Exam IV Study – Lecture 19
This notes set focused on developmental biology, particularly of vertebrates. The
entire developmental process is carried out by ligands, which bind to receptors on a
cell to trigger transduction. It’s important to note that the ligand doesn’t pass into
the cell; they are entirely extracellular signals. Also remember that membrane
bound receptors are not transducers, that happens inside the cell. Once bound to the
receptor, the ligand will bind to another ligand/receptor complex and dimerize,
triggering a secondary messenger cascade. The three ligands/receptors we HAVE to
know for the exam are as follows:
1. FGF/tyrosine kinase: limb growth
2. TGFβ/serine threonine kinase: epidermal specification
3. Hedgehog/patched receptor: ventral spinal cord
Possibly the most important of the above ligands is hedgehog (aka shh). It’s
responsible for the induction of the ventral spinal cord via the notochord. The
notochord secretes shh, causing the differentiation of certain structures to occur.
Cells closest to the notochord, thus in contact with the highest concentration of shh,
will develop into the floor plate. Membrane development will also occur here. Areas
with low shh will develop into ventral neurons. I’ll get into this more later.
Sexual development is determined by signal pathways, particularly testosterone.
Before sexual development there are two ducts present: the wollfian duct and the
mullerian duct. In the presence of testosterone the mullerian duct will degenerate,
while the wollfian duct will develop into male structures such as the vas deferens
and testes. However without exposure to testosterone, the wollfian duct will
degenerate while the mullerian duct will remain intact, eventually developing into
female structure such as the ovaries.
Another major topic discussed is oogenesis. This process begins with oogonium,
which as somatic cells destined to become eggs. First the oogonium undergoes
massive growth to become a primary oocyte. At this stage yolk, which is generate in
the liver, is imported into the primary oocyte. Know that the vitellogenin and VLDL
are associated with this, and that they are taken up by the LR8 receptor. It then
undergoes its first meiotic division to form a secondary oocyte. This splits the cell
into an egg and first polar body. It’s at this stage where the cell enters
developmental arrest until it’s ready for maturation. These secondary oocytes are
stored in the ovaries for up to 50 years. Once ready to mature, the egg will undergo
its second meiotic division to become a haploid ootid, and may undergo further
maturation to become a completely mature egg.
We need to know the process of chicken egg development for the exam (ugh).
Beginning in the ovary, a structure called the infundibulum acts as the site of
fertilization. The now fertilized egg, also called a blastodisc at this stage, travels
down the oviduct. The first half of the oviduct is called the magnum, which is where
egg white proteins such as albumen are added. The second half is called the isthmus, which is where a shell membrane is added. Once through the oviduct the egg passes
to the uterus, where a calcium carbonate shell is added over the initial shell was
Sperm development is another important topic discussed in class. The sperm cell,
called a spermatogonium, undergoes meiosis to become a spermatozoon, which is a
mature sperm. The spermatozoon contains several structures, including a sac of
enzymes called the acrosome, two centrioles, and protamine