Cleavage pattern of centrolecithal eggs

Question:

Centrolecithal eggs show

1. superficial cleavage
2. displaced radial cleavage
3. bilateral cleavage
4. discoidal cleavage

Rephrasing the question:

What type of cleavage pattern do centrolecithal eggs show?

Answer:

Centrolecithal eggs show superficial cleavage (option 1).

Explanation:

This question refers to the pattern of embryonic cleavage.

There are three broad groups of cleavage – holoblastic (complete cleavage), meroblastic (incomplete cleavage), and transitional (somewhere in the middle).

1. In holoblastic cleavage, the cleavage furrows divide the cells completely. So every cell division gives rise to 2n cells. This type of cleavage is seen in eggs with little yolk. Two types of eggs undergo holoblastic cleavage:
   
   • Isolecithal: sparse, evenly-distributed yolk
     Their cleavage patterns can be of four types:
     1. Radial (echinoderms, amphioxus)
        Radial cleavage is characteristic of the deuterostomes, which include some vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte. This gives the appearance of layers of cells stacked up on each other.
     2. Spiral (annelids, molluscs, flatworms)
        The cells divide at slight angles to the polar axis, so that they do not line up.
     3. Bilateral (tunicates)
        The first cleavage results in bisection of the zygote into left and right halves. The following cleavage planes are centred on this axis and result in the two halves being mirror images of one another.
     4. Rotational (mammals, nematodes)
        Rotational cleavage involves a normal first division along the meridional axis, giving rise to two daughter cells. In the second division, however, one of the daughter cells divides meridionally, whilst the other divides equatorially.
        
   • Mesolecithal: moderate amount of yolk, unevenly distributed
     These eggs are found in amphibians, and they divide in a radial pattern
     
2. In meroblastic cleavage, cell division divides only a small portion of the cell, since the cleavage portion doesn’t go through the entire cell. The portion with more yolk (vegetal pole) remains uncleaved. Two types of eggs use meroblastic cleavage:
   
   • Telolecithal: dense yolk concentrated at one end
     Such cells can divide in two patterns:
     
     1. Bilateral (cephalopod molluscs)
        Similar to what we see in the holoblastic tunicates (see 1.a.iii). The difference is that here, the blastomeres stay partially connected.
        
     2. Discoidal (fish, reptiles, birds)
        The cleavage furrows do not penetrate the yolk. The embryo forms a disc of cells, called a blastodisc, on top of the yolk. The layer of cells that have incompletely divided and are in contact with the yolk is known as the syncytial layer.
        
   • Centrolecithal: yolk concentrated in the centre of the egg
     These are found in arthropods. Here we see superficial cleavage: mitosis occurs but not cytokinesis, resulting in a polynuclear cell. With the yolk positioned in the center of the egg cell, the nuclei migrate to the periphery of the egg, and the plasma membrane grows inward, partitioning the nuclei into individual cells. The end result is a shell of cells with the yolk in the center. This type of cleavage can work to promote synchronicity in developmental timing, such as in Drosophila.
     
3. In transitional cleavage, the cleavage is atypical and assumes a transition between holoblastic and meroblastic.

Sources:

https://www.chegg.com/homework-help/definitions/cleavage-patterns-and-morphogenesis-14

https://en.wikipedia.org/wiki/Cleavage_(embryo)

https://www.lkouniv.ac.in/site/writereaddata/siteContent/202004080636590834shailie_Cleavage_types_and_patterns.pdf