What is neurulation?
-The development of the nervous system driven by a series of differentiation steps
At what stage is the embryo by the end of week 3?
- Gastrulation has occurred -> bilaminar now trilaminar with all three germ layers established
- The axis has been set
What is the important signalling role of the notochord?
- Releases signalling molecules which stimulate the overlaying ectoderm to become neuroectoderm;ectoderm which is committed to becoming nervous tissue
- The overlying tissue begins to thicken and forms the neural plate
Why does only the ectoderm which overlays the notochord becomes neuroectoderm?
-Notochord signalling molecules are diffusion limited and therefore only effects the tissues close to it
How does the neural plate develop into the neural tube?
-Thickening of the neural plate, relative to the rest of the ectoderm, results in the edges of the neural plate elevating out of the plane of the trilaminar disc and curling towards each other to form the neural tube
As the neural tube develops, the mesoderm also differentiates and develops into…
(draw if possible)
- Paraxial mesoderm (zone of mesoderm at the sides of the axis defined by the notochord)
- Intermediate mesoderm
- Somatic mesoderm
- Splanchnic mesoderm
What happens to the layers of the ectoderm which are not involved in the formation of the neural tube?
-They become suborganised into layers
What is the lateral plate during mesoderm differentiation?
-Somatic and splanchnic mesoderm
What are somites?
-Organised segments of paraxial mesoderm
In what type of pattern do somites appear?
-Regular and predictably, early in development
When do the first pair of somites appear?
-Day 20
In what type of sequence/direction do somites appear?
-Craniocaudal 3 pairs at a time until 42-44 pairs are present by the end of week 5
How many pairs of somites are present by the time they have finished developing?
-31 as some disappear
Where abouts in the embryo do somites lie?
-In pairs, one at both sides of the neural tube
What is the structure of somites?
-Regular block of mesoderm arranged around a small cavity
What happens in organised degeneration of somites?
- Ventral wall (wall facing endoderm) of somite breaks down, leading to the formation of the scleotome
- Organisation of the dorsal portion of the somite forms the dermomyotome
What is sclerotome?
-Hard tissue zone which develops into bone
What is the dermomyotome?
-Tissue which develops into muscle and skin-like tissue
What eventually happens to the myotome?
-Proliferates and migrates eventually forming the muscles
What eventually happens to the dermotome?
-Disperses widely to the ectodermal covering and forms the dermis of the skin
What does the medial demomyotome develop into?
-Back muscles
What does the lateral dermomyotome develop into?
- Ventral body wall muscles
- Contributes to limb muscles
What does the medial sclerotome develop into?
- Vertebral body
- Proximal portion of the ribs
What does the lateral sclerotome develop into?
- Vertebral arch
- Distal portion of the ribs
What is the epimere?
-Dorsal myotome which is innervated by dorsal rami
What is the hypomere?
-Ventral myotome innervated by ventral rami
How does 31 somites relate to full grown human structure?
- 31 somites
- 31 segments
- 31 pairs of spinal nerves
What is the clinical use of the term dermatome?
-A strip of skin supplied by a single spinal nerve
What is the clinical use of the word myotome?
-A muscle or group of muscles supplied by a single spinal nerve
List the derivatives of the paraxial mesoderm
- Axial skeleton (Vertebrae column and ribs)
- Dermis
- Muscles of body wall
- some limb muscles
List the derivatives of the intermediate mesoderm
-Urogenital system (kidneys, ureters and gonads)
List the derivatives of the somatic mesoderm
-Connective tissue of the limbs
List the derivatives of the splanchnic mesoderm?
- Smooth musculature
- Connective tissue
- Vasculature of the gut
What drives the folding of the embryo?
-Processes of differentiation such as the formation of the neural tube
In what two directions does the embryo fold?
- Cephalocaudal (Head and tail folding)
- Lateral folding
Before folding, where is the cardiogenic area located?
-Above the buccopharangeal membrane
Describe cephalocaudal folding
- Growth of the ectoderm (thickening and neural tube) leads to a top-heavy ectoderm layer
- This results in folding at the head and tail end of the trilaminar disc in such a way that endoderm becomes less visible and the entire embryo is covered in ectoderm, except one opening for the umbilical cord
What effect does cephalocaudal folding have on the cardigenic field?
-After folding the cardiogenic field lies within the embryo itself
What effect does cephalocaudal folding have on the yolk sac?
-As folding occurs the amniotic cavity also folds in the same direction and some yolk sac is pulled up into the embryo
Describe lateral folding
- Differentiation of the neural tube, somites and mesoderm drive lateral folding
- There is folding of the amniotic sac ventrally in such a way that the ectoderm and amniotic sac are beginning to surround the embryo. The space between the splanchnic and somatic mesoderm (lateral plate) becomes wider as the two lateral plates move towards each other, beginning to create the coelom.
- The endoderm and yolk sac fold in such a way that it begins to be drawn up into the embryo and forms the gut tube and umbilical cord
- The two lateral plates, with ectoderm overlying somatic mesoderm, fuse together forming the embryonic cavity (coelom) and ectoderm now surrounds the entire embryo
- The amniotic cavity also now surrounds the entire embryo
