Early Developmental Stages

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Early Developmental Stages

• Fertilization
• Several sperm penetrate, but not all survive
• One sperm enters the egg and their nuclei fuse

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Fertilization
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Embryonic Development

• During first stages of development, an organism
  is called an embryo
• Following fertilization, zygote undergoes
  cleavage
• Morula forms blastula with a hollow blastocoel

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Lancelet Early Development
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Neurulation and the Nervous System

• Nervous system
• Develops from midline located just above the
  notochord
• Thickening of neural plate is seen along dorsal
  surface of the embryo
• Neural folds develop on either side of neural
  groove becomes the neural tube

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Development of Neural Tube and Coelomin a Frog
Embryo
Ribet Ribet
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Vertebrate Embryo, Cross Section
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Developmental Processes

• Development requires
• Growth
• Morphogenesis
• Cellular Differentiation
• Each contains all the instructions needed by any
  other specialized cell in the body

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Cytoplasmic Influence on Development
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Induction

• The ability of one embryonic tissue to influence
  the development of another tissue
• Developmental path of cells is influenced by
  neighboring cells

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Model Organisms

• Roundworm, Caenorhabditis elegans
• Fruit Fly, Drosophila melanogaster
• Mouse, Mus musculus

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Development of C. elegans, a Nematode
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Homeotic Genes

• Homeotic Genes control pattern formation
• Organization of differentiated cells into
  specific three-dimensional structures
• Certain genes control whether a particular
  segment will bear antennae, legs, or wings
• Homeotic genes all contain the same particular
  sequence of nucleotides, the homeobox

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Extraembryonic Membranes
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Human Development Before Implantation
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Human Embryonic andFetal Development

• Human gestation time - time from conception to
  birth - approximately nine months
• Embryonic Development - Months 1-2
• Fetal Development - Months 3-9
• Extraembryonic Membranes
• Chorion
• Amnion
• Allantois
• Yolk Sac

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Human Embryonic Development
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Embryonic Development

• First Week
• Morula transformed into blastocyst
• Second Week
• Implanting begins
• Gastrulation occurs
• Ectoderm and Endoderm differentiate

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Embryonic Development

• Third Week
• Nervous system and circulatory system appear
• Fourth and Fifth Weeks
• Umbilical cord is fully formed
• Limb buds appear Head enlarges
• Sense organs more apparent

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Human Embryo at Beginning of Fifth Week
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Embryonic Development

• Sixth Through Eighth Weeks
• Head achieves normal relationship with the body
• Nervous system is developed enough to permit
  reflex actions

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Structure and Functionof the Placenta

• The placenta
• A mammalian structure that exchanges materials
  between maternal and embryonic circulations
• Gases
• Nutrients
• Wastes

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Anatomy of the Placenta in a Fetusat Six to
Seven Months
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Three Stages of Parturition