Developmental Bio
BTHS
Introduction to Life Cycles
Today’s Objectives
• Identify 3 model organisms used in Developmental
Biology and the advantages and disadvantages of each.
• Describe the basics of a mammalian life-cycle including:
– the diploid state and the haploid state, cleavage, implantation,
embryo and fetus.
• Identify the location of human embryonic development,
and fertilization.
• Define several stages of animal development including:
– fertilization, embryogenesis, cleavage, blastula, gastrulation,
organogenesis, metamorphosis, larva, gamete, somatic cell,
gametogenesis
Let’s take a brief look at
humans first
• Important points:
– Diploid vs. haploid
• Somatic cells vs. gametes
– Where does fertilization take place?
– Terms: Uterus, fallopian tube, morula,
blastocyst, implantation, placenta
Figure 2.9(1) Summary of
Meiosis
Figure 2.9(2) Summary of
Meiosis
Figure 2.9(3) Summary of
Meiosis
Figure 11.27 - Early Human
Development
Stages of Animal
Development
• The mere coordination to create offspring is a
major consideration in the life cycles
• Let’s take a look at stages within a typical
animal life cycle
– Can be generalized - most of the same steps, but
with variation
Basic Stages, in order
• Fertilization - > makes a zygote
• Embryogenesis
– Cleavage
– Blastula
– Gastrulation
– Organogenesis
– Larva -> metamorphosis -> adult
– Gametogenesis
Fertilization
• Joining of gametes
• Haploid to diploid
– Restore the diploid number (2n)
• Different from species to species!
Cleavage
• Period of rapid cell divisions
– Really fast! 1 per hour in or faster in some
cases
– What has to happen before every cell
division?
• Cytoplasm of zygote is divided into
many small cells
– Blastomeres
– This then becomes the blastula stage
Gastrulation
• Massive cell rearrangements of blastula
to form gastrula
– Set up axis (Anterior-posterior, dorsalventral, proximal-distal)
– Forms 3 germ layers
• Ectoderm (outer coating)
• Mesoderm (middle layer)
• Endoderm (inner lining)
Organogenesis
• Cells interact with each other, rearrange
to produce organs and tissues
– Cells can migrate great distances!
• (Ex. Blood, gametes, pigment cells, bones of
face)
Maturation
• Some species undergo metamorphosis
– Emerge from the egg as a larva
– Mature within the pupa to become an adult
– The larva can look very different from the
adult
Gametogenesis
• In some species the cells that are going
give rise to sperm and eggs are set
aside immediately after fertilization
– Germ line
• Migrate to the gonad, undergo meiosis
• Eventually -> if fertilization occurs, start
all over!
Frog Life Cycle (Xenopus)
• Let’s examine a specific example of a
life cycle, and observe the various
stages
• Xenopus laevis or Rana pipiens
• Seasonal mating
Figure 2.2(1) Early Development of the Frog Xenopus laevis
Figure 2.2(2) Early Development of the Frog
Xenopus laevis
Figure 2.3(1) Continued Development of Xenopus
laevis
Figure 2.3(2) Continued Development of Xenopus
laevis
Figure 2.4 Metamorphosis of the Frog
Eggs of Rana and Xenopus
• 2 hemispheres - one is yolky, one is not
– (vegetal and animal - we’ll learn more later)
• At fertilization, egg completes meiosis II
• Then sperm and egg nuclei fuse to
become one nucleus
• Cleavage begins
– What does this mean?
• As Cleavage occurs
– Fertilized egg (zygote) stays the same size
– Tens of thousands of cells are made from
the large egg cytoplasm
– On one side of the Blastula, a hollow cavity
forms called a blastocoel
• Then gastrulation begins
– 3 germ layers begin to form
• What are they?
– Can you see a head or tail end yet?
• Next the nervous system begins to form
in a process called Neurulation
– We’ll discuss in detail . . . . Neural tube
• The organism becomes segmented into
repeating segments
– Seen in SOMITES
– We’ll discuss more
• Mouth and anus develop
• Neurons start to make connections
• Finally larva hatches, undergoes
metamorphosis
• Quiz on Life Cycles - FRIDAY