Holoblastic Cleavage: Definition & Patterns

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  • 0:03 Background on Cellular…
  • 1:12 The Zygote's Poles
  • 1:52 Isolecithal Cells
  • 4:16 Mesolecithal Cells
  • 5:00 Lesson Summary
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Lesson Transcript
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.

Once an egg is fertilized, it rapidly divides into many smaller cells. This is called cleavage. But just as there are many different animals in the world, there are also many cleavage patterns. Here you'll learn about the patterns specific to holoblastic, or complete cleavage.

Background on Cellular Cleavage

I bet you didn't stop to think about the biology of the eggs you had for breakfast this morning, but there's some really cool stuff there to learn about. For example, you also came from an egg, though admittedly yours was a bit different than that of a chicken. If that chicken egg had been fertilized, it would have eventually produced a little chick, just like you were once a small baby. And if that chicken egg had been fertilized it would have also undergone cleavage, just like you did. Cleavage is the rapid cell division that leads to a multicellular embryo (to cleave something is to split or slice it).

But that's where the similarities stop because you and the chicken would undergo different types, or patterns, of cleavage. One of the main reasons for this is the amount of yolk present in the egg: mammal eggs have little to none, while chicken eggs have a lot. Yolk is very dense, and the more yolk there is, the more difficult it is to splice all the way through the cell. If the whole cell does divide, we call this holoblastic, or complete, cleavage. Think 'whole' for 'holo'-blastic.

The Zygote's Poles

Before we dive into the division itself, we need to review a few things about the egg we're set on cleaving. First is that this newly fertilized egg is called a zygote. Second is that this zygote has two poles called animal and vegetal. The animal pole is where the nucleus and the majority of the cytoplasm are found, whereas, the vegetal pole is the end with the highest concentration of yolk. Since the animal pole holds the nucleus, which contains the DNA and genetic instructions for the animal, we can think of this pole as the core of the animal, while the vegetal pole is just 'vegging out,' providing nutrients to the growing organism.

Isolecithal Cells

Okay, back to cleavage. Remember how I said that the amount of yolk influences the type of cleavage a zygote will undergo? What's also important is how that yolk is distributed. If the yolk is fairly evenly distributed throughout the egg, we call these isolecithal cells. 'Iso-' means 'same' or 'equal' - think of an isolateral triangle that has three sides of equal length.

There are four different patterns of holoblastic cleavage for isolecithal cells. The first is bilateral cleavage. This is when cleavage activity is the same on both sides, creating left and right halves. We can see the same type of symmetry in ourselves: look in a mirror and draw a line down your middle, and you'll find that each side is a mirror image of the other. We see this type of cleavage in animals such as tunicates, which, although they look nothing like us, are our primitive chordate ancestors.

The second pattern is radial cleavage, which is when cleavage occurs radially around a central axis. Think of the spokes on a wheel radiating from the center, or slices of pizza in a whole pie, and you'll get the idea here. We see this type of cleavage in animals such as echinoderms (like sea stars, sea urchins, etc.), which interestingly enough, are radially symmetric as fully developed organisms.

The third pattern for isolecithal cells is called rotational cleavage, and this happens when there is a rotation of the cleavage plane. This is quite interesting because the first division occurs normally. But then things change a bit because one of these new cells splits length-wise while the other splits width-wise. It's rotated 90 degrees to the first one, hence the name rotational cleavage. This is the very cleavage that we humans (and other mammals) undergo.

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