Vertebrate Development

1. Vertebrate Development Chapter 51

2. Material to be covered (for Tri-C) • Describe the events of each of the three stages of fertilization in an advanced vertebrate. • Compare the cleavage patterns, describe the appearance of the blastula and indicate how gastrulation proceeds in primitive chordates, aquatic vertebrates and reptiles/birds/mammals. • State the tissues produced by the three germ layers: endo-, meso- and ectoderm. • Understand the developmental processes that occur during neuralation. • Explain Haeckel’s “biogenic law”, ontogeny recapitulates phylogeny.

3. Material to be covered (for Tri-C) • Understand the importance of extra-embryonic membranes in terrestrial vertebrate development. • Describe the characteristic events of each trimester of human pregnancy and of postnatal development. • Use vertebrate models to understand embryonic development. • Discussion and evaluation of bioethical issues related to embryology: cloning, stem cells and in vitro fertilization. • Describe the ways that cells can signal each other. • Differentiate between intracellular receptors and cell surfaces receptors.

4. Outline • Stages of Development • Cell Cleavage Patterns • Gastrulation • Developmental Process During Neurulation • How Cells Communicate During Development • Embryonic Development-Vertebrate Evolution • Extraembryonic Membranes • Human Trimesters • Birth and Postnatal Development

5. Stages of Development • Fertilization • combination of gametes • Cleavage • series of extremely rapid mitotic divisions • Gastrulation • series of extensive cell rearrangement • Neuralation • the process where tissue forms a neural tube • Organogenesis • the process where cells interact with one another and rearrange themselves to produce tissues and organs • Gametogenesis • the development of gametes • often not complete until the organism matures • varies greatly within animal kingdom • Maturity • Larvae – pupae – adult • metamorphosis

6. Fertilization • Penetration • glycoprotein-digesting enzymes in acrosome of sperm head • Activation • events initiated by sperm penetration • chromosomes in egg nucleus complete second meiotic division • triggers movement of egg cytoplasm • sharp increase in metabolic activity

7. Stages of Development • Nuclei fusion • The third stage of fertilization is fusion of the entering sperm nucleus with the haploid egg nucleus to form the diploid nucleus.

8. Mammalian Reproductive Cells

9. Vertebrate Development Review • Formation of blastula • water drawn into cell mass forming a hollow ball of cells - blastula or blastocyst • Gastrulation • some cells of blastula push inward, forming a invaginated gastrula • invagination creates main axis of vertebrate body • Has an animal pole and a vegetal pole • Animal pole end forms external tissues • Vegetal pole form internal tissues • embryo now has three germ layers

10. Vertebrate Development Review • Neurulation • zone of ectoderm thickens on dorsal surface of embryo • neural tissue rolls and forms neural tube • cell migration • variety of cells migrate to form distant tissues

11. Vertebrate Development Review • Organogenesis • basic body plan established • tissues develop into organs • embryo will grow to be a hundred times larger

12. Vertebrate Development

13. Vertebrate Development

14. Cell Cleavage Patterns • Initial cell division, cleavage, is not accompanied by an increase in the overall size of the embryo. • morula - mass of 32 cells • Each cell is a blastomere. • eventually a blastula is formed • The pattern of cleavage is influenced by the presence of yolk • Animal Pole – small amount of yolk • Vegetal pole – large amount of yolk

15. Cell Cleavage Patterns • Primitive chordates • holoblastic cleavage - egg contains little or no yolk, and cleavage occurs throughout the whole egg • Amphibians and advanced fish • Eggs contain much more cytoplasmic yolk in one hemisphere than the other. • large cells containing a lot of yolk at one pole, and a concentrated mass of small cells with very little yolk at the other pole.

16. Holoblastic Cleavage

17. Cell Cleavage Patterns • Reptiles and birds • eggs composed almost entirely of yolk • cleavage only occurs in polar cytoplasm • meroblastic cleavage • Mammals • contain very little yolk • holoblastic cleavage • inner cell mass forms developing embryo • outer sphere, trophoblast, enters endometrium

18. Meroblastic Cleavage

19. Cell Cleavage Patterns • Blastula • Each cell is in contact with a different set of neighboring cells. • Interactions are a major factor influencing developmental fate.

20. Gastrulation • Certain groups of cells invaginate and involute from the surface of the blastula during gastrulation. • By the end of gastrulation, embryonic cells have rearranged into three primary germ layers: • ectoderm • mesoderm • endoderm

21. Gastrulation • Gastrulation in primitive chordates • surface of blastula invaginates into the blastocoel • eventually inward-moving wall pushes up against the opposite side of the blastula • produces embryo with two cell layers: • outer ectoderm • inner endoderm • mesoderm forms later between the ectoderm and endoderm

22. Gastrulation in a Lancet

23. Gastrulation • Gastrulation in most aquatic vertebrates • Yolk-laden cells of the vegetal pole are fewer and much larger than the yolk-free cells of the animal pole.

24. Frog Gastrulation

25. Gastrulation • Gastrulation in reptiles, birds, mammals • no yolk separates two sides of embryo • lower cell layer differentiates into endoderm and upper layer into ectoderm without cell movement • primitive streak

26. Mammalian Gastrulation

27. Developmental Processes During Neurulation • Tissue differentiation begins with the formation of the notochord and the hollow dorsal nerve cord. • neurulation • After the notochord has been laid down, ectodermal cells above the notochord invaginate, forming the neural groove down the long axis of the embryo. • edges move toward each other and fuse creating neural tube

28. Mammalian Neural Tube Formation

29. Developmental Processes During Neurulation • On either side of the developing notochord, segmented blocks of mesoderm tissue called somites form. • Ultimately, somites give rise to muscles, vertebrae, and connective tissues. • Mesoderm in the head region remains connected as somitomeres and form striated muscles of the face, jaws, and throat.

30. Developmental Processes During Neurulation • Neural crest • Edges of neural groove pinch off and form the neural crest. • Nearby clusters of ectodermal cells thicken into placodes. • Gill chamber • Some of the neural crest cells form cartilaginous bars between the embryonic pharyngeal slits. • forms efficient pump

31. Developmental Processes During Neurulation • Elaboration of the nervous system • Some neural crest cells migrate ventrally toward the notochord and form sensory neurons in the dorsal root ganglia. • others become specialized Schwann cells

32. How Cells Communicate During Development • Inductions between the three primary tissue types are referred to as primary inductions. • Inductions between tissues that have already been differentiated are called secondary inductions.

33. How Cells Communicate During Development • Nature of development decisions • Some cells become determined early in development. • At some stage, every cell’s fate becomes fixed (commitment). • not irreversible, but rarely reverses under normal conditions

34. Embryonic Development - Vertebrate Evolution • Ontogeny recapitulates phylogeny • Embryological development (ontogeny) involves the same progression of changes that have occurred during evolution (phylogeny). • Homework (due by wednesday) find out if this makes any sense. (turnitin.com) • Do you agree or disagree with this statement? Why? • Does science agree with this statement? Why?

35. Vertebrate Embryonic Development

36. Extraembryonic Membranes • Fluid-filled amniotic membrane: an adaptation to terrestrial life • amniotic membrane an extraembryonic membrane • Extraembryonic membranes, later to become fetal membranes, include the amnion, chorion, yolk sac, and allantois.

37. Extraembryonic Membranes

38. First Trimester • First trimester • fourth week - organ development • organogenesis • most women not yet aware of pregnancy • Fetal Alcohol Syndrome

39. First Trimester • Second month - morphogenesis • limbs assume adult shape • major organs become evident • embryo is about one inch in length • Third month - completion of development • now referred to as fetus • nervous system and sense organs develop • all major organs established

40. Second and Third Trimesters • Second trimester - growth • bone formation occurs • covered with fine hair (lanugo) • by the end of the sixth month, baby is one foot in length • Third trimester - pace of growth accelerates • weight of fetus more than doubles • most major nerve tracts formed within brain • by end, fetus is able to survive on own

41. Birth and Postnatal Development • Uterus releases prostaglandins • begin uterine contractions, but then sensory feedback from the uterus stimulates the release of oxytocin from the mother’s pituitary gland • rate of contraction increases to one contraction every two or three minutes • strong contractions, aided by the mother’s pushing, expels the fetus

42. Birth and Postnatal Development • Nursing • Milk production, lactation, occurs in the alveoli of mammary glands when they are stimulated by prolactin. • milk secreted in alveolar ducts which are surrounded by smooth muscle and lead to the nipple • first milk produced after birth called colostrum - rich in maternal antibodies • Milk synthesis begins about three days following birth.

43. Birth and Postnatal Development • Postnatal development • Babies typically double their birth weight within a few months. • Neuron production occurs for six months. • allometric growth