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- AuthorDenise DeCooman
Denise DeCooman was a teaching assistant for the General Zoology course at California University of Pennsylvania while she earned her Master's of Science in Clinical Mental Health Counseling from fall semester of 2015 and spring of 2017. She also has a Bachelor's of Science in Biological Sciences from California University. She has been writing instructional content for an educational consultant based out of the greater Pittsburgh area since January 2020.
View bio - InstructorElizabeth Friedl
Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.
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What is cleavage in biology? Understand how cleavage of a zygote results in an embryo. Learn about the types of cleavage & laws to explain the cleavage model.Updated: 11/21/2023
Frequently Asked Questions
What is the process of cleavage?
The process of cleavage is the first stage of development during embryogenesis. Cleavage begins not long after the egg is fertilized and ends when an embryoblast forms.
What are the blastomeres?
Blastomeres are the smaller daughter cells which result from cleavage post-fertilization. Blastomeres begin to differentiate in the morula stage of embryogenesis.
Which type of cleavage occurs in humans?
The type of cleavage that occurs in human beings is meroblastic, or incomplete cleavage. This type of cleavage is observed in most mammals and many other vertebrates.
Table of Contents
- Cleavage in Biology
- Cell Cleavage Mechanism for the Embryo
- Types of Cleavage
- Laws of Cleavage
- Lesson Summary
The term "cleave" means to split, or slice apart, an entity. When discussing cleavage in biology, or subdisciplines such as embryology or zoology, what is being referred to is the process by which cellular division occurs during the zygotic phase of development that ends with a multicellular embryo. A zygote is a recently fertilized egg that is ready to begin many rounds of incredibly fast cellular division. The process of DNA cleavage consists of cellular division without the growth or differentiation of cells. Cleavage ends when the embryo becomes a blastula, or hollow ball of cells via a series of rapid, mitotic divisions.
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Cleavage in developmental biology was first observed and recorded by the Dutch microscopist and naturalist, Jan Swammerdam. Swammerdam was studying frogs due to his interest in the developmental stages that led from the egg to adult forms of these amphibians and noticed cleavage in a newly fertilized tadpole zygote. Prevost and Dumas were the first to observe and record the entire process of cleavage in frog embryos during 1824.
The process of cleavage in embryology was changed forever by the philosopher and developmental biologist, Hans Adolf Eduard Driesch. While Driesch was studying the sea urchin species Echinus microtuberculatus, his findings had shown that the current school of thought in embryogenesis was incorrect. Driesch was able to isolate a cell from one of the sea urchin embryos and the cell was able to form a new, genetically identical sea urchin. Until this point, it was accepted that a separated cell would go on to die and that an incomplete embryo would form. However, Driesch showed that some zygotes were capable of producing a fully-formed embryo even if some of the cells were missing.
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Once fertilization takes place and a new organism begins to develop, the zygote begins cleavage. The site of the beginning of cleavage is known as the cleavage furrow . The zygote begins a series of very fast mitosis-like divisions, but instead of creating cells the same size, and growing, the cell divisions result in much smaller cells so the zygote does not experience growth during cleavage. The resulting blastula ends up the same size as the zygote was before cleavage had commenced. Factors that can regulate the development of cleavage include the amount of yolk as well as the distribution of yolk.
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Animal Embryonic Development Stages
There are many stages undergone by the zygote as it develops into an embryo, and then into a fetus after it is initially fertilized. All embryogenesis in the animal kingdom involves post-fertilization cleavage of the zygote. The stages involved in embryogenesis are as follows:
- Zygote - This is the first stage of cleavage which occurs after the egg is fertilized by the sperm. Once cleavage begins the derived from the process are referred to as blastomeres. The first division occurs in this stage, and then two cells become four, and four become eight, and so on. During this time, normal cellular processes are still taking place, but at a much quicker rate than normal, including cytokinesis and the synthesis of nucleic acids.
- Morula - The morula stage begins once there are thirty-two cells formed. The morula is still the same same size as the zygote was post-feralization, and growth is inhibited by the zona pellucida, which is a membrane that surrounds the blastomeres. The morula stage is when cell differentiation begins, which means that cells are starting to become specialized to carry out specific functions.
- Blastocyst - The next stage in embryogenesis is the blastocyst stage. At this point in development, the blastomeres are differentiating and an outer lining of cells of the blastocyst has formed and is known as the trophoblast. The inner portion has become a fluid-filled cavity called the blastocoel. An inner cell mass starts forming along the trophoblast of the blastocyst, which is referred to as the embryoblast.
- Embryoblast - At this point in embryogenesis, the term "embryoblast" is used, and a disc with two layers is observed, as well as outer cells referred to as the epiblast (ectoblast) as well as inner cube-shaped cells called the hypoblast (endoblast). The amniotic cavity begins developing in the epiblast (ectoblast) at this point as well.
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There are two main types of cleavage, which can be either complete or incomplete. Complete cleavage is also referred to as holoblastic (or total) cleavage and incomplete cleavage is also known as meroblastic cleavage. The type of cleavage exhibited by the organism is heavily dependent on the amount of yolk, and also the distribution of yolk, which is present within the zygote. Organisms that have more yolk tend to exhibit incomplete, or meroblastic, cleavage. Those with less yolk usually exhibit complete, or holoblastic, cleavage.
Total or Holoblastic Cleavage
This type of cleavage is typically exhibited in animals with less yolk available to the developing new life. Holoblastic cleavage is also referred to as complete cleavage. This is because the cell is completely cleaved during the cleavage process. Holoblastic cleavage can be equal or unequal. Equal cleavage gives way to blastomeres of the same size while unequal cleavage gives way to blastomeres of unequal sizes. Members of the animal kingdom which exhibit holoblastic cleavage include mammals (including human beings), Planarians (flatworms), echinoderms (such as sea stars and sea urchins), as well as amphibians.
Meroblastic Cleavage
Meroblastic cleavage is also known as incomplete cleavage and is observed in animals which tend to have more yolk available in early development. Incomplete cleavage means that the cell does not cleave entirely during the process of cleavage. Meroblastic cleavage can be either discoidal or superficial. In discoidal cleavage, the yolk is not penetrated. In superficial cleavage, only the periphery of the blastomere undergoes cleavage and the blastomeres that are undergoing division do not go through cytokinesis. Animals that tend to exhibit meroblastic or incomplete cleavage include insects and other arthropods as well many vertebrates, such as reptilians, fish, and birds.
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There are two biological laws which pertain to cleavage, which are Sach's laws and Balfour's law. Sach came up with two laws of cleavage, and the first states that the developing blastomeres divide into equally sized cells. His second law states that the preceding plane is intersected in cellular division at right angles. Balfour's law posits that the amount of yolk contained within the zygote is proportional to the speed at which development occurs.
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The focus of this lesson was cleavage in animal development. Cleavage is a process that occurs immediately after fertilization in a zygote, or newly fertilized egg, that leads to the development of a multicellular embryo. Cleavage encompasses multiple stages, including the development of the initial cleavage furrow in the zygote, or newly fertilized egg. The next stage is the morula, which has thirty-two blastomeres. The blastocyst then develops and the trophoblast and blastocoel are visible. The last stage of cleavage is the embryoblast stage, where the amniotic cavity is visible.
There are two main types of cleavage, which are complete, or holoblastic, and incomplete, or meroblastic, or incomplete. The two main types of holoblastic cleavage are equal and unequal while meroblastic cleavage may be either discoidal or superficial. This lesson also touched on Sach's laws which state cleavage occurs at the right angles of developing blastomeres and that the blastomeres should be observed at equal size and proportion. Balfour's law states that the amount of yolk available is directly proportional to the speed of the cleavage process.
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Video Transcript
Embryonic Development
You've come a long way from when you were just a zygote. You probably don't remember this stage of your life because you were only a newly fertilized egg in the first stage of being a unique individual. But think about how much has happened to you since then - you developed and grew, you were born, and now here you are, learning about it all!
As a human you are made up of many trillions of cells, but you weren't always this way. The first major stage of development a zygote goes through to help you reach this great cellular height is called cleavage. This is the rapid cell division that leads to a multicellular embryo (to cleave something is to split or slice it). And, this is such an important stage that we've devoted an entire lesson to it! Ready to get started?
Major Events During Cleavage
There are some really important things that happen during cleavage, as well as some processes that are essentially put on hold. The cell is dividing incredibly fast during cleavage, which means that the processes that go along with cell division, such as DNA synthesis, mitosis, and cytokinesis, also occur at rapid-fire speed. But during this time, very few new proteins are made. The embryo also doesn't do much growing in terms of size during cleavage - it stays the same size as the zygote! What happens is that, as the cells continue to divide, they divide into smaller and smaller cells instead of just building up into a larger embryo.
This makes sense if you think about it. If you cut an apple in half, you have divided the apple into two equal segments. Divide each of those two pieces in half and you now have four. But you don't have four larger pieces, just four pieces that add up to the same size as the original apple.
In an embryo, this process continues on and on, with each new cell dividing into smaller cells until a hollow cell ball called a blastula is formed. And inside this ball is a fluid-filled cavity called the blastocoel. Unlike our apple, though (which is now just many equal-sized pieces of one apple), each new cell that is formed from division has its own nucleus and is its own independent cell.
Patterns of Cleavage
Just like animals come in all shapes and sizes, so does the cleavage that creates them. There are two main patterns of cleavage to be familiar with: complete and incomplete. Complete cleavage is also called holoblastic cleavage, and incomplete cleavage is called meroblastic cleavage.
The type of cleavage depends on how much yolk is present as well as how that yolk is distributed. A cleavage furrow is the indentation at which cleavage begins. Think of a cleavage furrow as being similar to that indentation that appears on your forehead when you 'furrow' your brow.
Have you ever tried to cut through the middle of a hard-boiled egg? You might have noticed that it is far easier to cut through the white than the dense yolk in the center. A lot of yolk will lead to incomplete cleavage because the cleavage furrow can't pass through that yolk, while a smaller amount of yolk allows the cleavage furrow to pass through completely. To remember the difference, it might help to think of 'holoblastic' cleavage as cutting through the 'whole' cell.
Animals that tend to have complete cleavage are echinoderms (sea stars, urchins, and the like), flatworms, mammals, and amphibians. These guys don't have a lot of yolk in their eggs. On the other hand, animals that do have a lot of yolk and that have incomplete cleavage are things like fish, reptiles, birds, and even insects.
Lesson Summary
You've learned a lot today about a very early, but very important stage in your life. Animal cells undergo something called cleavage, which is the rapid cell division that leads to a multicellular embryo. You started this stage as a zygote, or newly fertilized egg, and ended as a blastula, or hollow cell ball.
Depending on what type of animal you are you, would have undergone a different type of cleavage. If you didn't have much yolk (say if you are a mammal, amphibian, or sea star), you would have gone through holoblastic or complete cleavage. The little amount of yolk allows the cleavage furrow to pass through the whole cell. But if you did have a lot of yolk (perhaps you are a bird, reptile, or fish) you would have gone through meroblastic or incomplete cleavage. Having so much yolk would have prevented the cleavage furrow from passing through entirely.
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