Principles of Development

1. Principles of Development Chapter 8

2. Key Events in Development • Development describes the changes in an organism from its earliest beginnings through maturity. • Search for commonalities.

3. Key Events in Development • Specialization of cell types occurs as a hierarchy of developmental decisions. • Cell types arise from conditions created in preceding stages. • Interactions become increasingly restrictive. • With each new stage: • Each stage limits developmental fate. • Cells lose option to become something different • Said to be determined.

4. Key Events in Development • The two basic processes responsible for this progressive subdivision: • Cytoplasmic localization • Induction

5. Fertilization • Fertilization is the initial event in development in sexual reproduction. • Union of male and female gametes • Provides for recombination of paternal and maternal genes. • Restores the diploid number. • Activatesthe egg to begin development.

6. Fertilization • Oocyte Maturation • Egg grows in size by accumulating yolk. • Contains much mRNA, ribosomes, tRNA and elements for protein synthesis. • Morphogenetic determinants direct the activation and repression of specific genes later in post-fertilization development. • Egg nucleus grows in size, bloated with RNA. • Now called the germinal vesicle.

7. Fertilization • Most of these preparations in the egg occur during the prolonged prophase I. • In mammals • Oocytenow has a highly structured system. • After fertilization it will support nutritional requirements of the embryo and direct its development through cleavage. • After meiosis resumes, the egg is ready to fuse its nucleus with the sperm nucleus.

8. Fertilization • A century of research has been conducted on marine invertebrates. • Especially sea urchins

9. Contact Between Sperm & Egg • Broadcast spawners often release a chemotactic factor that attracts sperm to eggs. • Species specific • Sperm enter the jelly layer. • Egg-recognition proteins on the acrosomal process bind to species-specific sperm receptors on the vitelline envelope.

10. Fertilization in Sea Urchins • Prevention of polyspermy – only one sperm can enter. • Fast block • Depolarization of membrane • Slow block • Cortical reaction resulting in fertilization membrane

11. Fertilization in Sea Urchins • The cortical reaction follows the fusion of thousands of enzyme-rich cortical granules with the egg membrane. • Cortical granules release contents between the membrane and vitelline envelope. • Creates an osmotic gradient • Water rushes into space • Elevates the envelope • Lifts away all bound sperm except the one sperm that has successfully fused with the egg plasma membrane.

12. Fertilization in Sea Urchins

13. Fertilization in Sea Urchins • One cortical granule enzyme causes the vitelline envelope to harden. • Now called the fertilization membrane. • Block to polyspermy is now complete. • Similar process occurs in mammals.

14. Fertilization in Sea Urchins • The increased Ca2+ concentration in the egg after the cortical reaction results in an increase in the rates of cellular respiration and protein synthesis. • The egg is activated.

15. Fusion of Pronuclei • After sperm and egg membranes fuse, the sperm loses its flagellum. • Fusion of male and female pronuclei forms a diploid zygote nucleus.

16. Cleavage • Cleavage – rapid cell divisions following fertilization. • Very little growth occurs. • Each cell called a blastomere. • Morula – solid ball of cells. First 5-7 divisions.

17. Polarity • The eggs and zygotes of many animals (not mammals) have a definite polarity. • The polarity is defined by the distribution of yolk. • The vegetal pole has the most yolk and the animal pole has the least.

18. Body Axes • The development of body axes in frogs is influenced by the polarity of the egg. The polarity of the egg determines the anterior-posterior axis before fertilization. At fertilization, the pigmented cortex slides over the underlying cytoplasm toward the point of sperm entry. This rotation (red arrow) exposes a region of lighter-colored cytoplasm, the gray crescent, which is a marker of the dorsal side. The first cleavage division bisects the gray crescent. Once the anterior-posterior and dorsal-ventral axes are defined, so is the left-right axis.

19. Amount of Yolk • Different types of animals have different amounts of yolk in their eggs. • Isolecithal – very little yolk, even distribution. • Mesolecithal – moderate amount of yolk concentrated at vegetal pole. • Telolecithal – Lots of yolk at vegetal pole. • Centrolecithal – lots of yolk, centrally located.

20. Cleavage in Frogs • Cleavage planes usually follow a specific pattern that is relative to the animal and vegetal poles of the zygote. • Animal pole blastomeres are smaller. • Blastocoel in animal hemisphere. • Little yolk, cleavage furrows complete. • Holoblastic cleavage

21. Cleavage in Birds • Meroblastic cleavage, incomplete division of the egg. • Occurs in species with yolk-rich eggs, such as reptiles and birds. • Blastoderm – cap of cells on top of yolk.

22. Direct vs. Indirect Development • When lots of nourishing yolk is present, embryos develop into a miniature adult. • Direct development • When little yolk is present, young develop into larval stages that can feed. • Indirect development • Mammals have little yolk, but nourish the embryo via the placenta.

23. Blastula • A fluid filled cavity, the blastocoel, forms within the embryo – a hollow ball of cells now called a blastula.

24. Gastrulation • The morphogenetic process called gastrulation rearranges the cells of a blastula into a three-layered (triploblastic) embryo, called a gastrula, that has a primitive gut. • Diploblastic organisms have two germ layers.

25. Gastrulation • The three tissue layers produced by gastrulation are called embryonic germlayers. • The ectoderm forms the outer layer of the gastrula. • Outer surfaces, neural tissue • The endoderm lines the embryonic digestive tract. • The mesoderm partly fills the space between the endoderm and ectoderm. • Muscles, reproductive system

26. Gastrulation – Sea Urchin • Gastrulation in a sea urchin produces an embryo with a primitive gut (archenteron) and three germ layers. • Blastopore – open end of gut, becomes anus in deuterostomes.

27. Gastrulation - Frog • Result – embryo with gut & 3 germ layers. • More complicated: • Yolk laden cells in vegetal hemisphere. • Blastula wall more than one cell thick.

28. Gastrulation - Chick • Gastrulation in the chick is affected by the large amounts of yolk in the egg. • Primitive streak – a groove on the surface along the future anterior-posterior axis. • Functionally equivalent to blastopore lip in frog.

29. Gastrulation - Chick • Blastoderm consists of two layers: • Epiblast and hypoblast • Layers separated by a blastocoel • Epiblast forms endoderm and mesoderm. • Cells on surface of embryo form ectoderm.

30. Gastrulation - Mouse • In mammals the blastula is called a blastocyst. • Inner cell mass will become the embryo while trophoblast becomes part of the placenta. • Notice that the gastrula is similar to that of the chick.

31. Suites of Developmental Characters • Two major groups of triploblastic animals: • Protostomes • Deuterostomes • Differentiated by: • Spiral vs. radial cleavage • Regulative vs. mosaic cleavage • Blastopore becomes mouth vs. anus • Schizocoelous vs. enterocoelous coelom formation.

32. Deuterostome Development • Deuterostomes include echinoderms (sea urchins, sea stars etc) and chordates. • Radialcleavage

33. Deuterostome Development • Regulative development – the fate of a cell depends on its interactions with neighbors, not what piece of cytoplasm it has. A blastomere isolated early in cleavage is able to from a whole individual.

34. Deuterostome Development • Deuterostome means second mouth. • The blastopore becomes the anus and the mouth develops as the second opening.

35. Deuterostome Development • The coelom is a body cavity completely surrounded by mesoderm. • Mesoderm & coelom form simultaneously. • In enterocoely, the coelom forms as outpocketing of the gut.

36. Deuterostome Development • Typical deuterostomes have coeloms that develop by enterocoely. • Vertebrates use a modified version of schizocoely.

37. Protostome Development • Protostomes include flatworms, annelids and molluscs. • Spiral cleavage

38. Protostome Development • Mosaic development – cell fate is determined by the components of the cytoplasm found in each blastomere. • Morphogenetic determinants. • An isolated blastomere can’t develop.

39. Protostome Development • Protostome means first mouth. • Blastopore becomes the mouth. • The second opening will become the anus.

40. Protostome Development • In protostomes, a mesodermal band of tissue forms before the coelom is formed. • The mesoderm splits to form a coelom. • Schizocoely • Not all protostomes have a true coelom. • Pseudocoelomates have a body cavity between mesoderm and endoderm. • Acoelomates have no body cavity at all other than the gut.

41. Two Clades of Protostomes • Lophotrochozoan protostomes include annelid worms, molluscs, & some small phyla. • Lophophore – horseshoe shaped feeding structure. • Trochophore larva • Feature all four protostome characteristics.

42. Two Clades of Protostomes • The ecdysozoan protostomes include arthropods, roundworms, and other taxa that molt their exoskeletons. • Ecdysis – shedding of the cuticle. • Many do not show spiral cleavage.

43. Building a Body Plan • An organism’s development is determined by the genome of the zygote and also by differences that arise between early embryonic cells. • Different genes will be expressed in different cells.

44. Building a Body Plan • Uneven distribution of substances in the egg called cytoplasmic determinants results in some of these differences. • Position of cells in the early embryo result in differences as well. • Induction

45. Restriction of Cellular Potency • In many species that have cytoplasmic determinants only the zygote is totipotent, capable of developing into all the cell types found in the adult.

46. Restriction of Cellular Potency • Unevenly distributed cytoplasmic determinants in the egg cell: • Are important in establishing the body axes. • Set up differences in blastomeres resulting from cleavage.

47. Restriction of Cellular Potency • As embryonic development proceeds, the potency of cells becomes progressively more limited in all species.

48. Cell Fate Determination and Pattern Formation by Inductive Signals • Once embryonic cell division creates cells that differ from each other, • The cells begin to influence each other’s fates by induction.

49. Induction • Induction is the capacity of some cells to cause other cells to develop in a certain way. • Dorsal lip of the blastopore induces neural development. • Primary organizer

50. Spemann-Mangold Experiment • Transplanting a piece of dorsal blastopore lip from a salamander gastrula to a ventral or lateral position in another gastrula developed into a notochord & somites and it induced the host ectoderm to form a neural tube.