Embryology

[msamadia1231]

Before we start talking to embryology, already getting a picture of an embryo in mom's womb, we need to know how we get an embryo. This is simple. One spermatozoon + ovum = one embryo

So fertilization took place with the spermatozoon and ovum joining. Fertilization is pretty important and tells us if it is a boy or girl. The sperm is the determining factor that will be a boy or girl. If the male gives an X chromosome it is girl, or Y, than a boy. Also remember that spermatozoon (or spermatozoa, plural) is haploid and the ovum is haploid. Joining them together makes the embryo a diploid. What does this "diploid" term mean? The embryo has 2 copies of DNA.

The uterus many proteolytic enzymes that break down glycoproteins attached to the head of the sperm, upon entering. This must occur before the egg and sperm can fuse. Note that the epididymis is the place above the testes that "matures" the sperm by adding the glycoproteins.

The removal of glycoproteins from the sperm is called capacitation.
Next, here is a picture of a spermatozoon: http://en.wikipedia.org/wiki/Image:Complete_diagram_of_a_human_spermatozoa.svg

Take note of the head, b/c the second part of fertilization is called the acrosome reaction. This reaction is basically giving the answer away. The acrosome is the cap strucure in the picture. So, this is a reaction that is occuring in the acrosome. The sperm needs to be in close proximity of the outer layer of the egg, corona radiata. Once the cap is released, it exposed hydrolytic enzymes that break down the corona radiata (outer layer of the egg). So think of this process as a construction machine trying to drill a hole. The inside layer is the zona pellucida, and thru a zona reaction, breaks this as well, and fuses its plasma membrane with the plasma membrance of the oocyte (female version of the spermatozoon).

Fusing together completes the process of Meiosis II by forming female and male pronucleus. The chromosomes (1 copy each) come to gether and make 2N, diploid. Once you get a 2N, you can tell yourself the process of fertilization is done.. Why? It is important that when studying processes, you set "markers" of when things end and begin. High importance. The 2N is pretty nice to remember.

You should know that since the sperm has to get close to the corona radiata b/c it might "die" there needs to be a lot of spermatozoon. The pH of the uterus is highly acidic. About the breaking apart the corona radiata, imagine this process as the more help or the more spermatozoon there is to help break down the "wall" the better. Basically, the more spermatozoa (plural) there is the higher chance of fertilization. If you don't break down the wall, no chance of fertilization.

Do not worry about multiple spermatozoa getting in, only one sperm is allowed for each ovum. Simple. There is a mechanism in place called the cortical reaction, a set up a protective layer around the zona pellucida. Remember that there will be hyperpolarization occuring once one spermatozoon enters the membrane of zygote. ??? Creates a negative charge and keeps further spermatozoa from approaching. The negative charge around the membrane of the zygote "repels" the others away.

Practice the above, explaining to yourself or imaginary patient, or if your parents will allow it, explain it to them.

We have spoken about the first process: Fertilization. What is the 2nd process? Cleavage.

[msamadia1231]

For cleavage, think mitosis. If things are going to keep dividing, the metabolic and O2 consumption increases. Once the zygote reaches 16-cells, it is renamed to a morula. Strange names. The cells that are increasing are not getting bigger in size, but smaller in size.

So, now the number is no longer 16 is no longer the cost of 3 watermelons plus tax (estimate), but the name morula, which has 16 small cells. About these small cells: there is an inner cell mass and the then the outer cell mass which will make the trophoblast. The trophoblast will be the provide food to the embryo as well as make up a big part of the placenta.

The third process after cleavage? Blastulation.[/b]

[msamadia1231]

You are probably wondering what we should be "thinking" when blastulation comes to mind? Well, we had the the inner cell mass and outer cell mass layer; in between that is going to fill with fluid and create a cavity called the blastocoele. So think filling a balloon with water.

Inner and outer cell mass = blastocyst (Important) Once you have a blastocyst, implantation occurs.
The inner cell mass will stick to one section and be called the embryoblast. The trophoblast makes contact with the layer of the uterus in creating the placenta.

Next stage? Gastrulation.

All gastrulation is about is the development of the 3 germ layers: endoderm, mesoderm, ectoderm. All 3 layers fit into a disk, not a CD-ROM, or floppy disk, but a trilaminar disk.

Note: before you get a trilaminar disk (gastrulation) the hypoblast and epiblast line up parallel to each other. What starts gastrulation? The epiplast (just layer of cells) invaginates (kind of like the process of phagocytosis. Once invagination occurs, this creates a layer in between the 2 layers; epiblast and hypoblast. The hypoplast becomes the endoderm (differentiate), the epiblast to the far side is the ecto derm, and the epiblast that created the layer in between is the meso derm.

How am I going to remember all these layers and what goes with what? Think big.
Endoderm: epithelium lining of GI tract, Digestive organs
Mesoderm: Liver, heart, lung and muscle tissue, including bone, blood vessels, etc.
Ectoderm (outside): skin, and CNS & PNS. Remember that the eye is part of the nervous system.

Last process of embryology is Neurulation. All chordates have a notochord, important, but not all chordates have a vertebrae. So neurulation is the 2 processes.

First notochord>vertebrae (vertebrates)
Second neural plate>neural groove (easy to remember)> brain and spinal cord.

[msamadia1231]

Need to know that differentiation is immature to mature cells. Think of the a rebellious teenage youth years and coming to a mature and responsible adult.


As options become smaller and smaller, and the captain of the soccer team is picking which one he wants on his team, let's say there is 4 left, there is less room for options. Determination is the "determination" that a cell takes as the options to develop into something else becomes less. The cell must really make sure "being a doctor is really what he wants to do: determined"

Induction is a MCAT concept you should have down. A germ layer next to another layer can release certain chemicals that will cause the "next door neighbor" to become a certain type of tissue or cell. Think of the eye (ectoderm) with blood vessels (mesoderm). How can that be? Induction... induction.

[kazuma]

are you sure it isn't depolarization that blocks polyspermy?

http://biology.kenyon.edu/courses/biol114/Chap13/fastblock.gif