Transcript Chapter 60
Vertebrate Development Biology II: Form and Function The six stages of vertebrate development • • • • • • Fertilization Cleavage Gastrulation Neurulation Neural crest formation Organogenesis Stage of vertebrate development (I) Stage of vertebrate development (II) Fertilization (I) • Entry of sperm cell induces activation – prevents other sperm from entering – Intitiates second meiotic division of egg nucleus – Induces polarity Fertilization (II) Fertilization in sea urchins Sperm penetration Polarity in early embryos Cleavage • Division of first cell to many within ball of same volume (morula) is followed by hollowing of that ball to a blastula. Form of cleavage and blastulation depends on orientation of yolk and nucleus – In primitive chordates, division is even, towards a symmetrical blastula composed of cells of equal size – In amphibians, holoblastic cleavage leads to assymetrical blastula – In reptiles and birds, meroblastic cleavage occurs, resulting in a cap of cells on top of the yolk – In mammals, holoblastic cleavage occurs, creating a trophoblast containing a blastocoel, with inner disc of cells equivalent to a blastodisc Yolk distribution in amniotic eggs affects blastula development Holoblastic cleavage • Cells with little yolk, and central nucleus, develop evenly Uneven cleavage • In frog cells, there is more yolk, and nucleus of fertilized egg is to one side: – Yolk slows division, so areas of low yolk content divide quicker, and create smaller cells (see here, front) – Areas of high yolk content divide more slowly, and give rise to larger cells Meroblastic cleavage • Occurring in reptiles, birds and mammals, an uneven division of cells causes a cap of cells on top of the yolk Blastula of mammals and birds • Cap of cells develops into a blastodisc • Blastocoel develops in mammals, surrounded by trophoblast Gastrulation • Invagination of outer layer of cells to inside of the blastula is known as gastrulation, resulting in the formation of the gastrula • Type of gastrulation is a function of type of blastula… • End result is three types of germ layer tissue - endoderm, mesoderm and ectoderm Gastrulation in the lancelet Gastrulation in the frog Gastrulation in birds Gastrulation in mammals Neurulation and neural crest formation • Formation of neural fold (primitive streak) above notocord, begins a channel that eventually seals on the dorsal surface, forming neural groove – Mesoderm derived tissue close to notocord develop into somites, giving rise to muscles, connective tissue and vertebrae • Layer of cells on dorsal surface of groove form neural crest, responsible for formation of several important organs – Associated patches of ectoderm tissue derive into placodes, which evetually result in important neurally related organs Neural tube formation (I) Neural tube formation (II) Induction • Proximity of a cell to certain other cells controls its development - Spemann and Mangold • Cells whose fate can be predicted are termed ‘determined’ • Cells whose fate cannot be reversed are termed ‘committed’ • Since all cells initially can become any tissue... ...Development is a process of progressive restriction of gene expression Spemann and Mangold’s dorsal lip transplant experiment Induction of the vertebrate eye Organogenesis • Ontogeny recapitulates phylogeny • (and a quick word about extraembryonic membranes) Derivation of major tissue types Embryonic development of vertebrates (I) Embryonic development of vertebrates (II) Extraembryonic membranes - Chick embryo Extraembryonic membranes mammalian embryo (I) Extraembryonic membranes mammalian embryo (II) The placenta Human development Developing human at 4 weeks Developing human at 7 weeks Developing human at 3 months Developing human at 4 months Ultrasound at 5 months Delivery position of foetus