Animal Development

1
Animal Development

• Chapter 47

2
Development

• Preformation Idea that egg contains a miniature
  adult that grows only in size during devel.
• Epigenesis Animal emerges from a formless egg
  (grows and differentiates).
• Microscopy has led to the overwhelming acceptance
  of epigenesis as the pattern of development in
  animals.

3
Fertilization

• Combination of egg nucleus and sperm nucleus.
• Activation of egg to begin development of zygote.
• Internal ? Fert. takes place inside body of
  female (ie humans)
• External ? Fert. takes place in external
  environment (ie fish)

4
The Acrosomal Reaction

• When it contacts the jelly coat of the egg, the
  sperm releases digestive (hydrolytic) enzymes
  from acrosome.
• Sperm digest jelly coat and membranes fuse.
• Sperm nucleus is released into the egg.
• Fast block to polyspermy ? egg depolarizes (Na
  in) to prevent double fertilization.

5
Cortical Rxn (Slow block to polyspermy)

• Progressive release of Ca2 by the eggs ER
  causes cortical granules to release enzymes
• The enzymes in effect harden the eggs membrane
  (fertilization membrane)
• This hard membrane is not penetrable by any more
  sperm, so only 1 sperm fertilizes the egg

6
Fertilization in Sea Urchin (external)
7
Egg Activation

• High levels of Ca2 also trigger the egg to
  increase cellular respiration and protein
  synthesis
• As this takes place, the nuclei of the egg and
  the sperm fuse with one another
• Diploid zygote nucleus is formed
• Cell division and DNA replication begin as the
  zygote develops

8
Mammal Fertilization (internal)

• Capacitation ? Secretions by female enhance the
  function of sperm in internally fertilizing
  mammals
• Zona pellucida recognizes the binding sperm
• Acrosomal and cortical reactions both occur in
  mammals
• Fast and slow polyspermy blocks similar to sea
  urchins

9
Mammalian Fertilization

• Entire sperm enters egg and the flagella divide
  to produce spindle of dividing egg
• Egg and sperm nuclei do not fuse immediately
• Share spindle in zygotes initial division.
• Appear first as diploid zygote as daughter of 1st
  mitotic division
• Sea urchins (external) fused DNA immediately

10
Mammalian Fertilization
11
Cleavage

• Rapid cell division that changes the large,
  single-cell zygote into a large ball of much
  smaller cells (blastomeres)
• Cells are smaller because the G1 and G2 phases
  are generally skipped
• Blastomeres each contain different cytoplasmic
  molecules (determine fate of cells?)

12
Cleavage in Sea Urchins
13
Polarity

• Most animals (not mammals) exhibit a patterned
  distribution of materials
• Vegetal Pole ? High yolk
• Animal Pole ? Low yolk, often anterior of animal
• Cleavage is more rapid in animal pole

14
Cleavage

• In sea urchins and frogs, first two divisions
  vertical, third horizontal
• Eight-celled embryo with two tiers of four cells
  is what continues with cleavage

15
Cleavage ? Morula Stage

• Cleavage continues and the morula is formed
• The morula is a solid ball of cells

16
Cleavage ? Blastula Stage

• A fluid filled cavity (blastocoel) forms within
  the morula
• Creates a hollow ball of cells (blastula)

17
Meroblastic/Holoblastic Cleavage

• Meroblastic Cleavage
• Birds (plentiful yolk) restrict cleavage to the
  animal pole of the zygote
• Holoblastic Cleavage
• Sea urchins, frogs (less yolk) show complete
  division of the egg

18
Gastrulation

• Development of a three-layered embryo (gastrula)
  with a primitive gut from the blastula
• Three Embryonic Germ Layers
• Ectoderm
• Endoderm
• Mesoderm

19
Sea Urchin Gastrulation

• Begins at the vegetal pole where individual cells
  enter the blastocoel as mesenchyme cells
• Rest of cells buckle in to form the archenteron
  (invagination)
• Open end of archenteron will form the anus and
  the other end the mouth
• Thus, the archenteron becomes the digestive tube
  (mouth to anus)

20
Sea Urchin Gastrulation
21
Organogenesis (differentiation)

• Ectoderm germ layer gives rise to
• Epidermis of skin, and its derivatives
• Epithelial lining of the mouth and rectum
• Cornea and lens of the eyes
• The nervous system adrenal medulla tooth
  enamel epithelium of the pineal and pituitary
  glands

22
Organogenesis

• Endoderm germ layer becomes
• The epithelial lining of the digestive tract
  (except the mouth and rectum).
• The epithelial lining of the respiratory system.
• The pancreas thyroid parathyroids thymus the
  lining of the urethra, urinary bladder, and
  reproductive systems

23
Organogenesis

• Mesoderm germ layer becomes
• The notochord
• The skeletal and muscular systems
• The circulatory and lymphatic systems.
• The excretory system
• The reproductive system (except germ cells)
• And the dermis of skin lining of the body
  cavity and adrenal cortex

24
Organogenesis
25
Amniotic Eggs

• Allows terrestrial organisms to reproduce in a
  dry environment
• Organisms is kept in a fluid filled environment
• Mammals and birds both have amniotic eggs

26
Avian Development
27
Avian Development
28
Avian Development
29
Extraembryonic Membranes

• yolk sac? providing nutrients to the embryo
• Amnion? encloses the embryo in a fluid-filled
  amniotic sac (protects the embryo from
  dessication)
• Chorion? cushions the embryo
• allantois ?disposal sac for uric acid (urine)

30
Amniotic Egg (Bird/Reptile)
31
Human Amniotic Development

• Human embryos develop in amniotic eggs the way
  birds and reptiles do, except they are implanted
  in the uterus, not externally developing
• There is more exchange of materials b/t mother
  and offspring (blood cells for immunity,
  nutrients, waste)
• ie placental mammals (humans)
• ie marsupial mammals (kangaroos)