Biol 61 – Animal Development
For this section we’re going to be talking about a little material from Chapter 18 in addition to Chapter 47.
should read pages 366-369, and Figs. 18.15, 16 and 19 from Chapter 18 (this most relates to the lecture material).
For Chapter 47, you should read pages 1021-1033.
The first step in forming a vertebrate organism is fertilization, when a haploid (n) sperm fuses with a haploid (n) egg
to form a diploid (2n) zygote.
This zygote then divides to form the morula, blastula, gastrula, etc. of the developing
OK, so sperm fuses with egg, the two nuclei fuse, and that’s it, right? Nope – fertilization is actually a little
more complicated, in part because one of the worst things that can happen in fertilization is for multiple sperm to
fuse with the egg at the same time.
This could result in triploid (3n) or worse zygotic nuclei, which would disrupt
both cell division (the process of meiosis especially) and embryonic development (because animal cells don’t like to
be aneuploid, with weird chromosome numbers).
Your book shows what happens with a sea urchin egg, but it really isn’t much different for any vertebrate (see Fig.
An egg is a very large cell (usually the largest in the body) that contains a haploid nucleus.
It is covered by a
protective layer of proteins and sugars outside its plasma membrane, and this is the
A sperm is composed
of a long flagella for swimming attached to a compact head that contains a haploid nucleus, a bunch of actin
monomers, and an acrosomal vesicle (along with mitochondria to power its swimming).
membrane-bound vesicle that contains enzymes that can digest through the outer glycoprotein jelly coat of the egg.
The sperm head touches and binds to this jelly coat, and the contents of the acrosomal process begin digesting the
At the same time, the actin monomers that were carried in the sperm head begin to form polymers of
actin (basically, ‘microfilaments’ form as the single actin polypeptides bind together into a polymer that gets longer
This actin rod pushes the plasma membrane of the sperm out like a little nose (think about Pinocchio
lying and his nose extending, for those of you who saw any Disney movies as kids).
The sperm head is pretty much
stuck in place within the jelly coat, but this acrosomal process – this little nose that gets pushed out during actin
polymerization – extends the plasma membrane of the sperm until it reaches the plasma membrane of the egg.
sperm contains the correct proteins in its plasma membrane that fit, like a lock and key, with membrane proteins on
the egg surface, then the plasma membrane of the sperm fuses with the plasma membrane of the egg and the sperm
nucleus is sent inside.
The biggest danger to the egg cell at this point is