Hey it’s Professor Dave, I wanna tell you about cell specialization.
By now we know a lot about cells.
We have a decent idea of how the first prokaryotic cells formed, we know how they merged with
one another to form more complicated eukaryotic cells, and we know about a variety of unicellular life.
But humans, and all the other animals that have ever existed, are multicellular.
We are made of trillions of eukaryotic cells.
So how did multicellular life come about, and how did it eventually transform into the
wide variety of species we see today?
Also, how is it that so many species seem so well-suited for their environment, able
to swim, and jump, and camouflage themselves?
Why do so many different species have such similar features?
Later we will talk about evolution on the grandest scale, but first let’s ask a simple
question.
At some point in biological history, a single cell divided into two identical cells, but
instead of staying separate, they stuck together.
How did they do this, and how do different cells in the same organism become different
from one another?
We can learn a lot about this by looking at an animal zygote.
Animals reproduce sexually, with meiosis in the reproductive organs yielding haploid gametes,
those being a sperm and an egg.
When the sperm fertilizes the egg, this yields a zygote, a single cell with all of the genetic
information from both parents.
This will undergo cleavage due to mitosis, which we have learned about, to yield two
cells, then four cells, then eight cells.
This eventually results in a blastula, which is a hollow ball of cells surrounding a cavity
called a blastocoel.
This is where things start to get specialized.
Gastrulation is the process by which these cells will fold into layers of embryonic tissue
that will eventually line the animal’s digestive tract, and all the other tissues and organs
will later form in a similar manner.
So what happened over half a billion years ago when animals first evolved?
Researchers look to unicellular protists like choanoflagellates and see that they are incredibly
similar to collar cells on certain sponges.
This and other data leads us to believe that ancestors of animals looked like this.
In order for these to become multicellular, they had to evolve new ways for cells to stick
together and communicate with one another.
We don’t yet know with certainty precisely how this happened, but with what we know about
gene expression, it’s not much of a stretch to see how novel proteins could come about
in choanoflagellates that would serve these functions.
So just the way endosymbiotic theory showed us how unicellular organisms came together
to form more complex cells by way of a symbiotic relationship, we can imagine how individual
cells produced during mitosis could also become an individual organism through the advent
of cell specialization and the mutual benefits that are then enjoyed by all the cells in
the organism.
In order for cell specialization to occur, all that has to happen is for different cells
to receive different signals that each guide gene expression in that particular cell, because
gene expression dictates all of the activities of the cell.
We know a bit about the regulation of gene expression, so we can imagine that certain
cells are receiving signals that result in the expression of particular genes that produce
a particular shape and set of functions for that cell.
In this way, cell specialization accounts for all of the different types of cells in
an animal, from muscle tissue, to a neuron, to any other type of cell.
Then, this mass of different specialized cells must assume a specific three-dimensional shape,
and this process is called morphogenesis.
So one might ask, how is that the different chemical signals reach the correct cells for
all of this to work out just right?
Much of this comes from cytoplasmic determinants that are already present in the cytoplasm
of the mother’s egg cell.
This cytoplasm is not homogenous, and there are various RNA molecules, organelles, and
proteins distributed unevenly.
This means that after a couple rounds of cell division, each resulting cell will have a
different quantity of these determinants, and this plays a large part in determining
the fate of each cell, as these play a part in cell differentiation.
Then, once the number of embryonic cells grows to a certain extent, the environment surrounding
a particular cell will also play a huge role.
Some cells will secrete signals to neighboring cells, so this has the effect of coordinating
the activity of all the cells in a particular area, through a process called induction.
Once a cell is determined to be a particular type of cell, it will become that type of
cell even if removed and placed in another part of the fetus.
This is because events have been set into motion whereby certain genes are expressed
and others are not, which results in the production of proteins that are specific to a particular
type of tissue.
So it really is the presence or absence of various signaling molecules early on that
results in widespread and directed cell differentiation within an organism.
But an organism must also establish a body plan and superimpose this on the developing cells.
This begins early on in the embryo when the general axes of the organism are established.
This means that left and right, top and bottom, and back and front are decided.
This is all controlled by positional information, again provided by cytoplasmic determinants
and inductive signals.
There are also genes that code for substances called morphogens that help develop these
and other features of an organism, like where a head or tail will go.
Now that we know how cell differentiation happens for embryos, it’s just a small leap
to understand how chance mutations in the genome of unicellular organisms could have
resulted in the first basic multicellular life, which blossomed into a huge variety
of species during the Cambrian explosion around half a billion years ago.
From there, life evolved in countless ways to eventually produce human beings that can
think and feel and wonder about how they got here.
Evolution is something of a hot-button issue in certain circles, and to those who don’t
study biology, it seems like a gigantic leap of faith.
But the field of evolutionary biology comprises a mountain of evidence for our current model
of the descent of man, so let’s get through some of this information next.