Gastrulation in Frogs
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- Gastrulation is the process in which the blastula rearranges itself into a three-layered gastrula.
- This is an important stage in embryonic development in which a single layer of the cell is transformed into three layers i.e., ectoderms, mesoderm, and endoderm.
- Sea urchins, frogs, and chicks are mostly studied by comparative embryologists and developmental biologists.
- In this article, we will see the process of gastrulation in frogs.
Gastrulation in Frog embryo
In frogs, animals and vegetal poles determine body axes. There are three types of movements during gastrulation in frogs. These movements create two layers i.e., outer ectoderm and inner endoderm.
Invagination
This is the apical constriction initiated by the pigmented depression at the future dorsal side in the frog embryo. This is the dorsal lip of the blastopore. The blastopore cells elongate to change their shape and become flask-shaped.
Involution
As invagination proceeds, there is the involution of marginal zone cells. The cells at the animal pole undergo epiboly and converge blastopore. The endodermal cells are first to invaginate and form archenteron.
Formation of Archenteron
The invaginating endodermal cells move to the interior. This is followed by other cells. The involuting cells gradually cover blastocoel. Blastocoel is reduced and the new cavity is formed by involuting cells, this is called Archenteron. It is in a cylindrical form which is much longer on the dorsal than the ventral side.
These cells, later on, end up being the pharyngeal cells of the foregut. As these very first cells enter the interior of the embryo, the dorsal blastopore lip ends up being made up of cells that involute into the embryo to become the prechordal plate (the precursor of the head mesoderm).
The next cells involuting into the embryo through the dorsal blastopore lip are called the chordamesoderm cells. These cells will form the notochord, a transient mesodermal “backbone” that plays an important role in differentiating and pattern the nervous system.
Epiboly
Expansion of ectoderm is by epiboly. Blastocoel is displaced to the opposite side of the dorsal blastopore lip by new cells entering the embryo. The widening blastopore finally develops the ventral lip over which mesodermal and ectodermal precursors cells pass. Due to the formation of the ventral lip, there is a ring formed around endodermal cells. This is exposed to the surface.
Yolk Plug
The blastopore is gradually covered by specific endoderm cells. The closing cells of the blastopore constitute the yolk plug. The remaining patch of the endoderm cells is called the yolk plug, and it also ultimately internalized.
At this point, all the endodermal precursors have been brought into the interior of the embryo, the ectoderm has actually surrounded the surface and the mesoderm has been brought between them.
Positioning the blastopore
The vegetal cells are crucial in identifying the place of the blastopore, as is the point of sperm entry. The microtubules of the sperm direct cytoplasmic movements that empower the vegetal cells opposite the point of sperm entry to cause the blastopore in the mesoderm above them. This area of cells opposite the point of sperm entry will form the blastopore and become the dorsal portion of the body.
MCQs about Gastrulation in Frogs
- 1. What is gastrulation?
- A) Process of forming blastula
- B) Transformation of blastula into a three-layered gastrula
- C) Development of the notochord
- D) Formation of the neural tube
- Answer: B) Transformation of blastula into a three-layered gastrula
- 2. Which germ layers are formed during gastrulation in frogs?
- A) Ectoderm and Mesoderm
- B) Ectoderm, Mesoderm, and Endoderm
- C) Mesoderm and Endoderm
- D) Endoderm and Ectoderm
- Answer: B) Ectoderm, Mesoderm, and Endoderm
- 3. What initiates invagination during gastrulation in frog embryos?
- A) Pigmented depression at the future ventral side
- B) Pigmented depression at the future dorsal side
- C) Blastocoel
- D) Archenteron
- Answer: B) Pigmented depression at the future dorsal side
- 4. Which movement involves the involution of marginal zone cells during gastrulation in frogs?
- A) Invagination
- B) Ingression
- C) Epiboly
- D) Involution
- Answer: D) Involution
- 5. What is formed by the invaginating endodermal cells during gastrulation in frogs?
- A) Blastocoel
- B) Archenteron
- C) Blastopore
- D) Ectoderm
- Answer: B) Archenteron
- 6. Which structure do the first involuting cells become in frog embryos during gastrulation?
- A) Pharyngeal cells
- B) Notochord
- C) Prechordal plate
- D) Neural tube
- Answer: C) Prechordal plate
- 7. What is the process of expansion of ectoderm during gastrulation in frogs called?
- A) Invagination
- B) Ingression
- C) Epiboly
- D) Delamination
- Answer: C) Epiboly
- 8. What constitutes the yolk plug during gastrulation in frog embryos?
- A) Invaginating endodermal cells
- B) The closing cells of the blastopore
- C) Pigmented depression
- D) Mesoderm
- Answer: B) The closing cells of the blastopore
- 9. What is the significance of vegetal cells in positioning the blastopore during frog gastrulation?
- A) They form the ectoderm layer
- B) They initiate invagination
- C) They determine the site of the blastopore
- D) They develop into the notochord
- Answer: C) They determine the site of the blastopore
- 10. Which type of movement involves the expansion of ectoderm and displacement of blastocoel during frog gastrulation?
- A) Invagination
- B) Epiboly
- C) Ingression
- D) Convergent extension
- Answer: B) Epiboly
- 11. What is the primary determinant of body axes during frog gastrulation?
- A) Mesoderm
- B) Ectoderm
- C) Vegetal and animal poles
- D) Blastocoel
- Answer: C) Vegetal and animal poles
- 12. Which cell type forms the pharyngeal cells of the foregut during frog gastrulation?
- A) Ectodermal cells
- B) Invaginating endodermal cells
- C) Chordamesoderm cells
- D) Pigmented cells
- Answer: B) Invaginating endodermal cells
- 13. What is the end result of epiboly during frog gastrulation?
- A) Formation of archenteron
- B) Displacement of blastocoel
- C) Formation of the neural tube
- D) Closure of the blastopore
- Answer: B) Displacement of blastocoel
- 14. During frog gastrulation, which part of the embryo ends up becoming the dorsal portion of the body?
- A) Blastocoel
- B) Vegetal cells
- C) Pigmented depression
- D) Point of sperm entry
- Answer: D) Point of sperm entry
- 15. What is the primary function of invagination during frog gastrulation?
- A) Formation of archenteron
- B) Development of the neural tube
- C) Closure of the blastopore
- D) Formation of the pharyngeal cells
- Answer: A) Formation of archenteron
- 16. What term is used for the process where endodermal cells move to the interior during frog gastrulation?
- A) Invagination
- B) Epiboly
- C) Ingression
- D) Involution
- Answer: C) Ingression
- 17. What is the function of the dorsal blastopore lip during frog gastrulation?
- A) Formation of the neural tube
- B) Formation of archenteron
- C) Development of the notochord
- D) Closure of the blastopore
- Answer: C) Development of the notochord
- 18. What term describes the process of endodermal cells converging to form the archenteron during frog gastrulation?
- A) Epiboly
- B) Invagination
- C) Involution
- D) Ingression
- Answer: C) Involution
- 19. Which cell type forms the precursor of the head mesoderm during frog gastrulation?
- A) Invaginating endodermal cells
- B) Prechordal plate cells
- C) Chordamesoderm cells
- D) Neural tube cells
- Answer: B) Prechordal plate cells
- 20. What is the outcome of involution during frog gastrulation?
- A) Formation of archenteron
- B) Development of the pharyngeal cells
- C) Closure of the blastopore
- D) Formation of the notochord
- Answer: D) Formation of the notochord
- 21. What is the function of the yolk plug during frog gastrulation?
- A) Formation of archenteron
- B) Closure of the blastopore
- C) Protection of the embryo
- D) Development of the notochord
- Answer: B) Closure of the blastopore
- 22. What term describes the process of cells elongating and changing their shape during frog gastrulation?
- A) Epiboly
- B) Invagination
- C) Convergent extension
- D) Involution
- Answer: C) Convergent extension
- 23. What determines the site of the blastopore during frog gastrulation?
- A) Vegetal cells
- B) Pigmented depression
- C) Mesoderm
- D) Neural tube
- Answer: A) Vegetal cells
- 24. What structure is formed by the invaginating endodermal cells during frog gastrulation?
- A) Blastocoel
- B) Archenteron
- C) Blastopore
- D) Ectoderm
- Answer: B) Archenteron
- 25. What is the primary role of epiboly during frog gastrulation?
- A) Formation of archenteron
- B) Displacement of blastocoel
- C) Development of the neural tube
- D) Closure of the blastopore
- Answer: B) Displacement of blastocoel
Summary:
Gastrulation in frogs is a critical phase of embryonic development, where the blastula transforms into a three-layered gastrula comprising ectoderm, mesoderm, and endoderm. This process involves intricate movements and cellular rearrangements.
In frog embryos, gastrulation begins with invagination, where cells at the future dorsal side undergo apical constriction, forming the blastopore’s dorsal lip. Subsequently, involution occurs, involving the inward movement of marginal zone cells and the initial invagination of endodermal cells to form the archenteron.
Formation of the archenteron progresses as endodermal cells move interiorly, covering the blastocoel and giving rise to a cylindrical structure longer on the dorsal side. These cells eventually differentiate into pharyngeal cells and contribute to the formation of the prechordal plate and chordamesoderm.
Epiboly expands the ectoderm, displacing the blastocoel and widening the blastopore. The closure of the blastopore involves the gradual covering by specific endodermal cells, forming the yolk plug, which is ultimately internalized.
Positioning the blastopore is crucial, determined by vegetal cells and the point of sperm entry. Microtubules direct cytoplasmic movements, leading to the formation of the blastopore in the mesoderm opposite the point of sperm entry, shaping the dorsal portion of the body.
Overall, gastrulation in frogs involves coordinated cellular movements and transformations, laying the foundation for the subsequent stages of embryonic development.