Frog Development Quiz: Fertilization and Embryo

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Questions and Answers

What structure forms the dorsal (back) surface of the frog embryo during early development?

The pigmented layer of the egg

The grey crescent region (correct)

The oocyte cortex

The sperm entry point

What happens to the egg cortex during fertilization in frogs?

It disintegrates immediately upon sperm entry

It rotates 30° to create the grey crescent (correct)

It becomes completely transparent

It remains unchanged

How does the sperm determine the point of entry into the oocyte?

It binds specifically to the pigmented layer

It is directed by gradients of mRNA

It can enter anywhere, but density of receptors affects binding (correct)

It always enters at the equatorial region

Which embryonic stage follows the blastula in frog development?

Gastrula Signup and view all the answers

What is the role of mRNA localization in frog oocyte asymmetry?

It influences the establishment of the dorsal-ventral axis Signup and view all the answers

What is the significance of the single sperm entry point (SEP) in frog fertilization?

It helps determine the dorsal side of the embryo Signup and view all the answers

Which aspect of the frog embryo's development is indicated by the generation of the grey crescent?

Establishment of body plan polarity Signup and view all the answers

Why is the density of receptors significant in the fertilization process of frogs?

It increases the chances of successful fertilization Signup and view all the answers

What is the significance of the animal pole in frog eggs?

It is the pigmented top half that is more active. Signup and view all the answers

How is the dorsal/ventral axis established in frog development?

At the moment of fertilization. Signup and view all the answers

Which of the following describes the oocyte asymmetry in frogs?

The oocyte shows asymmetry with distinct animal and vegetal poles. Signup and view all the answers

What role do mRNA localization play in frog development?

They help establish polarity and developmental axes. Signup and view all the answers

What aspect of frog eggs facilitates the easy observation of embryo development?

Their rapid development and transparency. Signup and view all the answers

What characterizes the transition from blastula to gastrula in frog development?

Cell movements and changes in cell fate occur. Signup and view all the answers

Which of the following best describes the characteristics of the vegetal pole?

It is less active due to having more yolk. Signup and view all the answers

What is a defining characteristic of the blastula stage in frog development?

It has a single cell layer surrounding a yolk. Signup and view all the answers

What characterizes the arrangement of cells during the transition from blastula to gastrula?

Cells undergo rearrangement and differentiation to form tissue layers. Signup and view all the answers

Which layer of cells develops into structures such as cartilage, muscles, and bone in an embryo?

Mesoderm Signup and view all the answers

What is the role of VegT protein in embryonic development?

It serves as a transcription regulator. Signup and view all the answers

What does the term 'ectoderm' refer to in embryonic development?

The outermost layer, including epidermis and nerve tissue. Signup and view all the answers

Which process initiates the formation of distinct embryonic tissues from the blastula stage?

Gastrulation Signup and view all the answers

During the development of the frog embryo, what aspect does the localization of mRNA influence?

It affects the generation of embryonic asymmetry. Signup and view all the answers

What is the result of cell division in the blastula stage of embryo development?

Cells remain identical and unspecialized. Signup and view all the answers

Which axis is established during the development of the frog embryo as a result of asymmetry?

Dorsal-ventral axis Signup and view all the answers

Signup and view all the answers

Study Notes

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

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Description

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.