Embryology Flashcards
What is the most critical period of pregnancy and why?
Embryonic period
All the major structures and systems are formed
Why is week 3 called the ‘week of 3s’?
3 cavities form (amniotic, yolk and chorionic) and 3 germ layers form in trilaminar disk (ectoderm, mesoderm, endoderm)
Define gastrulation
The process of establishing the 3 germ layers and hence the origin of all tissues of the body. Also setting the axes.
What does the primitive streak do?
Drives and coordinates gastrulation
Where is the primitive node located?
At the cranial end of the primitive streak
Where does the primitive streak appear?
The dorsal surface of the epiblast
In which direction does development proceed?
Cranial to caudal
How are the 3 germ layers formed?
By migration and invagination: epiblast divides and migrates up to edges of primitive streak, some new cells push through epiblast layer to create a new layer.
What defines the phylum Chordata?
Having a notochord
What is the notochord for?
basis of axial skeleton, drives formation of nervous system (drives neurulation), defines the midline and has an important signalling role
Define the axes in an embryo
Anterior - towards head
Posterior - towards feet
Dorsal - back
Ventral - front
Which tissues are derived from the ectoderm?
Organs and structures that maintain contact with the outside world.
Eg. Nervous system, epidermis
Which tissues are derived from the mesoderm?
Supporting tissues
Eg. Muscle, cartilage, bone, vascular system
Which tissues are derived from the endoderm?
Internal structures
Eg. Epithelial linings, parenchyma of glands
What is situs inversus?
A congenital birth defect that means that the body is the mirror image of what it should be.
This results from immobile cilia during the embryonic period but has no associated morbidity.
How do monozygotic and dizygotic twins arise?
Monozygotic - a single fertilised oocyte
Dizygotic - 2 fertilised oocytes
What is teratogenesis?
The process through which normal embryonic development is disrupted.
Name some teratogenic agents
Thalidomide, Rubella, Alcohol and Therapeutic drugs (eg. Warfarin)
How is the neural plate formed?
The notochord releases signals that affect the ectoderm. The ectoderm thickens and forms a slipper-shaped neural plate.
How is the neural tube formed?
The edges of the neural plate elevate out of plane and curl towards each other forming a tube.
Where is the paraxial mesoderm?
Around the notochord and neural tube
What are the two types of lateral plate mesoderm?
Somatic and splanchnic
What does somatic mesoderm go on to form?
Connective tissues of limbs
What does splanchnic mesoderm go on to form?
Smooth musculature, connective tissues and vasculature of gut
What are somites?
The organisation of paraxial mesoderm into segments. The segments appear in craniocaudal sequence at 3 pairs per day.
Where does dermomyotome form and what will it become?
Dorsally
Skin and muscle tissue
What happens to the myotome?
It proliferates and migrates
What happens to the dermatome?
It disperses
What does the segmentation organisation of the mesoderm give rise to?
Repeating structures
Eg. Vertebrae, ribs, intercostal muscles
What innervates the epimere?
Dorsal branch of spinal nerve
What innervates hypomere?
Ventral branch of spinal nerve
What is a dermatome?
A strip of skin supplied by a single spinal nerve
What is a myotome?
A muscle/group of muscle supplied by a single spinal nerve
What does the paraxial mesoderm go on to form?
Vertebral column and ribs, Dermis and Muscles
What does the intermediate mesoderm go on to form?
Urogenital system
What will the buccopharyngeal membrane be?
The future mouth
What are the two steps of embryonic folding and what is each step driven by?
Cephalocaudal folding (head and tail) - driven by neural tube
Lateral folding - driven by somites
Why does embryonic folding occur?
To: create an inside and an outside, create a opening for the umbilical cord, draw the amniotic sac around the embryo and create a new cavity within the embryo.
When is the pre-embryonic period?
The first 2 weeks
When are pregnancy weeks calculated from?
The date of the last menstrual period.
Where is the oocyte usually fertilised?
In the ampulla
Where is the ideal site for implantation?
Posterior uterine wall
What is the uterus lined with?
Glandular epithelium
How long is an oocyte viable for?
1 day
How long are sperm viable for?
Up to 3 days
When does cleavage begin?
30 hours after fertilisation
What is the zona pellucida?
A glycoprotein shell that surrounds the zygote immediately after fertilisation to exclude the sperm and prevent polyspermy.
What is the result of the first cleavage?
2 blastomeres of equal size
How does compaction occur?
The morula secretes tissue fluid and collects to create a cavity. The cells get pushed to one side.
What is another name for the inner cell mass?
Embryoblast
What is another name for the outer cell mass?
Trophoblast
Why does the blastocyst hatch?
So it is free to enlarge and interact with the uterine surface to implant.
What 2 layers does the outer cell mass split into?
syncytiotrophoblast and cytotrophoblast
What 2 layers does the inner cell mass split into?
epiblast and hypoblast
Which is the top layer of the bilaminar disc?
Epiblast
What does the change from histiotrophic to haemotrophic mean?
Changes from relying of simple tissue diffusion to relying on the maternal blood supply
What is an ectopic pregnancy?
Implantation occurring at another site than the uterine body.
Can very quickly become life threatening due to haemorrhage.
What is placenta praevia?
Implantation in the lower uterine segment. There is risk of haemorrhage due to enlargement of the foetus. Will always require a C section.
What is the connecting stalk?
A column of mesoderm that will be the umbilical cord.
How does the amniotic sac form?
From spaces within the epiblast
How does the primitive yolk sac form?
Formed by the hypoblast lining of the blastocyst.
How does the secondary yolk sac form?
Forms within the primitive yolk sac
When is the embryonic period?
Weeks 3-8
What has happened by the end of the fourth week?
nervous system has started to form, segments have appeared and the embryo has folded so that everything is in the right place
What are the hypomere and epimere?
The ventral and dorsal parts of the myotome.
Epimere = dorsal. Hypomere = ventral
What is the sclerotome?
The hard tissue section that gives rise to bones eg. Vertebrae and ribs
How do the vertebrae develop in the sclerotome?
Medial = vertebral body Lateral = vertebral arch
How do the ribs develop in the sclerotome?
Medial = proximal rib Lateral = distal rib
What arises from the dermomyotome?
Dermis and muscles
How do the muscles develop in the dermomyotome?
Medial = back muscles Lateral = ventral body wall muscles and limb muscles
What are neural crest cells?
A temporary group of cells unique to vertebrates that arise from ectoderm. They give rise to a diverse cell lineage.
What is a nucleus pullout and what is it formed from?
It is the inner core of the vertebral disc that is jelly-like but has collagen fibres so can withstand compression. It arises from the notochord.
How does the chorionic cavity form?
From spaces within the extra embryonic reticulum and mesoderm.
What is the intra embryonic coelum?
The space between the splanchnic and somatic mesoderm that forms from the separation of the lateral plate mesoderm.
What is the future anus called?
Cloacal membrane
When does gastrulation occur?
Week 3
