Copyright

Holoblastic Cleavage: Definition & Patterns

Holoblastic Cleavage: Definition & Patterns
Coming up next: Meroblastic Cleavage: Definition & Patterns

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
 Replay
Your next lesson will play in 10 seconds
  • 0:03 Background on Cellular…
  • 1:12 The Zygote's Poles
  • 1:52 Isolecithal Cells
  • 4:16 Mesolecithal Cells
  • 5:00 Lesson Summary
Add to Add to Add to

Want to watch this again later?

Log in or sign up to add this lesson to a Custom Course.

Log in or Sign up

Timeline
Autoplay
Autoplay
Speed

Recommended Lessons and Courses for You

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.

To unlock this lesson you must be a Study.com Member.
Create your account

Register to view this lesson

Are you a student or a teacher?

Unlock Your Education

See for yourself why 30 million people use Study.com

Become a Study.com member and start learning now.
Become a Member  Back
What teachers are saying about Study.com
Try it risk-free for 30 days

Earning College Credit

Did you know… We have over 200 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it risk-free for 30 days!
Create An Account
Support