CHAPTER 27 Reproduction and Embryonic Development Modules 27.9 – 27.15

The shape of a human sperm cell is adapted to its function PRINCIPLES OF EMBRYONIC DEVELOPMENT 27.9 Fertilization results in a zygote and triggers embryonic development The shape of a human sperm cell is adapted to its function Middle piece Neck Head Plasma membrane Tail Mitochondrion (spiral shape) Nucleus Acrosome Figure 27.9B

Only one of these sperm will penetrate this human egg cell to initiate fertilization Fertilization is the union of a sperm and an egg to form a diploid zygote Figure 27.9A

Process of fertilization 1 The sperm approaches the egg 2 The sperm’s acrosomal enzymes digest the egg’s jelly coat 3 SPERM Proteins on the sperm head bind to egg receptors 4 The plasma membranes of sperm and egg fuse Sperm head 5 The sperm nucleus enters the egg cytoplasm Nucleus Acrosome Acrosomal enzymes Plasma membrane 6 A fertilization envelope forms Receptor protein molecules Plasma membrane Sperm nucleus Vitelline layer Cytoplasm Egg nucleus Jelly coat 7 The nuclei of sperm and egg fuse EGG CELL Zygote nucleus Figure 27.9C

27.10 Cleavage produces a ball of cells from the zygote Cleavage is the first major phase of embryonic development It is the rapid succession of cell divisions It creates a multicellular embryo from the zygote It partitions the multicellular embryo into developmental regions

BLASTULA (hollow ball) Cross section of blastula Cleavage in a sea urchin ZYGOTE 2 cells 4 cells 8 cells Blastocoel Many cells (solid ball) BLASTULA (hollow ball) Cross section of blastula Figure 27.10

27.11 Gastrulation produces a three-layered embryo Gastrulation is the second major phase of embryonic development It adds more cells to the embryo It sorts all cells into three distinct cell layers The embryo is transformed from the blastula into the gastrula

The three layers produced in gastrulation Ectoderm, the outer layer Endoderm, an embryonic digestive tract Mesoderm, which partly fills the space between the ectoderm and endoderm

Development of frog gastrula Animal pole Blastocoel Development of frog gastrula 1 Vegetal pole BLASTULA GASTRULATION 2 Blastopore forming Blastopore forming Blastocoel shrinking Archenteron 3 Archenteron Ectoderm Mesoderm 4 Endoderm Yolk plug Yolk plug GASTRULA Figure 27.11C

27.12 Organs start to form after gastrulation Embryonic tissue layers begin to differentiate into specific tissues and organ systems In chordates the notochord develops from the mesoderm the neural tube develops from the ectoderm The neural tube becomes the brain and spinal cord

Neural plate Neural fold Neural fold Neural plate Notochord Ectoderm Mesoderm Endoderm Archenteron Neural folds Outer layer of ectoderm Neural tube Figure 27.12A, B

The body cavity, or coelom, also develops from the mesoderm Neural tube Somites are blocks of mesoderm that will give rise to segmental structures Notochord Somite Coelom Archenteron (digestive cavity) The body cavity, or coelom, also develops from the mesoderm Somites Tail bud Eye Figure 27.12C

Table 27.12

The tissues and organs of a tadpole emerge from cells of the ectoderm, mesoderm, and endoderm Figure 27.12D

Tissues and organs take shape in a developing embryo as a result of 27.13 Changes in cell shape, cell migration, and programmed cell death give form to the developing animal Ectoderm Tissues and organs take shape in a developing embryo as a result of cell shape changes cell migration Figure 27.13A

programmed cell death (apoptosis) Cell suicide Dead cell engulfed and digested by adjacent cell programmed cell death (apoptosis) Figure 27.13B

27.14 Embryonic induction initiates organ formation Induction is the mechanism by which one group of cells influences the development of tissues and organs from ectoderm, endoderm, and mesoderm Adjacent cells and cell layers use chemical signals to influence differentiation Chemical signals turn on a set of genes whose expression makes the receiving cells differentiate into a specific tissue

Induction during egg development Optic cup Lens ectoderm Cornea Future brain Lens Optic vesicle Future retina Optic stalk 1 2 3 4 Figure 27.14

27.15 Pattern formation organizes the animal body Pattern formation is the emergence of a body form with structures in their correct relative positions It involves the response of genes to spatial variations of chemicals in the embryo

Wing development ANTERIOR Bird embryo VENTRAL Normal wing Limb bud DISTAL Limb bud develops DORSAL PROXIMAL POSTERIOR Figure 27.15A

Pattern-forming zone Graft of cells from pattern- forming zone Wing with duplication Graft Donor limb bud Host limb bud Host limb bud develops Host pattern- forming zone Donor cells Figure 27.15B