18 Flashcards
When does organogenesis begin in humans? How long does it last?
Around 4 weeks following neurulation in the 3rd week. It lasts from around the 4th to the 8th week.
What changes happen during organogenesis?
Folding of the embryo into a 3D structure, most organs form their rudiments, the heart begins to beat and the neural tube forms.
What does neurulation involve?
The closing of folds from the head to the tail, which isn’t uniform.
How many pores are there in neurulation? Which closes last?
The anterior neuropore, the posterior neuropore, and 5 intermediate neuropores. Posterior closes last.
What happens if the different kinds of neuropores fail to close? Why does this happen?
Anterior: The embryo is in direct contact with wastes and the brain won’t develop. This is lethal. Anencephaly.
Intermediate-Posterior: The more posterior the failure to closure is, the higher the likelihood of a live birth. Spina bifida is the failure of the posterior neuropore to close.
The whole tube: craniorachischisis. Fatal.
Many reasons, some genetic.
Define the difference between primary and secondary neurulation
Primary: The neural tube is formed by invagination and pinching off. Anterior.
Secondary: The neural tube is formed by coalescence of mesenchymal cells (EMT) to form a rod, cavitation forms a tube. Posterior.
Note: intermediate areas in the junction region show a mix of the two, junctional neurulation.
What is the role of medial hinge point cells?
To anchor to the notochord and facilitate closure of the neural tube anteriorly (primary neurulation). This is driven by differential expression of protein products, with most actin apically.
Around what days do the main neuropores close?
Anterior: day 24
Posterior: day 27
What is a supplement that can prevent incorrect neural tube closure if taken early?
Folic acid (Vitamin B9). Not sure why. Must be taken before organogenesis begins, so by start of week 4.
How do Shh, Pax6, and Nkx6.1 interact to determine the dorsal/ventral patterning of the neural tube?
Shh inhibits Pax6 and activates Nkx6.1. Shh is expressed ventrally from the floor plate, BMP4 (which activates Pax6) from the roof plate. Where Shh concentrations are high enough to stimulate Nkx6.1 but too low to inhibit Pax6, motor neurons form.
How does tissue with high BMP activity wind up above the neural tube during neurulation?
Neurulation infolds the Smad1 inactive cells, thus BMP4 expressing epidermis is dorsal to the neural tube.
Where are the cell bodies of most neurons located?
In the spinal cord.
What neuron types are found in the ventral and dorsal spinal cord?
Ventral: motor
Dorsal: Sensory
What is the order of the layers of the spinal cord?
From outermost to innermost, marginal, mantle, ependymal.
How was it proven experimentally that the notochord induces formation of the ventral neural tube?
A donor notochord floor plate was taken and transplanted. This resulted in a secondary floor plate and set of motor neurons, thus notochord must be driving.
