Selected Invertebrates Early Development Selected Invertebrates

Cleavage What characterizes this process? rapid cell division; no overall growth; no increase in volume

Cleavage How does the cell cycle of blastomeres compare with that of somatic cells? biphasic versus multiphasic MPF responsible for biphasic cycle; controlled by presence of cyclin B chromosome condensation, nuclear envelop breakdown and spindle formation all depend on kinase activity

Cleavage What happens to the embryo during the mid-blastula transition? When does this occur? G1 and G2 are added to cell cycle; varies with organism (12-17 divisions); loss of cell synchrony

Cleavage What is karyokinesis and cytokinesis and how do they compare?

Cleavage What influences the pattern of cleavage in a particular organism? amount of yolk and position and timing of spindle formation

Cleavage inherited pattern of cell division also influences pattern

Cleavage

Cleavage How are cell fates specified during cleavage? cell-cell interactions asymmetric distribution of morphogenic determinants morphogenic determinants – transcription factors- cytoskeleton plays a role in distribution; also centrosome

Gastrulation What characterizes gastrulation in organisms? What kinds of movements occur? invagination – sea urchin endoderm involution – amphibian mesoderm ingression – sea urchin mesoderm delamination – mammalian hypoblast formation epiboly – ectoderm formation in amphibians

Axis Formation What are the three major body axes?

Sea Urchin Development What does the cleavage pattern look like?

Sea Urchin Development What characterizes the blastula stage? expanding blastocoel,cilla develops, embryo rotates in fertilization membrane, formation of vegetal plate, formation of hatching enzyme

Sea Urchin Development At what stage are the fates of individual cells determined? 60-cell fate map; most fates by conditional specification; micromeres via autonomous transplanted micromeres will alter fate of adjacent cells causing them to gastrulate

Sea Urchin Development What role does β-catenin play in specification? specifies micromeres (accumulates in nuclei of cells fated to become mesoderm and endoderm) and vegetal half of embryo lithium chloride causes accumulation of catenin in every cell; needed for inductive effects of micromeres β-catenin works through Pmar I genef delta-notch pathway also involved in specification of vegetal cells to become secondary mesenchyme

Sea Urchin Development

Sea Urchin Development When does axis specification occur? animal-vegetal axis established before fertilization anterior-posterior axis determined by a-v axis vegetal – determinants for posterior development dorsal-ventral and left-right after fertilization d-v by first cleavage plane

Sea Urchin Development How does gastrulation begin? ingression of primary mesenchyme (descendents of micromeres) – formation of filapodia

Sea Urchin Development What appears to be responsible for the ingression of primary mesenchyme? detachment from hyaline layer and adhersion to fibronectin and laminin associated with extracellular matrix primary mesenchyme form spicules

Sea Urchin Development What appears to be responsible for the initial invagination that occurs during gastrulation? shape changes in vegetal plate cells and in extracellular matrix

Sea Urchin Development secretory granules secrete CSPG into inner lamina of hyaline layer – absorbs water, swells – buckling of hyaline layer foregut, midgut and hindgut What is the fate of these vegetal plate cells?

Sea Urchin Development What happens during later stages of invagination? convergent extension

Sea Urchin Development secondary mesenchyme and filapodia; dispersion of secondary mesenchyme

Snail Development What kind of cleavage pattern characterizes these animals? spiral holoblastic – no blastocoel after second cleavage get micromeres in animal pole – displaced to right or left of macromere

Snail Development Orientation of cleavage plane determines right or left coiling snails determined by cytoplasmic factors in oocyte direction of shell coiling is controlled by single pair of genes and right is dominant to left direction of cleavage is determined by genotype of snail’s mother not its own genotype – factors come from egg cytoplasm

Snail Development What appears to be responsible for the mosaic development seen in molluscs? in situ hybridization shows accumulation of mRNA for BMP- paracrine factor dpp in B and C distribution of transcription factors to certain cells

Snail Development What is the polar lobe and why is it important? Removing polar lobe produces abnormal larvae – no endoderm, lacks mesodermal organs and eyes lobe contains endodermal and mesodermal determinants – localization established just after fertilization and associated with cytoskeleton or cortex

Snail Development Why does removal of the D blastomere or its first or second derivatives result in incomplete larvae? If D blastomeres don’t directly contribute cells to formation of many structures why are they so important to the formation of the same structures? receives determinant from polar lobe involved in inductive interactions

Snail Development What other aspect of snail development does the polar lobe influence? How do we know this to be true? dorsal-ventral axis formation; exp. that force lobe material into AB as well as CD blastomere

Snail Development How does gastrulation take place in snails? epiboly of micromeres from animal pole overgrows embryo

Tunicate Development What makes these organisms rather unique? What type of cleavage pattern do they have? invertebrate chordates because of larval form bilateral holoblastic; first cleavage plane establishes symmetry

Tunicate Development In what way does the pigmentation in Styela partita provide developmental information? cell fate information; A)before fertilization B)5 min after fert c)migration of sperm pronucleus clear cytoplasm = ectoderm; yellow cytoplasm = mesoderm; dark-gray = endoderm; light gray = neural tube and notocord

Tunicate Development What evidence is there of autonomous specification in tunicate blastomeres? transplant experiments RNA hybridization experiments altering β-catenin levels in cells transplants of larval nuclei into enucleated egg fragments produces cell characteristic of cytoplasm not nucleus Macho-1 mRNA found in vegetal hemisphere of unfertilized egg, migrates with yellow crescent into posterior vegetal region and eventually only in blastomeres giving rise to muscle cells inhibition of B-catenin results in loss of endoderm and increased B-catenin caused ectoderm to develop as endoderm

Tunicate Development What evidence is there for conditional specifiction? BMP signal from endoderm induces anterior cell to become notocord works through activation of Brachury gene FGF signal induces posterior cell to become mesenchyme

Tunicate Development When are the embryonic axes established? dorsal-ventral – prior to first cleavage anterior-posterior – prior to first cleavage left-right – first cleavage cap of cytoplasm at vegetal pole – future dorsal side – initiation of gastrulation posterior vegetal region (yellow crescent) – posterior side

Tunicate Development What is gastrulation like in these organisms? invagination, involution, and epiboly

Caenorhabditis elegans What does C. elegans look like? 959 cells; embryonic development in 16 hours; 1 mm long

C. elegans What pattern of cleavage in seen in this nematode? rotational holoblastic – each division produces founder cell and stem cell

C. elegans

C. elegans What determines anterior-posterior axis? What is the importance of the P-granules? elongated axis of egg and sperm pronucleus enters posterior P-granules specify germ cells

C. elegans When is dorsal-ventral and left-right axes established? d-v from division of AB cell; Abp becomes dorsal side; EMS becomes ventral side l-r at 12cell stage

C. elegans In what way is autonomous specification demonstrated in this organism? P1 develops without presence of AB cell In what way is conditional specification demonstrated? AB cell requires cell-cell interactions EMS requires signal from P2 maternal proteins acting as transcription factors include SKN-1, PAL-1, and PIE-1 determine fates of cells descended from P1. SKN-1 needed for fates of EMS cells (pharyngeal mesoderm and endoderm) PAL-1 needed for somatic cell derivatives and PIE-1 needed for germ cell derivatives AB alone only develops posterior parts of embryo

C. elegans cell-cell signaling APX-1 –delta; GLP-1 –notch MOM-2 wnt; MOM-5 frizzled

C. elegans Where in this organism is autonomous and conditional specification integrated? pharynx development

C. elegans When does gastrulation begin in this organism? just after formation pf P4 cell – 24 cell embryo