Gastrulation and Neurulation Flashcards
Describe the 1st week of embryological development
- Ovulation
- Fertilisation- conception in uterine tube
- Migration - to uterus
- Cleavage - cell division to morula then blastocyst
- Implantation - in uterine mucosa, complete by day 9
Describe the state by the end of the 2nd week of embryological development
Bilaminar disc with connecting stalk
Within the chorionic sac
Amniotic sac on the dorsal side
Yolk sac on the ventral side
Epiblast and Hypoblast are in contact
Day 14 primitive streak appears and the cells begin to move.
Gastrulation has begun
In the next 3-4 days a lot happens
When does the primitive streak form ?
Around day 14-15
Gastrulation
Bilaminar disc becomes the trilaminar disc
2 layers ————-> 3 layers
Describe early week 3
Formation of the primitive groove.
A groove with raised edges and a pit, formed on the epiblast along ahead-tail axis.
Groove – Primitive Streak
Pit - Primitive pit, associated with the primitive node which is a key signalling region in gastrulation.
Where does the primitive streak develop ?
Develops at the caudal end of the embryo
Primitive node function
A key signalling region in gastrulation
Sends out signalling factors
Describe what happens in cell migration 1
The epiblast cells divide and migrate through the primitive streak.
The epiblast cells displace and replace the hypoblast cells.
The epiblast cells start becoming endoderm.
Describe what happens in cell migration 2
Second wave of epiblast migration “fills in” between 2 layers.
This establishes a trilaminar disc, except at 2 locations.
The epiblast is now ectoderm.
Describe the location of the notochord
From the primitive pit rostrally, into the mesoderm, grows a tube-shaped structure.
Describe development of the notochord
Changes form a tube to a disc to a solid rod (17-20 days)
Describe the dorsal/ventral axis
The primitive node/notochord organise dorsal-ventral axis.
Skin/ ventral (belly) development uses signals from bone morphogenic proteins (BMP)
BMP
Bone Morphogenic Protein
Function of the notochord
Notochord is essential for setting up dorsal and ventral axes and inducing neurulation.
Node and notochord release factors (chords, noggin, follistatin) that block BMP.
Function of BMP
BMP would drive formation of skin on the back, but notochord drives back/neural tissue development instead.
Neurulation
The neural plate converts to a groove then a tube.
Forming of neural tissue, induced by the bar shaped tissue-notochord (deep to the neural epithelium)
epithelial cells
Describe neurulation
Epithelial cells become columnar in an area called the neural plate.
The neural plate converts to a groove then a tube (neurulation)
Middle neural groove becomes apparent after day 19
Neurulation - fold and groove
Cells on the edge of the plate thicken, forming a grove and then a fold.
Occurs between day 20-21
Neurulation - tube
The edges of the fold roll over to make a tube.
Occurs from day 22 onwards
Neurulation - closure
The folds close like a zip in cephalic and caudal directions, the open ends being the anterior and posterior neuropores.
They close to form a complete tube.
Anterior neuropore - 25 days
Posterior neuropore - 28 days
What forms the brain ?
Three dilations within the rostral tube - primary brain vesicles - go onto form the brain.
Describe secondary neurulation
The neural tube is lengthened caudally by a process of secondary neurulation
The medullary cord forms and develops a lumen before fusing with the caudal end of the neural tube.
Explain the formation of neural tube during neurulation
Fold and groove
Tube
Closure
Neural tube defects causes
Failure of the neuropores to close, leads to various levels of :
- Brain/skull defects : anterior neuropore
- Spina bifida : posterior neuropore
