CHAPTER 8

1. CHAPTER 8 Principles of Development

2. Organizing cells during development

4. Development • Development • Series of progressive changes in an individual from its beginning to maturity • Begins when a fertilized egg divides mitotically • Specialization occurs as a hierarchy of developmental “decisions”

6. Fertilization • Fertilization and Activation • A century of research has been conducted on marine invertebrates • Especially sea urchins • Contact and Recognition Between Egg and Sperm • Marine organisms • release enormous numbers of sperm in the ocean to fertilize eggs • Many eggs release a chemical molecule • Attract sperm of the same species

7. Fertilization • Sea urchin sperm • Penetrate a jelly layer surrounding egg • Next, contacts the vitelline envelope • Thin membrane above the egg plasma membrane • Egg-recognition proteins bind to species-specific sperm receptors on the vitelline envelope • Ensures an egg recognizes only sperm of the same species • In the marine environment • Many species may be spawning at the same time • Similar recognition proteins are found on sperm of vertebrate species

8. Fertilization • Prevention of Polyspermy • Fertilization cone forms where the sperm contacts the vitelline membrane • Sperm head drawn in and fuses with egg plasma membrane • Important changes in the egg surface block entrance to any additional sperm • Polyspermy, the entry of more than one sperm • In the sea urchin, an electrical potential rapidly spreads across the membrane • “fast block”

10. Fertilization • The cortical reaction follows • 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 • 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

13. Binding Sperm to Sea Urchin Egg

15. Fertilization After sperm and egg membranes fuse • Sperm loses its flagellum • Enlarged sperm nucleus migrates inward to contact the female nucleus • Fusion of male and female nuclei forms a diploid zygote nucleus

16. Fertilization • Fertilization • Sets in motion important changes in the egg cytoplasm • Fertilized egg called a zygote • Zygote now enters cleavage

17. Cleavage and Early Development • Cleavage • Embryo divides repeatedly • Large cytoplasmic mass converted into small maneuverable cells: blastomeres • No cell growth occurs, only subdivision until cells reach regular somatic cell size • At the end of cleavage • Zygote has been divided into many hundreds or thousands of cells • Blastula is formed

18. Types of Cleavage is Determined by Yolk

19. Cleavage Types • Holoblastic • Cleavage extends entire length of egg • Egg does not contain a lot of yolk, so cleavage occurs throughout egg • Example: mammals, sea stars, worms • Meroblastic • Cells divide sitting on top of yolk • Too much yolk and yolk can’t divide • Examples: birds, reptiles, fish

20. Egg Types and Cleavage • Isolecithal • Very little yolk, evenly distibuted • Use Holoblastic cleavage- full cleavage • Mesolecithal • Moderate yolk • Use Holoblastic - full cleavage • Telolecithal • Have an abundance of yolk • Use Meroblastic cleavage - partial cleavage

21. Development of Sea Urchin

22. An Overview of Development Following Cleavage • Blastulation • Cleavage creates a cluster of cells called the blastula • Blastula stage typically consists of a few hundred to several thousand cells • During blastula stage, first germ layer forms • In most animals • Cells are arranged around a fluid-filled cavity called the blastocoel (blas-to-seal)

23. An Overview of Development Following Cleavage • Gastrulation and Formation of Two Germ Layers • Gastrulation • Results in the formation of a second germ layer • Involves an invagination of one side of blastula • Forms a new internal cavity • gastrocoel • Opening into the cavity: Blastopore • Gastrula has an outer layer of ectoderm and an inner layer of endoderm

24. Generalized Development showing germ layers Incomplete/ Blind Gut Complete Gut Blastopore (Opening) Gastrocoel (Cavity) 8-24

25. An Overview of Development Following Cleavage • The only opening into embryonic gut is the blastopore • Blind or incomplete gut • Some animals retain the blind gut - the opening does not fully extend to other side (flatworms, sea anemones) • Most develop a complete gut - in which the opening extends and produces a second opening, the anus

26. Generalized Development showing germ layers Incomplete/ Blind Gut Complete Gut

27. An Overview of Development Following Cleavage • Formation of Mesoderm • Animals with two germ layers • Diploblastic (Endoderm and Ectoderm) • Most animals add a 3rd germ layer • Triploblastic • Mesoderm • 3rd germ layer • Forms between the endoderm and the ectoderm • Mesoderm arises from endoderm

28. Developmental Characteristics Germ Layer Outcomes: • Ectoderm • Epithelium and nervous system • Endoderm • Epithelial lining of the digestive and respiratory tract, liver, pancreas, • Mesoderm • Muscular system, reproductive system, bone, kidneys, blood

29. Germ Layer Outcome in mammals 8-29

30. An Overview of Development Following Cleavage • Formation of the Coelom (see-lom) • Coelom • Body cavity surrounded by mesoderm • The method by which the coelom forms is an inherited character • Important in grouping organisms based on developmental characters • Upon completion of coelom formation • Body has 3 tissue layers and 2 cavities • Animals Without a Coelom are called Acoelomates (Ex. flatworms)

32. Developmental Characteristics • Two major groups of triploblastic animals (animals with 3 germ layers) • Protostomes and deuterostomes • The groups are identified by four developmental characters • Cleavage Patterns (radial or spiral) • Fate of Blastopore (mouth or anus) • Coelom Formation (split mesoderm or outpocketing mesoderm) • Embryo Type (Regulative or Mosaic)

34. Protostomes and Deuterostomes Blastopore Fate • Fate of Blastopore • Deuterostome embryos • Develop a complete gut • Blastopore becomes the anus • Second opening becomes the mouth • Protosome embryos • Blastopore becomes the mouth • Anus forms from a second opening

35. Schizocoely Coelom forms from Endodermal cells move to blastopore and develop into mesoderm Mesoderm seperates or splits to form cavity (coelom) Occurs in Protostome (Earthworms, snails) Coelom Formation - mesoderm movement Enterocoely • Mesoderm sides push outward and expand into a pouch-like coelomic compartment • Pouch-like compartment pinches off and forms a mesoderm bound space surrounding the gut • Occurs in Deuterostomes ( Sea stars, fish, frogs, etc.)

37. Blastula and Gastrula Of Embryos

38. Vertebrate Development • The Common Vertebrate Heritage • All vertebrate embryos share chordate hallmarks • Dorsal neural tube • Notochord • Pharyngeal gill pouches with aortic arches • Ventral heart • Postanal tail

40. Vertebrate Development • Amniotes and the Amniotic Egg • Reptiles, birds, and mammals • Embryos develop within the amnion • Fluid-filled sac that encloses the embryo • Provides an aqueous environment in which the embryo floats • Protection from mechanical shock • Amniotic egg contains 4 extraembryonic membranes including the amnion

41. Vertebrate Development • In the shelled amniotic egg: • Yolk sac • Stores yolk • Allantois • Storage of metabolic wastes during development • Respiratory surface for gas exchange

42. Vertebrate Development • Chorion • Lies beneath the eggshell • Encloses the embryo and other extraembryonic membrane • As embryo grows • Need for oxygen increases • Allantois and chorion fuse to form a respiratory surface, the chorioallantoic membrane

43. Chick Embryo

44. A. Fish Larvae - 1 day old, has large yolk sac B. 10 day old fish larva, developed mouth, yolk sac smaller

45. Vertebrate Development • The Mammalian Placenta and Early Mammalian Development • Most mammalian embryos do not develop within an egg shell • Develop within the mother’s body • Most retained in the mother’s body • Monotremes • Primitive mammals that lay eggs • Large yolky eggs resembling bird eggs • Duck-billed platypus and spiny anteater

46. Vertebrate Development • Marsupials • Embryos born at an early stage of development and • Continue development in abdominal pouch of mother • Placental Mammals • Represent 94% of the class Mammalia • Evolution of the placenta • Required reconstruction of extraembryonic membranes • Modification of oviduct • Expanded region formed a uterus

47. Extraembryonic membranes of a mammal

48. Vertebrate Development • Early Stages of Mammalian Development (Human) Germinal Period (1st two weeks) • Blastocyst transported by oviduct to the uterus • Propelled by ciliary action • Around 6th day • Blastocyst = 100 cells • Contacts uterus • By the twelfth day • Implantation is complete • Embryo surrounded by pool of maternal blood • Chorion thickens, sends out tiny fingerlike projections • Chorionic villi

49. Early Development of the human embryo

50. Vertebrate Development • Amnion • Remains unchanged • Surrounds embryo • Secretes fluid in which embryo floats • Yolk sac • Contains no yolk • Source of stem cells that give rise to blood and lymphoid cells • Stem cells migrate to into the developing embryo