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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.

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Immunogenicity vs Antigenicity – What is the Difference??

Immunogenicity vs Antigenicity – What is the Difference??

In this post we will see the difference between immunogenicity and antigenicity. These two terms are related to antigen which can be confusing sometimes. In the end of the post as always, I will share my fun trick that will help you to remember the difference in super easy way. So let’s get started 🙂

Before we start with the definitions of these terms, one thing we need to understand clearly is whether it is immunogenicity or antigenicity both are properties of an antigen.

  • Immunogenicity – is the ability to induce humoral and /or cell mediated immune response. We know when B and T cells get activated by antigen, it results in effector cells and memory cells. For example, if B cell gets activated, it will differentiate into plasma B cell which secrets antibodies.

B cell + antigen = Plasma B cells (secrets antibody) + memory B cells

Such antigens are called immunogen more appropriately. These immunogens are large molecules with molecular weight at least >10,000 D and that is why they can induce the immune response.

  • Antigenicity – is the ability to combine with the final products of the humoral and/or cell mediated immune response. It is not activating the immune response rather it combines with the final products of the immune response. For example, as we just saw when B cells get activated it secrets antibodies and the antigens which have the property of antigenicity will combine to these antibodies.

Such antigens are called haptens. Haptens are small molecules with very low molecular weight and that is why they are not capable of inducing the immune response. An example of such hapten is drugs like penicillin.

Now, here is more very important point. Haptens can be immunogenic if they bind with carrier proteins because once they bind with carrier protein, it increases the molecular weight (becomes larger molecule) and thus it can induce the immune response.

Haptens + Carrier protein = Immunogen

One sentence you might come across every time you read about immunogenicity and antigenicity is : All immunogens are antigenic but all antigens are not immunogenic. This means all immunogens are antigenic meaning it can react to the end product of immune response. It makes sense right that if immunogen can induce the immune response it can also bind with the final product of immune response. But all the antigens cannot be immunogenic because we saw only high molecular weight antigen can induce the immune response right, so they are antigenic but not immunogenic.

Now it is time to share my fun trick with you guys to remember this concept easily 🙂

Imagine you want to eat pasta. You convince mom to make pasta and she makes it for you. So here, you are immunogenic because you convinced (inducing) mom to make pasta. Mom here is immune system who gives you the final product that is pasta. Now when you enjoying the yummy pasta, your brother walks in and he also eats the pasta. So brother is antigenic because he didn’t convince mom to make pasta, he is just eating the final product (like haptens only binding with final product). Now since you convinced mom to make is and you are also eating the pasta, you are both immunogenic as well antigenic but brother is only antigenic because he is just eating it and thus we say “All immunogens are antigenic but all antigens are not immunogenic”.

I hope this was helpful 🙂

Watch a video on this topic here.