1 Chapter 54. Development

2 What’s the most complex problem in biology?

3 The most complex problemHow to get from here to there

4 Development: cellular levelCell division Differentiation cells become specialized in structure & function if each kind of cell has the same genes, how can they be so different shutting off of genes = loss of totipotency Morphogenesis “creation of form” = give organism shape basic body plan polarity one end is different than the other symmetry left & right side of body mirror each other asymmetry pssst, look at your hand…

5 Development: step-by-stepGamete formation Fertilization Cleavage (cell division, mitosis) Gastrulation (morphogensis) Organ formation (differentiation) Growth & tissue formation (differentiation)

6 Fertilization Joining of egg nucleus & sperm nucleushow does sperm get through egg cell membrane? how to protect against fertilization by multiple sperm how is rest of development triggered?

7 Fertilization Joining of sperm & egg sperm head enters egg

8 Blocking polyspermy Triggers opening of Na+ channels in egg cell membrane depolarizes membrane “fast block” to polyspermy Triggers signal transduction pathway release of Ca+2 from ER causes cascade reaction across egg “fertilization envelope” forms like bubble around egg “slow block” to polyspermy

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10 “Fast block” to polyspermyRelease of Na+ causes depolarization wave reaction across egg membrane

11 Cleavage: start of multicellularityRepeated mitotic divisions of zygote may be unequal divisions of cytoplasm cleavage pattern determined by amount of yolk in egg leaves different contents in each cell seals development fate of each cell & its descendants vegetal pole = yolk-rich end animal pole = nearest the nucleus

12 Early embryonic stagesMorula solid ball stage Blastula hollow fluid-filled ball stage by time human embryo reaches uterus Gastrula development of primitive digestive tract (gut) & tissue layers

13 Egg   zygote   morula

14 Cell signaling Regulating the expression of genes that affect the developmental fate of the cell

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16 Egg   morula   blastulaBlastula formation successive divisions result in a blastula hollow single-layered sphere enclosing a space = blastocoel

17 Gastrulation zygote  blastula  gastrularearranges the blastula to form 3-layered embryo with a primitive gut

18 Gastrulation zygote  blastula  gastrulaHow you looked as a blastula…

19 Primary tissue or “germ” layersectoderm external surfaces: skin epidermis (skin); nails, hair & glands; tooth enamel; eye lens; epithelial lining of nose, mouth & rectum; nervous system endoderm internal lining epithelial lining of digestive tract & respiratory systems; reproductive system & urinary tract; digestive organs mesoderm middle tissues: muscle, blood & bone notochord; skeletal, muscular, circulatory, lymphatic, excretory & reproductive systems; lining of body cavity

20 blastopore: forms at sperm penetration pointGastrulation archenteron: forms endoderm embryonic gut blastopore: forms at sperm penetration point mesoderm dorsal lip: organizing center for development tissue formation

21 Organogenesis organization of differentiated cells into tissues & organs cell migration by changes in shape cells fold inward as pockets by changing shape cell movements cells move by pseudopods projecting from the cell body signals from cues guided by following chemical gradients respond to adhesive cues from recognition proteins on adjacent cells

22 Cells move by pseudopods

23 Cells change size & shape: sheets of cells expand & fold inward & outwardChanges in cell shape involve reorganization of cytoskeleton

24 Organogenesis Organ development begins with the formation of:neural tube future spinal cord & brain notochord primitive skeleton, replaced by vertebrate spinal column somites bands of tissue that will become muscles & bones

25 Neural tube developmentNeural tube & notochord embryonic structures that will become spinal chord & vertebrae

26 Apoptosis Programmed cell death Sculpts body partsGenetically programmed elimination of tissues & cells that were used for only short periods in embryo or adult human embryos develop with webs between toes & fingers, but they are not born that way!

27 Apoptosis