Fourth Week of Devlopment Flashcards
Which stage of embryonic development is the fourth week considered to be?
Phylotypic stage
State the main events taking place in the fourth week.
A positional inversion of the organs buds.
Formation of the neural tube and regionalization.
Specification of the neural crest population.
Somite differentiation: dermatome, myotome and sclerotome.
How does the embryo appear at the end of the fourth week?
The heart of the embryo is already formed and is relatively large compared to the size of the embryo.
The bud of the forelimbs and hind limbs are starting to form, a tail and the balloon which is what’s left of the yolk sac.
The segmentation of the body plan can be observed.
Describe lateral folding.
The ectoderm and the roof of the amniotic cavity grows faster than the other 2 germ layers. This forces the amniotic sac to slide down. While this is occuring, there is also the formation of the neural tube.
Eventually the wall of the amnion completely fuses, forming an envelope inside as well as the gut.
The fusion of the 2 sides of amnion remains inside the embryo, therefore connection between amnion and yolk sac remains.
What happens at the cranial end of the embryo during craniocaudal folding?
In the cranial end, part of the endoderm of the yolk sac is incorporated into the embroy as the foregut.
The developing forebrain grows beyond the oropharyngeal membrane to overhang the developing heart.
The septum transversum, primordial heart, pericardial coelum and oropharyngeal membrane move into the ventral part.
What happens to the yolk sac during craniocaudal folding?
The yolk sac is constricted.
What happens at the caudal end of the embryo during craniocaudal folding?
Part of the endoderm layer is incorporated as the hindgut.
The allantois becomes the most cranial part of the caudal portion, followed by the cloacal membrane and then the end of the neural tube.
How does nodal cillia help establish symmetry?
A clockwise rotation of the cilia in the nodal cells are important to establish the left and right symmetry of the body.
How does the development of the nervous system begin?
The notochord releases important factors which signals the ectoderm to differentiate into the neural ectoderm.
A more flat cell type is transformed into a columnar one. These columnar epithelial cells are the ones that characterise the neural plate, which is shaped in a way that can be foreseen which parts give rise to the brain and which gives rise to the spinal cord.
State the key steps of neurulation with regards to changes to the neural plate.
Neural plate forms.
Shaping, neural plate narrows in the transverse plane and elongates in the longitudinal plane.
Bending, neural folds form.
The neural groove closes.
Describe neurulation events from the beginning of 4th week to the formation of the neural tube.
The formation of the neural plate and tube begins at week 4, where the neural folds begin to form.
They are neural eithelial cells which create 2 lateral folds that start fusing in the center. This proceeds in both cranial and caudal direction until only 2 areas at both ends remain open - cranial neuropore, caudal neuropore.
The lumen of the neural tube is the neural canal.
What layer cover the the neural tube?
The ectodermal layer covering the neural skin is called the epi ectoderm, which delineates when the folds close. So there is a covering of the neural tube
Describe the elongation of the spinal cord throughout neurulation.
At the initial stages where fusion of the folds begin, the spinal cord is only 25% of the length of the neural plate.
At around 26 days, the spinal cord takes up almost 60% of the length of the entire neural plate.
What process is the elongation of the spinal cord due to? Describe this process.
Convergent extension. This describes a structure that is originally wide but not long, that eventually lengthens and narrows.
This is mainly emboly and epiboly.
Emboly is cell invagination, while epiboly is the expansion of one cell layer over another.
How do hinge points organise for neural plate bending?
The hinge points are wedgings of cells.
The medial hinge point (MDH) is present at the midline of the neural plate. Microtubules reorganise to form columnar cells which characterise the neural epithelium.
Lateral hinge points are also organised, the actin cytoskeleton at these hinge points are responsible for organising the tip of the cell into a wedge.
