Developmental Stages in an Amphibian

LE 21-4 Animal development Zygote (fertilized egg) Eight cellsBlastula (cross section) Gastrula (cross section) Adult animal (sea star) Cell movement Gut Cell division Morphogenesis Observable cell differentiation Seed leaves Shoot apical meristem Root apical meristem Plant Embryo inside seed Two cells Zygote (fertilized egg) Plant development

Most animals proceed through these stages during embryonic development: 1.Zygote 2.Early cleavage stages Morula (solid ball) Blastula (hollow ball) 3.Gastrula

LE 47-7 Fertilized egg Four-cell stage Morula Blastula

Starfish development, unfertilized egg. 2 blastomeres. Starfish development, nonmotile blastula. 4 blastomeres. 16 blastomeres.32 blastomeres. morula

LE 21-11a Sperm Molecules of a cytoplasmic determinant Fertilization Nucleus Molecules of another cytoplasmic determinant Unfertilized egg cell Zygote (fertilized egg) Mitotic cell division Two-celled embryo Cytoplasmic determinants in the egg Egg provides proteins and mRNAs required for early development Cleavage asymmetrically divides cytoplasmic components; immediately establishing polarity

LE 47-8 Anterior Right Animal pole Gray crescent Dorsal Ventral Left Posterior Body axes Establishing the axes Future dorsal side of tadpole Point of sperm entry First cleavage Vegetal hemisphere Vegetal pole Point of sperm entry Animal hemisphere

LE 47-9 Zygote 2-cell stage forming 8-cell stage 4-cell stage forming Animal pole Blasto- coel Blastula (cross section) Vegetal pole Blastula (at least 128 cells) 0.25 mm Eight-cell stage (viewed from the animal pole) 0.25 mm

Cleavage in 3 different animal lineages

Gastrulation - Establishing Germ Layers (tissue development) Ectoderm gives rise to outer covering and nervous system Endoderm gives rise to the digestive tract Mesoderm gives rise to muscle tissue

Starfish development, gastrula during invagination. Starfish development, mid-gastrula. LM X75. Starfish, late bipinnaria.Starfish, young adult.

Neurulation The nervous system is the first organ system to develop –Notochord from mesoderm --> replaced with backbone –Neural tube from ectoderm --> spinal chord Establishes basic body plan and layout of body parts

Fig

LE 47-14c 1 mm Notochord Archenteron (digestive cavity) Neural tube Neural crest Eye Somites Tail bud SEM Coelom Somite Somites

LE Notochord Archenteron Endoderm Mesoderm Ectoderm Neural tube Eye Coelom Somite Somites Neural tube Lateral fold Yolk stalk YOLK Form extraembryonic membranes Yolk sac Early organogenesis Forebrain Heart Blood vessels Late organogenesis

Organogenesis is the formation of the organs. The layers are germ layers; they have specific fates in the developing embryo. Organogenesis

Organogenesis is the formation of the organs –Endoderm The innermost layer Goes on to form the gut –Mesoderm In the middle Goes on to form the muscles, circulatory system, blood and many different organs –Ectoderm The outermost Goes on to form the skin and nervous system

Mammalian Development Human Prenatal Development Gestation lasts 266 days from fertilization to birth Development begins in the oviduct –About 24 hours after fertilization, the zygote has divided to form a 2-celled embryo –The embryo passes down the oviduct by cilia and peristalsis –The zona pellucida (a vestige of the egg shell) has dissolved by the 5th day, when the embryo enters the uterus –The embryo floats free for several days, nourished by fluids from glands in the uterine wall At this point, it is called a blastocyst (same as blastula)

24 hrs 1 day 5 days 7 days

The trophoblast is the outermost layer of cells in the blastocyst The trophoblast forms the chorion and amnion The inner cell mass forms the embryo itself

Development of the Placenta

12-day Human Embryo

Organ Development Begins during the first trimester –Gastrulation occurs during the 2nd and 3rd weeks, followed by neurulation (formation of the neural tube) –The heart beats spontaneously after 3.5 weeks –After the first two months of development, the products of conception are called a fetus Week 5

At the end of the first trimester (first 3 months of development) –Fetus can be recognized as a human –~56 mm long, and ~14 g –The sexes can be differentiated –Ears, eyes becoming well- developed, –Skeleton starting to develop –Notochord replaced with the developing vertebral column –Moves, ‘breathes’, makes sucking motions with thumb 33-day embryo measuring 7 x 3.2mm. Human embryo at 40 days.

LE Egg cell developing within ovarian follicle Follicle cell Nucleus Egg cell Fertilization Nurse cell Fertilized egg Embryo Nucleus Laying of egg Egg shell Multinucleate single cell Early blastoderm Plasma membrane formation Late blastoderm Yolk Body segments Cells of embryo Segmented embryo 0.1 mm Hatching Larval stages (3) Pupa Metamorphosis Adult fly Head Thorax Abdomen 0.5 mm BODY AXES Dorsal Ventral PosteriorAnterior

LE 21-14a Head Tail Wild-type larva Mutant larva (bicoid) Drosophila larvae with wild-type and bicoid mutant phenotypes

LE 21-14b Developing egg cell Bicoid mRNA in mature unfertilized egg Nurse cells Egg cell bicoid mRNA Bicoid protein in early embryo Fertilization Translation of bicoid mRNA 100  Anterior end Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo m

LE Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse

LE Eye Antenna Leg Wild typeMutant

LE ThoraxAbdomen Genital segments ThoraxAbdomen