Transcript Animal Development - Volunteer State Community College

Animal Development
Nancy G. Morris
Volunteer State Community College
Stages of Embryonic Development
Two early views of how animals developed from an
egg competed for supporters until modern
techniques were developed.

Preformation – suggests that the embryo
contained all of its descendents as a series of
successively smaller embryos within embryos.
This was popular until about the 18th century.
(Figure 47.1)
Stages of Embryonic Development
The second of two early views of how animals
developed from an egg competed for supporters
until modern techniques were developed:
Epigenesis – from egg to organism, an animal’s
form develops gradually.
Proposed by Aristotle
Gained support in the 19th century as improved
microscopy permitted scientists to observe
embryos as they developed.
Fertilization…
activates the egg and brings
nuclei of sperm and egg together
restores diploidy (from haploid
sets of chromosomes from two
individuals)
triggers onset of embryonic
development
The Acrosomal Reaction
The acrosomal
reaction is
the discharge
of hydrolytic
enzymes from
a vesicle in
the acrosome
of a sperm
cell.
Sperm contact egg’s
jelly coat
Acrosome releases
hydrolytic enzymes
Penetration (Fig. 47.2)
Proteins attach to
specific receptors on
egg’s vitelline layer =
species specificity
The plasma membranes
of the egg and sperm
fuse
The Cortical Reaction
The fusion
of egg &
sperm
membranes
stimulates a
series of
changes in
the egg’s
cortex known
as a cortical
reaction.
Chemical reactions change
the egg’s cortical granules.
Granules fuse with the plasma
membrane releasing enzymes
separating the vitelline layer
from the plasma membrane.
Swelling “lifts” the vitelline
membrane forming the
fertilization membrane.
Prevents penetration by
other sperm
Activation of the Egg
Chemical
change
(increase in
Ca2+) results
in metabolic
changes that
activate the
egg cell.
Figure 47.3
Cellular respiration & protein
synthesis rates increase.
Syngamy - sperm nucleus
within the egg swells &
merges with the egg nucleus
to form the zygote.
DNA replication begins & first
division occurs about 90
minutes after syngamy.
Figure 47.5
Fertilization in Animals
Capacitation
(enhanced
sperm
function)
results from
secretion in
the female’s
reproductive
tract.
Certain molecules on sperm’s
surface are altered increasing
motility.
Capacitated sperm must reach
the zona pellucida, the extra
cellular matrix of the egg,
containing a 3-D network of
glycoprotein filaments.
Microvilli from the egg pull
the whole sperm cell into the
egg cell.
Basic Developmental Vocabulary
Fertilization – activates egg & brings
together the nuclei of the egg and sperm
Cleavage partitions the zygote into many
smaller cells.
Gastrulation rearranges the blastula to
form a three-layered embryo with a primitive
gut, the archenteron.
Organogenesis is the process by which the
organs in the animal body form from the three
embryonic germ layers.
Basic Developmental Vocabulary
Blastula – a hollow sphere of cells (128 cells)
formed by cleavage of the morula. The blastula
contains the blastocoel that is fluid-filled. The
blastopore is the place where gastrulation begins.
Gastrulation – the process leading to the creation
of the primitive gut or archenteron. Invagination at
the blastopore results in the gut. (Figure 47.9)
Gastrula – transformation of the blastula into an
embryo possessing 3 germ layers, ectoderm,
mesoderm, endoderm. (Table 47.1)
Morula a solid mass of 16-64 cells formed by
cleavage (Latin for mulberry).
Figure 47.10
Gastrulation
in the Frog
Development of Extraembryonic Membranes
YOLK SAC – develop blood vessels to carry
nutrients into the embryo
AMINON – encloses the embryo in a fluidfilled sac, protecting from desiccation &
absorbing shock
CHORION – cushions the embryo against
mechanical shock
ALLANTOIS – disposal sac for uric acid
Figure 47.14
Development
of extraembryonic
membranes
in a chick
Mammalian Development
Fertilization occurs in the oviducts of most
mammals & early development occurs while the
embryo travels down the oviduct to the uterus.
Cleavage is relatively slow with the first,
second, & third divisions occurring at 36, 60, &
72 hours.
At 7 days after fertilization, the embryo
consists of more than 100 cells arranged around
a central cavity forming the blastocyst.
The blastocyst stage reaches the uterus &
begins to implant in the endometrium.
Early Human Embryo Development
Figure 47.15
Development of in Human Embryo
Trophoblast forms the chorion & fetal
portion of the placenta (along with
mesodermal tissue)
Epiblast cell layer forms the three germ
layers & the amnion
Hypoblast forms the yolk sac
Early Human Embryo Development
Figure 47.15
Early Human Embryo Development
Figure 47.15
Figure 47.14
Convergent extension of a sheet of cells
A signal causes cells to elongate and crawl between each
other. This results in the extension of the cell sheet in
a direction perpendicular to the convergence.
Figure
47.16
Change
in shape
during
morphogenesis