Holoblastic Cleavage vs Meroblastic Cleavage -What is the Difference?

Holoblastic Cleavage vs Meroblastic Cleavage -What is the Difference?

When we are studying embryology, it becomes confusing sometimes if we don’t clearly remember what holoblastic and meroblastic means! So in this post we will see holoblastic and meroblastic cleavage and different type of eggs which come under these two categories. And at the end as always I will give you my trick which helps me remember this concept in super easy way. So let us begin 🙂

We know that when sperm fertilizes egg, the resultant zygote undergoes series of mitotic divisions. This division or cleavage results in 2 cells followed by 4 cells, 8 cells and so on and this embryonic cleavage can be either holoblastic or meroblastic which is specific for different species.

Holoblastic cleavage – is a complete cleavage meaning it completely penetrates the egg.

Meroblastic cleavage – is incomplete or partial cleavage meaning it does not penetrate the egg completely.

Now one question comes in mind is, why some egg follows holoblastic and some follows meroblastic cleavage? The answer is yolk content of the egg. Because yolk is bulky, it is difficult for the cleavage to take place. So eggs which contain high amount of yolk follows meroblastic cleavage (incomplete) and those which contain very low amount of yolk follow holoblastic cleavage (complete).

Based on yolk distribution in eggs, they are divided into 5 different categories. They are as follows 🙂

Holoblastic cleavage : (eggs contains low amount of yolk)

• 1. Alecithal – yolk content is almost negligible or very very low.
  2. Isolechithal – yolk content is very less and it is evenly distributed. (remember Iso = even)
  3. Mesolecithal – yolk content is moderate and it is unevenly distributed. (remember M = Moderate)

Meroblastic cleavage : (eggs contain high amount of yolk)

1. Telolecithal – high amount of yolk present in the vegetal pole of the egg. (vegetal pole is a pole where the yolk content is concentrated)
2. Centrolecithal – high amount of yolk present in the center of the egg. (remember Centro = center)

This classification is easy to understand as we know that eggs containing high yolk will follow meroblastic cleavage and egg containing low amount of egg follows holoblastic cleavage but it becomes really confusing to remember which types comes under what. So it is time to share my trick now.

I correlate holoblastic with the word whole, it rhymes too kind of. Holo – whole, isn’t it! Try and say it out loud. So for me holo is whole meaning holoblastic is whole or complete cleavage and the remaining type is mero that would be incomplete cleavage. Now look at the classification. If you take first letters of names of eggs under holoblastic cleavage it becomes AIM and when we have aim to achieve something we put our COMPLETE effort isn’t it? So AIM is for complete or holoblastic cleavage. AIM stands for Alecithal, Isolechithal and Mesolechithal egg. The remaining two types are TC meaning Telolecithal and Centrolecithal and they follow meroblastic cleavage. This is how I remember it 🙂

I hope this was helpful 🙂

For more explanation on this topic, watch this video.