Early Developmental Steps - Online Tutor, Practice Problems & Exam Prep

Fertilization initiates development, leading to 13 nuclear divisions in fruit flies, creating a single cell with multiple nuclei. By the 10th division, primordial germ cells form, which will develop into eggs and sperm. Following this, the blastoderm layer forms, and gastrulation creates three germ layers: endoderm, mesoderm, and ectoderm. Cell interactions are crucial for development, with asymmetric and symmetric eggs influencing cellular responses. Asymmetry in eggs, like in fruit flies, results from uneven protein distribution, while symmetry is determined by sperm entry, affecting anterior-posterior orientation.

Drosophilia Development

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 going to talk about the next few steps, but we're going to do so from the perspective of fruit flies. Because that's what your book typically is going to use. So, fruit flies, after fertilization, what happens is there are 13 nuclear divisions. What I mean by nuclear divisions is that the nucleus is dividing. It's 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. It'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. There are 13 divisions. After the 9th, 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 begin to be enclosed into membranes, and these nuclei become cells, actually end up becoming primordial germ cells. 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 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 interphase, the very beginning of this 14th division. What happens is all the nuclei now have membranes, and those membranes form what's called the blastoderm layer of embryonic tissue. What this layer is, is it's one cell deep, and it forms completely around the growing embryo, and it happens around 3 hours after fertilization, and I'll show you what that looks like in a second.

After the blastoderm forms a little bit later, after some more cells are developed, a process called gastrulation occurs. Gastrulation forms the 3 major germ layers, which you probably learned during some of your intro biology classes, most likely biology 2. But if you haven't taken that class or you're taking it now or you're not familiar, let me refresh you. There's the endoderm, mesoderm, and ectoderm. The endoderm is going to be the layer of tissue that ends up forming these inside cells, so the lining of the gut. You have the mesoderm forming inside tissues, like bone, blood, muscles, etcetera. Then you have the ectoderm, and this forms the outer layer of the organism, epithelial cells like skin or different linings of different organs, stuff like that. Then once, this happens, segmentation of the organism, the embryo begins. So segmentation begins after gastrulation. So this is when the embryo begins forming those different segments, like the antenna, the abdomen, the thorax, etcetera. And so this is when the HAWK’s genes are expressed. So this is what this looks like. We have an egg at this point. This is an egg with one nucleus. 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 is now an embryo, because now each of these are cells, with membranes around them. And, these form the blastoderm. Eventually, gastrulation will occur, and then the embryo can begin patterning itself to its happiest extent and developing correctly. So that's kind of the very early stages of development in fruit flies. Let's now turn the page.

Cell-cell interactions

Okay. So now let's talk about cell-to-cell interaction. Cell-to-cell interactions are 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. The cells are constantly like, "Yay, give me information so I know what I'm to be doing." Now, during very early development, I'm talking about 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 a single cell, then how are you supposed to respond? So during very early development, the different regions of that single cell, such as which portion is closest to the nucleus, which portion is closest to the plasma membrane, etc., can respond differently to the same molecule. The two examples I want to give you of this are actually an asymmetric egg and a symmetric egg. 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 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. 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.

These concentrations of molecules result in different developmental occurrences. An example of this is fruit fly eggs have cells called nurse cells that surround the embryo, the eggs. Nurse cells express bicoid, but they are only present on one side of the cell. So when those cells are expressing bicoid, they're pumping that bicoid into the egg, or into the zygote once the embryo once it's formed, that means that bicoid 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. In the symmetric egg, every molecule within it is expressed equally throughout the cell. There's no kind of concentration gradient that exists in that cell. It's all symmetrical. So the difference that determines that, actual, you know, which side becomes the interior, which becomes posterior, what develops where, is actually controlled through the sperm.

Because the sperm, you have this full egg, and the sperm can only enter in one location. So where the sperm enters actually will determine whether it is going to be anterior or posterior. This happens in C. Elegans, which, if you remember, 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. The 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 one side of the organism. This concentration will determine the asymmetry of the egg, and it also determines the development of that region of the cell or the embryo. For the symmetric egg, the molecules are actually distributed equally. There's no kind of concentration gradient or anything that's going to lead to the development of one region of the cell differently from other regions of the cell. 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. There's nothing different about any of these locations, but it's just now that the sperm's there, there's more genetic material, there's extra proteins, and that determines the asymmetry, not the egg itself. So, with that, let's now 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

Here’s what students ask on this topic:

What are the initial steps of early development in fruit flies after fertilization?

After fertilization in fruit flies, the first step is 13 nuclear divisions, where the nucleus divides but the cell membrane does not. By the 10th division, nuclei at the posterior region are enclosed in membranes, forming primordial germ cells, which will develop into eggs and sperm. By the 14th division, all nuclei have membranes, forming the blastoderm layer. This layer is one cell deep and surrounds the embryo. Gastrulation follows, creating three germ layers: endoderm, mesoderm, and ectoderm. These layers will form various tissues and organs in the developing embryo.

How do asymmetric and symmetric eggs influence early development?

Asymmetric eggs have unevenly distributed proteins and molecules, leading to different developmental outcomes based on their concentration. For example, in fruit flies, nurse cells express bicoid only on one side, creating an anterior-posterior gradient. Symmetric eggs, on the other hand, have evenly distributed molecules. In organisms like C. elegans, the entry point of the sperm determines the anterior-posterior axis. The sperm's entry introduces additional genetic material and proteins, creating asymmetry and influencing the developmental fate of the regions within the egg.

What is the role of cell-to-cell interactions in early development?

Cell-to-cell interactions are crucial for proper development as cells need to respond to signals from other cells or their environment. These interactions guide cells to develop into specific tissues and organs. During early development, even a single cell can respond differently to the same molecule based on its position relative to the nucleus or plasma membrane. This differential response helps in the proper spatial organization and differentiation of cells, ensuring that the embryo develops correctly.

What are the three germ layers formed during gastrulation, and what do they develop into?

During gastrulation, three germ layers are formed: the endoderm, mesoderm, and ectoderm. The endoderm forms the lining of the gut and other internal organs. The mesoderm develops into tissues such as bone, blood, and muscles. The ectoderm forms the outer layer of the organism, including the skin and the linings of various organs. These germ layers are essential for the proper development of the embryo, as they give rise to all the tissues and organs in the body.

How does the blastoderm layer form in fruit fly embryos?

In fruit fly embryos, the blastoderm layer forms after the 13th nuclear division. Initially, the embryo is a single cell with multiple nuclei. By the 14th division, all nuclei are enclosed in membranes, forming individual cells. These cells create a single-cell-thick layer called the blastoderm, which surrounds the growing embryo. This process occurs approximately three hours after fertilization and is a critical step in early development, setting the stage for subsequent processes like gastrulation and segmentation.