Download Multicellularity

Multicellularity
Up to this point in the class we have been discussing individual cells, or, at most, populations of
individual cells. But some interesting life forms (for example, humans) consist not of individual
cells living in isolation but instead trillions of cells working together in a highly organized
fashion.
Essentially all multicellular organisms are eukaryotes, although many eukaryotes are not
multicellular. The three main multicellular lineages are plants, animals, and fungi, but here we
will focus only on plants and animals. Multicellular animals evolved about 1 billion years ago;
multicellular plants somewhat more recently. Eukaryotic cells, in contrast, evolved 2 billion
years ago, and life itself is somewhere around 3.5 billion years old. You do not need to
remember these numbers, but think about what they mean: life is fundamentally unicellular.
Multicellularity is a more recent adaptation. Most biological complexity is found at the single
cell level, and all of the things that you are learning in this course about single cells also apply to
the cells in a multicellular organism. In this lecture we will look at some of the adaptations that
make multicellular life possible, and how these derive from the basic mechanisms already in
place in unicellular organisms.
Outline:
I.
II.
III.
Specialization
Autonomous signals
A. Lineage
B. Asymmetric division
Cell-cell communication
A. Signals from adjacent cells
B. Short-range diffusible signals
C. Long range transported signals (hormones)
Guideline questions for reading:
1. What is an autonomous signal? How is it different from a signal that comes from a
neighboring cell?
2. Why does a cell’s lineage matter?
3. What does it mean to say that a cell division is asymmetric, and what purpose might that
serve?
4. How does a cell receive signals from other cells?
5. What is a hormone?
I. Specialization
Every cell in your body has the same DNA. But, your cells differ greatly in form and function.
However can this be, given that DNA is the information-storage molecule in the cell, and
ultimately determines function (via the central dogma of molecular biology: DNA  RNA 
protein  function)?
The answer is regulation: at every step along that central dogma hierarchy, the transfer of
information to the next step can be blocked or modified. We will study regulation in more detail
later in the semester. There, we will talk about it in the context of a single cell responding to its
environment, but multicellular organisms have adapted the same mechanisms for cell-cell
communication. For a cell in a multicellular organism, its environment consists primarily of the
cells around it, so it responds to the signals that they give off in the much the same way that a
unicellular organism responds to the environmental signals that it encounters.
So, how does a cell in a multicellular organism know what sort of cell it is supposed to be? How
do such cells arrange and organize themselves in such a way to make a more complex structure,
such as an eye, a liver, or a flower? There are two sorts of regulatory signals that work together
to control multicellular development: autonomous signals, which depend on lineage, and signals
which depend on cell-cell communication.
II. Autonomous signals
An autonomous signal is a signal that comes from inside the cell itself, not from other cells or
the surrounding environment. An autonomous signal could, for example, be a particular protein
that is found only in that specific kind of cell and helps determine the shape or function of that
cell. The source of autonomous signals is a cell’s lineage.
A. Lineage
Lineage refers to a cell’s ancestry. A cell that divides is called a mother cell, and the two new
cells that are formed after that cell division are called daughter cells. Lineage is the record of
which type of mother cell gave rise to this particular daughter cell, and which type of cell that
mother cell’s mother was, and so on back through each division to the single-celled embryo.
Lineage matters because a cell inherits not only its DNA from its mother cell, but also the
proteins in its cytoplasm and on its plasma membrane. Every cell in your body has the same
DNA, so that can’t be what determines a cell’s lineage, but not every cell has the same proteins.
The proteins that a cell contains are what directly determine its structure and its function.
Therefore, a cell will tend to have the same sort of structure and function as its mother, because it
inherited the same proteins. So cells of a given lineage will share certain similarities.
B. Asymmetric division
Given that a cell inherits both its DNA and its proteins from its mother cell, you might be
wondering how it is that different lineages are formed. One key way that this happens is through
asymmetric cell division. If a cell divides symmetrically, it gives rise to two identical daughter
cells. A division is called asymmetric if the two daughter cells are different in some important
way. For example, if a particular protein in the mother cell’s cytoplasm were inherited by only
one of the two daughter cells, that would be an asymmetric cell division. If that protein were a
key autonomous signal, then its asymmetric inheritance would lead to those two daughter cells
becoming two separate cell types.
For example, there is a particular animal that has special protein/RNA complexes called “P
granules” in its egg cells. (You do not have to remember these details; this is just an example to
help you understand). When the egg is fertilized, the P granules move to the end of the cell
where the sperm entered. When the fertilized egg goes through its first division, only one of the
two cells in the embryo inherits the P granules. For each the next 3 cell divisions, these Pgranules continue to be asymmetrically segregated into one daughter cell, so that at the 16-cell
stage, they are present in only one of the cells in the embryo (called the P4 cell). This one cell
then gives rise to all of the sperm or eggs in the adult animal. If you were to destroy the P4 cell,
then the adult animal would have no sperm or eggs—there is no other way for the other cells in
that animal to fill in and take over the job of producing sperm or eggs because those cells do not
have P granules.
P bodies
P4 cell
sperm cells
fertilized egg
adult male
The establishment of lineage by asymmetric cell division. The cells colored gray are all from
the same lineage, and are marked by the presence of P bodies, an autonomous signal. All of
the sperm cells in the adult male will share this same lineage. The first cell division
undergone by the fertilized egg (as well as many of the subsequent ones) is asymmetric
because the P bodies are segregated into only one of the two daughter cells.
Learning Goal: If I gave you a figure sort of like this one (but perhaps from a different
organism), you should be able to label a symmetric and an asymmetric cell division, identify
a set of cells sharing a given lineage, and point out the autonomous signal.
P granules, then, are an example of an autonomous signal, which is present only in cells that can
trace their lineage back to the P4 cell. The P4 cell was generated by a series of asymmetric cell
divisions in which the P granules were only inherited by one of the two daughter cells. Again,
you do not need to remember the terms “P granule” or “P4 cell” or the details of how this animal
develops; this is only an example. But you do need to know what an autonomous signal is, what
lineage means, and the role played by asymmetric cell division.
III. Cell-cell communication
In addition to autonomous signals, a cell’s fate can be controlled by signals that it receives from
other cells in the organism. Cell-cell communication plays a crucial role in the development of
all multicellular organisms. It allows more flexibility in development than autonomous signals,
because a cell’s type can be controlled by the nature of the cells around it, instead of strictly by
lineage. For example, the tip of a plant root contains many different cell types with specialized
roles in controlling the root’s growth. If you cut off the root tip and remove those specialized
cells, the remaining cells near the cut site will (over time) change their identity to recreate all of
those specialized cell types and form a new, fully functional root tip. Generally speaking, plants
rely completely on cell-cell communication to control their development, while animals use a
combination of cell-cell communication and autonomous signals. In both cases, many different
signals work together to specify the fate of a particular cell. How a given cell responds to a
specific signal depends on what other signals it is receiving at the same time and what signals it
has received and responded to in the past.
The means by which a cell receives a signal and responds to it are quite varied and complex, and
we will not get into them here. But one commonality that you should learn is that signals are
sensed by proteins called receptors (remember that we already talked about the LDL receptor).
When a signal comes into contact with its specific receptor, the two stick together, and this
causes a change in the receptor that leads to a change in the cell. We will learn more about
receptors and some ways in which they can act during the unit on regulation of metabolism.
Right now we will talk a little bit more about some of the types of signals that cells use to
communicate with each other.
A. Signals from adjacent cells
The shortest-range signals are those that are sent just from one cell to an adjacent cell. For
example, animal cells have many different transmembrane proteins sticking out of their plasma
membrane. Some of these proteins can interact with specific receptors on the surface of adjacent
cells. These interactions tell the cell what types of cells its neighbors are, and it responds
accordingly. This is how cells cooperate together to build up a larger structure such as a tissue.
(Both plants and animal cells also have other methods of communicating with adjacent cells
which we will not go into; the critical thing is to realize that each cell in a multicellular organism
knows who its neighbors are, and this is part of what controls its fate.)
B. Short-range diffusible signals
Another class of signals that work over the short- to medium-range is diffusible signals. These
signals are produced in a particular location and then diffuse outwards. This forms a
concentration gradient, with the highest concentration of the signal close to the source, and lower
concentrations further away from it. (You have already learned about concentration gradients
and diffusion when we were studying energy metabolism, so this concept should be familiar)
The concentration of the signal in a given region then affects the development of the cells in that
region.
C. Long-range transported signals (hormones)
Hormones are a class of long-range signaling molecules that allow communication between cells
in very different parts of the organism. A hormone is usually produced in a given organ, then
actively transported around the body in the bloodstream. For example, estrogen is a hormone that
helps control sexual development in females and regulates the menstrual cycle. It is produced
primarily in the ovaries, but is transported around the body and affects many different tissues.
Different tissues respond in different ways to estrogen, and some tissues do not respond at all.
Why do you think that might be? (That is, what characteristics might allow cells to respond or
not respond to estrogen?)
Finally, remember that cell-cell communication takes place not only during development, but
throughout the entire life span of a multicellular organism. Your cells are constantly telling each
other what they should be doing, so that everything stays coordinated and works together. For
example, after you eat a meal, there is a hormone produced in your gut that stimulates the release
of digestive enzymes to processes the food, and also tells your brain that you are full. As the
meal is digested, the amount of glucose in your bloodstream rises. This leads to higher levels of
ATP in specialized cells in your pancreas (you already understand how glucose leads to ATP).
The rise in ATP levels stimulates those cells to secrete a different hormone, insulin, which
signals your fat and muscle cells to take up more glucose from the blood, thus preventing your
blood sugar from getting too high. These and other hormones are necessary to coordinate your
body’s metabolism, so that every cell gets the just amount of nutrients that it needs.
Learning Goal: Realize that cells signal to each other over a range of distances, and understand
the key properties of different signals and their transport that allow them to work on those
different scales (adjacent vs short range vs long range).
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long range signals
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