Animal DevelopmentBy Natasha Guenther, Brea Altoya, and

Bianca (I can’t spell her last name so I’m leaving it out)

Key ConceptsAfter fertilization, embryonic development

proceeds through cleavage, gastrulation, and organogenesis

Morphogenesis in animals involves specific changes in cell shape, position, and adhesion

The developmental fate of cells depends on their and on inductive signals

Body-Building Plan for animalsOrganism’s dev. Is determined by the genome of the zygote

and also by the differences that arise b/w early embryonic cellsThese differences set the stage for the expression of different

genes in different cellsCytoplasmic determinates: maternal substances. Affect dev.

Of the cells that inherit them during early mitotic divisions of the zygote

Cell division continuesCell Differentiation: the specialization of cells in their

structure and function (timely communication is necessary)Morphogenesis: the process by which an animal takes shape

and the differentiated cells end up in the appropriate locations

4 Main Stages: 1. FertilizationMain function: To combine haploid sets of

chromosomes from two individuals into a single diploid cell (the zygote)Another important function: Activation of the

egg (contact of the sperm with the egg initiates metabolic reactions that trigger embryonic dev.)

Acrosomal Reaction and Corticole Reaction

DefinitionsAcrosomal reaction: Hydrolic enzymes

released from the acrosome make a hole in the jelly coat, while growing actin filaments. This structure potrudes from the sperm head and penetrates the jelly coat, binding to receptors in the egg cell membrane that extend through the vitelline layer.

Cortical Reaction: Fusion of the gamete membranes trigger a release of Ca creating a fertilization envelope.

Internal FertilizationSecretions in the female mammalian reproductive

tract alter certain molecules on the surface of sperm cells and increase sperm motilityZona pellucida: extracellular matrix of the egg.

Functions as a sperm receptor , binding to a complimentary molecule on the surface of the sperm head.

Key Difference: The haploid nuclei of mammalian sperm and egg do not fuse immediately (only after the 1st division do the chromosomes from the two parents come together.) Much slower!

2. Cleavage

Important DefinitionsMorula: After further cleavage divisions, the

embryo is in a multicellular ball that is still surrounded by the fertilization envelope. The blastocoel has begun to form.

Blastula: A single layer of cells surrounds a large blastocoel. The fertilization envelope is still present; the embryo will soon hatch from it and begin swimming.

Yolk! Yummyyyyyy stored nutrientsEggs and zygotes of sea urchins (and other

species) have definite polarity. During cleavage the planes of division follow a specific pattern relative to the poles of the zygoteThe distribution of yolk is a key factor

influencing the pattern of cleavage Yolk is more concentrated toward the vegetal pole Yolk decreases toward the animal pole

Amphibian Development1. The polarity of the egg

determines the anterior-posterior axis before fertilization.

2. 2. At fertilization the pigmented cortex slides over the underlying cytoplasm toward the point of sperm entry. This rotation exposes a region of lighter colored cytoplasm (which is a mark of the dorsal side)

3. 3. The first cleavage division bisects the gray crescent.

More Yolk!Has a pronounced effect on cleavage in the eggs

of birds, other reptiles, many fishes, and insects.Meroblastic cleavage: Cleavage of the fertilized egg

is restricted to the small disk of yolk-free cytoplasm and cannot penetrate through the dense yolk. Incomplete division of a yolk-rich egg. (birds)

Holoblastic cleavage: The complete division of eggs having little yolk (as in sea urchins) or a moderate amount (frogs)

Blastoderm (birds): a cap of cells formed in early cleavage divisions in birds, that rests on the undivided egg yolk.

3. GastrulationA dramatic rearrangement of the cells of the blastula to form a

three-layered embryo with a primitive gut.The process is driven by the same mechanisms in all species

Changes in cell motility Changes in cell shape Changes in cellular adhesion to other cells and to molecules of the

extracellular matrixGastrula: The three-layered embryoGerm Layers: The three layers produced (embryonic tissue)Ectoderm: Forms the outer layer of the gastrulaEndoderm: Lines the embryonic digestive tractMesoderm: Partly fills the space between the ectoderm and the

endodermEventually these 3 cell layers develop into all the tissues and

organs of the adult animal

Gastrulation in Sea Urchins

Gastrulation in a sea urchin embryo. Gastrulation begins with the migration of mesenchyme (mesoderm) cells from the vegetal pole into the blastocoel. The vegetal plate invaginates (buckles inward). Endoderm cells form the archenteron. Filopodia (made of mesenchyme cells) drag the archenteron across the blastocoel. Fusion of the archenteron with the blastocoel wall forms a digestive tube with a mouth and an anus.

Gastrulation in Frogs

4. OrganogenesisVarious regions of the 3 embryonic germ

layers develop into the rudiments of organsInvolves more localized morphogenetic

changes in tissue and cell shape 1st evidence of organ building is the appearance of

folds, splits, and dense clustering of cells