Embryology - Part 1
Welcome back to a brand new revision post I'm writing to procrastinate on doing actual work! (of course it's biochemistry, what else would it be?) Today we're going to be reviewing my first lecture on embryology which goes right up to something called neurulation, and I'll be attempting to explain, complete with rubbish hand drawn diagrams, how we go from boring little egg cells to something resembling...not a human.
Just so you know, for my own revision I'm including the scientific evidence that supports lots of these ideas in red. If you have no interest in this, skip over - it's a bit weird anyway.
The purpose of development is:
1) Pattern - cells need to know where to form the correct structures
2) Specialisation - cells become more specialised for certain jobs due to decision hierachy
3) Growth - a huge increase in cell number without an increase in overall size until later
Broadly, we can fit this first lecture into 4 subheadings: cleavage, blastulation (no, I'm not swearing at you), gastrulation (no, it's not farting) and neurulation.
Sperm and egg (the gametes, each with 23 chromosomes) meet, fuse, and form what's known as a zygote. This has 46 chromosomes, and has a 'zona pellucida' around it (in red)
2. Zygote now goes through a series of regulated cleavages (massive growth in cell number not size) to form a morula
3. This is the first specialisation of cells. Inside the morula, the cells clump together in a process know as compaction and then differentiate (this means to become specialised in order to be able to do a job) into trophoblasts (blue) and embryoblasts (orange).
- trophoblasts will eventually form the placenta
- embryoblasts will eventually form the embryo proper (that's just a fancy term for
Trophoblasts move to the outside and embryoblasts move inside because they have specific surface proteins on their membranes that 'tell' them where to go. We know this because if we transplanted trophoblast cells to the middle, then they would simply move back to the outside.
4. The embryoblasts (orange) then begin to gather on one side of the morula, and their group becomes known as the inner cell mass (ICM). This leaves a cavity on the otherside known as the blastocoel (green). At the same time, the zona pellucida disintegrates and disappears (no one cares about where it goes apparently!) and this is known as 'hatching', and when this has completed the mass is officially a blastocyst!
4a. The blastocoel implants in the uterus and proliferates (trophoblasts and
embryoblasts replicate) into the uterus lining.
5. This is the point of the next specialisation of cells. Embryoblasts (orange) differentiate (change) into epiblasts (purple) and hypoblasts (pink). At this point, the epiblasts are columnar cells (remember my post on epithelium?) and they will form the embryo eventually! You can see that there is now a line across the middle of the trophoblasts (blue) made up of the hypoblasts and epiblasts. This is known as the bilaminar disc, and its presence also causes another cavity to form, the amniotic cavity (yellow).
The definitive pattern (i.e. where the body will place certain things) isn't present in the blastocyst. We know this because:
- we can divide the blastula and so long as both have ICM, we develop two embryos
- we can transplant cells between blastocysts (i.e. put cells from a different blastocyst in) and still form a perfect embryo
- but sperm entry point does affect cleavages and organisation of embryo
All embryo cells come from the epiblast: this can be seen by placing markers and analysing their subsequent place in the embryo. Markers must not harm the cell.
6. Now we're into the most interesting bit, and we have to do some imagining. We have to cut a transverse section through the bilaminar disc in order to be able to see what's going on here, so say good bye to the nice, straight forward diagrams!
What happens here is that the primitive streak forms in the epiblast, caudal end first, although you don't have to worry about this in the diagram. Just focus on the fact that the epiblasts are purple, the hypoblasts are pink, and the primitive streak is green. In real life, this would run all the way through the bilaminar disc, not just be a single point like I've drawn, so I've tried to show what it would look like from above as well.
The primitive streak organises the body axis because if you transplant a second streak, then two body axes form. Means that the primitive streak must be the signal source, perhaps secreted proteins.
7. Then what happens is the epiblast layer migrates down and out, displacing the hypoblast layer (again, no one really cares where this goes. Poof, vanished). This forms three layers: the ectoderm, mesoderm, and endoderm. Collectively, they are known as the trilaminar disc.
The diagram of the trilaminar disc is not strictly accurate, because there are areas where there are no mesoderm. These areas will form the oral and cloacal membranes in the proper embryos (the mouth, nose, and genital opens). The diagram below is therefore more accurate, although I've left out the remaining primitive streak:
The position of the ICM in the blastocyst is known as inside out information, because the cells in contact with the trophoblasts will form the epiblast which ultimately defines which layer is the ectoderm.
Note: trophectoderm = trophoblast
This is where stuff gets a bit fiddly, both to remember and to think about what's happening. In reality, this stuff would be happening in 3D, so it's really difficult to draw in 2D. The diagrams might therefore look slightly dodgy (I say this as if they haven't been dodgy for the entireity of this blog!) but I hope you'll put up with my long enough to understand what I'm on about!
8. Now technically, I drew the diagrams above as if the primitive streak had disappeared. It definitely hasn't it's still there, and it's really important for this next step! Beneath the primitive streak, the mesoderm differentiates (changes) to form the notochord. I've drawn the primitive streak in yellow this time, and the notochord is a light green.
9. Almost simultaneously, the formation of the notochord induces (causes) thickening in the ectoderm to form a structure called the neural plate. This happens because the notochord secretes proteins that diffuse over a limited range and signal that part of the ectoderm to form the neural plate.
10. The nerual plate/ectoderm cells then invaginate (fold inwards on themselves) so that from above, it looks like the neural plate is 'zipping up' and pushing the notochord downwards. I can't draw this because it would look God-awful, but just imagine a zipper zipping up a jacket and that's essentially what it does. However this zipper has to work in both directions, as it zips up both caudally (towards the bum) and rostrally (towards the head) at the same time.This results in the formation of the neural tube.
Because I understand that my diagram looks a bit shite, this is what wikipedia free images has to offer, and I think this is much better. It shows the process in 3D, and also shows the positioning of the neural crest. Take note of this for the final step.
We have this evidence that the mesoderm causes the ectoderm to develop the nerural tube from transplantation experiments.
- if a second notochord is inserted, then two neural tubes develop
- any ectoderm that is moved next to the axial mesoderm (the mesoderm under the neural tube) will form a neural tube.
11. Finally, whilst the neural tube is forming the neural crest (seen in green above), cells break off into the mesoderm and move away. These will eventually form their own tissues!
I hope you've enjoyed this whistle stop tour of the first part of my embryology lectures! I have another one coming up next week and since it took a week for me to feel confident about this lecture, expect another post on the second half in the upcoming weeks!