Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson   OUTLINE OF TWELFTH LECTURE: Feb 13, 2006, by Corey Johnson Avian/mammalian gastrulation and fish development By now, you should have completed reading the chapters on cleavage and gastrulation. We have covered cleavage and gastrulation for sea urchin, fish, amphibians, birds, and mammals. You may skip the information in the text relating to other animals... but it is interesting! We'll move on to neurulation and axial structures for the next few lectures. Avian and Mammalian gastrulation Bird: The region of the blastoderm that is above the subgerminal space is known as the area pellucida because it is relatively clear. The outer region of the blastoderm that lies directly on top of the yolk is the area opaca. The hypoblast forms from the posterior, multilayered region of the blastoderm, the posterior marginal zone. A second source of hypoblast forms by a delamination of a small number of cells from the epiblast. Formation of the primitive streak (bird & mammal). Within the posterior area pellucida, a region of the epiblast undergoes thickening. The thickening is the result of ingression of cells into the blastocoel. The thickening becomes longer (A-P) and laterally thinner. Cells converge from the lateral regions and a depression forms in the center or the primitive streak: the primitive groove. The groove acts somewhat like a blastopore since it is through the groove that cells pass into the blastocoel. At the anterior margin of the streak, Henson's node contains a depression through which cells pass to the blastocoel. This region is the functional equivalent of the amphibian dorsal lip of the blastopore. Cells move as individuals from the lateral regions, crawling toward the streak, into the groove, and finally into the blastocoel where they'll migrate in mostly lateral directions forming endoderm and mesoderm (ingression). The first cells to ingress are the endoderm that will form the foregut. The second group of cells is the prechordal plate and notochord. The movement of these populations to the anterior causes a bulge known as the head process. Other cells move in and move laterally, forming a lower layer: the endoderm. Endoderm displaces the hypoblast to the margins. The endoderm comes to lie closest to the yolk with mesoderm above it, and ectoderm above the mesoderm. There is no archenteron formation, but the endoderm will eventually proliferate and migrate all of the way around the yolk forming the gut. The mesoderm ingresses, and then migrates between the epiblast and endoderm. The epiblast/ectoderm, which has been proliferating all this time, also migrates around the yolk so that the yolk is surrounded by endoderm. The primitive streak moves posteriorly (known as "the regression of the node"). Anterior regions become more advanced than posterior ones. Neurulation completes in the head before the back end has finished gastrulating. This is sometimes called "cephalic precocity." Some pictures: mouse development 1 mouse development 2 mouse development 3 Fish Development (teleost fish, such as zebrafish and medaka): Zebra fish have been chosen to become a genetic model organism: Danio rerio (The same species that have long been sold in pet stores) Many are now concentrating their research on them, and thousands of mutant lines have been isolated. Advantages: Can be easily raised Many eggs produced per individual Eggs (embryos) develop rapidly (very!) Transparent embryos Embryos are permeable to chemicals (unlike other fish, and frogs) They are vertebrates Disadvantages: (are very different from humans in many ways) Evolutionarily, teleost fish have diverged greatly from mammals. Their pattern of gastrulation is very different from ours. Have evolved two extraembryonic membranes we don't have (the yolk syncytial layer and the enveloping layer) (Amniotes have 4 extraembryonic membranes) Note that Zebra fish are classified as teleost: fish, and are not closely related to sharks, rays, lampreys or hagfish, each of which have very different gastrulation, etc. patterns Previously, most research on teleosts was done using either trout or killifish (Fundulus heteroclitus). The trout eggs came from hatcheries, and Fundulus is the common minnow used for bait in salt and brackish water. The leading fish embryologist was J. P. Trinkaus, and was Dr. Harris' PhD Professor and friend. His autobiography "Embryologist: My Eight Decades in Developmental Biology" was published shortly after his death in 2003). [link to obituary] H.V. Wilson ( Wilson Hall.. .) also began as a fish embryologist, but worked on the Black Sea Bass. (in the Atlantic, not the Black Sea!) The teleost's macrolecithal, telolecithal egg, is somewhat like a bird's egg in that it has a small cytoplasmic component (blastodisc/germinal disc) that undergoes meroblastic cleavage while the cytoplasm containing yolk does not. Blastomeres divide synchronously forming a blastoderm. At 16 cells, some become separate from the yolk as 2 tiers of cells are formed. At the 32/64 cell stage cleavages become asynchronous and the blastoderm becomes multilayered. At the end of cleavage there are several cell types. Surface cells (enveloping layer) are those on the outside, and they take on epithelial characteristics. Deep cells lie beneath. Yolk syncytial layer (YSL) emerges as the layer in contact with the yolk. A syncytium occurs either when cytokinesis doesn't separate adjacent cells, or when cells fuse. The result is several nuclei within the same cytoplasm. The enveloping layer (superficial cells) undergoes epiboly, crawling over the surface of the embryo. When the cells of the blastoderm have migrated about half way around the yolk, gastrulation begins when the deep cells undergo involution. The deep cells are then divided into a superficial layer (epiblast) and a deep layer that has undergone involution (hypoblast). The rim of involuting cells is known as the germ ring. cleavage in fish The outer, enveloping layer becomes an extraembryonic structure: the periderm. It has no homology with the extraembryonic membranes that amniotes have. The epiblast becomes the ectoderm The hypoblast becomes the mesoderm and endoderm Cells of the deep layer converge along the dorsal side of the embryo forming the A-P axis, the embryonic shield. This is the fishy version of the organizer. epiboly and continued development in fish For some of the movies we saw in lecture and more: fish movies They even have some in 3D!!! You'll need glasses though.

 

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