Early DevelopmentEarly Development

Chapter 21Chapter 21

Early DevelopmentEarly Development Highly variable among Highly variable among

different organismsdifferent organisms Common genetic and Common genetic and

cellular mechanisms cellular mechanisms in developmentin development

Begins with Begins with gametogenesisgametogenesis

Proceeds in ordered Proceeds in ordered phasesphases

Phases of Human Phases of Human DevelopmentDevelopment

Gametogenesis Gametogenesis and Fertilizationand Fertilization

GametogenesisGametogenesis

Formation of gametes, sperm and Formation of gametes, sperm and eggegg

In the reproductive organs of In the reproductive organs of adult organismsadult organisms

Sperm and egg contribute an Sperm and egg contribute an equal number of chromosomes to equal number of chromosomes to the offspringthe offspring Egg is 100x larger, contributes more Egg is 100x larger, contributes more

cytoplasmcytoplasm

Sperm Structure and Sperm Structure and FunctionFunction

Animal sperm has Animal sperm has four major four major compartments: compartments: the head (the the head (the

acrosome)acrosome) the neck (a the neck (a

centriole)centriole) a midpiece packed a midpiece packed

with mitochondriawith mitochondria a tail (a flagellum) a tail (a flagellum)

Sperm Structure and Sperm Structure and FunctionFunction

In plants, sperm develop from pollen In plants, sperm develop from pollen grainsgrains

Pollen are several haploid cells from Pollen are several haploid cells from meiosismeiosis

Pollen cell comes into contact with Pollen cell comes into contact with the stigma and divides by mitosis to the stigma and divides by mitosis to produce two sperm nucleiproduce two sperm nuclei

Move down the pollen tube to the Move down the pollen tube to the egg cellegg cell

Sperm Structure and Sperm Structure and FunctionFunction

Egg Structure and Egg Structure and FunctionFunction

Large, contain the nutrients required Large, contain the nutrients required for the embryo’s early developmentfor the embryo’s early development

In species that lay eggs in the In species that lay eggs in the environment, stores in the egg are the environment, stores in the egg are the only source of nutrients until it hatchesonly source of nutrients until it hatches

In mammals need stores until the egg In mammals need stores until the egg implants in the placentaimplants in the placenta

Plants also provide endosperm to Plants also provide endosperm to nourish the embryonourish the embryo

Egg Structure and Egg Structure and FunctionFunction

CorticalCortical granulesgranules- vesicles filled - vesicles filled with enzymes that assist fertilization with enzymes that assist fertilization in egg-laying animalsin egg-laying animals

Vitelline envelopeVitelline envelope- fibrous, - fibrous, matlike sheet of glycoproteins that matlike sheet of glycoproteins that surrounds the eggsurrounds the egg

Jelly layerJelly layer (a large, gelatinous mass (a large, gelatinous mass that also encloses the eggthat also encloses the egg

FertilizationFertilization Fusing of a haploid Fusing of a haploid

sperm cell with a sperm cell with a haploid egg cell to haploid egg cell to form a diploid zygoteform a diploid zygote

Can be internal or Can be internal or externalexternal

Requires exact timing Requires exact timing and recognitionand recognition

Must start Must start developmentdevelopment

Starts with gamete Starts with gamete releaserelease

FertilizationFertilization

Enzymes from the acrosome digest Enzymes from the acrosome digest through the egg's jelly layerthrough the egg's jelly layer

Acrosomal processAcrosomal process contacts the contacts the vitelline envelopevitelline envelope

Plasma membranes fusePlasma membranes fuse Sperm nucleus, mitochondria, and Sperm nucleus, mitochondria, and

centriole enter the eggcentriole enter the egg Sperm and egg nucleus fuseSperm and egg nucleus fuse

Species RecognitionSpecies Recognition

FertilizinFertilizin is a compound on the is a compound on the surface of sea urchin egg cellssurface of sea urchin egg cells

Binds to Binds to bindinbindin, a protein on the , a protein on the head of sea urchin spermhead of sea urchin sperm

Binding occurs in a species-specific Binding occurs in a species-specific mannermanner

Fertilizin from the eggs of one Fertilizin from the eggs of one species binds to sperm of its own species binds to sperm of its own species but does not bind to sperm of species but does not bind to sperm of different speciesdifferent species

Blocking PolyspermyBlocking Polyspermy

Wide variety of mechanisms to Wide variety of mechanisms to block block polyspermypolyspermy

In sea urchins, fertilization In sea urchins, fertilization results in erection of a physical results in erection of a physical barrierbarrier

Generates a Generates a fertilization fertilization envelope envelope as cortical granules as cortical granules fuse with the plasma membranefuse with the plasma membrane

Fertilization in MammalsFertilization in Mammals Internal fertilization, so species Internal fertilization, so species

recognition is generally not an issuerecognition is generally not an issue Acrosome still breaks down zona Acrosome still breaks down zona

pellucidapellucida Egg cells have a binding site for spermEgg cells have a binding site for sperm Glycoprotein ZP3 in the zonabinds to Glycoprotein ZP3 in the zonabinds to

the head of spermthe head of sperm Enzymes released from cortical Enzymes released from cortical

granules modify ZP3 to prevent binding granules modify ZP3 to prevent binding by additional spermby additional sperm

Fertilization in Flowering Fertilization in Flowering PlantsPlants

Takes place inside ovuleTakes place inside ovule Double fertilization-one sperm nucleus Double fertilization-one sperm nucleus

fuses with an egg to form a zygote, fuses with an egg to form a zygote, and the other sperm nucleus fuses and the other sperm nucleus fuses with two polar nuclei to form the with two polar nuclei to form the triploid endospermtriploid endosperm

Interaction between the pollen grains Interaction between the pollen grains and the ovule involve species and the ovule involve species recognitionrecognition

May also prevent self-fertilizationMay also prevent self-fertilization

EndospermEndosperm

Endosperm Endosperm provides provides nutrients for nutrients for the embryotic the embryotic development, development, germination germination and early and early seedling growthseedling growth

Animal Cleavage Animal Cleavage and Gastrulationand Gastrulation

CleavageCleavage CleavageCleavage is the set of rapid cell is the set of rapid cell

divisions (without growth) that follows divisions (without growth) that follows fertilizationfertilization

Divides up the cytoplasm into cells, no Divides up the cytoplasm into cells, no growthgrowth

Cells are called blastomeresCells are called blastomeres Forms a blastula, sphere of cellsForms a blastula, sphere of cells Pattern of cleavage varies among Pattern of cleavage varies among

speciesspecies Sometimes makes cells around yolkSometimes makes cells around yolk

CleavageCleavage

CleavageCleavage

Cells can divide at right angles to Cells can divide at right angles to one another, forming tiers in a one another, forming tiers in a pattern called pattern called radial cleavageradial cleavage

CleavageCleavage

Divide at oblique angles so that they Divide at oblique angles so that they pile up in a pattern called pile up in a pattern called spiral spiral cleavagecleavage

What Determines Cleavage What Determines Cleavage and Development?and Development?

Cytoplasmic determinantCytoplasmic determinant is a molecule found in the is a molecule found in the egg that helps direct early egg that helps direct early developmentdevelopment

Affect development Affect development independently of sperm or independently of sperm or zygote genotypezygote genotype

Involved in Involved in differentiationdifferentiation—the generation of different —the generation of different cell types from a single cellcell types from a single cell

Activating the Zygotic Activating the Zygotic GenomeGenome

Zygotic Genome not active Zygotic Genome not active during cleavageduring cleavage

In most animals it is not In most animals it is not transcribed until after cleavage transcribed until after cleavage is well under wayis well under way

Mammals are the exceptionMammals are the exception Transcribe from the zygotic Transcribe from the zygotic

genome at the two-cell stagegenome at the two-cell stage

Embryonic TissuesEmbryonic Tissues

Animal embryos develop three types Animal embryos develop three types of tissues, called of tissues, called germ layers:germ layers:

EctodermEctoderm forms the outer covering and forms the outer covering and nervous systemnervous system

MesodermMesoderm gives rise to muscle, gives rise to muscle, internal organs, and connective tissues internal organs, and connective tissues such as blood and cartilagesuch as blood and cartilage

EndodermEndoderm produces the lining of the produces the lining of the digestive tract or gut, along with some digestive tract or gut, along with some of the associated organsof the associated organs

GastrulationGastrulation After cleavage is complete After cleavage is complete

a bastula is the result a bastula is the result Hollow ball of cellsHollow ball of cells

Gastrulation rearranges Gastrulation rearranges cells cells

Results in the gastrula Results in the gastrula that contains the three that contains the three embryonic tissue typesembryonic tissue types Each gives rise to different Each gives rise to different

tissue typestissue types

GastrulationGastrulation

Zygote

Cleavage

Eight-cell stage

Cleavage

Blastula Cross section of blastula

Blastocoel

Blastocoel

Gastrula Gastrulation

Endoderm

Ectoderm

Blastopore

In most animals, cleavage results in theformation of a multicellular stage called a blastula.The blastula of many animals is a hollow ball of cells.

3

The endoderm ofthe archenteron de-

velops into the tissuelining the animal’s

digestive tract.

6

The blind pouchformed by gastru-

lation, calledthe archenteron,

opens to the outsidevia the blastopore.

5

Most animals also undergo gastrulation, a rearrangement of the embryo in which one end of the embryo folds inward, expands, and eventually fills the blastocoel, producing layers of embryonic tissues: the ectoderm (outer layer) and the endoderm (inner layer).

4

Only one cleavagestage–the eight-cellembryo–is shown here.

2 The zygote of an animal undergoes a succession of mitotic cell divisions called cleavage.

1

GastrulationGastrulation

At the end of gastrulation, the three At the end of gastrulation, the three embryonic tissues are arranged in embryonic tissues are arranged in layers, the gut has formed, and the layers, the gut has formed, and the major body axes have become visiblemajor body axes have become visible

Plant DevelopmentPlant Development

Plant Life CyclePlant Life Cycle

EmbryogenesisEmbryogenesis

First division is First division is asymmetricasymmetric The large basal cell The large basal cell

generates the generates the suspensor structure; suspensor structure;

Apical cell will give rise Apical cell will give rise to the to the shoot apical shoot apical meristemmeristem and the and the root apical meristemroot apical meristem

Meristem is area of Meristem is area of rapidly dividing cellsrapidly dividing cells

EmbryogenesisEmbryogenesis

An embryo contains an:An embryo contains an: EpidermisEpidermis- an outer covering of cells - an outer covering of cells

that protect the individual. that protect the individual. Ground tissueGround tissue- a mass of tissue that - a mass of tissue that

may later differentiate into cells for may later differentiate into cells for specialized functionsspecialized functions

Vascular tissueVascular tissue- that will differentiate - that will differentiate into specialized cells that transport food into specialized cells that transport food and water between root and shoot.and water between root and shoot.

Invertebrate Invertebrate DevelopmentDevelopment

Embryogenesis in Fruit Embryogenesis in Fruit FlyFly

Fertilized egg undergoes mitoses Fertilized egg undergoes mitoses without cytokinesis without cytokinesis Produces a multinucleate cell with a Produces a multinucleate cell with a

cytoplasm filled with nutrient-rich yolkcytoplasm filled with nutrient-rich yolk Each nucleus migrates to the outside Each nucleus migrates to the outside

of the embryo cell and receives a of the embryo cell and receives a plasma membrane plasma membrane

Embryo becomes an outer sheet of Embryo becomes an outer sheet of cells surrounding the original cells surrounding the original cytoplasmcytoplasm

Embryogenesis in Fruit Embryogenesis in Fruit FlyFly

Embryogenesis in Fruit Embryogenesis in Fruit FlyFly

Gastrulation starts with the formation Gastrulation starts with the formation of a cleft or furrow, followed by of a cleft or furrow, followed by formation of furrows that define the formation of furrows that define the head region and the series of body head region and the series of body regions called regions called segmentssegments

Embryo hatches to from larvaEmbryo hatches to from larva Larva forms pupa after a few daysLarva forms pupa after a few days Pupa goes through metamorphosis to Pupa goes through metamorphosis to

become flybecome fly

Vertebrate Vertebrate DevelopmentDevelopment

Embryogenesis in FrogEmbryogenesis in Frog

Frog embryo goes from one large cell Frog embryo goes from one large cell to a ball of cells (blastula) through a to a ball of cells (blastula) through a series of cleavage eventsseries of cleavage events Does not increase in sizeDoes not increase in size

Neural tubeNeural tube, which becomes the , which becomes the spinal cord and brain, forms at the end spinal cord and brain, forms at the end of gastrulationof gastrulation

Embryo hatches into tadpoleEmbryo hatches into tadpole Tadpole goes through metamorphosisTadpole goes through metamorphosis

Embryogenesis in FrogEmbryogenesis in Frog

Early Development in Early Development in HumansHumans

Human eggs are released into the Human eggs are released into the fallopian tubefallopian tube by the by the ovaryovary

After fertilization zygote goes After fertilization zygote goes through cleavage as it goes through cleavage as it goes down the fallopian tubesdown the fallopian tubes

Embryo undergoes Embryo undergoes implantationimplantation into the wall of the uterus, and the into the wall of the uterus, and the placenta formsplacenta forms PlacentaPlacenta carries nutrients to and waste carries nutrients to and waste

from the developing fetusfrom the developing fetus

Early Development in Early Development in HumansHumans