Do Now Identify each of the cells in the diagram and indicate the Human chromosome number of each. Sperm (23) Egg (23) Zygote (46)

How do we go from a zygote (a single cell) to a multicellular organism? (hint: think about a process used to increase cell#)

STAGES OF EMBRYONIC DEVELOPMENT Series of steps leading from zygote to organism 1.Cleavage – increase in cell# by rapid mitosis, no cell growth 2. Differentiation – cells get their jobs by “reading” different parts of DNA, form tissues and organs 3. Growth

Stage 1 Egg and sperm are produced by MEIOSIS (gametogenesis). They contain half as much genetic material as parent cells (in humans = 23) The joining of the two nuclei is called fertilization This can occur internally or externally

The sperm approaches the egg The sperm’s enzymes digest the egg’s jelly coat 4 The sperm nucleus enters the egg cytoplasm The plasma membranes of sperm and egg fuse SPERM Nucleus Sperm head Jelly coat EGG CELL Sperm nucleus Egg nucleus 5 The nuclei of sperm and egg fuse Zygote nucleus Stage 2 The fertilized egg or Zygote now has how many chromosomes? 46 Normal fertilization: edu/group/Urchin/nf ert.htm edu/group/Urchin/nf ert.htm

Stage 3 The fertilized egg divides into two new cells by MITOSIS to form identical cells This is the beginning of cleavage: rapid increase in cell #’s by mitosis (stages 3-4) NO CELL GROWTH! # of chromosomes in each cell: 46 2-cell stage

Stage 4 Four Cell Stage: still increasing in numbers by mitosis (cleavage) Number of chromosomes in each cell: 46

Stage 5 Blastula: hollow ball stage Number of chromosomes in each cell: 46 The layer of cells is generally one cell thick and the cavity is filled with fluid Cell growth starts

Stage 6 Gastrulation: inward pushing of blastula to form two layers (indentation) The cell is beginning to grow before dividing

Stage 7 Differentiation: formation of three layers which will develop into organs and organ systems Development can take place internally or externally

Twins Fraternal Twins – two separate eggs fertilized by two separate sperm (dizygotic) Identical Twins – single egg is fertilized, zygote splits early (monzygotic), have identical DNA

Indian girl - born fused at the pelvis to a "parasitic twin" that stopped developing in the womb She had absorbed the organs and body parts of the other fetus, a condition that occurs once in 50,000 conjoined twin births.

Fertilization and Cleavage Video Clip 0v/media/ch27/cleavage_implant.ht mlhttp://trc.ucdavis.edu/biosci10v/bis1 0v/media/ch27/cleavage_implant.ht ml lResearch.htmhttp:// lResearch.htm /celldynamics/events/workshops/arc hive/2003/cytomod_abstracts/GvD_ VDF/images/GvD-VEF-fig1.jpghttp://raven.zoology.washington.edu /celldynamics/events/workshops/arc hive/2003/cytomod_abstracts/GvD_ VDF/images/GvD-VEF-fig1.jpg s.com/adam htmhttp:// s.com/adam htm

Stem cells – cells without a “job”, removed from blastocyst before they differentiate