Animal Development

LE 47-1 1 mm

Morphogenesis process by which an animal takes shape determined by genome and differences between embryonic cells

The Acrosomal Reaction Triggered when sperm reaches egg releases hydrolytic enzymes that digest material surrounding the egg

LE 47-3 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 Initiated by the fusion of egg and sperm induces a rise in Ca2+ stimulates cortical granules to release their contents outside the egg cause formation of a fertilization envelope block to polyspermy

LE 47-4 500 µm 1 sec before fertilization 10 sec after fertilization Spreading wave of calcium ions Point of sperm entry

Activation of the Egg sharp rise in Ca2+ increases the rates of cellular respiration and protein synthesis by the egg cell = activation of egg cell

Urchin Egg Cell 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 the cortical reaction modifies the zona pellucida as a slow block to polyspermy

LE 47-6 Follicle cell Sperm basal body Zona pellucida Cortical ganules Sperm nucleus Egg plasma membrane Acrosomal vesicle EGG CYTOPLASM

Cleavage a period of rapid cell division without growth partitions the cytoplasm of one large cell into many smaller cells blastomeres

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

Polarity eggs and zygotes of many animals (except mammals) Animal pole Vegetal pole Most of yolk

LE 47-8 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 follow a pattern relative to the zygote’s animal and vegetal poles

LE 47-9 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 Ie. reptiles and birds

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 Ie. sea urchins and frogs

Gastrulation rearranges the cells of a blastula into a three-layered embryo a gastrula 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

Urchin Key Future ectoderm Future mesoderm Future endoderm Animal pole Blastocoel Mesenchyme cells Vegetal plate Vegetal pole Blastocoel Filopodia pulling archenteron tip Archenteron Mesenchyme cells Blastopore 50 µm Blastocoel Ectoderm Archenteron Blastopore Mouth Mesenchume (mesoderm forms future skeleton) Digestive tube (endoderm) Anus (from blastopore)

Frog 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

Video: Frog Embryo Development Organogenesis regions of the germ layers develop into rudimentary organs the notochord forms from mesoderm the neural plate forms from ectoderm curves inward, forming the neural tube Video: Frog Embryo Development

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

LE 47-14b Neural fold Neural plate Neural crest Outer layer of ectoderm Neural crest Neural tube Formation of the neural tube

Somites Coelom blocks Formed from mesoderm lateral to the notochord Lateral to the somites mesoderm splits to form the coelom

LE 47-14c Eye Somites Tail bud 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

Adaptations of Amniotes develop in a fluid-filled sac in a shell or the uterus Birds Reptiles mammals Germ layers give rise to membranes surrounding embryos

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 eggs of placental mammals Small store few nutrients Exhibit holoblastic cleavage Show no obvious polarity Gastrulation and organogenesis similar to birds and reptiles Early cleavage is relatively slow

Cleavage forms the blastocyst forms 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

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.

Fate Maps general territorial diagrams of embryonic development

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

Development Differentiation Signal molecules Influence gene

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