Early Developmental Steps

Hi in this video we're gonna be talking about early developmental steps. So you start off with an egg. A sperm encounters it and that is called fertilization. And that's obviously the very first step of development. Now we're gonna talk about the next few steps but we're gonna do So from the perspective of fruit flies Because that's what your book typically is going to use. So um fruit flies after fertilization, what happens is there's 13 nuclear divisions. So what I mean by nuclear divisions is that the nucleus is dividing, dividing 13 times but the rest of the cell isn't. So this is one cell with one membrane and has 13 nuclei in it. So there's a lot of genetic information. But this is super important. Eventually they will all get membranes. But this is the very first step of development. So there's 13 divisions after the ninth. What happens is the nuclei begin to migrate to one side of the embryo. And by the 10th division the nuclei that are present at the posterior region of this cell Are begin to be enclosed into membranes. And these nuclei become cells actually end up becoming primordial germ cells. So these are the primordial cells that will eventually create the egg and the sperm of this new organism. So that's one of the very, I think that's so surprising to me is one of the very first things that happen in an organism is the egg and the sperm are created or the primordial germ cells that will create the eggs and the sperm are made. That happens in the 10th division. Then the rest of the divisions are carried out. Then we enter into interface at the very beginning of this 14th division, what happens is all the nuclear I now have membranes and those membranes form what's called the blasted Durham layer of embryonic tissue. So what this layer is is it's one cell deep and it forms completely around the growing embryo and it happens around three hours after fertilization. And I'll show you what that looks like in a second after the boston term forms a little bit later after some more cells are developed. A process called gas relation occurs. And gas relation forms the three major germ layers which you probably learned during some of your intro bio classes. Most likely bio too. Um But if you haven't taken that class or you're taking it now or you're not familiar, you just need a refresher. Let me refresh you. There's the encoder, Mesut, Dermot, ectodermal the term. It's gonna be the layer of tissue that ends up forming forming these inside cells in the lining of the gut. You have the method um forming inside tissues like bone, blood muscles et cetera. Then you have the ectodermal and this forms the outer layer of the organism. Epithelial cells like skin or different linings and different organs, stuff like that. Then once this happens segmentation of the embryo begins. So segmentation begins after gas relations. So this is when the embryo begins forming those different segments like the antenna, the abdomen, thorax, et cetera. And so this is when the hawks genes are expressed. So this is what this looks like. We have a an egg at this point, this is an egg with one nuclei. All these divisions happen. Some of these cells will become primordial germ cells. They aggregate at one end, some of them will surround the which which is now an embryo because now each of these are cells with membranes around them and these form the blasted term. Eventually gas relation will occur and then the embryo can begin patterning itself to its happiest extent and developing correctly. Um So that's kind of the very early stages of development and fruit flies that let's now turn the page.

Okay, so now let's talk about cell to cell interactions. So cell cell interactions super important for proper development because the cells have to respond to signals from either other cells or their environment so that they properly develop into whatever they're supposed to develop into. So the cells are constantly like yeah, give me information. So I know what I'm supposed to be doing now. During very early development, I'm talking about like a single cell or a couple of cells. There's not a lot of information going on right. If you just have one other cell to interact with, it's not going to give you a lot of information. And if you're single cell then you then how are you supposed to respond? So doing very early development, the different regions of that single cell, so which portion is closest to the nucleus, which portion is closest to the plasma membrane etcetera etcetera etcetera. These cellular regions actually can respond differently to the same molecule. So the two examples I want to give you of this is actually an asymmetric egg and a symmetric egg. So if the egg is asymmetric, what that means is that there's a bunch of different proteins and molecules being expressed either inside the egg or surrounding the egg. And um those molecules are not equally distributed. Some of them sit maybe closer to the posterior end and some of them sit closer to the anterior end and because of that they're not equally distributed. We say that the egg is asymmetric because those molecules are actually concentrated in one region of the cell. And so and and so those concentration of molecules result in different developmental um occurrences. So an example of this is a fruit fly eggs have these cells called nurse cells that surround the embryo. The eggs and nurse cells expressed by coid but they are only present on one side of the cell. So when those cells are expressing by coid they're pumping that bike oid, you know into that egg or into the zygote. Once the embryo once it's formed, that means that Bitcoin is only going to be present at the anterior end. And so that itself is an asymmetric egg. The alternative to that is the symmetric egg and the symmetric egg. It means that every molecule that's in it is expressed equally throughout the south. There's now kind of concentration gradient that exists in that cell is all symmetrical. So the difference that determines that actual, you know which side becomes interior which becomes posterior. What develops where is actually controlled through the sperm. And so because the sperm, you know you have this full egg and the sperm can only enter in one location. So where the sperm enters actually will determine what is that going to be anterior. Is it going to be posterior? And so this happens in c elegans which if you remember what c elegans are those are the worms. So if I back up for a second here we have the two examples of this, we have the asymmetric egg and the symmetric egg. And these eggs are expressing some kind of black molecule. This black molecule in the asymmetric egg is going to be asymmetrically distributed or not distributed equally. And you can see that it's concentrated on this side of the organism. And this concentration will determine the asymmetry of the egg. And it also determines development of that region of the cell or the embryo. For the symmetric egg, the molecules are actually distributed equally. So there's no kind of concentration gradient or anything that's going to lead to development of one region of the cell, different from other regions of the cell. So the thing that determines asymmetry in this situation is actually the sperm. It can only enter into one region it enters here and that will determine which end is anterior and which end is posterior. If the sperm entered here, it would do the same, It could enter here, it would do the same. And there's nothing different about any of these locations. But it's just now that the sperms there, there's more genetic material, there's extra proteins and that determines the asymmetry, not the egg itself. So with that let's not move on.

Which of the following is not a germ layer formed during gastrulation?

• A. Endoderm
• B. Mesoderm
• C. Exoderm
• D. Ectoderm

Which of the following characteristics describes the blastoderm embryonic tissue?

• A. Contains 13 cells
• B. Is one cell layer deep
• C. Will develop into primordial germ cells
• D. Forms outer later of the organism

Gastrulation occurs after segmentation

• A. True
• B. False

Patterning of asymmetric eggs is first controlled through contact with sperm.

• A. True
• B. False