Animal Development

The question of how a zygote becomes an animal has been asked for centuries As recently as the 18th century, the prevailing theory was called preformation Preformation is the idea that the egg or sperm contains a miniature infant, or “homunculus,” which becomes larger during development

WHAT DETERMINES DEVELOPMENT Development is determined by the zygote’s genome and differences between embryonic cells Cell differentiation is the specialization of cells in structure and function Morphogenesis is the process by which an animal takes shape

Big ideas Gametes (fertilizaiton)  Zygote (cleavage)  Blastula (gastrulation)  Gastrula (neurulation)  Organogenesis Role of genes & protein concentration gradients Induction: communication from an inducer to a competent responder

Fertilization 2 major events: Fertilization brings the haploid nuclei of sperm and egg together, forming a diploid zygote The sperm’s contact with the egg’s surface initiates metabolic reactions in the egg that trigger the onset of embryonic development Most info comes from sea urchin studies External fertilization Problems of external fertilization: Dilution/protection of gametes in the enormous volume of the ocean Correct species fertilization Blocking polyspermy

The Acrosomal Reaction The acrosomal reaction is triggered when the sperm meets the egg This reaction releases hydrolytic enzymes that digest material surrounding the egg Acrosomal process adheres to receptors on vitelline layer (species specific) Sperm/egg membranes fuse, sperm nucleus enters Na+ influx, depolarization Depolarization sets up fast block to polyspermy

Fast block polyspermy Contact and fusion of sperm and egg membranes Entry of sperm nucleus Acrosomal reaction Sperm plasma membrane Sperm nucleus Cortical reaction Contact Acrosomal process Basal body (centriole) Sperm head Fertilization envelope Fused plasma membranes Cortical granule Actin Acrosome Hydrolytic enzymes Perivitelline space Jelly coat Vitelline layer Cortical granule membrane Sperm-binding receptors Egg plasma membrane EGG CYTOPLASM

The Cortical Reaction Fusion of egg and sperm also initiates the cortical reaction This reaction induces a rise in Ca2+ in cytoplasm that stimulates cortical granules to release their contents outside the egg Cortical granules fuse w/ membrane Enzymes Polysaccharides Fertilization envelope formed = slow block to polyspermy (follows repolarization) These changes cause formation of a fertilization envelope that functions as a slow block to polyspermy

Fast block polyspermy 500 µm 1 sec before fertilization 10 sec after Spreading wave of calcium ions Point of sperm entry

Activation of the Egg The sharp rise in Ca2+ in the egg’s cytosol increases the rates of cellular respiration and protein synthesis by the egg cell Chemical signals from cortical rxn cause H+ to be transported out --> increase in pH Nuclei fuse Egg/sperm differences Egg contains proteins, mRNA not found in sperm Ca2+ injection, temperature shock can cause artificial activation With these rapid changes in metabolism, the egg is said to be activated

Acrosomal reaction: plasma membrane LE 47-5 1 Binding of sperm to egg 2 Acrosomal reaction: plasma membrane depolarization (fast block to polyspermy) 3 4 6 Seconds 8 10 Increased intracellular calcium level 20 Cortical reaction begins (slow block to polyspermy) 30 40 50 1 Formation of fertilization envelope complete 2 Increased intracellular pH 3 4 5 Increased protein synthesis Minutes 10 20 Fusion of egg and sperm nuclei complete 30 40 Onset of DNA synthesis 60 90 First cell division

Fertilization in Mammals In mammalian fertilization, the cortical reaction modifies the zona pellucida as a slow block to polyspermy

Follicle cell Sperm basal Cortical Zona pellucida body ganules Sperm nucleus Egg plasma membrane Acrosomal vesicle EGG CYTOPLASM

Cleavage Fertilization is followed by cleavage, a period of rapid cell division without growth Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres

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

The eggs and zygotes of many animals, except mammals, have a definite polarity The polarity is defined by distribution of yolk, with the vegetal pole having the most yolk The development of body axes in frogs is influenced by the egg’s polarity

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

Cleavage planes usually follow a pattern that is relative to the zygote’s animal and vegetal poles

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

Meroblastic cleavage, incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds

Disk of cytoplasm Fertilized egg Zygote Four-cell stage Blastoderm LE 47-10 Disk of cytoplasm Fertilized egg Zygote Four-cell stage Blastoderm Cutaway view of the blastoderm Blastocoel BLASTODERM YOLK MASS Epiblast Hypoblast

Holoblastic cleavage, complete division of the egg, occurs in species whose eggs have little or moderate amounts of yolk, such as sea urchins and frogs

Gastrulation Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut

Video: Sea Urchin Embryonic Development The three layers produced by gastrulation are called embryonic germ layers The ectoderm forms the outer layer The endoderm lines the digestive tract The mesoderm partly fills the space between the endoderm and ectoderm Video: Sea Urchin Embryonic Development

The mechanics of gastrulation in a frog are more complicated than in a sea urchin-INVAGINATION OTHERS- INVOLUTION

LE 47-12 SURFACE VIEW CROSS SECTION Animal pole Blastocoel Dorsal tip of blastopore Dorsal lip of blastopore Vegetal pole Blastula Blastocoel shrinking Archenteron Ectoderm Mesoderm Blastocoel remnant Endoderm Key Future ectoderm Future mesoderm Yolk plug Yolk plug Future endoderm Gastrula

Gastrulation in the chick and frog is similar, with cells moving from the embryo’s surface to an interior location During gastrulation, some epiblast cells move toward the blastoderm’s midline and then detach and move inward toward the yolk. INVOLUTION

Epiblast Primitive Future streak ectoderm Endoderm Migrating cells LE 47-13 Epiblast Future ectoderm Primitive streak Endoderm Migrating cells (mesoderm) Hypoblast YOLK

Organogenesis During organogenesis, various regions of the germ layers develop into rudimentary organs organs

Video: Frog Embryo Development Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm Video: Frog Embryo Development

LE 47-14a Neural folds LM 1 mm Neural fold Neural plate Notochord Ectoderm Mesoderm Endoderm Archenteron Neural plate formation

The neural plate soon curves inward, forming the neural tube LE 47-14b Neural fold Neural plate The neural plate soon curves inward, forming the neural tube Neural crest Outer layer of ectoderm Neural crest Neural tube Formation of the neural tube

Mesoderm lateral to the notochord forms blocks called somites LE 47-14c Eye Somites Tail bud Mesoderm lateral to the notochord forms blocks called somites Lateral to the somites, the mesoderm splits to form the coelom SEM Neural tube 1 mm Notochord Neural crest Coelom Somite Archenteron (digestive cavity) Somites

LE 47-15 Eye Neural tube Notochord Forebrain Somite Coelom Heart Archenteron Endoderm Lateral fold Mesoderm Blood vessels Ectoderm Somites Yolk stalk YOLK Yolk sac Form extraembryonic membranes Neural tube Early organogenesis Late organogenesis

Many structures are derived from the three embryonic germ layers during organogenesis

Developmental Adaptations of Amniotes Embryos of birds, other reptiles, and mammals develop in a fluid-filled sac in a shell or the uterus Organisms with these adaptations are called amniotes In these organisms, the three germ layers also give rise to the four membranes that surround the embryo

Amnion Allantois Embryo Amniotic cavity with amniotic fluid Albumen LE 47-17 Amnion Allantois Embryo Amniotic cavity with amniotic fluid Albumen Shell Yolk (nutrients) Chorion Yolk sac

Mammalian Development The eggs of placental mammals Are small and store few nutrients Exhibit holoblastic cleavage Show no obvious polarity Gastrulation and organogenesis resemble the processes in birds and other reptiles Early cleavage is relatively slow in humans and other mammals

At completion of cleavage, the blastocyst forms The trophoblast, the outer epithelium of the blastocyst, initiates implantation in the uterus, and the blastocyst forms a flat disk of cells As implantation is completed, gastrulation begins The extraembryonic membranes begin to form By the end of gastrulation, the embryonic germ layers have formed-ECTODERM, MESODERM AND ENDODERM

Endometrium (uterine lining) Inner cell mass Trophoblast Blastocoel LE 47-18a Endometrium (uterine lining) Inner cell mass Trophoblast Blastocoel Blastocyst reaches uterus. Expanding region of trophoblast Maternal blood vessel Epiblast Hypoblast Trophoblast Blastocyst implants.

LE 47-18b Expanding region of trophoblast Amniotic cavity Amnion Epiblast Hypoblast Chorion (from trophoblast Yolk sac (from hypoblast) Extraembryonic membranes start to form and gastrulation begins. Extraembryonic mesoderm cells (from epiblast) Allantois Amnion Chorion Ectoderm Mesoderm Endoderm Yolk sac Extraembryonic mesoderm Gastrulation has produced a three-layered embryo with four extraembryonic membranes.

The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and develop in a similar way

Morphogenesis in animals involves specific changes in cell shape, position, and adhesion Morphogenesis is a major aspect of development in plants and animals But only in animals does it involve the movement of cells

The Cytoskeleton, Cell Motility, and Convergent Extension Changes in cell shape usually involve reorganization of the cytoskeleton Microtubules and microfilaments affect formation of the neural tube

LE 47-19 Ectoderm Neural plate

The cytoskeleton also drives cell migration, or cell crawling, the active movement of cells In gastrulation, tissue invagination is caused by changes in cell shape and migration Cell crawling is involved in convergent extension, a morphogenetic movement in which cells of a tissue become narrower and longer

LE 47-20 Convergence Extension

Roles of the Extracellular Matrix and Cell Adhesion Molecules Fibers of the extracellular matrix may function as tracks, directing migrating cells along routes Several kinds of glycoproteins, including fibronectin, promote cell migration by providing molecular anchorage for moving cells

Direction of migration 50 µm LE 47-21 Direction of migration 50 µm

Cell adhesion molecules contribute to cell migration and stable tissue structure One class of cell-to-cell adhesion molecule is the cadherins, which are important in formation of the frog blastula

LE 47-22 Control embryo Experimental embryo

The developmental fate of cells depends on their history and on inductive signals Coupled with morphogenetic changes, development requires timely differentiation of cells at specific locations Two general principles underlie differentiation: During early cleavage divisions, embryonic cells must become different from one another After cell asymmetries are set up, interactions among embryonic cells influence their fate, usually causing changes in gene expression

Fate Mapping Fate maps are general territorial diagrams of embryonic development Classic studies using frogs indicated that cell lineage in germ layers is traceable to blastula cells

Fate map of a frog embryo LE 47-23a Epidermis Central nervous system Epidermis Notochord Mesoderm Endoderm Neural tube stage (transverse section) Blastula Fate map of a frog embryo

Techniques in later studies marked an individual blastomere during cleavage and followed it through development

Cell lineage analysis in a tunicate LE 47-23b Cell lineage analysis in a tunicate

Establishing Cellular Asymmetries To understand how embryonic cells acquire their fates, think about how basic axes of the embryo are established

The Axes of the Basic Body Plan In nonamniotic vertebrates, basic instructions for establishing the body axes are set down early, during oogenesis or fertilization In amniotes, local environmental differences play the major role in establishing initial differences between cells and, later, the body axes

Restriction of Cellular Potency In many species that have cytoplasmic determinants, only the zygote is totipotent That is, only the zygote can develop into all the cell types in the adult

Unevenly distributed cytoplasmic determinants in the egg cell help establish the body axes These determinants set up differences in blastomeres resulting from cleavage

Left (control): Fertilized salamander eggs were allowed to divide normally, resulting in the gray crescent being evenly divided between the two blastomeres. Right (experimental): Fertilized eggs were constricted by a thread so that the first cleavage plane restricted the gray crescent to one blastomere. Gray crescent Gray crescent The two blastomeres were then separated and allowed to develop. Normal Belly piece Normal

As embryonic development proceeds, potency of cells becomes more limited

Cell Fate Determination and Pattern Formation by Inductive Signals After embryonic cell division creates cells that differ from each other, the cells begin to influence each other’s fates by induction

The “Organizer” of Spemann and Mangold Based on their famous experiment, Spemann and Mangold concluded that the blastopore’s dorsal lip is an organizer of the embryo The organizer initiates inductions that result in formation of the notochord, neural tube, and other organs

Nonpigmented gastrula (recipient embryo) LE 47-25a Pigmented gastrula (donor embryo) Dorsal lip of blastopore Nonpigmented gastrula (recipient embryo)

Secondary (induced) embryo LE 47-25b Primary embryo Secondary (induced) embryo Primary structures: Neural tube Notochord Secondary structures: Notochord (pigmented cells) Neural tube (mostly nonpigmented cells)

Formation of the Vertebrate Limb Inductive signals play a major role in pattern formation, development of spatial organization

The molecular cues that control pattern formation are called positional information This information tells a cell where it is with respect to the body axes It determines how the cell and its descendents respond to future molecular signals

The wings and legs of chicks, like all vertebrate limbs, begin as bumps of tissue called limb buds

Anterior AER Limb bud ZPA Posterior Apical ectodermal ridge 50 µm LE 47-26a Anterior AER Limb bud ZPA Posterior Apical ectodermal ridge 50 µm Organizer regions

The embryonic cells in a limb bud respond to positional information indicating location along three axes

Digits Anterior Ventral Proximal Distal Dorsal Posterior LE 47-26b Digits Anterior Ventral Proximal Distal Dorsal Posterior Wing of chick embryo

One limb-bud organizer region is the apical ectodermal ridge (AER) The AER is thickened ectoderm at the bud’s tip The second region is the zone of polarizing activity (ZPA) The ZPA is mesodermal tissue under the ectoderm where the posterior side of the bud is attached to the body

Tissue transplantation experiments support the hypothesis that the ZPA produces an inductive signal that conveys positional information indicating “posterior”

LE 47-27 Anterior New ZPA Donor limb bud Host limb bud ZPA Posterior

Signal molecules produced by inducing cells influence gene expression in cells receiving them Signal molecules lead to differentiation and the development of particular structures