Animal Development

1. 1 Animal Development

2. 2 An embryo is not preformed in the egg The embryo develops from epigenesis, the gradual, gene-directed acquisition of form

3. 3 Fertilization The first step in development is the union of male and female gametes … called fertilization External – in fish and amphibians, reproduce in water Internal – sperm introduced into female reproductive tract

4. 4 Functions of Fertilization To combine two haploid gametes ls into a single diploid cell, the zygote To activate the egg Contact of sperm on egg’s surface initiates metabolic reactions within the egg that trigger the onset of embryonic development Contact of sperm with the egg’s surface initiates metabolic reactions within the egg that trigger the onset of embryonic development Contact of sperm with the egg’s surface initiates metabolic reactions within the egg that trigger the onset of embryonic development

5. 5 Three Stages of Fertilization Penetration Sperm penetrates egg Activation Series of events initiated by sperm penetration called egg activation Nuclei fusion Entering sperm fuses with haploid egg nucleus to form diploid nucleus of zygote In some species of frogs, reptiles, and birds, more than one sperm may penetrate the egg, but only one is successful in fertilizing it. In mammals, penetration of the first sperm initiates changes in the egg membrane that prevent the entry of ohe rsperm. In some species of frogs, reptiles, and birds, more than one sperm may penetrate the egg, but only one is successful in fertilizing it. In mammals, penetration of the first sperm initiates changes in the egg membrane that prevent the entry of ohe rsperm.

6. 6 Acrosomal and cortical reactions during sea urchin fertilization Fertilization varies with different animal groups. Sea urchins used a lot as good general model. 1) Sperm cell contacts egg’s jelly coat 2) Hydrolytic enzymes from acrosome make a hole in jelly coat, while growing actin filaments create a protrusion from the sperm head, the acrosomal process 3) Acrosomal binds to receptors on egg’s vitelline layer 4) Fusion of plasma membranes of sperm and egg 5) Entry of sperm nucleus. Fusion of membranes causes electrical change in egg’s plasma membrane and the cortical reaction, blocking entry by other sperm 6) Cortical reaction. Cortical granules in egg fuse w/ plasma membrane and discharge molecules that separate the vitelline layer from plasm membrane and harden it. V layer becomes sperm-proof fertilization envelope.Fertilization varies with different animal groups. Sea urchins used a lot as good general model. 1) Sperm cell contacts egg’s jelly coat 2) Hydrolytic enzymes from acrosome make a hole in jelly coat, while growing actin filaments create a protrusion from the sperm head, the acrosomal process 3) Acrosomal binds to receptors on egg’s vitelline layer 4) Fusion of plasma membranes of sperm and egg 5) Entry of sperm nucleus. Fusion of membranes causes electrical change in egg’s plasma membrane and the cortical reaction, blocking entry by other sperm 6) Cortical reaction. Cortical granules in egg fuse w/ plasma membrane and discharge molecules that separate the vitelline layer from plasm membrane and harden it. V layer becomes sperm-proof fertilization envelope.

7. 7 Acrosomal and Cortical Reactions and Activation of the Egg Sperm cell contacts egg’s jelly coat Acrosome releases hydrolytic enzymes, make hole in jelly coat Growing actin filaments protrude from sperm head, called acrosomal process Tip of process coated with protein that binds to receptors located under jelly coat, on vitelline layer (just external to plasma membrane)

8. 8 Sperm nucleus enters egg Fusion of sperm and egg plasma membranes occurs, depolarizes plasma membrane Termed fast block to polyspermy - prevents multiple sperm from fusing with membrane In cortical reaction, cortical granules in egg fuse with plasma membrane and discharge molecules to separate vitelline layer from membrane, which hardens into fertilization envelope. Termed slow block to polyspermy Voltage across membrane returns to normal, fast block no longer functions At about the time fertilization envelope forms, the membrane voltage returns to normal and fast back to polyspermy no longer functions At about the time fertilization envelope forms, the membrane voltage returns to normal and fast back to polyspermy no longer functions

9. 9 Sharp rise in egg’s cytosolic concentration of Ca2+ triggers the cortical reaction and incites metabolic changes within the egg cell An unfertilized egg has slow metabolism, within a few minutes of fertilization, rates of cellular respiration and protein synthesis increase Egg cell is thus activated Sperm nucleus merges with egg nucleus, creating the diploid nucleus of the zygote DNA synthesis begins, and the first cell division occurs about 90 minutes

10. 10 Timeline for fertilization of sea urchins

11. 11 Fertilization in Mammals Test tube babies have made it possible to study fertilization process in mammals Test tube babies have made it possible to study fertilization process in mammals

12. 12 Fertilization in Mammals Fertilization is internal Mucous secretions alter surface of sperm cells and increase motility Sperm must migrate through outer layer of follicle cells on egg to reach zona pellucida Sperm head binds to receptor on zona pellucida, induces acrosome to release contents similar to sea urchin

13. 13 Fingerlike extensions of egg cell take in whole sperm, basal body of flagellum divides to form two centrosome (w/ centrioles) in zygote Haploid nuclei of sperm and egg do not fuse immediately in mammals Envelopes of both gametes disperse and chromosomes each share common spindle during first mitotic division Become diploid in two daughter cells

14. 14 Stages to Animal Development Fertilization is followed by three successive stages: Cleavage – rapid cell division into larger number of smaller cells Gastrulation - produces three-layered embryo called the gastrula Organogensis - generates rudimentary organs from which adult structures grow Cleavage – cell division w/o growth Cleavage – cell division w/o growth

15. 15 Cleavage Succession of rapid cell divisions that transform the zygote into a ball of much smaller cells, called blastomeres, each with its own nucleus Cleavage in an echinoderm (sea urchin) embryo A) Two-stage cell, following first cleavage division, occurs about 45-90 minutes after fertilization. Fertilization envelope still present. B) Second cleavage division produces four-cell stage C) In a few hours, further cleavage divisions have formed a multicellular ball. The embryo is still inside the fertilization envelope, from which the larva will hatch Cleavage in an echinoderm (sea urchin) embryo A) Two-stage cell, following first cleavage division, occurs about 45-90 minutes after fertilization. Fertilization envelope still present. B) Second cleavage division produces four-cell stage C) In a few hours, further cleavage divisions have formed a multicellular ball. The embryo is still inside the fertilization envelope, from which the larva will hatch

16. 16 Hemispheres of zygoteIn many organisms, distribution of yolk is a key factor in pattern of cleavage With exception of mammals, most animals have both eggs and zygotes with a definite polarity. During cleavage, planes of division follow a specific pattern relative to poles of the zygote. Polarity is determined by distribution of substances in cytoplasm: mRNA, proteins, yolk. In many frogs and other animals, the distribution of yolk is a key factor in influencing pattern of cleavage. Yolk is most concentrated at vegetal pole, while opposite pole, animal pole, has lowest. Hemispheres of zygote named for poles. With exception of mammals, most animals have both eggs and zygotes with a definite polarity. During cleavage, planes of division follow a specific pattern relative to poles of the zygote. Polarity is determined by distribution of substances in cytoplasm: mRNA, proteins, yolk. In many frogs and other animals, the distribution of yolk is a key factor in influencing pattern of cleavage. Yolk is most concentrated at vegetal pole, while opposite pole, animal pole, has lowest. Hemispheres of zygote named for poles.

17. 17 Cleavage in a frog embryo Continued cleavage produces a solid ball of cells known as the morula. Each individual cell is a blastomere. Final picture - morula - solid ball of cells. Each individual cell in morula is referred to as a blasatomere Final picture - morula - solid ball of cells. Each individual cell in morula is referred to as a blasatomere

18. 18 Blastula Blastocoel forms within morula, creating a hollow ball called the blastula In sea urchins, blastocoel is centrally located. In frogs, because of unequal cell division, the blastocoel is located in the animal hemisphere. Yolk is most plentiful and has its most pronounced effect on cleavage in the eggs of birds, reptiles, many fishes, and insects. In sea urchins, blastocoel is centrally located. In frogs, because of unequal cell division, the blastocoel is located in the animal hemisphere. Yolk is most plentiful and has its most pronounced effect on cleavage in the eggs of birds, reptiles, many fishes, and insects.

19. 19 Gastrulation Rearranges blastula to form a three-layered embryo with a primitive gut Some cells near surface of blastula move to an interior location and establish into a three-layered embryo called the gastrula. Three layers are: ectoderm endoderm mesoderm Differs in detail among animals, but a common set of cellular changes drives this spatial rearrangement of an embryo. Differs in detail among animals, but a common set of cellular changes drives this spatial rearrangement of an embryo.

20. 20 Gastrulation in a sea urchin

21. 21 Gastrulation in a frog embryo

22. 22 Organogenesis Regions of the three germ layers develop into organs of the animal body Neural tube and notochord are first to begin to develop in chordate embryos Notochord Formed from dorsal mesoderm Neural tube Originates as plate of dorsal ectoderm, above developing notochord Becomes CNS – brain and spinal cord Somites give rise to vertebrae and form muscles of axial skeleton Three kinds of morphogenetic changes – folds, splits, and dense clustering (condensation) of cells – are the first evidence of organ building. Three kinds of morphogenetic changes – folds, splits, and dense clustering (condensation) of cells – are the first evidence of organ building.

23. 23 Early Organogenesis in a Frog Embryo Neural plate formation – notochord has developed from dorsal mesoderm, and dorsal ectoderm has thickened, forming neural plate, in response to signals from notochord. Neural folds are two ridges that form lateral edges of neural plate – see light micrograph Formation of neural tube – infolding and pinching off of neural plate generates neural tube. Note neural crest cells, which migrate and give rise to nemerous structures. Somites – Shows embryo after completion of neural tube. Lateral mesoderm begun to separate into two tissue layers that line coelom; somites formed from mesoderm and flank notochord. Side view micrograph – somites give rise to vertebrae and skeletal muscleNeural plate formation – notochord has developed from dorsal mesoderm, and dorsal ectoderm has thickened, forming neural plate, in response to signals from notochord. Neural folds are two ridges that form lateral edges of neural plate – see light micrograph Formation of neural tube – infolding and pinching off of neural plate generates neural tube. Note neural crest cells, which migrate and give rise to nemerous structures. Somites – Shows embryo after completion of neural tube. Lateral mesoderm begun to separate into two tissue layers that line coelom; somites formed from mesoderm and flank notochord. Side view micrograph – somites give rise to vertebrae and skeletal muscle

24. 24 Cleavage, gastrulation, and early organogenesis in a chick embryo Cleavage: b/c of large amount of yolk, cleavage is meroblastic (incomplete). Cell division is restricted to small cap of cytoplasm at animal pole. Gastrulation: some cells migrate into interior of embryo through primitive streak. Some move laterally to form mesoderm, while others migrate downward to form endoderm. Early organogenesis: archenteron is formed when lateral folds pinch embryo away from yolk. Embryo remains attached to yolk by yolk stalk. Notochord, neural tube, and somites form much as they do in frog. Three germ layers and hypoblast cells contribute to system of extraembryonic membranes that support further development Cleavage: b/c of large amount of yolk, cleavage is meroblastic (incomplete). Cell division is restricted to small cap of cytoplasm at animal pole. Gastrulation: some cells migrate into interior of embryo through primitive streak. Some move laterally to form mesoderm, while others migrate downward to form endoderm. Early organogenesis: archenteron is formed when lateral folds pinch embryo away from yolk. Embryo remains attached to yolk by yolk stalk. Notochord, neural tube, and somites form much as they do in frog. Three germ layers and hypoblast cells contribute to system of extraembryonic membranes that support further development

25. 25 Late Organogenesis in a Chick Embryo Rudiments of most major organs have already formed in this chick, which is about 56 hours old. (2 1/4 days old) Can you locate: eye, forebrain, heart, neural tube, somites? Rudiments of most major organs have already formed in this chick, which is about 56 hours old. (2 1/4 days old) Can you locate: eye, forebrain, heart, neural tube, somites?

26. 26 Development Fates of Primary Germ Layers

27. 27 “Quick List” Ectoderm – epidermis, central nervous system, sense organs, neural crest Mesoderm – skeleton, muscles, blood vessels, heart, gonads Endoderm – lining of digestive and respiratory tracts; liver, pancreas

28. 28 Morphogenesis and cellular differentiation refine organs that arise from three embryonic germ layers Morphogenesis development of body shape and organization during embryonic development Cellular differentiation structural and functional differentiation of cells as they become specialized during development dependent on control of gene expression

29. 29 Extraembryonic Membranes Provide Support Services to Embryo Yolk sac Expands over yolk, blood vessels carry nutrients into embryo Amnion Encloses embryo in fluid-filled amniotic sac Protects from drying Chorion Along with amnion, cushions embryo against mechanical shocks Allantois Sac extends into extraembryonic coelom, Functions as disposal sac for uric acid Along with chorion, form blood vessels to transport oxygen to embryo Lateral folds of extraembryonic tissue extend over top of embryo and fuse to form two more memranes: amnion and chorion. Lateral folds of extraembryonic tissue extend over top of embryo and fuse to form two more memranes: amnion and chorion.

30. 30 Development of extraembryonic membranes in a chick Four major membranes are: yold sac, amnion, chorion, allantois Four major membranes are: yold sac, amnion, chorion, allantois

31. 31 Early development in human embryo Blastocyst – forms, about 100 cells, reaches uterus about 7 days after fertilization Trophoblast secretes enzymes and initiates implantation of endometrium Extraembryonic membranes start to form Gastrulation produces three-layered embryo with four extraembryonic membranes Trophoblast – outer epithelium surrounding cavity Trophoblast – outer epithelium surrounding cavity

32. 32 Early development of a human embryo and its extraembryonic membranes No polarity in contents of cytoplasm Cleavage of yolk-lacking zygote is holoblastic and slow. First division occurs about 36 hours after fertilization, second division at about 60 hours, and third division at about 72 hours. About 7 days, has over 100 cells No polarity in contents of cytoplasm Cleavage of yolk-lacking zygote is holoblastic and slow. First division occurs about 36 hours after fertilization, second division at about 60 hours, and third division at about 72 hours. About 7 days, has over 100 cells