Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson
OUTLINE OF TENTH LECTURE: Feb 8, 2006, by Corey JohnsonInduction and GastrulationHans Spemann won the Nobel Prize in 1935 for his work with Hilde Proschhold (Mangold) who died in a freak accident. They demonstrated that grafting the organizer region to a host embryo induced a second body axis! Newts were produced with 2 heads, or even 2 whole bodies joined at the belly. link to news story from UC Berkeley What was happening, exactly? It caused cells to reorient and undergo gastrulation where the second organizer was. Not only was this the death to preformationism (which, believe it or not was still lingering in the early 1900s), but was an amazing example of regulative development. It appears that the organizer region is capable of attracting cells towards it, regardless of their fate. Cells that would ordinarily become part of the host embryo are rearranged to become part of the 2nd axis. (One other explanation was that the 2nd embryo axis was derived entirely of the grafted organizer, but experiments using different colored newts demonstrated that the new axis was derived from host cells). Even heat-killed organizers could be implanted in host embryos and result in the same effect. What was this "un-dead" substance?!?!? Many investigators have spent their careers trying to answer that. We still don't know the whole story. How do these cells become the organizer? It is believed to be a result of cortical rotation. At fertilization, cortex that was in contact with the yolk cytoplasm suddenly comes in contact with animal pole cytoplasm. The opposition somehow is thought to trigger a signal that resides in the cortex of the gray crescent until gastrulation. Spemann and Mangold used the term induction to describe the power of one group of cells to cause another group of cells to become something different. The induction of a primary axis is called primary induction. An even earlier form of induction takes place: mesodermal induction. Cleavage and the formation of the blastula is generally viewed as a time of little specialization, however that view is changing. Why is there a blastocoel? It has been suggested that the blastocoel limits the contact between pre-ectoderm and pre-endoderm cells. Where they meet turns out to be the future mesoderm. A region of macromeres in the amphibian (vegetal pole cells) induces the surrounding animal pole cells to become mesoderm. This is called 'mesodermal induction.' Cells of the animal pole when isolated become ectoderm. Those from the vegetal pole form endoderm in culture. The cells in the middle do not form mesoderm unless they're exposed to the vegetal cells or factors from with vegetal cells. If taken from later embryos (just before gastrulation) they'll form mesoderm. This middle group of cells, called the marginal zone, require the inductive influence of the vegetal pole cells (or something within them). The cells responsible for this activity are found in the dorsal, vegetal cells where the ventral lip of the blastopore will eventually form (see figure for orientation). The mesodermal inducing center is often called the 'Nieuwkoop center' for the fellow who found it (1973). The inducing "strength" or potency of this center is dependent upon the degree of cortical rotation. If the cortex only rotates a little, there's a little bit of induction. If there's a lot, there's a lot of induction. The connection is unclear. Like the Primary organizer, a graft of the Nieuwkoop center can induce a second axis. It does so, however, by inducing a second Primary organizer. This dorsal signal breaks the radial symmetry of the embryo. If separated, ventral cells will produce an abnormal, radially symmetric embryo. Dorsal cells can produce a slightly abnormal but recognizable embryo, missing only a few ventral structures. So a ventral fate might be viewed as a default state, while the dorsal signal breaks such symmetry forming a polarized embryo. This is a very important observation. The "dorsalizing" influence of the organizer breaks the radial symmetry to only a single plane of symmetry on the same axis. Observation: if the Nieuwkoop center is needed to position the organizer so that the embryo can break its radial symmetry, what sets up the asymmetric position of the Nieuwkoop center? How can symmetry break? Random chance is not a causal force of nature... something must determine the asymmetry, right? The stages setting up the fate of mesodermal cells is as follows:
(2) Nieuwkoop center specifies the Spemann organizer (3) Organizer dorsalizes the adjacent mesoderm by inhibiting the ventralizing (default) signal.
(b) a division of that mesoderm into 2 or more sub-types: dorsal and ventral mesoderm.
gastrulation is characterized by drastic cell rearrangements to accomplish several goals:
2) oppose tissues not previously in contact - making other inductions possible 3) create a 3 dimensional structure (for the 1st time in some animals) with a gut extending from one end to another
In newts, cells of the animal and vegetal pole involute and move into the interior of the embryo from all directions. Mesoderm moves in from the lateral regions. Endoderm moves in from the vegetal region. Vital dyes are used as methods to trace cell movements and have been used to determine the fate of cells. This is known as fate mapping:
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