Development Chapter 47

Big Questions: How does a multicellular organism develop from a zygote? How is the position of the parts of an organism determined? How does differentiation of cell type occur in organisms? How are genetics and development connected? Environment?

Developmental Biologists noted common features of early embryonic stages among animals demonstrated specific patterns of gene expression that direct cells in a developing embryo to adopt distinctive fates embryonic development involves common stages that occur in a set order

Development is Regulated The development of an organism is coordinated by sequential changes in gene expression. Multicellular organisms must regulate: Cell division Differentiation Morphogenesis

Fertilization The formation of a diploid zygote from a haploid egg and sperm Sperm penetrates Changes at the egg surface prevent polyspermy. Ca2 spread across the egg correlates with the appearance of the fertilization envelope

Fertilization envelope Figure 47.4 Experiment Fertilization envelope 10 sec after fertilization 25 sec 35 sec 1 min 500 μm Results Spreading wave of Ca2+ Point of sperm nucleus entry Figure 47.4 Inquiry: Does the distribution of CA2+ in an egg correlate with formation of the fertilization envelope? 1 sec before fertilization 10 sec after fertilization 20 sec 30 sec 500 μm

Video: Calcium Release Following Egg Fertilization

Cleavage Rapid division of cells without G1 or G2 stages So what is physically different about cells? Q: How do cells know where they are in the embryo, and what they should become? (a) Fertilized egg (b) Four-cell stage Figure 47.6 Cleavage in an echinoderm embryo (c) Early blastula (d) Later blastula

Cytoplasmic Determinants Unequal distribution of mRNA and proteins from egg cell Starts process of cell fate and polarity of organism

Cytoplasmic determinants 1 Newly fertilized egg 3 Two-cell embryo Figure 47.21 Partitioning of P granules during C. elegans development 2 Zygote prior to first division 4 Four-cell embryo Unequal distribution of proteins determining cell fate Ex: more proteins in anterior end will signal “head region” to cells

Pattern Formation How do organs develop in their proper positions? How do cells "know" where they are within a developing organism? Proteins released by cells Morphogens form gradients establishing patterns.

Embryonic Induction Signaling by nearby cells send adjacent cells down a specific developmental path.

Presence of the inducer/signaling molecule triggers gene expression (signal transduction) in cell. From translation of that protein starts differentiation of cell.

Morphogenesis “Gastrulation” 3 germ lines established Differentiation accomplished by the expression of cell type-specific proteins. “Organogenesis” Formation of organs

Differentiation Control Genes must be both turned on and turned off at appropriate times and locations. Regulated by internal and external cues

Internal Cues DNA packing, micro RNA’s and epigenetics can switch genes ON or OFF. Transcription factors must be present and can be either stimulatory or inhibitory.

External Cues (outside the cell) External cues signal to cells, causing cells to alter their gene expression.

Environmental Cues The presence of particular molecules and conditions in the local environment is required for development to proceed properly. Ex. The role of temperature and moisture in seed development. Why do you water a seed after you plant it? Hmm…

Hydrangea flowers can change from pink to blue based Environmental Cues Changing the environment around the cells can alter gene expression. Ex: Himalayan rabbits carry the C gene, which is required for the development of pigments in the fur, skin, and eyes, and whose expression is regulated by temperature. Why? Hydrangea flowers can change from pink to blue based on soil pH

Apoptosis “Programmed cell death”: Important role in defining borders and openings in the developing organism.

Homeobox (HOX) Genes Genes that serve as master regulators of animal development. Similar HOX genes found in all animals Ex: the “head” hox gene regulates head development

Hox Gene Ancestry

Experimental Evidence Mutations in similar hox genes caused mutations in same structures.

Experimental Evidence Swapping “leg” hox genes for “antenna” hox gene of a developing fly embryo. Weird!