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Embryonic Development – Chapter 44
I. Stages of Development
A. Fertilization
1.sperm (n) fertilizes the ovum
(n) to produce a zygote (2n)
a.sperm are
b.
produced during
spermatogenesis in the
testes and the ovum is
produced during oogenesis
in the ovaries – both are
forms of meiosis
2.sperm and ovum must
“recognize” each other
through protein receptors on
the cell membrane that are
species specific
3.enzymes secreted by the
sperm digest the outer coat of
the ovum until one sperm
reaches and fuses with the
ovum’s cell membrane
4.sperm’s nucleus is pulled
inward – electrical and
chemical changes occur
modifying the egg cell
membrane so that no more
sperm can enter
B. Cleavage – cell division
(mitosis)
1.zygote divides rapidly to
produce 2,4,8,16, 32 . . . cells
 ball of cells or morula
2.fluid forms inside the ball and
a cavity (blastocoel) appears
in the center  blastula
3.a yolk develops providing
“food” for the embryo
4.two basic types of cleavage:
radial (zygote is divided into
radial segments like those of
an orange) and spiral (lie in an
offset, spiral pattern)
5.cells in the blastula are still
identical and undifferentiated
(stem cells)
C.
Gastrulation
1.cells differentiate and arrange
themselves into 3 distinct
layers: endoderm, mesoderm,
and ectoderm
2.ectoderm forms the outer
covering or epidermis of the
skin, also forms the
neuroectoderm that later
invaginates and becomes the
nervous system (nerve cord,
brain, nerves, etc.); lining of
the oral cavity and rectum
3.endoderm forms the inner
lining of the digestive and
respiratory systems,
associated glands
4.mesoderm forms most of the
body’s internal organs:
skeletal system, muscles,
circulatory, respiratory;
dermis of skin;
5.coelom – body cavity
surrounded by mesoderm may
form in which the organs are
held
6.an opening appears on one
side of the gastrula, then
extends the length of the
gastrula and opens to the
opposite side forming the
digestive tract
a.protostomes – first opening
becomes the mouth
(arthropods, mollusca,
annelida)
b. deuterostomes – second
opening becomes the mouth
(chordata, echinodermata)
D. Neurulation – formation of the
central nervous system
1.two parallel folds rise up and
join together forming the
neural tube (brain and spinal
cord)
2.neurula
3.muscles (somites) begin
forming in “blocks” around
both sides of the spinal cord
E. Organogenesis – formation of
the organs and organ systems
1.in humans after three weeks
following fertilization – most
major organs and systems are
fully formed, heart begins
beating
2.limb “buds” form legs and
arms in human, apotosis helps
form digits (fingers and toes) –
4th and 5th weeks
3.after 3 months (1st trimester) –
all major organs and systems
are fully formed and
functional
4.in Drosophila, as the adult
develops inside the pupa,
undifferentiated cells called
imaginal discs form specific
adult body structures – eyes
and antennae, legs, wings,
head, thorax, and abdomen,
etc.
II. Differentiation and Determination
A. Control of cleavage
1.chemicals present in the ovum
cytoplasm (mRNA from the
mother) may regulate early
development – maternal
determinants: genes of the
mother control cleavage
2.Gradients of proteins
diffusing from each end of the
ovum determine the head
(anterior) and tail ends
(posterior)of the embryo, top
(dorsal) and bottom (ventral)
3.These proteins come from
bicoid genes in the nurse cells
within the ovary.
a.bicoid diffuses down the
ovum determining the
anterior end of the embryo
and switching on
transcription of other
proteins made by gap genes
which determine the
embryo’s overall pattern of
development
B. Embryonic genes – homeotic
genes produce transcription
factors that control development of
major body features, organ
systems, or structure in
multicellular animals
C. Determinination – progressive
process which restricts the fate of
cells as time goes on and genes are
activated, a cell’s ultimate fate
may be decided fairly early in
embryonic development, however,
all somatic cells in the body are
totipotent (contain an entire set of
genes)
1.indetermininate cleavage
(echinoderms and chordata) –
determination begins at the 8
cell stage or later – embryo
develops normally if a cell is
removed during these early
divisions
2.determinate cleavage
(mollusca, nematoda,
arthropoda) – determination
is shown from the first cell
division, embryo is abnormal
if any cell is removed during
these early divisions
3.fate maps have been
developed in simple organisms
showing progressive
development and fate of
specific body cells in simple
organisms (round worm)
D. Apotosis (programmed cell
death) – may be important in
forming digits (toes and fingers)
E. Induction – one cell alters the
fate of another (the position and
environment of a cell affects its
genetic fate)
1.example: lens of the eye is
formed from ectoderm of the
head as a result of contact
with part of the brain, the
optic vesicle
2.example: transplanted tissue
form the dorsal lip of the
blastopore caused the
recipient embryo to develop a
second head-tail axis; the
dorsal lip is called the primary
organizer of the embryo
F. Pattern and position –
determination is a result of a cell’s
position
1.regeneration in animals
(amphibians, planaria) starts
with the formation at the site
of injury of a blastema ( a
mass of similar cells
resembling each other).
2.later the blastema cells
differentiate into various cell
and tissue types reforming the
missing organ
a.one vital stimulus to this
process is the presence of
nerve tissue
III. Maturation and aging – complex
processes
A. Aging = sum of changes that
accumulate with time and make an
organism more likely to die
1.aging and death seem to be
genetically programmed –
most species have a distinct
“life span”
2.What are the causes?
3.Characteristics
a.bodies cope less effectively
with stress/disease
b.
immune system
becomes less effective
c. oxidation damages
biological molecules and
tissues
d. glucose undergoes
uncatalyzed reactions with
some of the body’s proteins
and causes stiffening of
connective tissue and heart
muscle
e. accumulation of mutations