BIOL 4330 1st Edition Lecture 6 Outline of Last Lecture I. Drosophila ContinuedII. Gap GenesIII. Early Anterior-posterior Axis SpecificationOutline of Current Lecture I. Amphibian GastrulationII. Amphibian Axes FormationIII. The OrganizerIV. SummaryCurrent LecturePhylogenetic tree of the chordates shows that the amphibians were the first to require a wet environment. Reorganization of the cytoplasm and cortical rotation produce the gray crescent in frog eggs. After the sperm enters, the entire pole will rotate 30 degrees. The vegetal pole has rotated above the equatorial plane and into the animal pole area. The area of overlap is known as the gray crescent. It also marks the presumptive dorsal area of the frog. Also referred to as the “organizer”.The sperm centriole enters the egg and starts this movement. The dorsal is 180 degrees opposite of where the sperm enters. The area of first cleavage is opposite the where the first sperm occurs.Yolk is in the vegetal area, and inhibits/slow downs cleavage. At the stage where there is 128 cells, a blastula occurs and the blastocoels gives rise to the archenteron (primitive gut). When looking at the scanning electron micrographs, a morula is a 16 cell stage that looks like a mulberry.What keeps the cells together is the same thing that works in the adhesion junction, cadherin. Lack of EP-cadherin in organisms result in the failure of blastomere adhesion. Cell adhesion molecules (EP-cadherin) keep cleaving cells together and keep them bunched together and it will disintegrate and fall apart without it.Amphibian GastrulationThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.“Gastrulation is the time where a vertebrate takes its head out of its anus” –Ray KellerThree goals of gastrulation- Brings inside the embryo those areas destined to form the endodermal organs- To surround the embryo with cells capable of forming the ectoderm- To place the mesodermal cells in the proper positions between the endoderm and ectodermUltimately, gastrulation is to create the three layers (ectoderm, endoderm, mesoderm).Epiboly occurs at the the same time as gastrulation. It is the thinning and spreading of cell layers that It allows the embryo to make dramatic physical restructuring. [7.3 Watched a video of Xenopus gastrulation]. The blastopore becomes the anus. Spreads and surrounds other tissues. Grey crescent = blastopore = anus. The dorsal lip is also where the grey crescent was originally. Intercalate = they spread.Bottle cells are typical migratory cells that move. [7.4 Watched a video of gastrulation and neurulation in Xenopus].Early Movements of Xenopus GastrulationChordamesoderm forms the notochord (transient mesodermal rod important to inducing and patterningn.s.). The ectoderm starts to push and move around, the chordamesoderm becomes involuted and becomes part of the mesoderm. The Xbra is activated and starts to express in the chordamesoderm and allows it to form the spinal chord. The Gooscoid is a gene that is critical for the spinal cord to form down your back.How to cells move? By actin. Also, all cells have polarity. Electronegativity and adhesion (catenins, cadherins) caused by differential concentration. Adhesion molecules are anchored by catenin-cadherins that pull to/from the other cell. It’s not like electromagnetism. Its not gravity and simple nuclear pulling.Transcription Tranduction Cascades conserve energy across all organisms. The prechordal plate is the precursor to head mesoderm.Amphibian Axes FormationAmphibian axes formation combines autonomous specification (mosaic development) with confitional specification (regulative development). Even an unfertilized amphibian egg has polarity along animal-vegetal axis.Spemann’s demonstration of nuclear equivalence in newt cleavage – each cell exhibits autonomous specification to the 16 cell. He used a hair from his infant daughter. He tightened the embryo with the hair and tied it on a cleaving newt embryo. One cell slipped across the tightening throat and into the other embryo through that bottleneck. Until the 16 cell stage, each one can form its own full self.Because it was before the 16 cell stage, he was able to make two newts instead of one. This occurred around 1902. [Watched a video where a piece of the grey crescent is grafted Assymetry in the amphibian egg-importance of the gray crescent that gives rise to the dorsal lip cells. The Bauchstück is the belly piece that van occur. Similar to a polar body, not a normal development like the others.The organizer organizes a secondary axis by the dorsal blastospore lip tissue. Primary embryonic induction; organization of a secondary axis by dorsal blastospore lip tissue was studied by Hans Spemannand Hilde Mangold.Organizer is the dorsal lip cells & derivatives (notochord & head endomesoderm). 1) Organizer induces the host’s ventral tissues to change their fates to form a neural tube and dorsal mesoderm tissues (e.g. somites) 2) Organizer organizes the host and donor tissues into a secondary embryo with clear anterior-posterior and dorsal-ventral cellsPrimary embryonic induction - in which the progeny of the dorsal lip cells induce the dorsal axis and notochordWhat gives the dorsal most vegetal cells their special properties? Most likely β-catenin (beta catenin). \Properties of the Organizer Tissue 1) The ability to self-differentiate into dorsal mesoderm (prechordal plate, chordomesoderm etc)2) The ability to dorsalize the surrounding mesoderm into paraxial (somite forming) mesoderm when it otherwise would form ventral mesoderm 3) The ability to dorsalize the ectoderm and induce formation of the neural tube4) The ability to initiate the movements of gastrulationBMPs and the fate of the ectoderm as dictated by the organizerThe epidermis is induced–not neural tissue. Ectoderm is induced to become epidermal tissue by binding BMPs (bone morphogenetic proteins). The n.s. forms from ectoderm that is protected from epidermal induction by BMP-inhibiting molecules. To summarize:1) The default state of ectoderm is to become neural tissue.2) Certain parts of the embryo induce the ectoderm to become epidermal tissue by secreting BMPs.3) The organizer tissue acts by secreting molecules that block BMPs thereby allowing the ectoderm to be “protected” by these BMP inhibitors and become neural tissue.Chordin,