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Meroblastic Cleavage: Definition & Patterns

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

After an egg is fertilized it begins to rapidly divide through a process called cleavage. In this lesson you'll learn about the patterns specific to meroblastic, or incomplete cleavage.

Cleavage

Did you know that you and a snake have something in common? Believe it or not, you both came from an egg! And both you and the snake experienced something called cleavage. This is the rapid cell division that leads to a multicellular embryo. To 'cleave' means to slice or split something, so it makes sense that cleavage is the splitting of cells.

Yolk is very dense, so it makes sense that the more yolk present, the more difficult it is to cleave, or split, a cell all the way through. In this case there is an incomplete cleavage, which we call meroblastic cleavage. Here, cleavage doesn't extend into the yolk itself. The word 'meroblastic' doesn't exactly roll off the tongue, so it might help to also think of holoblastic cleavage, which is complete cleavage through the 'whole' cell. If it's not one, it must be the other, right?

Telolecithal Cells

If the yolk is dense and concentrated at one end of the cell we call these telolecithal cells ('telos' means 'end' - think of the two poles on Earth as being the different 'ends'). There are two different patterns of meroblastic cleavage for telolecithal cells. The first, bilateral cleavage, is when cleavage activity is the same on both sides. What this cleavage pattern does is creates left and right halves that are mirror images of each other. It's like how your left and right sides are mirror images of each other. In meroblastic cleavage, the blastomeres (new cells produced by cleavage) are still partially connected. We see meroblastic bilateral cleavage in animals called cephalopod molluscs, which are animals like octopi, squid, and cuttlefish.


An octopus zygote will undergo meroblastic bilateral cleavage
octopus


The second meroblastic pattern for telolecithal cells is discoidal cleavage, which is when cleavage creates a disc of cells called a blastodisc. This blastodisc forms at the animal pole, and because cleavage doesn't penetrate the yolk, the inner cells of this disc are continuous with the vegetal pole (instead of separated). Animals that go through this type of cleavage pattern are reptiles (including our good friend the snake!), birds, and even fish.

Centrolethical Cells

There's another type of cell that undergoes incomplete cleavage. These cells have a dense yolk concentrated in the middle and are therefore called centrolecithal cells (think 'centro' for 'center'). The cleavage pattern we see with this type of egg is called superficial cleavage or surface cleavage. What happens here is that cleavage is confined to the surrounding cytoplasm outside of that central yolk (remember the yolk itself isn't cleaved). We see this cleavage pattern in insect eggs (they're animals too!).


A centrolecithal insect cell will undergo superficial cleavage
insect


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