EmbryologyEmbryology

Cellular and Molecular Cellular and Molecular Mechanisms Involved in Mechanisms Involved in

FertilizationFertilizationand Developmentand Development

Sea Urchin Sea Urchin FertilizationFertilization

Contains digestive enzymes

unfertilized sea urchin egg

fertilized sea urchin egg

Vitelline envelopeEgg plasma membrane

Animation Animation 43-0143-01

Sea Urchin FertilizationSea Urchin Fertilization

Acrosomal enzymes digest

jelly coat

Bindin on acrosomal process contacts

receptors on vitelline envelope

Acrosomal process forms

by actin polymerization

Egg plasma membrane forms

fertilization cone to engulf sperm head

Preventing PolyspermyPreventing Polyspermy

• Polyspermy = fertilization of egg by more Polyspermy = fertilization of egg by more than one spermthan one sperm

• Blocked in two waysBlocked in two ways– Fast block to polyspermy: Change in Fast block to polyspermy: Change in

charge across the egg plasma membrane, charge across the egg plasma membrane, mediated by influx of sodium ionsmediated by influx of sodium ions

– Slow block to polyspermy: cortical Slow block to polyspermy: cortical granules release enzymes and water-granules release enzymes and water-absorbing materials to form fertilization absorbing materials to form fertilization envelope around egg, mediated by envelope around egg, mediated by release of stored calcium from release of stored calcium from endoplasmic reticulumendoplasmic reticulum

Sea Urchin FertilizationSea Urchin Fertilization Cortical granules Cortical granules

fuse with plasma fuse with plasma membrane and membrane and releaserelease– Enzymes to catalyze Enzymes to catalyze

reactions thatreactions that• break bonds between break bonds between

vitelline envelope and vitelline envelope and plasma membraneplasma membrane

• remove bindin remove bindin receptorsreceptors

• harden vitelline harden vitelline envelopeenvelope fertilization envelopefertilization envelope

– Substances that Substances that absorb water and absorb water and raise vitelline raise vitelline envelopeenvelope

Applying Your KnowledgeApplying Your Knowledge

A.A. Which structure of the egg has a receptor for Which structure of the egg has a receptor for sperm?sperm?

B.B. Which structure is directly responsible for Which structure is directly responsible for bringing the sperm head into the egg ?bringing the sperm head into the egg ?

C.C. Which part of the sperm contains enzymes Which part of the sperm contains enzymes for digesting through the jelly coat for digesting through the jelly coat surrounding the egg? surrounding the egg?

1.1. BindinBindin2.2. Cortical GranuleCortical Granule3.3. AcrosomeAcrosome4.4. Vitelline envelopeVitelline envelope5.5. Fertilization coneFertilization cone

DevelopmentDevelopment• A series of progressive changes that A series of progressive changes that

generates the structures of the generates the structures of the organism throughout its life cycleorganism throughout its life cycle

• IncludesIncludes– growth: change in size due to increase in growth: change in size due to increase in

cell number and/or cell size cell number and/or cell size – differentiation: cellular specializationdifferentiation: cellular specialization– morphogenesis: establishing body and morphogenesis: establishing body and

organ shapesorgan shapes

Determination Precedes DifferentiationDetermination Precedes Differentiation

• Determination: commitment Determination: commitment of a cell to a particular fate, of a cell to a particular fate, occurs by the activation of occurs by the activation of specific genesspecific genes

• Affected by Affected by – Cytoplasmic Cytoplasmic

segregationsegregation– InductionInduction

• Differentiation isDifferentiation isthe result of the result of determinationdetermination

Induction: process in which a substance or tissue Induction: process in which a substance or tissue influences the fate of a group of adjacent cellsinfluences the fate of a group of adjacent cells

Optic vesicle induces lens placode

Optic cup induces lens

Lens induces cornea

Transplant of tissue from dorsal lip of blastopore induces second invagination

and formation of secondary embryo.

Embryonic StagesEmbryonic Stages

• Cleavage: series of rapid cell Cleavage: series of rapid cell divisions following fertilization, divisions following fertilization, leading to leading to – morula: ball of cellsmorula: ball of cells

– blastula: hollow sphere of cellsblastula: hollow sphere of cells

MorulaMorula BlastulaBlastula

Blastocoel = fluid-filled

space

Blastomere = individual cell

of blastula

Yolk Content Affects Cleavage PatternsYolk Content Affects Cleavage Patterns

Blastodisc= layer of cells on top of yolk

Cytoplasmic Determinants Influence Cleavage PatternsCytoplasmic Determinants Influence Cleavage Patterns

Mosaic Development

Regulative Development

Orientation of mitotic spindles in early cleavage is dependent on the organization

of cytoplasmic determinants in the egg

Cell Adhesion Molecules Cell Adhesion Molecules Influence Blastula OrganizationInfluence Blastula Organization

• Cell Adhesion Molecules (CAMs) are trans-Cell Adhesion Molecules (CAMs) are trans-membrane cell surface proteins that bind to CAMs membrane cell surface proteins that bind to CAMs on other cells.on other cells.

• CAMs are involved in cell migration and producing CAMs are involved in cell migration and producing stable tissue structure.stable tissue structure.

• Experiments have shown that cadherin (one type of Experiments have shown that cadherin (one type of CAM) stabilizes the frog blastula.CAM) stabilizes the frog blastula.

Embryonic StagesEmbryonic Stages Gastrulation: cellular movements Gastrulation: cellular movements

forming three tissue layers and distinct forming three tissue layers and distinct body axesbody axes

primitive gut

linings of digestive and respiratory tracts

nervous system, skin

mesoderm

blood vessels, muscle, bone, liver and heart

Sea Urchin

Invagination

Migration

Sea Urchin GastrulationSea Urchin Gastrulation

Cytoskeletal Components Directing InvaginationCytoskeletal Components Directing Invagination

Embryonic StagesEmbryonic Stages

Gastrulation in the FrogGastrulation in the Frog

Involution

Embryonic StagesEmbryonic Stages

Gastrulation in the ChickGastrulation in the ChickMigration

Neurulation in the FrogNeurulation in the Frog Notochord, derived from Notochord, derived from

mesoderm, induces the mesoderm, induces the overlying ectoderm to form overlying ectoderm to form the neural tubethe neural tube

Cytoskeletal Cytoskeletal ComponentsComponents

Involved Involved with with

NeurulationNeurulation

Tissues of the Tissues of the Frog NeurulaFrog Neurula

Neurulation in the Chick EmbryoNeurulation in the Chick Embryo

Applying Your KnowledgeApplying Your Knowledge

A.A. Which one represents a hollow ball of cells?Which one represents a hollow ball of cells?

B.B. Which tissue induces formation of neural Which tissue induces formation of neural structures?structures?

C.C. Which one involves invagination or involution Which one involves invagination or involution of cells? of cells?

1.1. GastrulaGastrula2.2. NeurulaNeurula3.3. BlastulaBlastula4.4. EctodermEctoderm5.5. MesodermMesoderm

MorphogenesisMorphogenesis

• Pattern Formation: organization of Pattern Formation: organization of differentiated tissues into specific differentiated tissues into specific structuresstructures– Body segmentation in fruit flies is Body segmentation in fruit flies is

controlled by controlled by • gradients of materials established in the gradients of materials established in the

egg egg • actions of a series of embryonic genesactions of a series of embryonic genes

Animation Animation 19-0319-03

Genes Affecting Body SegmentationGenes Affecting Body Segmentation Products of Maternal Effect Genes establish gradients in the egg

Gap genes define broad areas and regulate Pair-rule genes

Pair-rule genes refine segment locations and regulate Segment Polarity genes

Segment Polarity genes determine segment boundaries and orientations

Homeotic genes define the role of each segment

Mutation in a Homeotic Gene Causes Mutation in a Homeotic Gene Causes Legs to Grow Where Antenna Should BeLegs to Grow Where Antenna Should Be

Homology between Homology between Genes Controlling Genes Controlling

Segmentation in Fruit Segmentation in Fruit Flies and MiceFlies and Mice

Morphogenesis May Require ApoptosisMorphogenesis May Require Apoptosis

Apoptosis = Programmed Cell DeathApoptosis = Programmed Cell Death

Cellular and Molecular Mechanisms Cellular and Molecular Mechanisms Essential to DevelopmentEssential to Development

• Specificity of receptors and enzymes Specificity of receptors and enzymes

• Cytoplasmic organizationCytoplasmic organization

• Cell movement through the action of Cell movement through the action of cytoskeletal elements and cell adhesion cytoskeletal elements and cell adhesion moleculesmolecules

• Orientation of tissue layers leading to Orientation of tissue layers leading to InductionInduction

• Differential Gene Expression Differential Gene Expression

Applying Your KnowledgeApplying Your Knowledge

A.A. Which process leads to a morula?Which process leads to a morula?B.B. Which process leads to the formation of Which process leads to the formation of

somites, the precursors to vertebrae?somites, the precursors to vertebrae?C.C. Which process is directly preceded by Which process is directly preceded by

determination? determination?

1.1. DifferentiationDifferentiation

2.2. GrowthGrowth

3.3. MorphogenesisMorphogenesis