Centrolecithal eggs show
superficial cleavage
Centrolecithal Eggs and Cleavage
Embryonic development begins with fertilization, followed by a series of rapid cell divisions called cleavage. The pattern of cleavage is significantly influenced by the amount and distribution of yolk within the egg.
Centrolecithal eggs are characterized by having the yolk concentrated in the center of the egg cytoplasm. This central mass of yolk affects how the cell can divide.
Cleavage in Centrolecithal Eggs
In centrolecithal eggs, the cleavage is typically meroblastic, meaning that the cleavage furrow does not completely divide the entire egg. Because the yolk is in the center, the initial nuclear divisions occur within the central yolk mass without immediate cytoplasmic division.
The nuclei divide repeatedly, and these daughter nuclei then migrate outwards towards the periphery of the egg. Once they reach the surface layer of cytoplasm surrounding the central yolk, cell membranes form around them, creating a layer of cells (a blastoderm) at the surface.
This type of cleavage, where the cleavage is restricted to the peripheral layer of cytoplasm around a central yolk mass, is known as superficial cleavage.
Analyzing the Options
- superficial cleavage: This pattern is characteristic of centrolecithal eggs, where divisions occur at the periphery around the central yolk.
- displaced radial cleavage: Radial cleavage is typically seen in isolecithal or mesolecithal eggs. Displaced radial cleavage is a modification, but not associated with the central yolk distribution of centrolecithal eggs.
- bilateral cleavage: This pattern is found in certain animal groups and results in mirror-image halves. It's not the typical cleavage for centrolecithal eggs.
- discoidal cleavage: This occurs in telolecithal eggs (like bird or reptile eggs) where there is a large amount of yolk concentrated at one pole. Cleavage is limited to a small disc of cytoplasm (blastodisc) at the animal pole.
Therefore, the cleavage pattern observed in centrolecithal eggs is superficial cleavage.
Important Questions from Basic concepts of development
Following statements are made regarding animal development:
A. The cell is first specified towards a given fate, suggesting that it would develop into this cell type, even in a neutral environment.
B. Holoblastic rotational cleavage is observed in tunicates.
C. Infolding of sheet of cells is called ingression.
D. Conditional specification can be observed in sea urchin embryos.
Which one of the following options represents the combination of all correct statements?
C. elegans embryo uses both autonomous and conditional modes of specification. The following statements are about specification of cell lineages:
A. The signals from the P1 blastomere instructs the EMS cell for its further development.
B. In the absence of the instructive signal, the EMS cell will divide into two MS cells.
C. In the absence of the POP‐1 signal, the prospective MS cell can take up the E fate.
D. The MOM‐2 protein from the blastomere neighbouring the EMS cell instructs the dividing EMS blastomere to take up MS fate.
Which one of the following options represent both correct statements?
When 8-cell embryo of tunicates is separated into 4 blastomere pairs and allowed to grow independently in culture medium, then each blastomere pair can form most of the cell types; however, cells for nervous system are not developed. The following statements are formed from the above observations:
A. Nervous system development demonstrated autonomous specification.
B. The other tissue types are formed due to conditional specification.
C. All the tissue types, except nervous tissues that developed, demonstrated autonomous specification.
D. Nervous system development demon-strated conditional specification.
The correct combination of statements that explains the above result is:
The cells of inner cell mass of a blastocyst stage mammalian embryo are
Which one of the following statements about stem cells is correct?
