Animal Development
Outline
I.
II.
Early Stages of Embryonic Development
A. Intro
B. Fertilization
C. Cleavage
D. Gastrulation
Morphogenesis
Intro
• Make a tiny dot at the top of your
paper…
– In your first week of development, that’s
how big you were!
• Less than a month later, brains take
form & heart is beating
Intro
• Preformation
– Embryo contains all descendants
– Russian nesting dolls
– Is there any merit to this idea? DNA?
• Epigenesis
– Aristotle (350 B.C.)
– Animal emerges gradually from a
formless egg
– Favored idea as microscopy improved
Intro
• After fertilization, cell division
partitions the cytoplasm in such a
way that nuclei of different embryonic
cells are exposed to different
cytoplasmic environments
– Expression of different genes in
different cells
Fertilization
• Goal = combine haploid sets of
chromosomes from two individuals
into a single diploid cell
• Also, must activate the egg to trigger
development of the embryo
• Lab study = sea urchins
(deuterostomes)
Acrosomal Reaction
• Acrosome = vesicle at tip of sperm cell
• Releases hydrolytic enzymes that
enable penetration of the egg’s jelly
coat
• “Lock-and-key” fit
ensure fertilization
occurs solely from
within a species
Acrosomal Reaction
• Sperm nucleus into egg’s cytoplasm
• Fusion of the membranes opens
sodium ion channels
• Depolarization = fast block to
polyspermy
– Prevents an abnormal number of
chromosomes in the zygote
Acrosomal Reaction
Cortical Reaction
• Series of changes in the outer cortex
of the egg cytoplasm
• Release of Ca2+ into cytosol
• High Ca2+ concentration changes
cortical granules that result in the egg
cell separating from the vitelline layer
• Fertilization envelope resists the entry
of additional sperm (slow block)
Cortical Reaction
Egg Activation
• High Ca2+ concentrations also produce
metabolic changes
– Neither sperm, nor DNA are responsible
for activation!!!
– Parthenogenesis a possibility
• Sperm cell nucleus begins to swell & within
20 minutes has merged w/ the egg nucleus
• First cell division occurs within 90 min.
In Mammals
• Internal vs. external in urchins
• Female secretions alter the surface
of sperm cells & increase the
sperm’s motility
• Zona pellucida = extracellular matrix
of the female egg
In Mammals
In Mammals
• Microvilli take in the entire sperm cell
• Basal body of sperm’s flagellum
divides into two centrosomes (w/
centrioles for mitotic spindles)
Cleavage
• Rapid cell division after fertilization
• Cells skip G1 & G2 phases (S & M only)
Sea urchin: cleavage begins 45-90 min. after fertilization
Cleavage
1. Vegetal pole
– Yolk (stored nutrients) is concentrated
2. Animal pole
– More rapid cell division
– Typically the anterior end of the embryo
Cleavage
•
Plasma membrane & cortex rotate
towards point of
sperm entry exposing
a light-gray region
(gray crescent)
– Opposite of sperm
entry
– Marks the dorsal
side of an
amphibian egg
Cleavage
zygote  morula  blastula  gastrula
Frog cleavage
Zygote (1 cell)
8-celled embryo
Morula (16-64 cells)
Cleavage
Frog blastula
Gastrulation
• Rearrangement of blastula cells into
a three-layered embryo
1. Ectoderm
2. Endoderm
3. Mesoderm
• Cells can now interact w/ each other
in new ways
Gastrulation
Gastrulation
•
•
•
•
•
Invagination = vegetal plate buckles inward
Involution = cells rolling over the edge of a lip into
the interior
Blastocoel = fluid-filled cavity (hollow blastula)
Blastopore = open end of the archenteron (anus
in a deuterostome)
Archenteron = deep, narrow cavity that is the
continuation of invagination (digestive tract)
Gastrulation
Gastrulation
Organogenesis
• Three germ layers develop into
beginnings of organs
• First organs in chordates are the
notochord & neural tube
Organogenesis
1. Notochord
– From dorsal mesoderm above archenteron
– Core of the backbone in which vertebrae are
attached
2. Neural tube
– Dorsal ectoderm just above notochord
– CNS
Organogenesis
In Mammals
• Fertilization in oviduct (Fallopian
tubes)
• Journey to uterus implantation takes
up to 1 week
• At this point, embryo has over 100
cells & is known as a blastocyst
• Blastocyst has two parts:
1. Inner cell mass = developing embryo
2. Trophoblast = fetal portion of the placenta
In Mammals
Morphogenesis
• Movement of cell shape & position
– Reorganization of cytoskeleton
(microtubules & microfilaments)
Morphogenesis
• Convergent extension = cell
crawling in which a sheet of cells
becomes narrower to grow longer