What I'd like to do today, is to give you an epilogue,
the very end of the last lecture on cell type, that I did not want to
cram into the very last one minute, so I'm going to take five minutes
now to talk about that, and then I want to move on to a
topic that is very related, because everything, of course,
is very much related by the control of gene expression,
by cell-cell interactions, by cell fate, and so on, but I want
to talk to you about a particularly important set of interactions
between cells. Here we are, on your game board of
life, we're moving up through the formation module,
talked about cell type, and we're going to build on that
conversation in talking about fertilization today.
So let me go back and remind you where we were.
At the very end of last lecture we were talking about cell type,
and what I wanted to do with you, was to trace for you the ontogeny,
the development of a particular type of cell, and indicate to you how
many different interactions there were between cells --
-- how many different sets of genes had to be activated,
and had to interact with one another, to get you from a fertilized egg,
all the way to a differentiated cell type, and I used the paradigm of
skeletal muscle. And really, the point that I think
you got, was that this is very complicated, and if you go on in
your careers and think about, as engineers for example, as
biologists, engineering a particular kind of cell from another kind of
cell, you really have to know these steps along the way,
and ask whether or not in order to get from point a to point b,
you have to go through the whole set of 30 steps --
-- or whether you can circumvent those steps, and short-circuit,
and go on a much shorter route. And we'll talk more about this in
subsequent lectures, but this is very relevant for
thinking about how to engineer particular cell types.
What we were talking about was an extraordinary experiment that was
done in the 1920's, that demonstrated a piece of
embryonic tissue called the organizer of the dorsal mesoderm,
was able to activate the formation of a second --
[PAUSE[ -- axis, or a second conjoined
embryo, where most of the second embryo was derived from the host
tissue, into which this piece of organizer, or dorsal mesoderm,
had been transplanted. And this organizer was able to induce a
second embryo, including skeletal muscle,
must be made from the host cells.
OK, who does not have their handouts here from the last lecture?
I'm not trying to nail you, I just want to know if I need to
write something on the board. I need to write something on the
board, OK. So, the notion that we were left with
was that, until now, we had gotten to the point where
genetic information that had defined the dorsal part of the embryo,
synergized with genetic information that formed the dorsal mesoderm that
formed the mesoderm. That gave rise to this piece of
tissue called dorsal mesoderm, abbreviated meso, which is also
known as the organizer. And this piece of dorsal mesoderm,
or organizer, itself, as I said, a very powerful inducing center,
and it induces formation of a group of tissue blocks called somites.
These somites, as I'll show you in a moment,
are segments, or segmental units, that are present along the whole
axis of the body, the whole trunk region of the body
of vertebras. And they actually hark back to our ancestry,
derived from organisms that were fully segmented,
worms for example, earthworm type animals,
that had multiple segments. OK. The somites,
in turn, become subdivided, and I put this in just to indicate,
again, the complexity of the process, and part of the somite,
termed the myotome, activates the expression of the transcription
factor myoD. And it was pointed out to me by
Christine that I was abbreviating transcription factor,
TXN that is my abbreviation, many of you may remember from way
back, for transcription. So, if you have in your previous
notes, a TXN, and you're not sure what I meant, I meant transcription.
OK, so these somites get subdivided, they make the myotome that expresses
myoD, and as we talked about last time, we come in full circle to
where we wanted to be, which is the activation of the
skeletal muscle program. Let me show you this on this
PowerPoint slide. OK, so here are the organizers in
diagram, activating the formation, or inducing the formation, of these
somites. The somites get divided, myoD is expressed, myoD and other
family members, remember I made the point this was a
member of a redundant transcription factor, a redundant gene family,
myoD maintains its own expression through a positive alter
regulatory route. That means it binds to its own
promoter, and perpetuates, or activates, its own transcription,
so once myoD is expressed, it maintains its own expression,
a very popular way of stabilizing gene expression in particular cell
types. OK, here are some pictures of early human embryos.
Now, you should realize, the reason in this previous slide
that I went from a round circle representing a round ball of cells
as an embryo, to no circle, is that everything I've told you
until now, took place in a round ball of cells,
OK? Up to about the 4, 00th cell stage, or about the 10,
00th cell stage, the frog embryo is just a round ball of cells,
and most embryos are not very distinguished looking.
But the process of making somites and dividing them,
goes on until the embryo is a million cells or more,
and during that time, the embryo reorganizes its body plan,
so it's not longer a ball of cells, it starts to take on its
characteristic form. And some of the characteristic form
it takes on, is shown in this human embryo, about 19 days after
fertilization, this region here is going to be the
nervous system -- -- and on either side of the nervous
system, I've got in red here, boxed these little segmented blocks,
and you can see these segmented blocks in older embryos.
Those are the somites from which most of the skeletal muscle is going
to form. This is a movie to show you the segmentation
of the somites. So here they go,
here are the blocks of cells being divided one from another,
it's a very beautiful process, and you can see it's very ordered.
Down the middle here is the neural tube, which is starting to give rise
to the central nervous system, and on either side of the neural
tube is this future muscle tissue, or the future somites that include
the skeletal muscle, and you can see them being divided
symmetrically on either side of this midline here, in to these
chunks of tissue. And it's actually a very interesting
story in the regulation of gene expression, as to how you time
things in such a coordinated fashion. In all animals,
the somites become segmented first in the head region,
and then the segmentation moves down towards the tail,
and there is an intrinsic timer in the embryo, that I don't have time
to describe to you, that again, converges on the
regulation of gene expression, that allows this extraordinarily
coordinated segmentation of the body plan.
OK, so what I've done is to try and trace this ontology of the cell type.
What you'll realize if you think about this, is that I haven't
actually told you what transcription factors actually activate the myoD
promoter in the somites. This is an RNA expression pattern,
and the whole myoD regulatory loop, is controlled in a transcriptional
way. And we don't know what exactly activates myoD expression in the
somites, but we know a lot about what happens before.
And with that, I am going to move on to what I want
to spend most of today talking to you about, and that is the question
of reproduction. And I'm going to talk to you about
the molecular biology of reproduction, and particularly we're
going to talk about sexual reproduction --
-- as one of the most important
examples of cell-cell interactions. And in fact, the one without which
none of us would be here, although that, in a sense,
is a silly thing to say because without most of the interactions,
the cell interactions during development, none of us would
be here anyway. But, this is a very interesting
process, a very important process, and it exemplifies some points that
I want to bring up here. So the notion here is that two
haploid cells, the sperm, and N is my designation
for haploid, I presume you're all with me when I use N and 2N,
as haploid and diploid, OK, fine. Sperm and egg go through the
process of fertilization --
-- to form a single cell, a zygote that is diploid, and the
zygote will then go on to perform a multi-cellular embryo,
etc. Now, there are some really extraordinary things about this.
One of the extraordinary things is that the sperm and the egg are fully
differentiated cells. I have been telling you,
indeed really trying to hammer into you, the notion that cells progress
from less specialized states to more specialized states.
And this is the one instance during development, where the flip is true,
where the opposite is true. So the sperm and the egg are fully
differentiated cells, they have reached this final cell
type, but when they fuse, and they form the zygote, they now
make the most fully undifferentiated cell. So the zygote is uncommitted,
that may or may not be spelled correctly.
So the zygote is uncommitted, and I want to introduce you to a new
term, which is that of potency, where potency refers to the number
of possible fates that a cell can assume. We're going to talk a lot
about this when we talk about stem cells. The number of possible fates
that a cell can assume, and the sperm and the egg are
uni-potent, they have one possible fate, which is themselves.
And the opposite is true of the zygote. It is totipotent,
it can become everything, all possible cell types.
And that is the antithesis of what
happens during development in essentially, I would say,
in every single cell type of the body. And it's extraordinary,
and we don't understand, we don't understand, how you get this
complete reversal of fates, although I'll try to go through this
with you in a future lecture. All right, why go to all this
trouble? This is a lot of work for an organism to go through,
and the bottom line is that it leads to genetic re-assortment,
and it leads to genetic diversity. So the notion of sexual reproduction,
I'm going to write it up here, is that it engenders genetic
diversity, and it does so because genetic materials becomes
re-assorted during meiosis, and during recombination, or during
the joining together, of the sperm and the egg.
OK, I divided this lecture up into a few parts. The first is
gametal genesis --
-- where the gametes are the egg and the sperm, just a different name for
them, and gametogenesis is one of those developmental biology terms
that has the suffix "-genesis", the creation of the gametes. Both
the sperm and the egg start off from diploid, precursor cells,
and this is a very interesting story, but I'm not going to tell you.
But I'll remind you, well, I'm going to allude to it.
I'll remind you that long ago, I showed you these beautiful series
of pictures from worms. When we talked about determinants,
cell autonomous factors that control cell fate, and I showed you how
these determinants were segregated into a single cell,
of the 32 cell worm embryo, and that this cell was going to give
rise to the gametes, the egg and the sperm.
In fact, the exact same thing is probably true in all animals,
including ourselves. And certainly in frogs, and in fish,
we know that there are determinants that are segregated,
and we know what these are, they turn out to be proteins that
bind RNA and control translation of presumably specific target genes.
So, somewhere back in the dawn of time, during development,
the germ cells become determined -- -- but I am not going to talk about
this, I am going to talk about them when they actually already know that
they're germ cells, and when they start thinking about
assuming their final differentiated fate. And I want to make a couple
of contrasting points, and OK, well let me do my board work
and then we can look at that. OK. The precursor of the spermatozoa is
the spermatagonium. This is a diploid cell that
undergoes meiosis, to give rise to a haploid
spermatazoan. The spermatagonium is a moderate sized cell,
it's just a regular sized cell, and when it undergoes its conversion,
its differentiation, to a spermatazoan, or a sperm,
it becomes very small, it jettisons most of its cytoplasm, and
it becomes motile. So you go from a medium sized cell
to a tiny cell that has the very important property of being motile.
And this is true, really, universally, some,
and I'll talk about this in a moment. The way that spermatozoa are motile
is slightly different in different organisms, but they pretty much,
all spermatozoa are motile. In contrast, the egg arises from a
diploid oogonium and the oogonium also starts off as a medium sized
cell, but it does the opposite. It gets very big, OK? And the
oogonium moves to change into something called an oocyte,
or I prefer the term, egg, and this was a haploid cell. OK.
It's large, by necessity sessile, and it's large because it stores a
bunch of stuff, and the stuff that it stores is food,
to nourish the early embryo before the embryo can fend for itself,
or before it can be nourished by the mother. And it also contains
components that regulate early development.
So it's full of things like determinants. It's also full of
machinery that can make ribosomes and so on, and the size of the egg
varies from organism to organism. In frogs, for example, the egg is
about a millimeter in diameter, in humans the egg is about 50
micrometers in diameter, because the mother nourishes the egg
sooner than, or the embryo, sooner in humans than in frogs.
All right, so let's look at a couple of these diagrams that I gave you.
This is to remind you about meiosis and what meiosis is,
and I want to make a couple of points that are up here on the top,
some of which are more important than others. Spermatozoa,
and spermatogenesis, is featured by continuing mitosis throughout life
of the male. All the meiotic products become sperm,
but this continuing mitosis is what's really important.
The spermatogonium, the precursor of the spermatozoa,
persists throughout life, and so you get a lot of sperm produced.
So we can add to our list here, that you get many sperm. In humans
there are about five times ten to the seventh sperm per ejaculate.
If you go about five-fold lower than that, you're in the range of
infertility. We can talk about why that is, and so one can do
calculations, and it's been calculated that males can make up to
ten to the 13th sperm in a lifetime.
If you do the calculation, you will understand that not all of
those sperm are used, and some are them are resorbed,
so you can do that rather interesting calculation another time.
OK. Oogenesis, in contrast to the plentiful and
cheap sperm, and we'll come back to this point in a moment.
In females, mitosis of the oogonia, ends before birth, and in fact, by
the time a child is born, it is a female, it has about a
million or so, primary oocytes,
which are still diploid, they're along this pathway towards
formation of the egg. And by the time the child is about
five, there are only about 500, 00 primary oocytes sites. OK?
And those cells have withdrawn from the cell cycle,
they are not making anymore of them. So that is your pool from which to
make eggs, and in fact, most of those, 90% of those,
are going to die, and you're going to get a greater than 90% are going
to die, and only about 500 eggs mature during the lifetime of a
human female. OK? And it's actually not quite clear
why, but it's certainly harder to make a good quality egg,
probably, than to make lots of smaller sperm,
for reasons of energy expenditure of the animal.
All right, let's look through these slides a little more.
The testes is an extraordinary organ that comprises many,
many, many tubules, all coiled together. Within the seminiferous
tubules there is an array of sperm production that is architecturally
very interesting. Ignore this red up here for the
moment, and look at this architecture of the
wall of one of the The developing spermatozoa are
embedded in cells called Sertoli cells. Sertoli cells supports,
and nourish, and induce the formation of the mature spermatozoa.
So the spermatozoa that are most immature, are lying away from the
luminal, the cavity, of the seminiferous tubule,
and as these cells mature and undergo meiosis,
they move closer and closer to the lumen, until you get a bunch of
spermatozoa, with these tails pointing out into the lumen of the
tube, ready to be shed upon ejaculation.
So it's a very beautifully, architecturally very beautiful
process. Now what does the spermatazoan actually look like?
It's got three parts. It's got this head that's got a nucleus that
is highly condensed. I've told you previously that it's
a real feat to pack the meter of DNA that's in every cell,
into a little, tiny nucleus. You have to really pack it tightly.
Well, it's even worse for a sperm because the nucleus of a sperm is an
order of magnitude smaller, in many cases, than a somatic cell.
So you have to pack the DNA even more tightly, and there are special
proteins that overcome the charges on the DNA, to overcome the charge
repulsion on the deoxyribonucleic acid, OK? So these are basic
proteins that really pack the DNA tightly, to fit into this compact
nucleus. The other thing we'll talk about, in a bit,
is this structure called the acrosome.
It derives from the Golgi, which you remember is involved in
protein trafficking. Two other regions of the sperm that
are almost universal, and very important, are this thing
called the mid-piece, which is packed with mitochondria
that are going to provide energy for the sperm as it moves.
And finally, this tail region, which is a flagellum, now what's a
flagellum? So, a flagellum is one of these
biologically fantastic structures that uses polymers of proteins to
make something that works in a concerted fashion.
The proteins that it uses makes up the microtubules,
so they are the tubulin proteins, we'll have more to say about them on
Friday. We mentioned microtubules in the last lecture,
in this case, microtubules pack together in a very ordered way,
you can read this in your book. Microtubules pack together in a very
ordered way, to make this very long structure, that when supplied with
energy, will beat in a regular, often a sinusoidal fashion, and
propel the cell forward, or wherever it is going. All right,
female reproductive system. The ovary is the sight of egg
production. In mammals, the uterus is the site of embryo
growth, fertilization usually occurs somewhere between the ovary and the
uterus, in the tube, the oviduct, or the fallopian tube,
between the ovary and the uterus.
All right, this is a diagram from your book again.
In the developing, in the ovary, one can find eggs of
developing, or oocytes sites of different stages,
as they undergo development. They grow larger, and lie in a
fluid-filled cavity, and eventually, I'll talk a bit more
about this in a moment, this fluid filled cavity gets very
large, and it bursts, and releases the egg.
It's very interesting, in mammals, the egg is actually
released almost into the body cavity, and it's gathered up by so called
fimbrae, or folds of waving tissue that lie at the opening of the
oviduct, or fallopian tube, and these kind of make a current
that draws the released egg into the oviduct, so it can get on its way to
the uterus. Subsequent to the egg being
released, what's left of where the egg developed in the ovary,
forms this thing called the corpus luteum, which secretes substances,
hormones, that I'll talk about in a moment, that are very important for
maintaining pregnancy, if this occurs.
All right, you can see what I'm dwelling on here,
not dwelling on. I'll dwell on things that I feel are more
important, by writing things on the board, OK? So,
stay with me. Let's go now to a system that I do want to dwell on
more, which is the hormonal control of gametogenesis.
Professor Jacks talked to you previously about hormones.
Hormones are made in most animals,
and their characteristic is that they work at very long range.
So I want to characterize these as long-ranged, inducers,
and they can be very long-range inducers. The hormones that control
gametogenesis are made in the brain, in the hypothalamus, and the
pituitary and they travel way through the bloodstream to get to
the ovaries and the testes. This looks like a hermaphrodite here,
[LAUGHTER], OK and this, I just realized this, OK,
whatever. To get to the ovaries or the testes, or both,
whatever, {LAUGHTER], OK, anyway, that point I want to
make is that these hormones work at a very long distance.
And I want to contrast that with the kinds of inducers that wee have
been talking about in the preceding couple of lectures.
The kind of classical, embryonic inducers, which act at
very short range. So, for example, we talked about
nodal signaling --
-- last lecture. For example, nodal,
which is a ligand I mentioned, pivotal in making the mesoderm,
nodal can only work over one, or a few, cell diameters.
So if you were cells, you could have a conversation with
one another, or maybe, the signal could get all the way to
here, but it sure wouldn't get to the back of the room.
Whereas, if you were a hormone, your signal would get all the way
back to the back of the room, and probably all the way down the
infinite corridor. OK, so there's a real order of
magnitude different in the way hormones and other inducers can act.
So, one to a few cells. All right, there are a bunch of
different kinds of hormones that are involved in control of gametogenesis.
I want to distinguish the steroid hormones, which are derivatives of
cholesterol, and we mentioned them long ago in the lipid section.
Steroids that include testosterone, estrogen, and progesterone, and act
through a series of receptors that actually lies within the cell,
they're not membrane-bound receptors, they are so-called nuclear receptors,
so they act through nuclear receptors.
I'm actually not going to write that on the board. And the other
hormones I want to indicate are peptide hormones,
that as their name indicates, are small polypeptide chains, and
these include, I'm going to put abbreviations up,
and you can get the full names later. FSH, LH, and GNRH,
follicle-stimulating hormone, luteinizing hormone, and
gonadotropin-releasing hormone. One of the things that is similar
about the hormonal control of gametogenesis,
to many other things we've been talking about,
is the notion of feedback control. So, there is an enormous amount of
feedback, both positive and negative in the circuitry that gives rise to
the formation of egg and sperm. This is an example of the circuitry
leading to stomatogenesis. Here is GNRH, released from the
hypothalamus that stimulates the anterior pituitary,
to produce lutenizing hormone, or follicle stimulating hormone.
These act on cells within the testes, which act to,
one set of cells acts to activate testosterone, the major male steroid
hormone, and that acts back on the Sertoli cells in the seminiferous
tubules, and together, these orchestrate the production of
spermatozoa, and other various responses to these hormones.
Now, the green indicates positive loops, and the red indicate negative
feedback, so too much testosterone can have the effect of inhibiting
the anterior pituitary's production of lutenizing hormone,
or follicle-stimulating hormone. And there's a very careful balance
of these hormones in the normal animal. Now, in females,
things look similar. Again, GNRG, LH, and FSH, are produced to
stimulate, release, and maturation of oocytes,
and release from the ovary. The ovary, instead of producing
testosterone, although it does produce some of that,
produces estrogen and progesterone, and those are both required for
normal maturation of the egg, and also for subsequent pregnancy.
And also, as you will see, there is a feedback loop of both estrogen and
progesterone, onto the hypothalamus and the anterior pituitary.
And as this feedback loop that is exploited in the birth control pill,
where if one gives high levels of estrogen and progesterone,
you suppress the production of both GNRH from the hypothalamus,
and LH and FSH from the anterior pituitary and therefore suppress the
production of eggs from the ovary. Now actually looking, it's
interesting that there's a female birth control pill,
because really, there are certainly targets in the male for birth
control pills, but these have not yet been
exploited. But the notion is that you exploit
this feedback loop, and its well-enough understood that
you can. OK, so let me move on to talking about the process of
fertilization, per say.
The process of fertilization involves the egg and the sperm
coming together, and recognizing one another as being
from the same species. This is very important --
-- very important that you get
species-specific recognition, in order that you get correct
transmission of genetic material. So the first thing that we mention
is sperm-egg recognition --
-- and this has many levels. In some animals, the egg, or the
uterus, or the female sends out various chemo attractants to
attractants, to encourage the sperm to get to the egg.
Some of these have been isolated, and there are small peptides, for
example, sea urchins, which are the organism that have
been used to study fertilization. Most have very small peptides,
or the chemo attractants, in humans there is some data that there may be
chemo attractants released by the oviduct that causes the sperm to
swim in a directed fashion, but it's very interesting that they
can't be that powerful, those chemo attractants,
because one needs so many sperm to be released in order to fertilize
the egg. And it's very interesting that although there are five times
ten to the seventh, or more, spermatozoa in a human
ejaculate, if you actually look in the region of the oviduct,
where fertilization would take place, you only find a few hundred
sperm there. So most of them disappear on the way,
and so if there are chemo attractants, I say they're not very
good ones. OK, another process that is very
important in mammals, is something called capacitation.
The sperm, as I told you, have this capacity to be motile,
that they contain a lot of energy stores, or they contain mitochondria
that can convert energy to movement.
But they save that energy for a time when they think they might be in the
vicinity of an egg, and when sperm are exposed to fluid
from a mammalian oviduct, they undergo an increase in motility,
so capacitation leads to an increase in sperm motility.
This process is mediated by some kind of signal transduction
mechanism, that involves the second messenger, cyclic AMP that you
discussed with Professor Jacks, and this is very important.
If capacitation doesn't occur, fertilization doesn't occur either.
OK, so once the sperm are in the vicinity of the egg,
they then have the challenge of fusing with it.
This is an animation I showed you previously, of a sperm moving
towards an egg. Now this is a mammalian egg,
and it's surrounded by a bunch of stuff. It's surrounded by these
circly things, which are cells that released from
the ovary, along with the egg, and it's surrounded by a number of
so-called membranes. The most important one of which is
called the zona pellucida, and as the sperm goes towards the
egg, and approaches the egg, it has to burrow its way through a
bunch of membranes to get there, this animation doesn't finish
fertilization clearly. OK, so if you look at a mammalian
egg at fertilization, there is the egg per-say,
that is a cell surrounded by a plasma membrane,
and then there's this thick, yellow layer, that's called the zona
pellucida. Around that, is this layer of cells
called the cumulus, and the sperm have to borrow through
both the cumulus layer, and particularly the zona pellucida,
before they get to the plasma membrane, and before they can then
try to fuse with the plasma membrane, and enter the egg.
So, the second thing that we need to think about is contact,
and there's two steps to this. The first contact and they're
actually very interesting steps because there's a bunch of
reciprocal interaction, and reciprocal receptor ligand
interaction. The first interaction is between the sperm,
which provides a receptor, and the egg, and this layer called
the zona pellucida around the egg, and the zona provides the ligands
for the interaction. And it turns out that the zona
pellucida, although it's called a membrane very often,
is not a lipid by-layer, it's a glycol-protein layer,
OK? And the proteins in the zona are called zp1,
zp2, and zp3, and they form a very complex meshwork of
glycol-proteins. The sperm receptor in mammals
recognizes a protein called zp3 in the zona, and then subsequently,
there's a shift in carbohydrate arrangement of the zona proteins,
and it then recognizes something called zp2. Now,
this receptor-ligand interaction has a very interesting consequence.
It leads to activation of second messengers, it goes through a system
of proteins called G proteins, but what was most important is that
it leads to the release of calcium within the sperm.
And this calcium, let me move the calcium here,
so this leads to the release of calcium with the sperm,
and this causes something called the acrosome reaction.
Now the acrosome, as I said, derives from the Golgi, and it is
filled with Golgi-derived proteins, particularly proteases.
Upon the acrosome reaction, there is a massive exocytosis of the
acrosomal contents, to release proteases,
and these proteases digest the zona. All right, some diagrams. This is
a diagram you have on your handout. Here is the sperm moving through
the cumulus layer, getting through the zona,
at contact there is a reaction, such that the acrosome releases its
contents, and the next step can happen.
This is a movie of the acrosome reaction from the horseshoe crab,
limulus, it has an extraordinary acrosome. The acrosome in limulus
compromises acrosomal protein, the proteases, but a lot of
unpolymerized, and upon contact with the limulus
egg, the, let me stop this so you can watch it while I'm telling
you about it. The actin that is in the acrosome
polymerizes, this is a matter of seconds, and sends out this
incredibly long process, here it comes, look at this.
See this thing shooting out? That is a polymer of actin that is
coated with these acrosomal proteases. So this is the sperm
head, and so it's shooting out this process that is many,
many times longer than the sperm head, and it's allowing it to get
into the limulus egg. Mammalian sperm do not have such a
spectacular acrosome reaction, but they do have one that serves the
same purpose. OK, all right, so the next thing that is
going to happen is that there is a second reciprocal interaction,
and this time, the egg is going to provide the receptor,
and the sperm is going to provide the ligand.
So, we now have the sperm, whose acrosomal membrane that had
been the Golgi membrane, is now exposed, and is covered with
ligands, so the acrosome, the acrosome membrane, is covered
with ligand. This interacts with receptors on the
egg plasma membrane, and there is, as a result of this
receptor-ligand interaction, the activation of a
signal-transduction system, and the signal transduction system
does two things, so there is this interaction leads
within the egg, to activation of signal transduction,
that has the effect, eventually, of releasing calcium
with the egg. And this calcium released within the egg
does two things. The first thing that it does is to
prevent any additional sperm from entering the egg,
and I'll tell you about this in a second. So it blocks,
or activates the block, to polyspermy, and in particular it
activates the so-called slow block to polyspermy,
and it also activates, a little while later, the zygote,
to begin dividing, to enter the cell cycle, and to begin various other
processes required in the early embryo.
So cell cycle entry of the zygote. All right, so let's look at a movie
of the sperm entering the egg. OK, here is, I'll show you this
again. Here is the sperm, its tale is sticking out, there,
it's going right into the egg. The cell membrane reciprocates,
and buckles, and in some organisms, it engulfs the sperm as it's going
in, but the actual sense in most organisms, the membranes either fuse,
or somehow you get the whole sperm being pushed into the egg.
In most animals the sperm membrane doesn't actually enter the egg,
but in some animals it does, and in some cases it doesn't seem to matter.
OK, what is this block to polyspermy? This is something that
happens in the first 30 seconds after fertilization,
and it involves exocytosis of a set of granules called the cortical
granules, that contain enzymes, that do two things.
These enzymes firstly, when they are released,
break apart junctions between the outer membranes,
in this case, this is shown in frogs. The outer membrane in frogs is
called the vitelline envelope, its equivalent to the zona pellucida
in mammals. And before fertilization,
this outer envelope is tethered by proteins, to the plasma membrane,
so they're very closely opposed. At the release of the cortical
granules, these protein contacts are broken, and the fertilization
envelope lifts off the surface of the egg. A number of other things
happen, as well. In mammals the zona pellucida is
completely remodeled, so it no longer binds with the sperm,
the carbohydrates are stripped off the zona proteins so that they no
longer bind sperm. This is a movie of the block to
polyspermy, or the consequence of the block to polyspermy in the frog.
Here is the frog egg. At fertilization,
watch carefully, you can see there is a big halo that
arises above the frog egg, there it is, and this is a
consequence of cortical granule exocytosis, and the blocked
polyspermy. OK. OK. Before you go,
what I did not cover in lecture today, don't go for one second.