Animal Development—Part 2

Cleavage

• A succession of rapid cell divisions after fertilization.
• Think of it as having three stages:
  1. Beginning: zygote (a single cell—the fertilized egg)
  2. Middle: morula (a solid ball of cells)
  3. Ending: blastula (“basketball”)
     • The morula develops a fluid-filled cavity called the blastocoel, forming a hollow sphere.
• During cleavage, the cell does not grow in size.
  • An original zygote has its cytoplasm partitioned into many smaller cells called blastomeres.

Part of figure 32.2, page 590, Campbell's Biology, 5th Edition

Rearrangement of Cytoplasm: the Frog and Its Grey Crescent

• The distribution of cytoplasm in the frog is not uniform.
  • 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.
  • 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.
• 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.

Figure 20.2, Purves's Life: The Science of Biology, 7th Edition

Significance of the Gray Crescent

• The gray crescent establishes the bilateral symmetry of the frog.
  • Even before the first cleavage event, we can tell how the frog will develop.
• 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).

Figure 47.7, page 941, Campbell's Biology, 5th Edition

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).

Figure 20.4, Purves's Life: The Science of Biology, 7th Edition

The Frog Blastula

• Not all blastulas are perfectly symmetrical hollow spheres of cells.
• 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!

Figure 47.8(d), page 942, Campbell's Biology, 5th Edition; figure 20.5, Purves's Life: The Science of Biology, 7th Edition

Gastrulation

• 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”)

Part of figure 32.2, page 590, Campbell's Biology, 5th Edition

Gastrulation in Frogs

• 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”).

Unknown source

Three Tissue Layers

Organogenesis

• Cell differentiation continues such that the three germ layers develop into tissues and organs.
• Neurulation in Chordates:
  • 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.
  • 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.

Figure 47.11(a) and 47.11(b), page 946, Campbell's Biology, 5th Edition

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?
• Experiment: Spemman bisected the frog zygote with a loop he formed from a single baby hair.
  • If bisection splits the gray crescent:
    • Two normal embryos are produced.
  • 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.
• 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.

Figure 20.10, Purves's Life: The Science of Biology, 7th Edition

Embryonic Induction

• Spemman and Mangold experiment, 1924
• Question: can some cells induce other cells to follow a particular developmental path?
• 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.
• Conclusion: the dorsal lip of the blastopore is capable of inducing the formation of an embryo.
  • They called this tissue the primary embryonic organizer.

Figure 47.21, page 954, Campbell's Biology, 5th Edition