BIOC19: LECTURE 7
• Skeletal muscles a.k.a voluntary muscles
• Myofilaments are composed of muscle specific proteins: actin and myosin.
• Pink diagram shows the development of the neural tube. The ectoderm folds-in, forms the
neural groove, and then the neural tube pinches out. This diagram shows the cross-section
of the neural tube.
• Black diagram shows the top view of the neural tube. The early neural groove is not
zipped and the neural tube on its right is zipped up or closing neural tube in its late phase.
Its starts to zip at the brain and then to the posterior end of the embryo. Neural tube
zippers up before the development of muscles.
• The diagram on the lower left also shows a cross-section of a neural tube. In that
diagram, neural tube is pinched. Right under the neural tube is notochord and the red part
is mesoderm, which eventually develops into different regions. The red mesoderm cells
proliferate and surround the embryo. There is also endoderm in the middle and epidermis
on the outside.
• Diagram with labels show locations or action sites of mesoderm regions. Note: the
missing label is "Axial Mesoderm"
• Somites form into muscles
• The 3 main regions are found on both sides of the central neural tube (refer to the
• Upper right diagram shows a microscopic picture. It shows the neural tube in the middle
and sausage-shaped paraxial mesoderm on both its side
• Paraxial mesoderm starts diving into somites (the bumps in the upper right picture),
starting from head and moving down to the tail sequentially
• Different species of animals has different number of somites
• All vertebrates have division of paraxial mesoderm
• By counting the number of somites, you can tell what stage of development a particular
animal is at because as development progresses, the number of somites increase
• In the first diagram, you see the pre-segmented stage. Pre-segmentation stage is before
the division of paraxial mesoderm into somites when the neural plate is still open, the
neural groove is still there, notochord is present, and there is unsegmented mesoderm
(particularly paraxial mesoderm) • Then, in the second diagram, you can see that the neural tube closes and pinches off. The
unsegmented mesoderm starts cutting off individual somites. As you can ssee, somite is a
mass of cells
• Then, in the third diagram, you can see that the somite differentiates into two
components. The part closer to notochord and the bottom of the neural tube differentiates
into sclerotome. The part of somite that is more closer to the surface or more dorsal
differentiates into dermamyotome.
• Dermamyotome subdivides into 2 further regions: dermatome and myotome. Dermatome
is closest to ectoderm and it forms the dermis layer of skin or the deep layer of skin. The
more internal, larger mass of dermamyotome subdivides into myotome which forms the
• This experiment proves that the cells in the somites form muscles of the limb/chicken
• Note: The blue thick double line on the far right of the diagram is Neural Tube
• Diagram on the far right shows the limb bud of the chicken that forms the wing
• Surgically remove chick somites and replace them with quail somites which have a
different pigmentation than chick somites. Then look at what develops. Result: a wing
develops and we can see that the wing's cells have striated muscle which all have the
pigmentation of quail somite. So, skeletal muscles of the limb originate from somites
because as shown in the experiment, limbs of the chicken were derived from cells that
originated from the quail somites
• Myogenesis is the development of skeletal muscle tissue. It can be divided into 3 stages
• Commitment/Determination is when pluripotent commit to the myogenic pathway
because otherwise pluripotent cells can differentiate into many other things
• Differentiation steps involves turning on genes encoding skeletal muscle protein genes
• Skeletal muscles are the only type of cellular body that can contract. Their ability to
contract is controlled and triggered by nerve action. That's why nerves must grow into the
developed muscle cells to form a neuro-muscular junction. The firing of the motor neuron
controls, through the neuro-muscular junction, when the muscle is going to contract.
• #1 sentence refers to the diagram on the far left. In fact, each sentence refers to a diagram
on the top
• Note: On the diagram that is on the far left, label the blue line 'Neural Tube' and
the red circle 'notochord'
• Shh is an inducing factor, given off by neural tube and notochord, which triggers step# 2
• In step #2, Shh transforms myotome cells to myoblasts, triggering commitment to the
myogenic pathway • In step #3, myoblasts undergo rapid cell division and when they reach the right number,
they begin to align and undergo cell fusion so lots of myoblasts end up in one cell, giving
the cells its contractile ability and multi-nucleated characteristic
• Muscle specific proteins include actin and myosin
• Final step is when muscle contracts in a organized manner
• Bottom picture is of muscle fiber cells. See how the many nuclei get pushed to the edge.
• Slide 10:
• There are very distinct series of steps in muscle differentiation, which can be recognized
at the morphological with a microscope. They are also recognizable the individual steps
at molecular level because of muscle specific proteins actin and myosin that form the
myofilaments (the contractile element). At the molecular level, you can see when genes
of actin and myosin are turned on and when those genes make the actin and myosin
• Certain features of muscle cell differentiation are self-stimulating and we can look at the
them in tissue culture without having to add external inducer AS LONG AS WE START
WITH MYOBLASTS. In the last slide, we mentioned that myotome cells are pluripotent
and Shh transforms them into myoblasts and triggers commitment to the myogenic
pathway. That is why we would not have to add an external factor/inducer if we start with
those committed myoblasts because we know that they are already committed/restricted
to the myogenic pathway.
• In this experiment, we're going to look at the differentiation of myoblasts in mature
muscle fiber cells.
• We start with embryonic thigh muscle that was isolated at a step when myoblasts were
• So, we take a piece of the embryonic thigh muscle, cut it into tiny pieces, and then treat
with an enzyme called trypsin. Trypsin digests the cell adhesion proteins that are holding
the thigh muscle cells together and thus, the thigh muscle tissue is dissociated into
individual myoblast cells.
• Then you remove trypsin through centrifugation and you are left with single celled
• You put these myoblasts on a petri dish (along with culture medium) and they undergo
muscle cell differentiation without any external inducer.
• In vitro mean tissue culture
• What happens, step by step, when you put the myoblasts on that petri dish? (refer to
experiment on the previous slide)
• With such a tissue culture, you can recognize the different steps of differentiation at the
molecular and morphological level
• 2) Cell division stops when the cells reach a certain number/density of myoblast cells and
through regulation, they stop dividing • 4) Cell fusion signals the change in the pattern of gene expression to turn on genes
encoding actin and myosin
• 5) Increase in muscle-specific proteins that organize themselves in myofilaments
• 6) Twitching is spontaneous and not in an organized way because motor neurons have not
yet grown in the muscles and neuro-muscular junction has not yet formed
• Multinucleated cells form in 6-12 hours which is too little time for many many rounds of
DNA replication for nuclear division
• No mitotic figures or condensed chromosomes were observed under the microscope
which we would expect to find during nuclear division
• Multinucleated cells formed even if you put in DNA synthesis inhibitors which prevent
• Can different types of muscle cells fuse? No. Myoblasts of different organs or origins do
not fuse together to make multinucleated cells
• Note: the slide should say 'Thigh muscle myoblasts + heart muscle myoblasts = no
• In this experiment, we're trying to see whether myoblasts, that are from the same tissue
but different species, fuse.
• Rat thigh muscle myoblasts labelled with radioactive triated thymidine + Rabbit thigh