BIOL 4330 1st Edition Lecture 8 Outline of Last Lecture I. Intro to Zebrafish DevelopmentII. Axis SpecificationIII. Nodal SignalingIV. Bozozok ProteinV. Axis FormationOutline of Current Lecture I. Development of the ChickII. GatrulationIII. Epiboly of the EctodermIV. The Chick Organizera. FGFsCurrent LectureAmniotes allow the egg to be laid on land instead of in water where buoyancy compensates it. Aristotle was the first to document the details of the 3-week development of the domestic chicken Gallas galas. Chick embryos are a very inexpensive surrogate to human embryos. Chick Development- Fertilization occurs in the oviduct, before the albumen and shell are secreted to cover it.- Egg is telolectithal (like in fishes)- Cleavage is discoidal and meroblastic.- Cleavage does not extend into the yolk, but only occurs in a small disc of cells at the top of the animal pole (2-3mm in diameter)- By the time the hen has laid the egg, the blastoderm contains 50,000 cells.- The avian embryo comes entirely from the epiblast (like deep cells in a fish); the hypoblast does not contribute any cells to the embryo (unlike in the fish where it forms the mesoderm).Between the blastoderm and yolk is a space called the subgermal cavity formed when the blastoderm cells absorb water from the albumen and secrete it between themselves and the yolk.Deep-cells are shed, leaving the one-cell thick area pellucid (which forms most of the embryo). area opaca – perpherial ring of cells that have not shed their deep cells. marginal zone sits between the area These 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.pellucid and area opaca. All these structures absorb water and start to differentiate the yolk from the embryo itself.Gastrulation- Avian embryo comes entirely from the epiblast.- Hypoblast cells form portions of the external membranes, especially the yolk-sac and stalk linking the yolk mass to the endodermal digestive tube- Hypoblast cells also provide chemical signals that specify the migration of epiblast cells- The three germ layers of the embryo are formed from the epiblastAmniotes have evolved a set of extraembryonic tissues: the hypoblast, the yolk-sac, and the amnion.The Neiwokoop center in frogs sends the signals to the blastocore organizer area, and is analogous to thehypoblast in chicks.The major structure characteristic of avian, reptilian, and mammalian gastrulation is the primitive streak.Koller’s sickle will extend and eventually become Henson’s node.Cells converge to form the primitive streak by first forming the primitive groove which serves as an opening for migrating. Primitive streak defines the axes of the embryo and extends from the posterior to the anterior, migrating cells enter through the dorsal side and move to its ventral side. Henson’s node (primitive knot) and cells move through the funnel shaped depression (primitive pit) to form the notochord and prechordal plate; and separate the left portion of the embryo from the right. Henson’s node = dorsal lip, (the organizer in amphibians, the embryonic shield in fishes)At 19-22 hours:The axis of the streak is equivalent to the dorsal ventral axis of amphibians. Migrating cells enter through the dorsal side and move to the ventral. Cells moving through the anterior end of streak gives rise to the prechordal mesoderm, notochord, nd anterior/medial part of the somites. Cells moving through the middle give rise to the lateral part of the somites and to the heart and kidneys. Cells at the posterior end of the streak give rise to lateral plate and extraembryonic mesoderm. [Humans have a yolk sac, the placenta, but no yolk]At 23-24 hours: The four somite stageStereograms showing migration of endodermal and mesodermal cells through the primitive streak tracks are GFP-labeled cells at that position in the primitive streak.1st cells: migrating through Hensen’s node become pharyngeal endoderm of the foregut-once deep they migrate anteriorly and displace the hypoblast cells – germinal crescent- contain precursor to the germ cells- migrate through the blood vessels to the gonads.2nd set of cells to migrate move anteriorly but not as ventrally as the 1st set- remain between the endoderm and epiblast to form the prechordal plate mesoderm (the head forms rostrally of Henson’s node)3rd set: become chordomesoderm, has 2 components; 1) head process (most anterior will underlie the cells of the forebrain and mid brain 2) the notochord which will form as the primitive streak regresses.Centralization of the primitive streak is activated by the Wnt planar polarity pathway in the epiblast next to the Koller’s sickle at the posterior of the embryo. Wnt in the epiblast is activated by FGFs from the hypoblast. SEM show epiblast cells passing into the blastocoel and extending their apical ends to becomebottle cells.Hox gene activation begins when the mesodermal precursor cells are still in the epiblast. Hox genes lay down the segmentation of whether that segment will be limbs, the neck vertebrae, coccyx, and the that the somites are going to contain legs. They identify what each of those veterbraes will ultimately contain.Regression of the Primitive StreakChick gastrulation at 24-48 hours after fertilizationA) Primitive streak at full extensionB) 2-somite stage (25 hours) primitive streak begins to regressC) 4-somite stage 27 hoursD) 28 hours primitive streak has regressed to the caudal portionExhibit a distinct anterior to posterior gradient of development. Anterior laid down ahead of posterior having a “head start”Epiboly of the EctodermWhile streak is being formed and cells are moving inward, the ectodermal precursors proliferate and migrate to surround the yolk by epiboly (Herculean task). This takes 4 days to complete. At the end the ectoderm surrounds the yolk, the endoderm ahs replaced the hypoblast and the mesoderm is between these two regions.Anterior-posterior Axis Formation in the Chick EmbryoConversion of the radially symmetrical blastoderm into a bilaterally structure is determined by gravity.As the ovum passes through the reproductive tract it is rotated for about 20 hours in the shell gland (10-12 revs/h) – shifts the yolk such that the lighter components (stored maternal components) lie beneath one side of the blastoderm and that end becomes the posterior portion of the embryo. PMZ-posterior marginal zone –