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Chapter 11- Fish and mammals • Zebrafish are becoming the sweetheart of developmental biologists • • • • • • Large broods Breed year-round Easy and cheap Transparent embryos Develop outside mother Early development complete in 24 hours A. Cleavage 1st 12 divisions are sychronous to form blastoderm Blastoderm is perched on a large yolk cell 6 1 Three cell populations 1. Enveloping layer (EL) 2. Deep layergives rise to embryo proper Fig. 11.1 3. Yolk syncytial layer (YSL) Fig. 11.2 B. Gastrulation Epiboly Recall Epiboy from Ch 9 Deep cells migrate to outside then encase entire yolk Movement not by crawling, but by YSL cells expansion and pulling EL cells along 1. Enveloping layer (EL) Embryonic shield epiblast hypoblast 2. Deep cells Fig. 11.3 2. YSL cells YSL 6 hrs post-fertilization • A hypoblast is formed either by involution of superficial cells or by ingress • These combine with superficial epiblast cells to form the embryonic shield (function equivalent of the dorsal lip in amphibians) B. Gastrulation (cont.) Animal Ventral Head The hypoblast cells extend in both directions to form the notochord precursor Dorsal Ectoderm Trunk Tail Vegetal Fig. 11.3 Mesoderm Endoderm Fig. 11.2 -A zebrafish fate map C. Axis formation 1. Dorsal ventral axisAs with the amphibian dorsal lip (Organizer), the embryonic shield: 1. Establishes the dorsal-ventral axis Converts lateral/ventral medoderm to dorsal mesoderm (notochord) Convert ectoderm to neural rather than epidermal B-catenin 2. Forms the notochord precursor 3. Secretes proteins to inhibit Fig. 11.6 BMP from inducing ectoderm to become epidermis BMP2 Chordino •This inhibiting molecule is called Chordino Embryonic shield • If mutate chordino, no neural tube is formed 4. Acquires its function from B-catenin accumulation in nearby cells •B-catenin accumulates in YSL cells •Goosecoid is activated samois goosecoid BMP inhibitors e.g. Chordino C. Axis formation (cont.) 2. Anterior-posterior axisIn amphibians , the anterior-posterior axis is formed during oogenesis This axis is stabilized during gastrulation by two signaling centers Anterior neural inducing signal (from ectoderm cells) Fig. 11.6 Posterior neural-inducing signal ( from mesoderm cells) 3. Left-right axis Not much known, but involves TGF-b family signaling molecules Mammalian Development Tough to study!! • 100um diamater (1/1000th volume of frog egg! • Few in number (<10/female) • Develops within mother • Cleavage events take 12-24 hours each • Development occurs en route to uterus 3. Cleavage during migration down oviduct 4. Implant in uterus 2. fertilization 1. Egg released from ovary Fig. 11.20 Mammalian Development A. Cleavage Distinctions of mammalian cleavage 1. Slow- 12-24 hrs per cleavage 2. 2nd cleavage is rotational 3. Marked asychrony in early cell division 4. Cleavage at 2nd division requires Amphibians Mammals newly made proteins from zygote Fig. 11.21-rotational 5. Compaction (marked cell huddling) occurs at 8 cell stage cleavage in mammals compaction Fig. 11.23- compaction at 8 cell stage (day 4 in humans) A. Cleavage (cont.) 16 cell embryo is termed “morula” •external cells will become trophoblast, which will become the placenta •Internal cells will become inner cell mass (ICM), or the embryo proper This marks 1st differentiation event in mammalian development At 64 cell stage, an internal cavity appears and the embryo is termed a blastocyst, ready for implantation onto uterus wall The Zona pellucida (recall ch. 7) must be shed in order to implant • Blastocyst lyses a small hole in zona using the enzyme strypsin Note- attachment of embryo to oviduct wall is called a tubal pregnancy B. Gastrulation Similar to reptiles and birds •Mammalian embryo relies on mother for nutrients, not yolk •Thus, the embryo must have a specialized organ to accept nutrients- called the chorion •The chorion induces uterine cells to become a decidua (rich in blood vessels) Epiblasts form amnionic cavity epiblasts Hypoblasts (from ICM) line the blastoceol- these give rise to extraembryonic endoderm hypoblasts blastocoel Fig. 11.28- Day 15 human embryo B. Gastrulation (cont.) Mammalian mesoderm and endoderm cells arise from epiblasts that migrate through primitive streak E-cadherin attachment is mechanism Henson’s Node Primitive streak Direction of migration Fig. 11.28- Day 16 in human Fig.11.11- Chick gastrulation- similar to mammalian Those cells that migrate through the Henson’s node will become the notochord B. Gastrulation (cont.) Extraembryonic membrane Formation Trophoblast cells (originally termed “cytotrophoblast) gives rise to multinucleated syncytiotrophoblasts Uterine wall These syncytiotrophoblasts: • secrete proteolytic enzyme to invade uterine wall • Digest uterine tissue • Mothers blood vessels contact the syncytiotrophoblast cells • Embryo produces its own blood vessels Embryo’s blood vessels Chorion Villi Embryo chorion Mother’s Placenta Mothers blood vessels Fig. 11.27-Blastocyst invading uterus Blood vessels feed embryo, but blood cells do not mix Fig. 11.31 C. Anterior-posterior axis formation Two signaling centers 1. Anterior visceral endoderm (AVE) 2. Node (Organizer) These work together to form forebrain Fig. 11.34 These are on opposite sides of a “cup” structure Node produces Chordin and Noggin AVE produces Lim-1 and Otx-1 Knock-out of one of these results no forebrain C. Anterior-posterior axis formation The Hox genes specify anterior-posterior polarity These are homologous to homeotic gene complex (Hom-C) of drosophila Recall that the Hom-C genes are arranged in the same order as their expression pattern on anterior-posterior axis Mammalian counterparts are clustered on 4 chromosomes Equivalent genes (Hoxb-4 and hoxd-4) are called a paralogous group C. Anterior-posterior axis formation (cont.) Fig. 11.36- Hox genes are organized in a linear sequences that concurs with posterior to anterior structures This is referred to as the hox code Hox gene rules 1. Different sets of Hox genes are required for specification of any region of the anteriorposterior axis Hoxa-2 KO- stapes missing, duplicate incus Incus Hoxa-3 KO- thymus, neck cartilage malformed Stapes 2. Different members of a paralogous group may specify different organ subsets in a given region st Example Hoxd-3 KO deformed atlas (1 vertebra) Hoxa-3/Hoxd-3 double KO- atlas and neck cartilage nearly absent 3. A hox gene KO causes defects in the anterior-most region of that gene’s expression Retinoic Acid has a profound effect on development Recall amphibian development (Ch. 10) Structure of retinoic acid (not in textbook) Fig. 10.41 RA Retinoic acid activates mammalian hox genes Lacks all distal vertebra Wild-type mouse RA-treated embryo mouse embryo Hox gene Retinoic acid bind a receptor, then the complex binds promoter of a hox gene Retinoic acid is likely produced in the node, and perhaps more time spent in the node dictates more posterior specification D. Dorsal-ventral axis formation Dorsal axis forms from ICM cells near trophoblast Inner cell mass (ICM) Trophoblast Blastocoel Ventral axis forms from ICM cells near blastcoel Fig. 11.32 Fig. 11.42 E. Left-right axis formation Note that mammals are asymmetrical Two levels of regulation1. Global- a inv gene defect results in all organs on the wrong side 2. Organ-specific- an iv gene defect causes the axis of an organ to change Organs are located in specific locations