Early Development

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Early Development

• Gametes

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Sperm
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Egg

• Lecithal Yolk

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Microlecithal egg

• Small amount of yolk
• Amphioxus
• Eutherians

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Mesolecithal egg

• Medium amount of yolk
• Amphibians

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Macrolecithal egg

• Large amount of yolk
• Bird and reptiles
• Most fish

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Isolecithal

• Even yolk distribution
• In microlecithal eggs

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Telolecithal

• Uneven yolk distribution
• Macrolecithal and Mesolecithal eggs
• Vegetal Pole yolk region
• Animal Pole relatively yolk-free, high
  metabolic activity/embryo

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Amniote eggs
Amnion
Allantoic cavity
Amnionic cavity
Allantois
Albumin
Chorion
Yolk
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Layers around egg

• Vitelline membrane
• Jelly
• Capsule
• Shell
• Albumin
• Corona Radiata
• Zonal Pellucida

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Oviparous
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Viviparous
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Ovoviviparous
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Fertilization
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Internal Fertilization

• Apodans
• Urodeles
• Amniotes

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External Fertilization

• Fish
• Frogs

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Zygote
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Cleavage BlastulaMicrolecithal Eggs

• Cleavage Mitosis divisions
• Blastomeres
• Blastocoel

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Cleavage Blastula

• Blastocyst in mammals
• Inner cell mass in mammals
• Trophoblast cells in mammals

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Cleavage BlastulaMesolecithal eggs
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Cleavage BlastulaMacrolecithal Eggs

• Blastoderm
• Blastocoel

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Gastrulation

• Germ layers form from which ALL future organs
  form
• Notochord forms
• Bilateral symmetry established

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Gastrulation/Microlecithal eggs

• Involution
• Blastopore
• Archenteron

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Germ layers

• Ectoderm outer layer
• Mesoderm middle layer
• Forms notochord
• Splits to form coelom
• Endoderm inner layer around archenteron

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GastrulationMesolecithal eggs

• Epiboly

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Ectoderm

• Nervous System
• Sensory structures
• Neural crest cells that become melanocytes,
  adrenal gland
• Epidermis of skin
• Epithelium of mouth/nose and anus

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Endoderm

• Lungs Swim bladders
• Digestive viscera

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Mesoderm

• Chordomesoderm becomes notochord

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Mesoderm

• Dorsal Mesoderm Epimere
• Segmented bands called somites
• Divides into
• Dermatome
• Myotome
• Sclerotome

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Mesoderm

• Lateral plate mesoderm hypomere
• Splits into Somatic and Splanchnic layers
• Coelom between these layers

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Hypomere

• Somatic Mesoderm plus Ectoderm
• Somatopleure
• Splanchnic Mesoderm plus Endoderm
• Splanchnopleure

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Mesoderm

• Intermediate mesoderm Mesomere
• Kidney tubules and associated ducts

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Key Points

• Which germ layer (ectoderm, endoderm, mesoderm)
  gives rise to the following structures
• Heart
• Liver
• Lung
• Biceps muscle
• Notochord

• Brain
• Kidney
• Spinal cord
• Skin

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Gastrulation in Macrolecithal eggs

• Delamination
• Blastoderm forms upper sheet of cells called
  Epiblast and
• Lower sheet of cells called Hypoblast
• Epiblast becomes Ectoderm
• Hypoblast becomes Endoderm

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Gastrulation in Macrolecithal eggs

• Mesoderm forms by Primitive Streak
• Cells stream inward from posterior to anterior
• Gives rise to notochord

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Gastrulation in macrolecithal eggs

• Some mesoderm is unorganized and migrates, called
  MESENCHYME

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Gastrulation in macrolecithal eggs

• Body stalk
• Connection from body to yolk

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Gastrulation in mammal

• Blastoderm
• Delamination to form hypoblast epiblast
• Primitive streak forms mesoderm
• Notochord
• Mesenchyme
• Coelom from splitting of lateral plate mesoderm

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Neurulation

• Dorsal hollow nerve cord
• Neural crest cells branch off

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Organogenesis

• Beginning of all major organs of the body

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Extraembryonic membranes

• Fish
• Body stalk
• Yolk sac

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• Amphibians

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Extraembryonic membranes

• AMNIOTES
• Yolk sac (from splanchnopleure)

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Amniotes

• Amnion
• From somatopleure
• Amniotic Fluid

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Amniotes

• Chorion
• From somatopleure
• For communication with oxygen source
• Helps form placenta in mammals
• Against shell in birds

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Amniotes

• Allantois
• From splanchnopleure
• Gas exchange in reptiles
• Waste receptacle in eutherians

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Placenta in Eutherians

• Excellent waste removal nutrient uptake
• Moms uterus plus babys extraembryonic membrane
• Attaches to baby via umbilical cord
• Yolk sac functions as placenta in marsupials