Early Development

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Early Development

• Chapter 21

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Early Development

• Highly variable among different organisms
• Common genetic and cellular mechanisms in
  development
• Begins with gametogenesis
• Proceeds in ordered phases

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Phases of Human Development
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Gametogenesis and Fertilization
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Gametogenesis

• Formation of gametes, sperm and egg
• In the reproductive organs of adult organisms
• Sperm and egg contribute an equal number of
  chromosomes to the offspring
• Egg is 100x larger, contributes more cytoplasm

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Sperm Structure and Function

• Animal sperm has four major compartments
• the head (the acrosome)
• the neck (a centriole)
• a midpiece packed with mitochondria
• a tail (a flagellum)

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Sperm Structure and Function

• In plants, sperm develop from pollen grains
• Pollen are several haploid cells from meiosis
• Pollen cell comes into contact with the stigma
  and divides by mitosis to produce two sperm
  nuclei
• Move down the pollen tube to the egg cell

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Sperm Structure and Function
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Egg Structure and Function

• Large, contain the nutrients required for the
  embryos early development
• In species that lay eggs in the environment,
  stores in the egg are the only source of
  nutrients until it hatches
• In mammals need stores until the egg implants in
  the placenta
• Plants also provide endosperm to nourish the
  embryo

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Egg Structure and Function

• Cortical granules- vesicles filled with enzymes
  that assist fertilization in egg-laying animals
• Vitelline envelope- fibrous, matlike sheet of
  glycoproteins that surrounds the egg
• Jelly layer (a large, gelatinous mass that also
  encloses the egg

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Fertilization

• Fusing of a haploid sperm cell with a haploid egg
  cell to form a diploid zygote
• Can be internal or external
• Requires exact timing and recognition
• Must start development
• Starts with gamete release

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Fertilization

• Enzymes from the acrosome digest through the
  egg's jelly layer
• Acrosomal process contacts the vitelline envelope
• Plasma membranes fuse
• Sperm nucleus, mitochondria, and centriole enter
  the egg
• Sperm and egg nucleus fuse

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Species Recognition

• Fertilizin is a compound on the surface of sea
  urchin egg cells
• Binds to bindin, a protein on the head of sea
  urchin sperm
• Binding occurs in a species-specific manner
• Fertilizin from the eggs of one species binds to
  sperm of its own species but does not bind to
  sperm of different species

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Blocking Polyspermy

• Wide variety of mechanisms to block polyspermy
• In sea urchins, fertilization results in erection
  of a physical barrier
• Generates a fertilization envelope as cortical
  granules fuse with the plasma membrane

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Fertilization in Mammals

• Internal fertilization, so species recognition is
  generally not an issue
• Acrosome still breaks down zona pellucida
• Egg cells have a binding site for sperm
• Glycoprotein ZP3 in the zonabinds to the head of
  sperm
• Enzymes released from cortical granules modify
  ZP3 to prevent binding by additional sperm

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Fertilization in Flowering Plants

• Takes place inside ovule
• Double fertilization-one sperm nucleus fuses with
  an egg to form a zygote, and the other sperm
  nucleus fuses with two polar nuclei to form the
  triploid endosperm
• Interaction between the pollen grains and the
  ovule involve species recognition
• May also prevent self-fertilization

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Endosperm

• Endosperm provides nutrients for the embryotic
  development, germination and early seedling growth

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Animal Cleavage and Gastrulation
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Cleavage

• Cleavage is the set of rapid cell divisions
  (without growth) that follows fertilization
• Divides up the cytoplasm into cells, no growth
• Cells are called blastomeres
• Forms a blastula, sphere of cells
• Pattern of cleavage varies among species
• Sometimes makes cells around yolk

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Cleavage
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Cleavage

• Cells can divide at right angles to one another,
  forming tiers in a pattern called radial cleavage

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Cleavage

• Divide at oblique angles so that they pile up in
  a pattern called spiral cleavage

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What Determines Cleavage and Development?

• Cytoplasmic determinant is a molecule found in
  the egg that helps direct early development
• Affect development independently of sperm or
  zygote genotype
• Involved in differentiationthe generation of
  different cell types from a single cell

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Activating the Zygotic Genome

• Zygotic Genome not active during cleavage
• In most animals it is not transcribed until after
  cleavage is well under way
• Mammals are the exception
• Transcribe from the zygotic genome at the
  two-cell stage

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Embryonic Tissues

• Animal embryos develop three types of tissues,
  called germ layers
• Ectoderm forms the outer covering and nervous
  system
• Mesoderm gives rise to muscle, internal organs,
  and connective tissues such as blood and
  cartilage
• Endoderm produces the lining of the digestive
  tract or gut, along with some of the associated
  organs

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Gastrulation

• After cleavage is complete a bastula is the
  result
• Hollow ball of cells
• Gastrulation rearranges cells
• Results in the gastrula that contains the three
  embryonic tissue types
• Each gives rise to different tissue types

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Gastrulation
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Gastrulation

• At the end of gastrulation, the three embryonic
  tissues are arranged in layers, the gut has
  formed, and the major body axes have become
  visible

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Plant Development
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Plant Life Cycle
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Embryogenesis

• First division is asymmetric
• The large basal cell generates the suspensor
  structure
• Apical cell will give rise to the shoot apical
  meristem and the root apical meristem
• Meristem is area of rapidly dividing cells

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Embryogenesis

• An embryo contains an
• Epidermis- an outer covering of cells that
  protect the individual.
• Ground tissue- a mass of tissue that may later
  differentiate into cells for specialized
  functions
• Vascular tissue- that will differentiate into
  specialized cells that transport food and water
  between root and shoot.

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Invertebrate Development
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Embryogenesis in Fruit Fly

• Fertilized egg undergoes mitoses without
  cytokinesis
• Produces a multinucleate cell with a cytoplasm
  filled with nutrient-rich yolk
• Each nucleus migrates to the outside of the
  embryo cell and receives a plasma membrane
• Embryo becomes an outer sheet of cells
  surrounding the original cytoplasm

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Embryogenesis in Fruit Fly
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Embryogenesis in Fruit Fly

• Gastrulation starts with the formation of a cleft
  or furrow, followed by formation of furrows that
  define the head region and the series of body
  regions called segments
• Embryo hatches to from larva
• Larva forms pupa after a few days
• Pupa goes through metamorphosis to become fly

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Vertebrate Development
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Embryogenesis in Frog

• Frog embryo goes from one large cell to a ball of
  cells (blastula) through a series of cleavage
  events
• Does not increase in size
• Neural tube, which becomes the spinal cord and
  brain, forms at the end of gastrulation
• Embryo hatches into tadpole
• Tadpole goes through metamorphosis

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Embryogenesis in Frog
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Early Development in Humans

• Human eggs are released into the fallopian tube
  by the ovary
• After fertilization zygote goes through cleavage
  as it goes down the fallopian tubes
• Embryo undergoes implantation into the wall of
  the uterus, and the placenta forms
• Placenta carries nutrients to and waste from the
  developing fetus

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Early Development in Humans