Chapter 43 EMBRYOLOGY

STAGES OF ANIMAL REPRODUCTION & DEVELOPMENT 1) gamete formation 2) fertilization 3) cleavage 4) gastrulation 5) organ formation

1) GAMETE FORMATION Spermatogenesis forms sperm Oogenesis forms egg As egg matures different regions of the cell store different mRNA transcripts & enzymes Centrosome created to define poles of cell Prepared for first cell divisions to make 2 different cells

2) FERTILIZATION Sperm penetrates egg Sperm nucleus and egg nucleus fuse Polarity is already established at this time Animal pole is closest to nucleus Vegetal pole is closest to yolk

3) CLEAVAGE Cell divides but maintains same volume Cytoplasmic localization makes each new cell behave differently because of What mRNA and enzymes were stockpiled in that region of cytoplasm Indeterminate = deuterostomes Determinate = protostomes

CLEAVAGE PATTERNS Protostomes: spiral cleavage pattern Deuterostomes: radial cleavage pattern Both form a ball of cells called a blastula Individual cells = blastomeres Ball of cells becomes hollow now called blastocyst

4 GASTRULATION Cells differentiate by selective gene activation Cells become arranged into primary tissue layers (ecto, endo and mesoderm) Morphogenesis a) dispersal of cells-pseudopods move them to set locations identified by chemical gradients & adhesion proteins b) cells change shape, tissues fold c) apoptosis removes some cells d) coelome forms

5) ORGAN FORMATION 1) cytoplasmic localization gives cells molecules for signaling nearby cells to coordinate a) cell – to- cell signals b) morphogens = long range signals act on control elements of DNA, work by conc. Gradients 2) Master genes – activated by morphogens, interact with control elements to set body plan 3) homeotic genes & other master genes activate or block gene sets