Developmental Stages in an
Amphibian
LE 21-4
Animal development
Cell
movement
Zygote
(fertilized egg)
Eight cells
Gut
Blastula
Gastrula
Adult animal
(cross section) (cross section)
(sea star)
Cell division
Morphogenesis
Observable cell differentiation
Seed
leaves
Plant development
Zygote
(fertilized egg)
Two cells
Shoot
apical
meristem
Root
apical
meristem
Embryo
inside seed
Plant
Most animals proceed
through these stages
during embryonic
development:
1. Zygote
2. Early cleavage stages
• Morula (solid ball)
• Blastula (hollow ball)
3. Gastrula
LE 47-7
Fertilized egg
Four-cell stage
Morula
Blastula
Starfish development, unfertilized egg.
16 blastomeres.
2 blastomeres.
32 blastomeres.
morula
4 blastomeres.
Starfish development,
nonmotile
blastula.
LE 21-11a
Unfertilized egg cell
Sperm
Molecules of another
cytoplasmic determinant
Molecules of a
Nucleus
cytoplasmic
•Egg provides
Fertilization
proteins and
determinant
mRNAs required
for early
development
Zygote
(fertilized egg)
Two-celled
embryo
•Cleavage
asymmetrically
Mitotic cell division divides
cytoplasmic
components;
immediately
establishing
polarity
Cytoplasmic determinants in the egg
LE 47-8
Point of
sperm entry
Animal
hemisphere
Vegetal
hemisphere
Point of
sperm
entry
Anterior
Right
Ventral
Gray
crescent
Vegetal pole
Future
dorsal
side of
tadpole
First
cleavage
Dorsal
Left
Posterior
Body axes
Animal pole
Establishing the axes
LE 47-9
Zygote
0.25 mm
2-cell
stage
forming
4-cell
stage
forming
Eight-cell stage (viewed
from the animal pole)
8-cell
stage
0.25 mm
Animal pole
Blastula
(cross
section)
Blastocoel
Vegetal pole
Blastula (at least 128 cells)
Cleavage in 3 different
animal lineages
Gastrulation - Establishing Germ
Layers (tissue development)
• Ectoderm gives
rise to outer
covering and
nervous system
• Endoderm
gives rise to the
digestive tract
• Mesoderm
gives rise to
muscle tissue
Starfish development, gastrula during invagination.
Starfish, late bipinnaria.
Starfish development, mid-gastrula. LM X75.
Starfish, young adult.
Neurulation
• The nervous system is the first organ system to
develop
– Notochord from mesoderm --> replaced with
backbone
– Neural tube from ectoderm --> spinal chord
• Establishes basic body plan and layout of body
parts
Fig. 52.11
LE 47-14c
Somites
Eye
SEM
Neural tube
Notochord
Coelom
Archenteron
(digestive cavity)
Somites
Tail bud
1 mm
Neural
crest
Somite
LE 47-15
Eye
Neural tube
Notochord
Forebrain
Somite
Heart
Coelom
Archenteron
Endoderm
Mesoderm
Lateral fold
Blood
vessels
Ectoderm
Somites
Yolk stalk
YOLK
Yolk sac
Form extraembryonic
membranes
Early organogenesis
Neural tube
Late organogenesis
Organogenesis
Organogenesis is the
formation of the organs.
The layers are germ
layers; they have
specific fates in the
developing embryo.
• Organogenesis is the formation
of the organs
– Endoderm
• The innermost layer
• Goes on to form the gut
– Mesoderm
• In the middle
• Goes on to form the
muscles, circulatory
system, blood and many
different organs
– Ectoderm
• The outermost
• Goes on to form the skin
and nervous system
Organogenesis
Mammalian Development
Human Prenatal Development
• Gestation lasts 266 days from fertilization to birth
• Development begins in the oviduct
– About 24 hours after fertilization, the zygote has
divided to form a 2-celled embryo
– The embryo passes down the oviduct by cilia and
peristalsis
– The zona pellucida (a vestige of the egg shell) has
dissolved by the 5th day, when the embryo enters the
uterus
– The embryo floats free for several days, nourished by
fluids from glands in the uterine wall
• At this point, it is called a blastocyst (same as
blastula)
24 hrs
1 day
5 days
7 days
• The trophoblast is the outermost layer of cells
in the blastocyst
• The trophoblast forms the chorion and amnion
• The inner cell mass forms the embryo itself
Development of the Placenta
12-day Human Embryo
Organ Development
• Begins during the
first trimester
– Gastrulation occurs
during the 2nd and
3rd weeks, followed
by neurulation
(formation of the
neural tube)
– The heart beats
spontaneously after
3.5 weeks
– After the first two
months of
development, the
products of
conception are called
a fetus
Week 5
• At the end of the first
trimester (first 3 months of
development)
– Fetus can be recognized as
a human
– ~56 mm long, and ~14 g
– The sexes can be
differentiated
– Ears, eyes becoming welldeveloped,
– Skeleton starting to develop
– Notochord replaced with the
developing vertebral column
– Moves, ‘breathes’, makes
sucking motions with thumb
33-day embryo measuring 7
x 3.2mm.
Human embryo at 40
days.
LE 21-12
Follicle cell
Egg cell
developing within
Nucleus
ovarian follicle
Egg cell
Nurse
cell
Fertilization
Laying of egg
Fertilized egg
Egg shell
Nucleus
Embryo
Multinucleate
single cell
Early blastoderm
Plasma
membrane
formation
Yolk
Late blastoderm
Body
segments
Cells of
embryo
Segmented
embryo
0.1 mm
Hatching
Larval stages (3)
Pupa
Metamorphosis
Adult fly
Head Thorax
Abdomen
0.5 mm
Dorsal
BODY
AXES
Anterior
Posterior
Ventral
LE 21-14a
Tail
Head
Wild-type larva
Tail
Tail
Mutant larva (bicoid)
Drosophila larvae with wild-type and bicoid mutant
phenotypes
LE 21-14b
Nurse cells
Egg cell
Developing
egg cell
bicoid mRNA
Bicoid mRNA
in mature
unfertilized
egg
Fertilization
Translation of bicoid mRNA
100 m m
Bicoid protein
in early
embryo
Anterior end
Gradients of bicoid mRNA and Bicoid protein in normal
egg and early embryo
LE 21-23
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse
LE 21-13
Eye
Leg
Antenna
Wild type
Mutant
LE 21-24
Thorax
Thorax
Genital
segments
Abdomen
Abdomen