Developmental Patterns PDF

Developmental Patterns: Chapt 1: Stages of Early Animal Development fertilization cleavage gastrulation neurulation organogenesis Chapt 2: Commitment specification and determination types of specification Chapt 1: p. 7-11; Chapt 2: p. 3...

Developmental Patterns: Chapt 1: Stages of Early Animal Development fertilization cleavage gastrulation neurulation organogenesis Chapt 2: Commitment specification and determination types of specification Chapt 1: p. 7-11; Chapt 2: p. 36-45 (Chapt 1: p. 7-11; 14-18 Chapt 2: p. 39-50)  Early development in animals Example: Frog 1) Fertilization: entry of the sperm into the egg (oocyte) triggers: the completion of oocyte meiosis => haploid pronucleus the fusion of the egg and the sperm pronuclei => zygote the prevention of polyspermy the initiation of cell division cycles Embryonic cell cycles  Early development in animals 2) Cleavage is the rapid series of mitotic cell divisions. no embryonic gene expression and no cell growth during early cleavage (no G 1 or G2) → the size of the embryo remains unchanged and the size of the cells decreases with cell each division everything that the embryo needs for growth and survival was provided maternally (by expression of maternal genes during oogenesis) in amphibians: cell division rate is slower at the vegetal hemisphere than at the animal hemisphere, due to its high yolk density (mostly maternal protein products) morula: a solid cluster of cells (after a few cell divisions) blastula: a later stage in which the embryo becomes a hollow sphere of cells (blastocoel: a fluid-filled cavity) Early development in Xenopus  Early development in animals at the mid-blastula transition, cell growth and embryonic gene expression begins (G1 and G2) → the rate of cell division slows down and the embryo begins to grow in size 3) Gastrulation is the movement of cells that lead to the formation of the three germ layers. the archenteron (gastrocoel) extends inside and also becomes wider and the blastopore becomes circular Gastrulation Lab 1 handout, p. 3 Gastrulation Lab 1 handout, p. 3 Gastrulation Lab 1 handout, p. 3  Gastrulation in Xenopus Posterior view Amphibian gastrulation Lab manual, p. 95  Early development in animals Germ layers: ectoderm (outer): skin, nervous system endoderm (inner): digestive and respiratory systems mesoderm (middle): circulatory system, blood, bones, muscles, gonads, osmoregulatory and urinary systems Some organs have components of more than one layer (example: the amnion in birds and mammals).  Early development in animals 4) Neurulation (neurula stage in vertebrate embryos) is the separation of the central nervous system primordium from the rest of the ectoderm. a region of ectoderm (the neural ectoderm at the dorsal side of embryo) folds to become the neural tube (this is instructed by the underlying mesoderm) the rest of the ectoderm becomes the epidermal (skin) ectoderm the neural crest is also generated: when the neural tube pinches off, some cells at the junction between the neural and epidermal ectoderms migrate to different regions of the embryo Neurulation Early development in Xenopus Neurulation Ectodermal and mesodermal derivatives Neural tube (Central nervous system) Epidermal ectoderm (in birds and mammals)  Early development in animals 5) Organogenesis (morphogenesis): further cell differentiation the arrangement of cells into tissues and organs limb development Early development in Xenopus Summary of the Xenopus life cycle  Differentiation Differentiation is the generation of a specialized cell type from an undifferentiated precursor. Precursor 1. commitment Differentiated cell 2. differentiation A cell becomes committed to a specific fate before it differentiates into a specialized cell type.  Commitment Embryologists have divided the process of cell commitment into two steps: 1) Specification: the cell is capable of differentiating autonomously, but the fate is reversible 2) Determination: the cell is irreversibly committed to a specific fate.  Specification strategies The behavior of a specified cell depends on the type of specification (three types): 1) Conditional specification cell specification depends on interactions between the blastomeres reversible: a cell is specified to a cell type only because of its position in the embryo this type of development is regulative: - a cell can acquire a different fate, depending on its neighboring cells  Conditional specification Examples of conditional specification: Amphibian blastula if dorsal cells are transplanted to the ventral region of the embryo: → the cells will acquire a new specification according to their new environment and will become belly tissue if some cells are removed from the blastula → normal development will occur because other cells can be re-specified to replace the lost cells Sea urchin morula If blastomeres from the 2- or 4-cell stage are isolated, each isolated cell will develop into a complete organism. Conditional specification in amphibians Conditional specification in amphibians  Conditional specification in sea urchins  Specification strategies 2) Autonomous specification irreversible: cell specification depends on the cytoplasmic components present in different regions of the egg (the egg cytoplasm is not uniform) the cytoplasmic make-up of the blastomeres defines their specification this type of development is determinative: - a blastomere has the same fate whether it is isolated from or remains part of an embryo  Autonomous specification in Caenorhabditis elegans The anterior-posterior axis is established in the C. elegans zygote by the differential distribution of cytoplasmic morphogens. As a result, the blastomeres of the 2-cell stage are autonomously specified.  Autonomous specification Examples of autonomous specification: Patella (a shell mollusk): the 16-cell stage blastomeres are autonomously specified. if a blastomere that corresponds to trochoblast (ciliated cell) is removed and placed on a petri dish: → the cell will develop into trochoblast and the larva from which the cell was removed will lack the structure (none of the remaining cells can change their fate to replace it) Tunicate 8-cell stage blastomeres if cells are separated from the embryo → each will form its respective cell types, resulting in fragments, not complete embryos Autonomous specification in Patella Autonomous specification in Patella  Autonomous specification 8-cell stage in tunicates fate map Sea tulip  Specification strategies 3) Syncytial specification (in insects) multiple rounds of mitosis without cytokinesis (syncytial cleavage, the first 12 rounds in Drosophila) results in a single cell with many nuclei (a syncytium) after 13 rounds, the nuclei migrate to the periphery and become separated by cell membranes (cellularization) their specification is determined by gradients of morphogens of maternal origin present in the syncytium prior to cellularization Syncytial cleavage & cellularization in Drosophila  Syncytial specification in Drosophila Segments in the Drosophila larva  The bicoid gradient specifies anterior structures Bicoid: anterior morphogen a maternal gene product The bicoid gradient specifies anterior structures Bicoid is necessary and sufficient for inducing the formation of anterior structures.