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

Backtracking: Advantages and Disadvantages of Sexual Reproduction?

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.

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

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

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

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

Mammalian Gamete Production: Females

Mammalian Gamete Production: Males

Mammalian Gamete Production: Males

Preformation vs Epigenesis Aside: Fertilization Preformation vs Epigenesis

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

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

Fertilization: The Acrosomal Reaction External Fertilization in Sea Urchins 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

Fertilization: The Acrosomal Reaction What needs to happen once one sperm nucleus has entered the egg?!

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

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

Fertilization: Slow Block to Polyspermy The Cortical Reaction: 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

Fertilization: Slow Block to Polyspermy The Cortical Reaction: 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 57:743-750, 1997, Z. Xu, A. Abbott, G. Kopf, R. Schultz and T. Ducibella)

Fertilization: Slow Block to Polyspermy

Fertilization: Slow Block to Polyspermy Enzymes from the cortical granules separate the vitelline layer from the plasma membrane 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

Fertilization: Slow Block to Polyspermy

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

Sea Urchin Larva at 13 days Timing: Sea Urchin Larva at 13 days

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

Fertilization: Sea Urchins Vs. Mammals

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

Fertilization: Sea Urchins Vs. Mammals 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)

Fertilization: Sea Urchins Vs. Mammals

Unfertilized Star Fish Eggs Fertilization Reviewed Unfertilized Star Fish Eggs

Fertilized egg (zygote) - the nucleus is no longer visible; the fertilization envelope is present Let’s Watch: http://www.youtube.com/watch_popup?v=jp-RgIRgcYE#t=000

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

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

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

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

(Becomes the dorsal side) Yolk is concentrated at the vegetal pole. The opposite pole is the animal pole (Becomes the dorsal side)

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.

The third division is horizontal, producing eight cells. 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.

A fluid-filled cavity, the blastocoel forms within the morula, creating a hollow ball of cells, the blastula. Above right. Human- day 3-4 Note the difference the presence of yolk makes!

Sea Urchin Morulas - 8-16 cell stage Sea Urchin Blastula - 32-cell stage, blastocoel

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

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

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

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

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

Sea Urchin: Late Gastrula - endoderm, ectoderm, mesoderm

Blastula  Gastrula in a Frog

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

anatomy.med.unsw.edu

Zooming in on the Frog Gastrula

How do we know??

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

In chordates, the neural tube and notocord form first 3. Organogenesis Morphogenetic changes – folds, splits and clusterings (condensation) begin the process of organ building In chordates, the neural tube and notocord form first

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.

Earliest Organogenesis in Frog (Chordate) Embryos

Let’s Watch – http://www.youtube.com/watch_popup?v=-KC1CA_Os1g#t=000

Formation of the Neural Tube in a Chordate Species: Mouse Day Gestation: 8 www.med.unc.edu