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Today: Group Quiz 6 Group Quiz 6 Intro to Development Intro to Development Thursday: Review Paper Draft Due for Peer Review!

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Presentation on theme: "Today: Group Quiz 6 Group Quiz 6 Intro to Development Intro to Development Thursday: Review Paper Draft Due for Peer Review!"— Presentation transcript:

1 Today: Group Quiz 6 Group Quiz 6 Intro to Development Intro to Development Thursday: Review Paper Draft Due for Peer Review!

2 Backtracking: Advantages and Disadvantages of Sexual Reproduction?

3 Generalizations about Sexual Reproduction Offspring created by the fusion of haploid gametes to form a diploid zygote. The female gamete ( ovum ) is generally large and nonmotile. The male gamete ( spermatozoan ) is generally smaller and motile.

4 Some animals are Hermaphrodites (single individual has both male and female reproductive systems) Common in sessile or burrowing animals, and in parasites Generalizations about Sexual Reproduction Advantages? Does this generate genetic diversity?

5 Fertilization Overview Fertilization: the union of sperm and egg Can be EXTERNAL or INTERNAL

6 External Fertilization Photo by Scott Egan,; University of Rhode Island Dept. of Natural Resources; A pair of wood frogs, Rana sylvatica, in amplexus

7 Internal fertilization requires cooperative behavior and Sophisticated reproductive systems including organs to deliver sperm and receptacles for its storage and transport to eggs. Internal Fertilization

8 Mammalian Gamete Production: Females

9 Mammalian Gamete Production: Males

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11 Aside: Fertilization Preformation vs Epigenesis

12 Fertilization Combines haploid sets of chromosomes to form single diploid zygote Combines haploid sets of chromosomes to form single diploid zygote Activation of the egg Activation of the egg Model Organism = Sea Urchin Model Organism = Sea Urchin Why Urchins??

13 Studying Sea Urchin Fertilization Source: NASA- Photo credit - Joseph Tash

14 External Fertilization in Sea Urchins External Fertilization in Sea Urchins Sperm exposed to molecules from the jelly coat surrounding an egg, the acrosome discharges its contents by Exocytosis Sperm exposed to molecules from the jelly coat surrounding an egg, the acrosome discharges its contents by Exocytosis = Acrosomal Reaction The acrosomal process (elongating structure) then penetrates the jelly coat of the egg and adheres to a specific receptor molecule The acrosomal process (elongating structure) then penetrates the jelly coat of the egg and adheres to a specific receptor molecule Fertilization: The Acrosomal Reaction

15 What needs to happen once one sperm nucleus has entered the egg?!

16 Fusion of the sperm and egg membrane causes ion channels in the eggs membrane to open Fusion of the sperm and egg membrane causes ion channels in the eggs membrane to open Sodium ions flow into the cell, causing a membrane depolarization (change in membrane potential) Sodium ions flow into the cell, causing a membrane depolarization (change in membrane potential) Prevents more than 1 sperm from entering (1-3 seconds!) Prevents more than 1 sperm from entering (1-3 seconds!) Fertilization: Fast Block to Polyspermy

17 Membrane potential is restored within 2 minutes! (Uh oh…)

18 The Cortical Reaction: Fusion of sperm and egg triggers release of calcium from the ER into the cytosol Fusion of sperm and egg triggers release of calcium from the ER into the cytosol Calcium release begins at site of sperm entry and propagates across the fertilized egg Calcium release begins at site of sperm entry and propagates across the fertilized egg Fertilization: Slow Block to Polyspermy

19 The Cortical Reaction: High calcium concentration causes the cortical granules to fuse with plasma membrane High calcium concentration causes the cortical granules to fuse with plasma membrane Mouse oocyte stained to show cortical granules (small red dots). Genomic DNA in the metaphase plate of the secondary oocyte (top right) appears bluish-white. (From Biology of Reproduction 57: , 1997, Z. Xu, A. Abbott, G. Kopf, R. Schultz and T. Ducibella)

20 Fertilization: Slow Block to Polyspermy

21 Enzymes from the cortical granules separate the vitelline layer from the plasma membrane Enzymes from the cortical granules separate the vitelline layer from the plasma membrane Water is drawn into the perivitelline space by osmosis, swelling it Water is drawn into the perivitelline space by osmosis, swelling it The swelling pushes the vitelline layer away from plasma membrane where it is hardened by enzymes to form the Fertilization Envelope The swelling pushes the vitelline layer away from plasma membrane where it is hardened by enzymes to form the Fertilization Envelope Fertilization: Slow Block to Polyspermy

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23 Rise in Calcium also induces metabolic changes within the egg Rise in Calcium also induces metabolic changes within the egg Metabolism increases rapidly! Metabolism increases rapidly! Nucleus of sperm starts to swell Nucleus of sperm starts to swell Sperm nucleus merges with egg nucleus ~ 20 minutes) Sperm nucleus merges with egg nucleus ~ 20 minutes) First division ~90 minutes First division ~90 minutes Fertilization: Activation

24 Timing: Sea Urchin Larva at 13 days

25 Fertilization: Sea Urchins Vs. Mammals Internal Fertilization (terrestrial!) Key Initial Differences: 1. Capacitation : molecules in mammalian female reproductive tract alter surface of sperm and increase the motility (~ 6 hrs) 2. Mammalian egg cloaked by follicle cells- capacitated sperm cell must migrate through this layer to reach the zona pellucida

26 Fertilization: Sea Urchins Vs. Mammals

27 The Zona Pellucida = filamentous network of glycoproteins (extracellular matrix) One of the glycoproteins, ZP3 functions as a sperm receptor One of the glycoproteins, ZP3 functions as a sperm receptor Binding induces the acrosome to release its contents (as with sea urchins) Binding induces the acrosome to release its contents (as with sea urchins) Enzymes from acrosome help sperm penetrate the zona pellucida Enzymes from acrosome help sperm penetrate the zona pellucida Fertilization: Sea Urchins Vs. Mammals

28 As with sea urchin, binding of egg triggers depolarization of egg membrane (fast block to polyspermy) As with sea urchin, binding of egg triggers depolarization of egg membrane (fast block to polyspermy) A Cortical Reaction functions as the slow block (granules in cortex release contents; enzymes catalyze changes in the zona pellucida) A Cortical Reaction functions as the slow block (granules in cortex release contents; enzymes catalyze changes in the zona pellucida) Fertilization: Sea Urchins Vs. Mammals

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30 Unfertilized Star Fish Eggs Fertilization Reviewed

31 Fertilized egg (zygote) - the nucleus is no longer visible; the fertilization envelope is present Lets Watch:

32 Stages of Development Fertilization is followed by 3 stages: 1. Cleavage 2. Gastrulation 3. Organogenesis

33 1.CLEAVAGE Succession of rapid cell divisions Succession of rapid cell divisions Partitions the cytoplasm into smaller cells, BLASTOMERES, each with its own nucleus Partitions the cytoplasm into smaller cells, BLASTOMERES, each with its own nucleus Each region of cytoplasm contains different cytoplasmic components Each region of cytoplasm contains different cytoplasmic components

34 2-cell and 4-cell stage of Sea Urchin Cleavage (Blastomeres) What happened to the size of each blastomere??

35 Polarity of Zygote and Egg Most animals (except mammals) produce asymmetrical eggs and zygotes Most animals (except mammals) produce asymmetrical eggs and zygotes Distribution of yolk, mRNA and proteins is not uniform Distribution of yolk, mRNA and proteins is not uniform Sets the stage for subsequent developmental events!

36 Yolk is concentrate d at the vegetal pole. The opposite pole is the animal pole ( Becomes the dorsal side)

37 Polarity of Frog Eggs In the final stage, a frog oocyte is pigmented dark brown in one hemisphere ( animal pole ). The other hemisphere ( vegetal pole ) shows the yellow color of the egg yolk.

38 Cleavage The first two cleavage divisions are vertical producing four cells extending from animal to vegetal pole. The third division is horizontal, producing eight cells. Continued divisions (16-64 cells) produce a solid ball of cells, the morula.

39 A fluid-filled cavity, the blastocoel forms within the morula, creating a hollow ball of cells, the blastula. Note the difference the presence of yolk makes!

40 Sea Urchin Morulas cell stage Morulas Sea Urchin Blastula - 32-cell stage, blastocoel Blastula

41 Sea Urchin: Late Blastula - shows thickened layer of cells at one end, the vegetal pole Late BlastulaLate Blastula

42 Morula (16-64 cells) Blastula (>128 cells) A fluid-filled cavity, the blastocoel, forms within the morula creating a hollow ball Source: courtesy of Dr. J. Hardin, Univ. of Wisconsin

43 Blastula Gastrula Changes in cell motility, shape and adhesion result in the spatial rearrangement of an embryo Changes in cell motility, shape and adhesion result in the spatial rearrangement of an embryo Results in three familiar embryonic tissue layers: Results in three familiar embryonic tissue layers: ectoderm, endoderm and mesoderm ectoderm, endoderm and mesoderm

44 2. Gastrulation Source: raven.zoology.washington.edu/ embryosraven.zoology.washington.edu/ embryos

45 Sea Urchin: Early Gastrula - blastocoel, blastopore, primitive gut (archenteron) Early GastrulaEarly Gastrula

46 Sea Urchin: Late Gastrula - endoderm, ectoderm, mesoderm Late GastrulaLate Gastrula

47 Blastula Gastrula in a Frog

48 Frog Development Great model organisms: HUGE eggs! (> 1mm) HUGE eggs! (> 1mm) External fertilization and development External fertilization and development Vertebrates! Vertebrates! Fertilization Neuralation in 18 hours under good conditions Fertilization Neuralation in 18 hours under good conditions

49 anatomy.med.unsw.edu

50 Zooming in on the Frog Gastrula

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52 How do we know??

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54 Stages of Development Fertilization is followed by 3 stages: 1. Cleavage 2. Gastrulation 3. Organogenesis

55 Morphogenetic changes – folds, splits and clusterings (condensation) begin the process of organ building In chordates, the neural tube and notocord form first

56 Sea Urchin: the bipinnaria larva (bilateral symmetry) develops into a brachiolaria larva; this larva undergoes metamorphosis to become the adult starfish, which has radial symmetry.

57 Earliest Organogenesis in Frog (Chordate) Embryos

58 Lets Watch – pup?v=-KC1CA_Os1g#t=000

59 Formation of the Neural Tube in a Chordate Species: Mouse Day Gestation: 8


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