Animal Development By Natasha Guenther, Brea Altoya, and Bianca (I can’t spell her last name so I’m leaving it out)

Key Concepts After 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 animals Organism’s dev. Is determined by the genome of the zygote and also by the differences that arise b/w early embryonic cells These differences set the stage for the expression of different genes in different cells Cytoplasmic determinates: maternal substances. Affect dev. Of the cells that inherit them during early mitotic divisions of the zygote Cell division continues Cell 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. Fertilization Main 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

Definitions Acrosomal 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 Fertilization Secretions in the female mammalian reproductive tract alter certain molecules on the surface of sperm cells and increase sperm motility Zona 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 Definitions Morula: 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 nutrients Eggs 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 zygote The 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 Development The polarity of the egg determines the anterior-posterior axis before fertilization. 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. 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. Gastrulation A 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 matrix Gastrula: The three-layered embryo Germ Layers: The three layers produced (embryonic tissue) Ectoderm: Forms the outer layer of the gastrula Endoderm: Lines the embryonic digestive tract Mesoderm: Partly fills the space between the ectoderm and the endoderm Eventually 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. Organogenesis Various regions of the 3 embryonic germ layers develop into the rudiments of organs Involves 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