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

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 Rotation of cortex relative to inner cytoplasm creates bilateral symmetry.

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

molecular events during rearrangement Figure 20.3 -catenin GSK-3  molecular events during rearrangement Figure 20.3 Sperm centriole rearranges vegetal pole microtubules. Microtubules guide movement of cortical cytoplasm, proteins, organelles. Asymmetric distribution of developmental signals: b-catenin (0range), GSK-3 (blue)

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 blastodisc Figure 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 blastocyst Figure 20.5 32-cell stage = trophoblast + inner cell mass 32-cell stage leaves zona and implants

frog blastula fate map Figure 20.6 cleavage produces cells with different contents blastulation prevents cell contact undifferentiated cells of blastula have distinct fates

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 development Figure 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 involution Figure 20.8 vegetal cells form 1˚ mesenchyme sea urchin involution Figure 20.8 involution of a tube of cells vegetal pole flattens Vegetal pole cells detach - 1˚ mesenchyme Archenteron tip cells detach - 2˚ mesenchyme 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 gastrulation Figure 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 gastrulation Figure 20.13 Primitive streak Hensen’s node

early mammalian gastrulation Figure 20.14 resembles reptile/bird system inner cell mass produces several layers hypoblast lines trophoblast - chorion epiblast upper layer forms amnion primitive groove and Hensen’s node function as in chick system

Neurulation organogenesis formation of organs and organ systems caused by inductive interactions among germ layers

frog neurulation Figure 20.15 some cells determined to be mesoderm some determined to be chordomesoderm dorsal, nearest midline forms notochord notochord induces ectoderm to form begin nervous system development

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 segments of mesoderm = somites somites directs development of vertebrae, ribs, trunk muscles, limbs, outgrowth of nerves, blood vessels, etc .repeated segments are modifed along the anterior/posterior axis development is guided by Hox (homeobox) genes

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 clusters Figure 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 extraembryonic membranes Figure 20 chick extraembryonic membranes Figure 20.18 shell lining embryo compartment waste storage Yolk sac - hypoblast + endoderm Chorion - mesoderm + ectoderm Amnion - mesoderm + ectoderm Allantoic membrane - endoderm pantry

placenta: chorion + uterine tissues Figure 20.19