Frog Development Quiz: Fertilization and Embryo

Cell Differentiation and Development

• Cell differentiation is the process where cells become specialized, and this involves many cellular processes
• The fate of cells during development can be controlled by their position within the blastula
• The location of cells in the blastula can determine the final cellular fate in the adult frog

Somatic Cells, Gametes and Zygotes

• Somatic cells have a complete set of chromosomes whilst gametes have half the number
• Somatic cells (e.g. body cells) have the same DNA in a defined number of chromosomes, called the karyotype
• The karyotype varies between species

Model Organisms

• Models organisms are used in labs to study embryonic development as humans cannot be used for practical reasons
• Model organisms must be able to produce a large quantity of eggs and embryos easily
• They should also be easy to work with, be easily transparent, and be large and develop outside the body.

C. Elegans

• This organism is a transparent nematode
• It is around 1mm in length and has a lifespan of 2 to 3 weeks
• The generation time is 3 to 4 days
• Adult hermaphrodites have 959 somatic cells
• Males have 1031 cells

Fruit Fly - Drosophila melanogaster

• This organism is easy to work with in the lab
• The life cycle is 5 days
• Embryos and larvae are also easy to work with

Zebrafish - Danio rerio

• Zebrafish are easy to work with in the lab
• Their eggs and embryos are easy to work with
• Rapid development is another key feature

Xenopus laevis - African Clawed Frog

• Xenopus frogs are easy to handle and keep in the lab
• They produce eggs all year round
• Eggs are laid in water, and the embryo is easy to observe
• Another key characteristic is quick development

Asymmetry of Oocyte

• Frog eggs are radially asymmetrical
• A pigmented top half called the animal pole and a white bottom half called the vegetal pole
• The white side has less cytoplasm and more yolk
• Frogs have many planes of asymmetry (e.g. left/right, anterior/posterior, dorsal/ventral).

Generation of Polarity

• The dorsal/ventral axis is established at the point of fertilization
• Single sperm enters the egg to cause fertilization
• The opposite point to where the sperm enters becomes the dorsal (back)side of the organism
• The sperm can bind anywhere on the oocyte surface
• It requires binding to receptors and the density of the receptors is highest just above the equator.

Grey Crescent

• Sperm enters the oocyte and migrates to the oocyte pronucleus which is in the middle
• Egg cortex - a pigmented layer - rotates around 30 degrees
• Region around gray crescent is destined to become the dorsal surface

Development of the Embryo

• From single cell to blastula to gastrula
• The process is controlled by rearrangements of cells and their differentiation

Blastula to Gastrula

• Gastrulation is when cells rearrange and differentiate to form tissue layers
• Different types of cells form from the different layers during development

Role of mRNA Localization

• VegT mRNA is localised after fertilisation and produces VegT protein
• VegT protein acts as a transcription regulator
• It's located in the vegetal pole of the egg
• VegT protein activates genes that code for mesoderm and endoderm

Fate (Determination of Cell Fate)

• This is when the cell fate is fixed
• It becomes a particular part of the organism
• A cell type (like muscle) is pre-programmed to become another particular cell type (e.g. muscle) because of its position within the blastula

Becoming Different (Experiment for Cell Determination)

• Injecting a particular blastomere (cell in the blastula) with vital dye to determine cell fate
• Observing stained cells in fully developed animal; comparing those cells with their initial location in the blastula allows one to create a fate map

Fate Experiment 1

• Select blastomere, inject marker
• Allow the animal to develop
• Identify location of stained cell - same blastomere, same final tissue

Fate Experiment 2

• Select same blastomere as in experiment 1
• Inject marker
• Move cell in the blastula
• Identify location of stained cell in adult animal

Fate Experiment 2 - Result

• Stain is not in the same tissue as experiment 1
• Stain is located in different tissue
• Results differ from experiment 1

Fate Conclusion

• The final fate is determined by the position in the blastula

Blastula Fate Map

• Position of cells in the blastula can be mapped to their final fate in the adult frog
• Cell fate depends on position

Development Robustness

• Cells are 'blank canvases' until later in development
• Early location doesn't impact cell fate later
• Cell will be influenced by neighbours

Human vs Xenopus Development Comparison

• Comparing human and Xenopus development, it shows similar early characteristics, but humans have a more complicated and multilayered development process
• Humans also have many more steps and earlier breaking cells out of constrained areas
• This is important for understanding cell development

Developmental Defects

• Miss expression of VegT can lead to several defects, and this control process is critical to proper development
• These defects include suppressed head formation, mislocalised mesoderm position and growth abnormalities
• In humans, these types of problems can lead to embryonic lethality and abnormal limb growth.

Fate

• Moved cells come under the influence of new neighbouring cells.
• This is called induction of tissue-determining transcription factors.
• Cells have varying potentials to become certain cell types.
• Totipotent - can become all cell types (e.g. early embryonic stem cells)
• Pluripotent - can become most cell types (e.g. later embryonic stem cells)
• Multipotent - can become several cell types (e.g. fibroblasts)

Fixed Fate

• Later in development the fate becomes fixed.
• The potential becomes reduced and cell becomes terminally differentiated.

Differentiation

• All cells express housekeeping proteins, e.g. ATP synthase.
• Different cells have specific proteins (e.g. keratin in epithelial cells , haemoglobin in red blood cells or collagen in skin fibroblasts)

Methods to Reprogram Adult Cells

• The process of differentiation can be reversed by introduction of genes into cells.
• These genes can transform one differentiated cell type into another, like fibroblasts into iPS cells
• The process can be achieved using pluripotent stem cells like in Dolly the sheep

Weismann's Central Dogma

• The germ-line only inherits information (G).
• However, the somatic cells (S) can impact the germ-line through methylation of germ cell DNA ('epigenetics')

Summary

• Oocytes have cellular polarity
• Development and cell fate is determined by position in the blastula
• Cells differentiate terminally but can be reprogrammed

Suggested Reading

• Molecular Biology of the Cell by Alberts, 7th edition
• Chapters 7, 21 and 22

Test your knowledge on the early developmental stages of frogs, including fertilization, axis establishment, and the significance of various structures in the embryo. This quiz covers key concepts such as mRNA localization, the grey crescent, and oocyte asymmetry essential for understanding amphibian development.