Introduction to Cell Specialization

Introduction to Cell Specialization

by Chris Tindle

​Introduction to Cell Specialization

In the last class we talked about a whole host of specialized cells; from root hair cells to nervous tissues cells, to muscle cells. In keeping with that, I wanted to slow down and dive a bit deeper into how cells become specialized.

 

Key points:

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.​

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.

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​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.

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​In the next slide we will get to see the development of a zygote from fertilization to implantation.

Multiple Choice

Cells start off as a _____ which further develops into an eventual fetus.

• Zygote

• Blastocyst

• Stem

• Blob

​Development: The Big Picture P2

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.

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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.​

Development: The Big Picture P3

Different organisms develop in different ways, but there are some basic things that must happen during the embryonic development of almost any organism:

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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.

Open Ended

Why might cell division be the first step for most organism in the cell specialization process?

Consider the complexity of organisms when answering this question. 

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:

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​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.

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​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).

Multiple Select

Recall the two types of sources of information in development from the last slide.

More than one answer is correct.

• Extrinsic

• Intrinsic

• Extraneous

• Intraveinous

Open Ended

Why might it be beneficial for a stem cell to have two sources of information to pull from when deciding what cell to be?

​Differentiation, Determination, and Stem Cells

As development occurs, ​cells tend to become more and more restrictive in their "specialization capabilities". That is to say that as development advances, the amount of cell specialization that occurs is lessened.

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​For instance, a human zygote can give rise to all types of cells, but as those cells specialize further and further into tissues and other organs the need for further specialization is decreased. These changes are mainly due to alterations in gene expression as embryotic development occurs.

​Intrinsic: The Different Germ Layers

Eventually, these cells split into three distinct groups ​known as germ layers:

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1. Ectoderm: mainly consisting what will be the outer layer of our skin.

​2. Mesoderm: forms connective tissues, the heart, bone, and skeletal muscle.

​3. Endoderm: mainly consisting of what will become the liver, digestive system, etc.

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​Each germ layer will, under normal conditions, give rise to their respective sets of tissues and organs.

Multiple Select

What are the three germ layers that form all parts of our body?

• Endoderm

• Ectoderm

• Mesoderm

• Epiderm

• Mediderm

The Extrinsic Process

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.

1. At first, cells may be specified, earmarked for a certain fate but able to switch given the right cues.

2. 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.

3. 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.​

Open Ended

The Extrinsic Process can be summed up in 3 steps: 

The cell first becomes ________ meaning it will change given the right cues

Then the cell's fate is _______ in which it commits to a certain fate

Finally the cell fully ________ in which its shape is stable and further change is irreversible 

​The Extrinsic Process Continued

​As mentioned at the end of the previous slide; gene expression is responsible for the final differentiation step.

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The picture on the right should illustrate this point clearly. if the correct genes are not properly expressed then the cell will not complete the specialization process.

​Finale: 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

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For instance, hematopoietic stem cells in the bone marrow can give rise to all the cell types of the blood system (shown to the right) but not other cell types such as neurons or skin cells.​