Introduction to development
How does an organism go from a single cell to something as complex as a frog, fly, or human being? Learn the basic principles of development.
- A multicellular organism develops from a single cell (the zygote) into a collection of many different cell types, organized into tissues and organs.
- Development involves cell division, body axis formation, tissue and organ development, and cell differentiation (gaining a final cell type identity).
- During development, cells use both intrinsic, or inherited, information and extrinsic signals from neighbors to "decide on" their behavior and identity.
- Cells usually become more and more restricted in their developmental potential (the cell types they can produce) as development progresses.
You, my friend, are a walking, talking, thinking, learning collection of over cells. But you weren’t always that large and complex. In fact, you (like every other human on the planet) started out as a single cell – a zygote, or the product of fertilization. So, how did your amazing, complex body form?
Development: The big picture
During development, a human or other multicellular organism goes through an amazing transformation, one at least as dramatic as the metamorphosis of a caterpillar turning into a butterfly. Over the course of hours, days, or months, the organism turns from a single cell called the zygote (the product of sperm meeting egg) into a huge, organized collection of cells, tissues, and organs.
As an embryo develops, its cells divide, grow, and migrate in specific patterns to make a more and more elaborate body. To function correctly, that body needs well-defined axes (such as head vs. tail). It also needs a specific collection of many-celled organs and other structures, positioned in the right spots along the axes and connected up with one another in the right ways.
The cells of an organism's body must also specialize into many functionally different types as development goes on. Your body (or even the body of a newborn) contains a wide array of different cell types, from neurons to liver cells to blood cells. Each one of these cell types is found only in certain parts of the body—in certain tissues of certain organs—where its function is needed.
How does this intricate cellular dance unfold? Development is largely under the control of genes. Mature cell types of the body, like neurons and liver cells, express different sets of genes, which give them their unique properties and functions. In the same way, cells during development also express specific sets of genes. These patterns of gene expression guide cells’ behavior and allow them to communicate with neighboring cells, coordinating development.
In this article and the ones that follow, we’ll take a closer look at principles and examples of development.
Some basic processes of development
Different organisms develop in different ways, but there are some basic things that must happen during the embryonic development of almost any organism:
- The number of cells must increase through division
- Body axes (head-tail, right-left, etc.) must form
- Tissues must form, and organs and structures must take on their shapes
- Individual cells must acquire their final cell type identities (e.g., neuron)
To be clear, these processes aren’t separate events that happen one after another. Instead, they are going on at the same time as the embryo develops.
For instance, different body axes (such as head-tail and left-right) are set up at different times during early development, while the cells of the embryo are dividing away in the background. Similarly, formation of an organ requires cell division to build that organ, as well as differentiation (cells taking on their final identities) to ensure that the right cells make up the right parts of the organ.
Sources of information in development
How do cells know what they're supposed to do during development? That is, how does a cell know when and how to migrate, divide, or differentiate? Broadly speaking, there are two kinds of information that guide cells' behavior:
- Intrinsic (lineage) information is inherited from the mother cell, via cell division. For instance, a cell might inherit molecules that "tell" it that it belongs to the neural, or nerve cell-producing, lineage of the body.
- Extrinsic (positional) information is received from the cell's surroundings. For instance, a cell might get chemical signals from a neighbor, instructing it to become a particular kind of photoreceptor (light-detecting neuron).
During development, cells often use both intrinsic and extrinsic information to make decisions about their identity and behavior. Of course, they don't actually "decide" by thinking the problem over like you or me! Instead, cells make decisions in the way a calculator or computer would: by using genes and proteins to perform logic operations that calculate the best response.
Differentiation, determination, and stem cells
Over the course of development, cells tend to become more and more restricted in their "developmental potential." That is, the types of other cells they can make by cell division (or directly turn into) become fewer and fewer.
For instance, a human zygote can give rise to all the cell types of the human body, as well as the cells that make up the placenta. To use vocab from the stem cell field, this ability to give rise to all cell types of the body and placenta makes the zygote totipotent. However, after multiple rounds of cell division, the cells of the embryo lose their ability to give rise to cells of the placenta and become more restricted in their potential (pluripotent). These changes are due to alterations in the set of genes expressed in the cells.
Eventually, the cells of the embryo are split into three different groups known as germ layers: mesoderm, endoderm, and ectoderm. Each germ layer will, under normal conditions, give rise to its own specific set of tissues and organs.
As the cells of a germ layer continue to divide, interacting with their neighbors and reading out their own internal information, their cell fate “options” will get narrower and narrower. At first, cells may be specified, earmarked for a certain fate but able to switch given the right cues. Next, they may become determined, meaning that they are irreversibly committed to a certain fate. Once a cell is determined, even if it’s moved to a new environment, it will differentiate as the cell type to which it's become committed.
Eventually, most cells in the body differentiate, or take on a stable, final identity. Examples of differentiated cell types in the human body include neurons, the cells lining the intestine, and the macrophages that gobble up bacterial invaders in the immune system. Each differentiated cell type has a specific gene expression pattern that it maintains stably. The genes expressed in a cell type specify proteins and functional RNAs needed by that particular cell type, giving it the right structure and function to do its job.
Left panel: liver cell. The liver cell contains alcohol dehydrogenase proteins. If we look in the nucleus, we see that an alcohol dehydrogenase gene is expressed to make RNA, while a neurotransmitter gene is not. The RNA is processed and translated, which is why the alcohol dehydrogenase proteins are found in the cell.
Right panel: neuron. The neuron contains neurotransmitter proteins. If we look in the nucleus, we see that the alcohol dehydrogenase gene is not expressed to make RNA, while the neurotransmitter gene is. The RNA is processed and translated, which is why the neurotransmitter proteins are found in the cell.
For example, the diagram above shows two genes that are differently expressed between a liver cell and a neuron. One gene, encoding part of an enzyme that breaks down alcohol and other toxins, is expressed only in the liver cell (and not in the neuron). The other gene, encoding a neurotransmitter, is expressed only in the neuron (and not in the liver cell). Many other genes would also be expressed differently between these two cell types.
Adult stem cells
Not all cells in the human body differentiate. Some cells in the adult body retain the ability to divide and produce multiple cell types. These include adult stem cells, which are usually multipotent: they can produce more than one cell type, but not a large range of cell types. For instance, hematopoietic stem cells in the bone marrow can give rise to all the cell types of the blood system (shown below), but not other cell types such as neurons or skin cells.
The hallmark of stem cells is that they undergo asymmetric cell division, producing two daughter cells that are different from one another. One daughter remains a stem cell, a process called self-renewal (the dividing cell "renews" itself by making a functionally identical daughter). The other daughter cell takes on a different identity, either differentiating directly into a needed cell type or going through additional divisions to make more cells.
You can learn more about development and see more examples of its principles and processes in these articles:
- Frog development: learn about the early development of frogs. Bonus: see an experiment that makes a two-headed newt!
- Homeotic genes: learn about the "master regulator" genes that specify whole segments or structures of the body. Bonus: see a fly with legs growing out of its head!
Want to join the conversation?
- I could not find specific lesson for plant embryology so I am asking this question here.
In the unfertilized embryo sac in angiosperms, is the central cell considered to be haploid or diploid? though it has two polar nuclei but both of them are haploid, so I don't think it can be considered diploid.
(Actually, I encountered a question that specifically asks the number of haploid cells in an unfertilized embryo sac.)
Please help.(2 votes)
- 'Polygonum-type central cell is a binucleate cell that, upon fertilization with one of the two sperm cells, forms triploid endosperm to nourish embryo development. '
The fact that is it, binucleate counts, as it is diploid.
The central cell of the embryonic sac in Angiosperms is diploid. And triploid fertilization is what makes it unique and distinguished Angiosperms from other plants.