14.4 Experimental Analysis of Development
Key Takeaways
A fate map shows what a blastula region normally becomes, while determination is a commitment that persists even if the tissue is moved.
Differentiation is the later appearance of specialized structure, and induction is one tissue signaling another.
The optic vesicle induces the lens from overlying ectoderm, and the gray crescent of the frog egg marks the future dorsal side.
Transplanting the dorsal lip organizer to another gastrula induces a second embryonic axis that recruits host tissue.
Nuclear-transplant experiments show that a differentiated nucleus can still direct development because genes are regulated rather than discarded.
14.4 Experimental Analysis of Development
Normal development shows the order of stages. It does not show whether a cell was already committed, whether a neighbor induced that commitment, or whether specialization threw genes away. Those questions were answered by moving tissue and by moving nuclei. Each experiment asks one question, and mixing the conclusions is how these items are missed.
Fate maps record the normal path
A fate map shows what a region of the blastula normally becomes if it is left in place. Cells are marked with a dye or a genetic label, and their descendants are followed. One region normally becomes epidermis, another the nervous system, another mesoderm. The map describes the undisturbed embryo. It does not prove that the region is already committed.
Determination is stronger than a fate map
Determination is a commitment that persists even if the tissue is moved. If moved tissue still follows its fate map, it was determined. If it matches its new surroundings, it was not. Early in gastrulation, prospective neural ectoderm may become epidermis when placed in an epidermis region. Later, determined neural tissue still forms neural tissue after a move.
Differentiation is the later appearance of specialized structure, such as a lens cell making lens proteins or a neuron extending an axon. Determination can precede any visible specialization. A fate map, a determined state, and a differentiated cell are three different things.
Induction: one tissue signals another
Induction is a signal by which one tissue changes the fate of another. The optic vesicle, an outgrowth of the forebrain, contacts head surface ectoderm and induces the lens from that ectoderm. The lens is not a fragment of the vesicle. Remove the vesicle early, and the lens does not form. Place a vesicle under ectoderm that is still competent to respond, and a lens can form in the new place. Ectoderm that has already become epidermis will not answer.
The gray crescent marks the dorsal side
After fertilization in the frog, cortical cytoplasm shifts and a gray crescent appears opposite sperm entry. Sperm entry marks the future ventral side. The gray crescent marks the future dorsal side, including the future organizer. If the first cleavage splits the gray crescent between the two blastomeres, each can become a complete embryo. If only one blastomere receives the crescent, only that cell builds a dorsal axis. The other becomes a disorganized belly piece. The crescent marks dorsal fate. It is not the neural tube itself.
The organizer induces a second axis
The dorsal lip of the blastopore is the organizer. Spemann and Mangold transplanted it from one early gastrula to the ventral side of another. The host built a second embryonic axis. Because donor and host differed in pigment, the graft and the host could be told apart. The grafted lip became dorsal mesoderm, including notochord. Most of the second neural tube came from host cells that would have formed belly epidermis. The organizer recruited host tissue. That is induction.
The organizer experiment shows induction of a second axis. It does not show that differentiated cells have lost genes. Host cells that joined the second axis still had their genomes and changed which genes they used. Moving prospective neural ectoderm early may change its fate, because it is not yet determined, but the organizer can still recruit host tissue into a second axis. Neither result means specialization deletes DNA.
Nuclear equivalence means the genes are still there
Nuclear equivalence means a nucleus from a differentiated cell can still direct development. Destroy or remove the egg nucleus, insert a nucleus from a specialized cell, and ask whether development continues. Gurdon placed a nucleus from a tadpole intestinal cell into an enucleated frog egg. Some eggs became swimming tadpoles, and later transplants produced fertile adults. The intestinal nucleus had not discarded the genes for other tissues. Those genes were regulated.
Mammalian somatic-cell nuclear transfer repeated the test. A nucleus from an adult mammary-gland cell, the route that produced the sheep Dolly, was placed in an enucleated egg and directed development of a new animal. The body cell was specialized. Its genes were still there, turned on and off rather than thrown away. Organizer grafts and fate maps do not empty the nucleus.
| Experiment or description | Question it answers | Result to remember |
|---|---|---|
| Fate map of the blastula | What does this region normally become? | A normal destiny, not proof that the fate is locked |
| Transplant of prospective tissue | Is the tissue already determined? | Early neural ectoderm may change fate; determined tissue keeps its fate after a move |
| Optic vesicle placed against ectoderm | Can one tissue induce another? | Competent overlying ectoderm forms a lens |
| Dorsal lip transplanted to a host gastrula | What does the organizer do? | It induces a second axis and recruits host tissue |
| Nucleus of a differentiated cell put into an enucleated egg | Are genes discarded during specialization? | The nucleus can still direct development; genes are regulated |
Warning
The organizer experiment does not show that differentiated cells have lost genes. It shows that the dorsal lip can induce a second embryonic axis from host tissue. Gene retention is the result of nuclear transplantation, from Gurdon's frogs through mammalian somatic-cell nuclear transfer.
One scenario, five terms
Put the results in one picture. A fate map says a dorsal blastula region normally becomes neural ectoderm. Move that prospective neural ectoderm early and it may change fate, so it is not yet determined. The organizer, grafted ventrally, can still recruit host tissue into a second axis. The optic vesicle induces a lens from competent ectoderm. None of these moves discards genes. A differentiated nucleus placed in an enucleated egg can still direct development.
The dorsal lip of the blastopore is grafted from one early gastrula onto the ventral side of another. What does this Spemann and Mangold result show?
The gray crescent marks the future ventral side and blocks every inductive signal.
Cleavage enlarges the egg, and the graft stops both embryos from forming germ layers.
Differentiated cells delete the genes they no longer express, so the host cannot form any second axis.
The dorsal lip is an organizer that induces a second embryonic axis and recruits host tissue into that axis.
Prospective neural ectoderm is moved early and adopts the fate of its new surroundings. The same kind of tissue, moved after it is committed, still forms neural tissue. Which wording fits these results?
The optic vesicle prevents lens formation, so induction means a tissue blocks specialization in its neighbor.
A fate map means every blastula region has already discarded unused genes, so a move can never change fate.
The early tissue was not yet determined. Determination is a commitment that persists after a move, and differentiation is the later appearance of specialized structure.
Differentiation is only the drawing of a fate map, and an early move proves that the cells have lost their nuclei.
Nuclei from differentiated frog cells, and later nuclei from mammalian body cells, are placed into eggs whose own nuclei were removed. Some of those eggs still develop. Which conclusion fits?
Specialization permanently deletes every unused gene, so a body-cell nucleus cannot direct development.
Only the gray crescent contains DNA, so a transplanted body-cell nucleus never enters cleavage.
The organizer graft is the experiment that first proved differentiated cells have lost their genes.
A differentiated nucleus still contains the genome needed to direct development, because specialization regulates genes instead of discarding them.
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