypoblast make with the trophoblast?
Primitive yolk sac
What pushes away the primitive yolk sac membrane from the cytotrophoblast? What will it become?
Acellular extraembryonic reticulum which will later become extraembryonic mesoderm by cell migration
How is the chorionic cavity formed?
Spaces in the extraembryonic mesoderm merge
How is the embryo suspended in the chorionic cavity?
By a connecting stalk
How is the secondary yolk sac formed?
A little bit is pinched off from the primary yolk sac and whatever is not pinched off is now referred to as the secondary yolk sac
What is the connecting stalk made from and what is its future function?
Made from mesoderm that will become the umbilical cord
What can bleeding around the end of the second week be confused with?
Menstrual bleeding
List the main ectodermal derivatives (4 main points)
- Skin and derivatives
- Brain and spine
- Peripheral nerves
- Retina and iris of the eye
List the main mesodermal derivatives (5 main points)
- All types of muscle
- Connective tissue
- Bone + cartilage
- Blood and blood vessels
- Urinary system
List the main endodermal derivatives (3 main points)
- Digestive tract lining
- Glands associated with digestion
- Gut derivatives (lungs etc.)
What is the first thing that occurs in the 3rd week?
Formation of the primitive streak, the primitive pit and the primitive node
Which anatomical position does the primitive streak appear at?
Caudal end
Where does the primitive node appear?
The primitive node appears at the cranial end of the streak
What three things does the epiblast differentiate into?
- Ectoderm
- Mesoderm
- Endoderm
What is a germ layer?
A layer from which other developed tissues originated from
What is the mesoderm?
The middle germ layer which gives rise to muscle, bone, connective tissue etc.
What is the ectoderm?
The outermost germ layer giving rise to the epidermis and nerve tissue
What is the endoderm?
The innermost germ layer giving rise to the gut lining and derivatives
What is the process of gastrulation?
The reorganisation of the germ layers into the ectoderm, mesoderm and endoderm and hence establishing the origin of all tissues.
What is the primitive streak? Why is it important?
A narrow groove that develops on the dorsal surface of the epiblast. It has an important role in the orientation of the embryo, determining the front and the back.
What will the two holes in the mesoderm become?
The future mouth and anus
How is the trilaminar disc produced from the bilaminar disc with its primitive streak?
- Cells of the epiblast migrate towards the primitive streak
- They invaginate into the epiblast and displace the hypoblast
- More cells spread laterally and towards the cranial end creating ectoderm and mesoderm (with two holes)
Where is the notochord located?
The centre of the mesoderm
What are the functions of the Notochord?
- Sets axis for skeleton
- Drives neurulation
- Defines the midline
In what direction does gastrulation proceed?
cephalocaudally (head to tail)
How is the notochord formed?
- Prenotochordal cells of epiblast migrate through the cranial part of the primitive pit
- Forms a solid rod of cells running in the midline with an important signalling role.
What remains of the notochord in adults?
Nucleus pulposus of intervertebral discs.
What is the function of the notochord in neurulation?
Directs the conversion of overlying ectoderm to neurectoderm
How is the neural tube formed?
- Notochord signals ectoderm to thicken
- Edges of the neural plate elevate out and curl towards each other forming the neural groove and folds
- Folds come together to create neural tube with epidermis above it
What is the function of the neural tube in nerve innovation?
Neural tube produces the nerves that innovates the dermomyotome.
What are somites?
Segments of the paraxial mesoderm (next to the neural tube) that give rise to repeating structures (ribs, vertebrae, intercostal muscles)
When do the first somites appear?
Day 20 at the occipital region
How many pairs of somites are there?
31 pairs in total
What 4 types of mesoderm are produced in segmentation?
- Paraxial
- Splanchnic
- Somatic
- Intermediate
What space is formed while the 4 types of mesoderm are being produced?
Intraembryonic coelom
What are the derivatives of the paraxial mesoderm?
- Muscles of the axial body wall
- Dermis
- Axial skeleton
What are the derivatives of the splanchnic mesoderm?
- Smooth muscles
- Connective tissue of gut
What are the derivatives of the somatic mesoderm?
- Connective tissue of limbs
- Contributes to axial body wall
What are the derivatives of the intermediate mesoderm?
-Urogenital system (kidney, ureters, gonads)
How is sclerotome formed? What will it become?
Organised degeneration leads to the somite’s ventral wall breaking down creating sclerotome.
Sclerotome will become the hard tissue section - bones etc.
How is dermatome and myotome formed? What will they become?
Further organisation of the dorsal portion of the sclerotome forms the combined dermomyotome. Which has the myotome proliferating and migrating and the dermotome dispersing.
The dermatome will become the skin section (dermis) and the myotome the muscle section (muscles)
What is the clinical definition for a dermatome?
A strip of skin supplied by a single spinal nerve
What is the clinical definition for a myotome?
A muscle/group of muscles supplied by a single spinal nerve
Define gastrulation
The process of forming the 3 germ layers in the embryo and thus providing the origin in all the embryonic tissue. It also results in the axis being set in the embryo.
Describe the onset of gastrulation
Begins at approx day 16 with the formation of the primitive streak that proceeds caually–>cephallically. This is brought about by a relative invagination of the epiblast and reorganisation of the cells. This occurs in the epiblast which defines the dorsum.
Describe the primitive pit and node.
Most cephalic of the streak.
Has a key role in the defining the cephalocaudal polarisation of the embryo and also in signalling and co-ordination.
At the pit is a connection between the amniotic cavity and yolk sac called the neuroenteric canal.
What is the effect of FGF8 in the formation of the epiblast during gastrulation?
Downregulates E-cadherin enabling reducing the amount of cell surface binding and allowing the migrating cells to move unhindered.
Describe the cloacal membrane
Tightly adherent endoderm and ectoderm that has no mesoderm between them and it thought to be involved in several congenital bladder disorders.
What defines the embryo axis initially?
Cephalically: certain growth factors from the anterior visceral endoderm (AVE) such as lefty and cerberus inhibit nodal activity in the cranial end.
Midline: by the primitive streak which is maintained by NODAL (a TGF-beta).
Dorsal/ventral: in the mesoderm by a TGF-beta and a FGF cause the development of the intermediate and lateral plate mesoderm which is antagonised by other genes in the node.
Left/right-ness: by NODAL which is moved via cillia on the node.
