1. Cellular Mechanisms of Development Chapter 19

2. 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. 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). During cleavage, the zygote is divided into smaller & smaller cells called blastomeres: -Moreover, the G1 and G2 phases are shortened or eliminated.

4. 4 Cell Division Because of the absence of the two gap/growth phases, the rapid rate of mitotic divisions during cleavage is never again approached in the lifetime of an animal. Example: zebrafish blastomeres divide once every several minutes during cleavage to create an embryo with a thousand of cells in just under 3 hours!!! C. elegans- nematode with 959 somatic cells

5. 5 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.

6. 6 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)

7. 7 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.

8. 8 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.

9. 9 Donor No donor Recipient Before Overt Differentiation Recipient After Overt Differentiation NormalNot Determined (early development) Determined (later development) Tail cells are transplanted to head Tail cells develop into head cells in head Tail cells develop into tail cells in head Tail cells are transplanted to head TailHead Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

10. 10 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.

11. 11 Cell Differentiation Cytoplasmic determinants -Tunicates are marine invertebrates; -Tadpoles have tails, which are lost during metamorphosis into the adult; -Egg contains yellow pigment granules that become asymmetrically localized following fertilization: -Cells that inherit them form muscles - Female parent provides egg with macho-1 mRNA -Encodes a transcription factor that can activate expression of muscle- specific genes

12. 12 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.

13. 13 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:

14. 14 Continuing… 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.

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16. 16 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.

17. 17 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.

18. 18 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.

19. 19 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.

20. 20 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.

21. 21 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.

22. 22 Drosophila Embryogenesis 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.

23. 23 Nurse cells secrete maternally produced bicoid and nanos mRNAs into the oocyte that will be 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. Continuing…

24. 24 Establishment of the D/V axis Maternally produced dorsal mRNA is placed into the oocyte. -Not asymmetrically localized. After fertilization, a series of steps results in selected transport of Dorsal into ventral nuclei, thus forming a D/V gradient

25. 25 Production of Body Plan The body plan is produced by sequential activation of three classes of segmentation genes: 1. Gap genes -Map out the coarsest subdivision along the A/P axis. 2. Pair-rule genes -Divide the embryo into seven zones (Fig 19.13). 3. Segment polarity genes -Finish defining the embryonic segments.

26. 26 Morphogenesis Morphogenesis is the formation of ordered form and structure: -Animals achieve it through changes in: -Cell division -Cell shape and size -Cell death -Cell migration -Plants use these except for cell migration.

27. 27 Morphogenesis Cell division -The orientation of the mitotic spindle determines the plane of cell division in eukaryotic cells: -If spindle is centrally located, two equal-sized daughter cells will result; -If spindle is off to one side, two unequal daughter cells will result.

28. 28 Morphogenesis Cell shape and size -In animals, cell differentiation is accomplished by profound changes in cell size and shape: -Nerve cells develop long processes called axons; -Skeletal muscles cells are large and multinucleated.

29. 29 Morphogenesis Cell death -Necrosis is accidental cell death; -Apoptosis is programmed cell death: -Is required for normal development in all animals -“Death program” pathway consists of: -Activator, inhibitor and apoptotic protease

30. 30 Morphogenesis Cell migration -Cell movement involves both adhesion and loss of adhesion between cells and substrate; -Cell-to-cell interactions are often mediated through cadherins; -Cell-to-substrate interactions often involve complexes between integrins and the extracellular matrix (ECM).

31. 31 Environmental Effects Both plant and animal development are affected by environmental factors: -Germination of a dormant seed proceeds only under favorable soil and day conditions; -Reptiles have a temperature-dependent sex determination (TSD) mechanism; -The water flea Daphnia changes its shape after encountering a predatory fly larva.

32. 32 Environmental Effects

33. 33 Environmental Effects In mammals, embryonic and fetal development have a longer time course. Thus they are more subject to the effects of environmental contaminants, and blood-borne agents in the mother: -Thalidomide, a sedative drug. Many pregnant women who took it had children with limb defects.