Development

1
Development

• Chapter 37

2
Objectives

• Be familiar with the early stages of animal
  development
• Recognize the germ cell layers from which
  different tissues develop and the organs that
  result from those layers

3
What happens AFTER fertilization

• Fertilization results in the union of the egg and
  sperm nuclei resulting in a diploid cell (Zygote)
• Zygote undergoes Mitosis to increase cell numbers
  
• Between mitotic events there is no increase in
  cell size (resulting in smaller cells)

4
Cleavage

• During cleavage the cytoplasm pinches in with
  the
• help of microfilaments

• divides egg cytoplasm into numerous smaller
  cells
• called Blastomeres

5

• cleavage increases the number of cells but not
  the
• overall volume of the egg

...

• Different daughter cells receive different
  regions of
• the cytoplasm depending on where they form
• Therefore, they end up with different bits of
  information
• and the descendants of those cells develop
  differentially

6
Stages of Cleavage (in the mammalian egg)
Early cleavage
Late cleavage
Blastula hollow ball of cells surrounding a
fluid filled cavity
Blastomeres surround a fluid-filled cavity
called the blastocoel
7
Gastrulation

• cellular organization, invagination
• newly formed cells become arranged into
  different
• germ layers

1. Ectoderm

• outermost layer
• nervous system, outer layer of skin

2. Mesoderm

• middle layer
• muscle, skeleton, circulatory, reproductive,
• excretory, connective tissues

3. Endoderm

• innermost layer
• inner lining of gut and associated organs

8
Blastula
Early gastrula
Late gastrula
Cell differentiation occurs during gastrulation
9
Organogenesis
Primary tissue layers give rise to groups of
cells unique in structure and function (organs)
Morphogenesis

• cell division
• cell migration
• tissue growth, folding
• programmed cell death sculpts body parts