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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).
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Vertebrate Development Biology II: Form and Function
The six stages of vertebrate development • Fertilization • Cleavage • Gastrulation • Neurulation • Neural crest formation • Organogenesis
Fertilization (I) • Entry of sperm cell induces activation • prevents other sperm from entering • Intitiates second meiotic division of egg nucleus • Induces polarity
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
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
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
Organogenesis • Ontogeny recapitulates phylogeny • (and a quick word about extraembryonic membranes)
