Animal gametogenesis and embryology(chapters 25, 32, 47)

What is an animal?multicellular, heterotrophic, ingest food

no cell walls; organisms held together byextracellular matrix; intracellular junctions

nervous and muscle tissues are unique

distinctive stages of development

body development regulated by Hox genes

What is a Hox gene?

Contains a “homeobox” that enables it to bind toDNA as a transcription factor (3 -helices)

Remainder of the protein helps determine whichgene it regulates

Some of these gene products help regulate bodypart development (homeotic genes)

So far, homeotic genes have been found only inanimals

Model animals: Drosophila, nematodes

Animals were traditionally classified based onanatomy (body plans) and embryology

New approaches may modify old dichotomies:

I. parazoa vs metazoa (eumetazoa)presence or absence of true tissues

II. radial vs bilateral symmetrycephalizationgerm layers- radiata have two,bilaterals three

Germ layers seen during gastrulation

ectoderm- skin, nervous system

endoderm- (archenteron)- digestive system,liver, lungs

mesoderm- muscles, other internal organs

III. Body cavity (of bilaterials)

acoelomate (flatworms)

pseudocoelomate (rotifers and roundworms)

coelomates- body cavity completely linedby mesoderm tissue

Purpose of body cavity?hydrostatic skeleton in invertebratesgrowth, development and positioning of

internal organs

Protostomes vs deuterostomes

Stages of animal development

fertilization cleavagegastrulation (formation of germ layers)organogenesis

Cleavage distributes cytoplasm of zygoteinto many smaller cells

Cells in most animals have polarityvegetal pole (yolk)animal pole (very little yolk)

yolk is especially prominent in eggs ofbirds, reptiles, many fishes and insects

affects cleavage- incomplete (meroblastic)vs complete (trophoblastic)

Sea urchin gastrulation