Cellular Mechanisms of Development

1. Cellular Mechanismsof Development Chapter 19

2. Overview of Development Development is the successive process of systematic gene-directed changes throughout an organism’s life cycle -Can be divided into four subprocesses: -Growth (cell division) -Differentiation -Pattern formation -Morphogenesis

3. Cell Division After fertilization, the diploid zygote undergoes a period of rapid mitotic divisions -In animals, this period is called cleavage -Controlled by cyclins and cyclin- dependent kinases (Cdks) Duringcleavage,the zygote is divided into smaller & smaller cells called blastomeres -Moreover, the G1 and G2 phases are shortened or eliminated

4. Cell Division

5. Cell Division

6. Cell Division

7. Cell Division

8. Cell Division Caenorhabditis elegans -One of the best developmental models -Adult worm consists of 959 somatic cells -Transparent, so cell division can be followed -Researchers have mapped out the lineage of all cells derived from the fertilized egg

11. Cell Division Blastomeres are nondifferentiated and can give rise to any tissue Stem cells are set aside and will continue to divide while remaining undifferentiated -Tissue-specific: can give rise to only one tissue -Pluripotent: can give rise to multiple different cell types -Totipotent: can give rise to any cell type

12. Cell Division Cleave in mammals continues for 5-6 days producing a ball of cells, the blastocyst -Consists of: -Outer layer = Forms the placenta -Inner cell mass = Forms the embryo -Source of embryonic stem cells (ES cells)

15. Cell Division A plant develops by building its body outward -Creates new parts from stem cells contained in structures called meristems -Meristematic stem cells continually divide -Produce cells that can differentiate into the various plant tissues -Leaves, roots, branches, and flowers The plant cell cycle is also regulated by cyclins and cyclin-dependent kinases

16. Cell Differentiation A human body contains more than 210 major types of differentiated cells Cell determination commits a cell to a particular developmental pathway -Can only be “seen” by experiment -Cells are moved to a different location in the embryo -If they develop according to their new position, they are not determined

18. Cell Differentiation Cells initiate developmental changes by using transcriptional factors to change patterns of gene expression Cells become committed to follow a particular developmental pathway in one of two ways: 1) via differential inheritance of cytoplasmic determinants 2) via cell-cell interactions

19. Cell Differentiation Cytoplasmic determinants -Tunicates are marine invertebrates -Tadpoles have tails, which are lost during metamorphosis into the adult -Egg contains yellow pigment granules -Become asymmetrically localized following fertilization -Cells that inherit them form muscles

21. Cell Differentiation Cytoplasmic determinants -Female parent provides egg with macho-1 mRNA -Encodes a transcription factor that can activate expression of muscle- specific genes

22. Cell Differentiation Induction is the change in the fate of a cell due to interaction with an adjacent cell If cells of a frog embryo are separated: -One pole (“animal pole”) forms ectoderm -Other pole (“vegetal pole”) forms endoderm -No mesoderm is formed If the two pole cells are placed side-by-side, some animal-pole cells form the mesoderm

23. Cell Differentiation Another example of induction is the formation of notochord and mesenchyme in tunicates -Arise from mesodermal cells that form at the vegetal margin of 32-cell stage embryo -Cells receive a chemical signal from underlying endodermal cells -Anterior cells differentiate into notochord -Posterior cells differentiate into mesenchyme

26. Cell Differentiation The chemical signal is a fibroblast growth factor (FGF) molecule -The FGF receptor is a tyrosine kinase that activates a MAP kinase cascade -Produces a transcription factor that triggers differentiation Thus, the combination of macho-1 and FGF signaling leads to four different cell types

27. Cell Differentiation

28. Cell Differentiation

29. Cell Differentiation

30. Cloning Until very recently, biologists thought that determination and cell differentiation were irreversible in animals Nuclear transplant experiments in mammals were attempted without success -Finally, in 1996 a breakthrough Geneticists at the Roslin Institute in Scotland performed the following procedure:

31. Cloning 1. Differentiated mammary cells were removed from the udder of a six-year old sheep 2. Eggs obtained from a ewe were enucleated 3. Cells were synchronized to a resting state 4. The mammary and egg cells were combined by somatic cell nuclear transfer (SCNT) 5. Successful embryos (29/277) were placed in surrogate mother sheep 6. On July 5, 1996, Dolly was born

34. Cloning Dolly proved that determination in animals is reversible -Nucleus of a differentiated cell can be reprogrammed to be totipotent Reproductive cloning refers to the use of SCNT to create an animal that is genetically identical to another -Scientists have cloned cats, rabbits, rats, mice, goats and pigs

35. Cloning Reproductive cloning has inherent problems 1. Low success rate 2. Age-associated diseases Normal mammalian development requires precise genomic imprinting -The differential expression of genes based on parental origin Cloning fails because there is not enough time to reprogram the genome properly

36. Cloning In therapeutic cloning, stem cells are cloned from a person’s own tissues and so the body readily accepts them Initial stages are the same as those of reproductive cloning -Embryo is broken apart and its embryonic stem cells extracted -Grown in culture and then used to replace diseased or injured tissue

37. Cloning

38. Cloning

39. Cloning Human embryonic stem cells have enormous promise for treating a wide range of diseases -However, stem cell research has raised profound ethical issues Very few countries have permissive policy towards human reproductive cloning -However, many permit embryonic stem cell research

40. Cloning Early reports on a variety of adult stem cells indicated that they may be pluripotent -Since then these results have been challenged

41. Pattern Formation In the early stages of pattern formation, two perpendicular axes are established -Anterior/posterior (A/P, head-to-tail) axis -Dorsal/ventral (D/V, back-to-front) axis Polarity refers to the acquisition of axial differences in developing structures Position information leads to changes in gene activity, and thus cells adopt a fate appropriate for their location

42. Drosophila Embryogenesis Drosophila produces two body forms -Larva – Tubular eating machine -Adult – Flying sex machine axes are established Metamorphosis is the passage from one body form to another Embryogenesis is the formation of a larva from a fertilized egg

43. Drosophila Embryogenesis Before fertilization, specialized nurse cells move maternal mRNAs into maturing oocyte -These mRNA will initiate a cascade of gene activations following fertilization Embryonic nuclei do not begin to function until approximately 10 nuclear divisions later

44. Drosophila Embryogenesis After fertilization, 12 rounds of nuclear division without cytokinesis produces a syncytial blastoderm -4000 nuclei in a single cytoplasm Membranes grow between the nuclei forming the cellular blastoderm Within a day of fertilization, a segmented, tubular body is formed

47. Drosophila Embryogenesis Nüsslein-Volhard and Wieschaus elucidated how the segmentation pattern is formed -Earned the 1995 Nobel Prize Two different genetic pathways control the establishment of the A/P and D/V polarity -Both involve gradients of morphogens -Soluble signal molecules that can specify different cell fates along an axis

50. Establishment of the A/P axis Nurse cells secrete maternally produced bicoid and nanos mRNAs into the oocyte -Differentially transported by microtubules to opposite poles of the oocyte -bicoid mRNA to the future anterior pole -nanos mRNA to the future posterior pole -After fertilization, translation will create opposing gradients of Bicoid and Nanos proteins