Animal Embryonic Development

From Fertilization to Organogenesis

Early Stages of Development

•Fertilization•Cleavage•Gastrulation•Neurulation

Figure 20.1

Fertilization•unequal gamete contributions–egg contributes•nutrients•proteins, mRNAs•mitochondria•essential developmental genes (imprinted)

–sperm contributes•centriole•tubulin•essential developmental genes (imprinted)

Fertilization•rearrangements of egg cytoplasm–egg contents are distributed heterogeneously

–frog model system•animal hemisphere–contains nucleus–heavily pigmented cortical cytoplasm–lightly pigmented inner cytoplasm

•vegetal hemisphere–contains nutrients–unpigmented

formation of the gray crescent

Figure 20.2

Fertilization•rearrangements of egg cytoplasm–imposes bilateral symmetry on egg•site of sperm entry–head (anterior) end–ventral region

•gray crescent–tail end–dorsal region

•(hence) left-right axis

GSK-3

-catenin

molecular events during

rearrangementFigure 20.3

Cleavage - blastulation•rapid cell divisions

–divisions oriented in specific directions

•little gene expression•little cell growth•packaging of cytoplasmic heterogeneity

•final product is a hollow ball of cells = blastula–cells = blastomeres–hollow cavity = blastocoel

Cleavage - blastulation•yolky eggs alter pattern of

divisions–animal hemisphere divides normally

–vegetal (yolky) hemisphere•divides less often•produces larger cells

yolk affects the cleavage pattern

Figure 20.4

Cleavage - blastulation•amount of yolk affects cleavage

pattern–if yolk is divided into cells•complete cleavage

–if yolk is not divided•incomplete cleavage•embryo is a blastodisc atop the intact yolk

formation of blastodiscFigure 20.4

Mammalian Cleavage

•in oviduct•slow cell divisions•asynchronous cell divisions

mammalian cleavage

is rotational

Figure 20.5

Mammalian Cleavage

•in oviduct•slow cell divisions•asynchronous cell divisions•accompanied by gene expression•produces a blastocyst–inner cell mass - primordial embryo

–trophoblast - primordial placenta component

formation of mammalian blastocystFigure 20.5

frog blastula fate map

Figure 20.6

Fate Maps•undifferentiated cells of blastula have distinct fates–determination fixes fates of blastomeres•early determination yields mosaic development–a lost blastomere causes a lost body part

•later determination yields regulative development–a lost blastomere is compensated during development

humans exhibit regulative developmentFigure 20.7

Gastrulation-organizing the body plan•undifferentiated cells produce germ

layers–ectoderm - prospective epidermis, nervous system

–endoderm - prospective gut tissues–mesoderm - prospective organs, etc.

•germ layers migrate to new positions•contact between layers allows inductive interactions to direct differentiation

sea urchin involutionFigure 20.8

vegetal pole flattens

vegetal cells form 1˚ mesenchyme

involution of a tube of cells

primitive gut

(archenteron) is formed prospective ectoderm, endoderm

& mesoderm are formed

Gastrulation-organizing the body plan•blastopore becomes mouth or

anus–mouth in protostomes–anus in deuterostomes

Gastrulation-organizing the body plan•frog model system

–gastrulation begins at gray crescent

–“bottle cells” bulge into blastocoel & pull neighbors along

–initial involution forms the dorsal lip of the blastopore (d.l.b.)

–epiboly •surface cell layers migrate to blastopore•migrating cells form endoderm, mesoderm

frog gastrulati

onFigure 20.9

Figure 20.12

gray crescent

is necessary

for normal

development

Figure 20.10

Gastrulation-organizing the body plan•frog model system

–ß-catenin activates genes to produce proteins that cause bottle cells to initiate involution

–cells of the gastrula are determined during migration over the d.l.b.•dlb is necessary for normal development•dlb is sufficient for normal development

role of dlb in development in frog

Figure 20.11

Gastrulation-organizing the body plan•reptile/bird model

–two-layered blastodisc + large yolk mass•upper layer–epiblast–becomes embryo

•lower layer–hypoblast–becomes extra-embryonic membranes

chick gastrulationFigure 20.13

earlymammalia

n gastrula

tionFigure 20.14

Neurulation•organogenesis –formation of organs and organ systems

–caused by inductive interactions among germ layers

frog neurulationFigure 20.15

Neurulation•vertebrate body segmentation–alongside neural tube•segments of mesoderm = somites•somites direct development of vertebrae, ribs, trunk muscles, limbs, outgrowth of nerves, blood vessels, etc•repeated segments are modified along the anterior/posterior axis

somites contribute to

vertebrae, ribs & muscles

neural crest cells give rise to peripheral

nerves

Figure 20.16

HOX genes control anterior-posterior differentiation

•families of ~10 HOX genes are on different chromosomes

•HOX genes are expressed “in order”

•HOX genes guide differentiation from anterior to posterior

mouse HOX gene clustersFigure 20.17

vertebrate extraembryonic membranes

•reptiles, birds and mammals produce membranes that–surround the embryo–originate in the embryo–are not part of the embryo–provide nutrition, gas exchange and waste removal

chick extraembry

onic membranesFigure 20.18

shell lining

embryo compartmen

t

waste storage

pantry

placenta: chorion+

uterine tissuesFigure 20.19