Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson   OUTLINE OF ELEVENTH LECTURE: Feb 10, 2006, by Corey Johnson Induction and Gastrulation continued Primary induction continued. Lauri Saxén and Sulo Toivonen (two grown men collecting newts) found that some substances (including guinea pig liver) induced anterior structures to form at the end of gastrulation in the overlying neural ectoderm. Other substances (guinea pig bone marrow) induced caudal (tail) structures when implanted into the blastocoel. In addition to inducing tail and head structures, the dual implants also formed the intermediate spinal cord structures that neither did alone. The caused them to introduce the dual gradient hypothesis. This has been "tweaked" in recent years but seems to be generally true. Anterior signals (Follistatin, Chordin, and Noggin) act in a gradient fashion. Posterior signals have been identified but no one has definitively confirmed the dual gradient hypothesis - though it does explain the results of Saxén and Toivonen. A gradient in effect may not necessarily be a gradient in action. Some people think gradients can only be diffusible substances. Another idea may be cell-to-cell communication where a signal is "diffused" over space and time as a signal is received by one cell which in turn begins producing the same signal and so on. ..Kind of like that telephone game kids used to play where the original message gets weaker as it is propagated from one child (or cell) to the next. Only the message gets weaker rather than distorted. Maybe that's a little obtuse. Newt Gastrulation The movement of cells during newt gastrulation has shown that vegetal cells move inward becoming endoderm. As these cells are large and yolky, they move slowly and do not travel very far. Endoderm surrounds the archenteron; cells lining the top of the archenteron form a single layer of endoderm, while the bottom lining is several cells thick. They contain large amounts of yolk, and as development progresses, this yolk will be digested. The cells of the archenteron (gut) floor will eventually become small and a single layer like the others, as their contents are consumed. The movement of mesoderm is from lateral regions and anterior regions toward the dorsal lip. This epiboly is followed by involution at the lip. Eventually, the lateral mesoderm forms "lateral lips" and eventually a ventral lip that covers much of the endoderm. The circle of involuting cells has at its center, a region of yolk that is visible for a time but is "swallowed" by the narrowing of the blastopore. Mesoderm fills the space between the outer ectoderm and the inner endoderm that lines the primitive gut. Throughout this process the embryo lengthens along the A-P axis through convergent-extension movements. That is, animal pole cells migrate towards (converge on) the dorsal midline causing the embryo to lengthen (extension) in the perpendicular, A-P axis. Xenopus has a variation of this process. The animal pole is several layers thick, and instead of all of these cells undergoing involution, only the deeper cells participate. The outer cell layer (presumptive ectoderm) undergoes convergent extension but do not pass into the interior of the embryo. urodele and Xenopus fate map The region of the animal pole that becomes mostly mesoderm is called the marginal zone. The surface cells are referred to as the non-involuting marginal zone; the deeper cells are called the involuting marginal zone. The involuting cells migrate toward the lip, and when they finally get there, they turn around and migrate on the deep surface of more superficial cells. How strange that this region could attract cells to it, then just as they reach itÉ they migrate away from it! Xenopus gastrulation Xenopus deep fate map There is a group of cells at the dorsal lip called "bottle cells." These appear at the beginning of gastrulation and are formed by apical constriction. The function of these cells is not yet clearÉ it seems to vary among species. In newts they direct archenteron formation, but in Xenopus they only initiate the blastopore. Xenopus bottle cells Bottle cells "initiate" the blastopore and migrate anteriorly. They begin the formation of the archenteron. The very next cells that move in from the dorsal lip, will form the prechordal plate and notochord (chordamesoderm). These marginal zone cells have an unique ability to "burrow" into endoderm. Holtfreter (1940s) found that grafts of marginal zone tissue could sink into endoderm creating a mini-blastopore. Further more bottle cells migrate on glass, "carrying" cells in their wake. The dorsal involuting cells become notochord and sometimes somites, whereas the lateral and ventral regions of the marginal zone form the rest of the mesoderm.

 

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