Cleavage
- A succession of rapid cell divisions after fertilization.
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Think of it as having three stages:
- Beginning: zygote (a single cell—the fertilized egg)
- Middle: morula (a solid ball of cells)
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Ending: blastula (“basketball”)
- The morula develops a fluid-filled cavity called the blastocoel, forming a hollow sphere.
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During cleavage, the cell does not grow in size.
- An original zygote has its cytoplasm partitioned into many smaller cells called blastomeres.
Rearrangement of Cytoplasm: the Frog and Its Grey Crescent
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The distribution of cytoplasm in the frog is not uniform.
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Nutrient molecules (the yolk) are denser and, due to gravity, accumulate in the lower half of the egg. This is called the vegetal hemisphere.
- To remember this relationship, think “yolk” and “vegetal” are both at the end of the alphabet.
- The outermost layer of this cytoplasm (the cortex) is not pigmented.
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The upper hemisphere is known as the animal hemisphere since this is where the nucleus and polar bodies are located.
- The outermost layer of this cytoplasm (the cortex) is heavily pigmented.
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Sperm always enter the frog egg in the animal hemisphere.
- When this occurs the cortical cytoplasm (not all the cytoplasm) rotates toward the site of sperm entry.
- This creates a band of diffusely pigmented cytoplasm, called the gray crescent, on the opposite side of sperm entry.
Significance of the Gray Crescent
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The gray crescent establishes the bilateral symmetry of the frog.
- Even before the first cleavage event, we can tell how the frog will develop.
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The first cleavage plane always bisects the gray crescent.
- The site of sperm entry will become the ventral region of the frog (the belly).
- The gray crescent will become the dorsal region of the frog (the back).
Patterns of Cleavage: Due to Cytoplasm
- Often, the material in the cytoplasm of the zygote is not evenly distributed.
- When this happens, the pattern of cell division creates blastomeres of unequal size.
- This is particularly true with regards to yolk (the nutrients that were originally stored in the egg).
The Frog Blastula
- Not all blastulas are perfectly symmetrical hollow spheres of cells.
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The frog blastula is not symmetrical:
- The blastocoel is in the animal hemisphere.
- There is a greater number of cells in the vegetal hemisphere.
- Embryologists have developed fate maps that identify the tissues and organs that will develop from specific blastomeres. Development is a tightly controlled process!
Gastrulation
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Formation of the gastrula (word derivation: gaster, “stomach”)
- The process by which a group of cells move inward (invaginate) into the blastula.
- This forms a two-layered embryo with an opening called the blastopore.
- The new inner cavity is called the archenteron (word derivation: arch, “old”; enteron, “stomach”)
Gastrulation in Frogs
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Cells located where the gray crescent was begin to invaginate into the blastocoel, forming the blastopore.
- This area is now known as the dorsal lip of the blastopore and is a key player in the future development of tissue.
- Tissue rolls over the dorsal lip of the blastopore as it moves into the embryo. This process is called involution.
- The new cavity is called the archenteron (primitive gut) and will eventually become the digestive tract.
- The blastopore will become the anus in the frog, because the frog is a deuterostome. The mouth will form elsewhere (“second the mouth”).
Three Tissue Layers
| Tissue Layer |
Will Become… |
| Ectoderm |
- Brain and nervous system
- Epidermis of skin
- Hair and nails
- Corona and lens of eye
|
| Mesoderm |
- Skeletal system (bones, cartilage, notochord)
- Muscular system
- Circulatory system
- Excretory system
- Reproductive system
|
| Endoderm |
- Epithelial lining of digestive tract
- Epithelial lining of respiratory system
- Lining of reproductive system
- Urethra
- Urinary bladder
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Organs:
|
Organogenesis
- Cell differentiation continues such that the three germ layers develop into tissues and organs.
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Neurulation in Chordates:
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Notochord: cells along the dorsal surface of the mesoderm form a stiff rod.
- Provides a support function in lower chordates.
- Nearby mesoderm will become the vertebral column in higher chordates.
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Neural tube: a structure which develops from the ectoderm germ layer
- It will become the brain and spinal cord (central nervous system)
- Formed in a stepwise process in which the neural plate and neural fold migrate to form a neural groove and eventually the neural tube.
Important Experiments
Cytoplasmic Determinants
- Hans Spemann's experiments from early 1900s
- Question: can a nucleus remain totipotent (capable of directing the development of a complete embryo) during cleavage and blastula formation?
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Experiment: Spemman bisected the frog zygote with a loop he formed from a single baby hair.
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If bisection splits the gray crescent:
- Two normal embryos are produced.
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If bisection does not split the gray crescent:
- The cell with the gray crescent becomes a normal embryo.
- The cell with no gray crescent becomes a clump of undifferentiated cells.
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Conclusion: cytoplasmic material is unevenly distributed in the zygote.
- Certain substances in the cytoplasm, cytoplasmic determinants, are necessary for proper development.
- If a cell does not have these cytoplasmic determinants, it cannot proceed to develop in the proper fashion.
- In particular, the frog needs cytoplasmic determinants from the gray crescent for normal development.
Embryonic Induction
- Spemman and Mangold experiment, 1924
- Question: can some cells induce other cells to follow a particular developmental path?
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Experiment: Spemman and Mangold transplanted the dorsal lip of the blastopore from a nonpigmented frog gastrula to a new location on a pigmented frog gastrula.
- The recipient embryo formed a second notochord and neural tube in the region of the transplant and, eventually, almost a complete second embryo.
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Conclusion: the dorsal lip of the blastopore is capable of inducing the formation of an embryo.
- They called this tissue the primary embryonic organizer.
