Embrology 2 Flashcards
When is the embryonic development period?
Weeks 3-8 (inclusive) of a pregnancy
What are some key features of the embryonic period?
The period of greatest change
All major structures and systems formed
Greatest risk of major congenital malformation - due to environ exposure or drugs
How does drug therapy effect a growing baby through the difff stages of development?
In pre-embryonic stage- lethal effects- loose pregnancy
In embryonic stage- very sensitive-significant/catastrophic effect
In feral stage- less sensitive but can still cause damage
After embryonic period risk of a structure defect is lowered except for CNS which is still very sensitive to alcohol etc
What is the result of early embryonic development and what does this involve?
Result= foundations laid for development of body systems
Means sufficient cell number of the right type in the right place
List the key events/processes in early development. Why is each stage so key to the next?
Fertilisation & implantation Gastrulation Neurulation Segmentation Folding
All so key ad each step depends on the preceding step being completed successfully
What marks the start of gastrulation?
At the end of the 2nd week the epiblast is a uniform disk. After Gastrolation a new feature the “primitive streak” appears on the dorsal surface of the epiblast
What happens during gastrulation?
Cells in the epiblast layer divide and migrate upwards into the epiblast layer. The hypoblast is displaced and a 3rd layer is created.
What is the structure created after gastrulation?
A triluminar disc made up of an ectoderm (becomes outside of foetus), a mesoderm (is middle), and an endoderm (becomes inside of foetus).
All 3 are derived from the epiblast layer.
What does the ectoderm go on to become?
The organs and structures that maintain contact with the outside world
Eg nervous system, skins epidermis
What does the Mesoderm go on to form?
Supporting tissues
Eg muscle, cartilage, bone and vascular system (inc heart and vessels)
What does the endoderm go on to form?
The internal structures
Eg GI tract’s epithelial lining, respiratory tract and parenchyma of glands
What is situs inverses? What are the complications if any?
It’s when the viscera is a complete mirror image of what it should be.
Commonly results from immobile cilia spring embryogenesis
No usual associated morbidity
Problems arise if there is both normal and mirror-image present
How is left-right asymmetry achieved?
On the node of the streak there are cilated cells. The action of these results in left-ward flow of signalling molecules. This causes side-specific signalling to cascade.
IE on left hand side any signalling molecules arrive
Lack of signals on right side is a signal within itself
Summarise when, what, how and why for Gastrulation.
When-3rd wee of development (starts embryonic period)
What-bilaminar disk becomes trilaminar disk with 3 germ layers
How-primitive streak forms leading to migration & invagination of cells
Why-to ensure correct placement or precursor tissues to allow subsequent morph oh genesis to take place
What is a Notochord?
A solid rod of cells running in the midline with an key signaling role.
It directs conversion of overlying ectoderm to Neuroectoderm.
(Receptors for this on ectoderm only not endoderm)
How is the neural tube formed?
Notochord signals cause the overlaying ectoderm to thicken
Becomes a slipper shaped neural plate
The edges elevate out of the plane of the disk and curl towards each other. This creates the neural tube.
Once the neural tube has formed what happens to the mesoderm?
It forms 4 separate parts.
2 shelf like parts = somatic mesoderm (forms the body) and the splachnic mesoderm (forms the viscera)
The mesoderm inbetween these 2 is called the intraembryonic mesoderm
The mesoderm surrounding the neural tube is called the paraxial mesoderm
What is a somite and what can it be used for?
It a segment of the paraxial mesoderm.
Leave a total of 31
They appear at highly regular intervals and are predictable so allow us to age an embryo accurately
What is the organised degeneration of somites?
Originally appear as a regular block of mesoderm cells around a cavity.
They then degenerate (Ventral wall breaks down, formation of the sclerotium)
Then further organisation of dorsal portion forms the combined dermomyotome.
The myotome proliferates and migrates, dermatologists disperses.
What can be derived from a somite?
Dermatome-skin section -“dermis”
Myotome-muscle section-muscles
Sclerotme-hard tissue section-bones
Why is the final number of somites important and what is it?
31 is the key number. Also the number of pairs of spinal nerves.
What happens to the triluminar disk after the neural tube is formed?
It begins to fold so that the outside is covered entirely by ectoderm, and the endoderm is fully inside (other than a small hole to access yolk).
This begins with cephalocaudal folding (head and tail fold) then lateral folding (sides fold).
The entire embryo is then enveloped in the amniotic sac.
During folding what does the yolk help form?
The intraembryonic cavity.
What is folding designed to achieve?
Draws the margins of the disc together
Creates a ventral body wall
Pulls amniotic membrane around disc-suspends embryo
Pulls connective stalk centrally
