BIOL 303 Lecture Notes - Lecture 9: African Clawed Frog, Polarity In Embryogenesis, Blastula
14 May 2018
School
Department
Course
Professor
Announcements
• Soup for Science – Jan 15th-19th
• MSURJ Journal is looking for submissions
• MBSU is looking for note takers
Review from last class
• Origins and history of development theory
• Cell development and lineage
• Cell fate from the layers of the gastrula
• Cell proliferation and differentiation – Mosaic development (intrinsic factors) and
Regulative development (cell-cell interactions and extrinsic factors)
• Differentiation is not stimulated by Mosaic or Regulative development only, it is almost
always a combination of the two – they are not mutually exclusive but happen at the same
time
Today:
• Going over a typical life cycle
• Looking at difference approaches to developmental biology: The Anatomical approach
and the Experimental approach
A Typical Life Cycle
• Species of frog – Xenopus Laevis
• Frogs are not a good animal for screening
because they take a lot of time to go through
their life cycle BUT they have awesome
embryos so we like that about them
• The frog, when in the mood, will lay eggs
• They are fertilized outside of the body
• Cleavage occurs in the egg and it becomes a
blastula
• It goes through gastrulation (complicated,
covered later)
• It then becomes a neurula where it first forms
its nervous system – the groove becomes the
spinal cord
• Organogenesis – tissue divisions start to take
on more recognizable properties – There are
muscles along the spinal cord, gills are
formed, the digestive system is formed, etc.
• Metamorphosis occurs changing it from a
tadpole to a frog
• The entire cycle is shown in figure 1
Figure 1
find more resources at oneclass.com
find more resources at oneclass.com
Biol 303 Jan 11th
2
Fertilization
• Fusion of haploid (one copy of chromosomes) gametes to form a diploid zygote (two
copies of the chromosomes)
• Since this is a cycle, fertilization can be considered first or last
• An egg already has internal organization that is radial – it has two poles, the Animal pole
and Vegetal pole – see figure 2
• There is more mRNAs and proteins at
the Vegetal pole.
• The Animal pole also has mRNA and
proteins but they are different and
fewer.
• The cortex of the egg is the part of the
cytoplasm that is just inside the
membrane and the centrosome triggers
a reaction that causes the cortex to
rotate inside the egg – this defines the
next axis, the Dorsal Axis and Ventral
Axis of the egg.
Cleavage
• Many divisions of the cells – rapid increase in the numbers of cells
• The cell divisions are synchronous, each cell’s cycle is synchronized - As they keep
developing into smaller and smaller cells, they remain on the same clock
• The blastula is a ball of cells
• We end up with a blastocoel – a fluid filled cavity, then we enter the midblastula
transition and the cell divisions become asynchronized (not synchronized)
• There is a lot of gene expression involved in the cell cycle ex. Cyclins, that are translated,
used, broken down again
• There are internally loaded proteins and mRNA, mitochondria from the mother that make
all the events up to the midblastula transition happen
• After the midblastula transition, the zygote starts creating and using its own proteins
• Midblastula Transition
➢ Cell divisions become asynchronous
➢ Decondensation of chromatin
➢ Zygote gene expression begins
Gastrulation
• Embryos have to do gastrulation or they won’t make it
• A massive, complex, highly coordinated movement of cells that establishes the three
primary germ layers – see figure 3
• We are looking at a frog embryo at the vegetal pole, it is already divided into the gastrula
stage, it is forming a section called the Dorsal Lip – which will extend itself starting
dorsally until there are cells flowing into the embryo from all sides – all of the cells from
the outside flow in and curl in on themselves
Figure 2
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Announcements: soup for science jan 15th-19th, msurj journal is looking for submissions, mbsu is looking for note takers. Review from last class: origins and history of development theory, cell development and lineage, cell fate from the layers of the gastrula, cell proliferation and differentiation mosaic development (intrinsic factors) and. Regulative development (cell-cell interactions and extrinsic factors: differentiation is not stimulated by mosaic or regulative development only, it is almost always a combination of the two they are not mutually exclusive but happen at the same time. Today: going over a typical life cycle, looking at difference approaches to developmental biology: the anatomical approach and the experimental approach. Organogenesis: this is when the embryo shifts from intrinsic factors to signalling to each other (extrinsic, germ layers are established, neurulation: the induction and formation of the neural tube. Gametogenesis: meiosis occurs reduction of dna from diploid to haploid to form haploid gametes, they then fuse with other gametes to become zygotes.
