Title: The Continuity of Life II:
1Chapter 45
- The Continuity of Life II
- Development
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2The Continuity of Life II Development
- Each life began with the fusion of a tiny motile
sperm cell and a larger but structurally simple
egg. - The fertilized egg is transformed into a complete
organism which closely resembles its parents - This process involves growth (increase in size),
differentiation (specialization of cells,
tissues, and organs), and morphogenesis (shaping
of the adult body form). - Development is most clearly explained through
concrete examples.
http//www.stanford.edu/group/Urchin/path.html
3Development of the Sea Urchin
- Sea urchins have been a favorite of embryologists
to study because - their eggs are produced in large numbers
- they are fertilized and develop internally,
making it possible to study them under relatively
simple lab conditions - both egg and developing embryo are nearly
transparent, so its easier to observe early
events without with out disrupting them - the process is rapid
- In about 48 hours, the zygote develops into a
free swimming larval form, known as the pluteus.
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4Fertilization and Activation of the Egg
- development begins with fertilization of the egg
by the sperm - sperm cell of sea urchin consists of an acrosome
at anterior tip, tightly packed nucleus,
cytoplasm, and long flagellum. - egg cell is much larger than the sperm, and is
surrounded by an outer membrane known as the
vitelline envelope. - embedded in the vitelline envelope and on its
surface are species-specific protein receptors
that participate in the binding of sperm to egg. - ultimately the cell membranes of the sperm and
egg make contact, they fuse, and the sperm
nucleus enters the cytoplasm of the egg.
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mmon20Development20Stages.htm
5Fertilization and Activation of the Egg (cont)
- fertilization has at least four consequences
- changes take place on the surface of the egg to
prevent the entry of an additional sperm. the
increase in concentration of free Ca ions
triggers a series of reactions that cause the
vitelline envelope to lift off the surface of the
egg. The protective outer membrane that forms
from the vitelline envelope is know as the
fertilization membrane. - the egg is activated metabolically, evidenced by
increase in protein synthesis and rise in oxygen
consumption. - genetic material of the male is introduced into
the female gamete - egg begins to divide by mitosis, with the
centrioles of the sperm participating in the
organization of the mitotic spindle.
http//www.stanford.edu/group/Urchin/path.html
6From Zygote to Pluteus
- zygote divides once an hour for 10 hours, in a
process known as cleavage. the embryo at this
stage is called a morula. - Na ions are pumped from the cells into the
extracellular spaces, followed by this osmotic
flow of water. this creates a fluid filled cavity
called a blastocoel in the center of the embryo.
when the blastocoel is fully formed, the embryo
is now called the blastula, and its cells are
called blastomeres. - the formation of the blastula is followed by a
process known as gastrulation, which gives rise
to the primative gut. - gastrulation in the sea urchin begins with the
formation of the blastopore, an opening into the
blastula - primary mesenchyme-cells that break loose near
the blastopore and move over the interior surface
towards the opposite pole. - the entire new cell layer closest to the
blastopore turns inward, moving through the
blastocoel, forming a new cavity called the
archenteron. - the archenteron will develop into the digestive
tract, and the blastopore into the anus. - once gastrulation is complete, evidence of cell
differentiation can be seen.
http//www.stanford.edu/group/Urchin/path.html
7The Influence of the Cytoplasm
- the sea urchin egg contains a relatively small
amount of yolk which is concentrated in the lower
(vegetal) half of the egg. the upper half is
called the animal half - at cleavage the first two cell divisions run
perpendicular. these 4 cells are separated, and
each can develop into normal pluteus larva. - during the third cleavage division, the cytoplasm
of the animal and vegetal half are separated as
four cells split across the equator, producing an
eight cell embryo.
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g
8Development of the Amphibian
- the eggs of frogs and many other amphibians are
laid in shallow water and are fertilized
externally, hence they are easy to be readily
observed. - the amphibian however contains a large amount of
yolk. the animal half and vegetal half of the egg
differ markedly in appearance. - in experiments by Hans Spermann in early 1900s
it was shown that the cytoplasm associated with
the gray crescent is of critical importance in
the later development of the embryo. these
experiments provide further evidence that
differences in the cytoplasm and partially in
localized factors play a major role in
determining the course of early development. - when larger amount of yolk are present, the egg
divides unevenly, forming larger cells in the
vegetal hemisphere. - the amphibian blastocoel is small and usually
off-center. - the blastopore appears as a crescent shaped slit,
and always forms at the boundary between the gray
crescent and the vegetal hemisphere. - the formation of the blastopore marks the
initiation of gastrulation. cells at the dorsal
lip change shape and sink below the surface as
they move to the interior. - the direction in which the migrating cells move
is the future anterior-posterior axis of the
animal.
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re/chap10.html
9The Development of the Amphibian (cont)
- in the course of gastrulation, the primary
embryonic tissues (endoderm, mesoderm, ectoderm)
become arranged in a three-layered pattern. - floor of archenteron is composed of yolk laden
endodermal cells. its roof consists of endodermal
cells that have been pushed and pulled to the
interior by a sheet of mesodermal cells that lies
above it along the axis of the embryo. - this sheet of mesoderm includes cells destined to
form the notochord and is called the
chordamesoderm. - at the sides of the archenteron, other mesodermal
cells have slipped between the ectoderm and the
endoderm, forming the lateral plate mesoderm. - neural ectoderm-sheet of ectoderm that will give
rise to the brain and spinal cord - epidermal ectoderm-ectoderm covering rest of the
gastrula, will give rise to the epidermis of the
skin. - by end of gastrulation, chordamesoderm has formed
the notochord, and neural ectoderm has begun to
thicken, forming the neural plate. - neural plate ridges curve up and in, forming
neural groove. they meet to form the neural tube,
which pinches off from rest of ectoderm. - somites-blocks of tissue formed from
differentiated notochord split
10The Development of the Amphibian (cont)
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re/chap10.html
11The Development of the Amphibian (cont)
- differentiation is the result of the selective
activation and inactivation of specific genes in
the nucleus of a cell. - although cells have become differentiated, they
retain their full developmental potential. - in the amphibian, the first tissue to develop a
unique character is the dorsal lip of the
blastopore. it is the key to events that follow. - dorsal lip of the blastopore called the organizer
because of its effect on organizing tissues in
the embryo. - embryonic induction- occurs when two different
types of tissue come in contact with one another
in the course of development and one tissue
induces the other to differentiate. - induction takes place if chemical exchanges are
allowed, and if prevented it does not occur.
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anders/images/SalamanderSpotted_2.jpg
12Development of the Chick
- the hens egg is different then any of the
previous eggs examined. - it is surrounded by a shell, which permits it to
develop on land, as well as a system of
membranes. it also contains a large amount of
yolk. - the yolk of a hens egg is so large and dense
that cleavage doesnt involve most of the egg
mass. - the only part of the fertilized egg that cleaves
a thin layer of cytoplasm that sits on top of the
yolk and contains the nucleus. - cleavage of this thin layer produces a
lozenge-shaped blastula known as a blastodisc. it
is made up of many cells and the cells are
divided into two layers the upper layer
(epiblast) and the lower layer (hypoblast). - within a short period of time a visible line, the
primitive streak, appears on the surface of the
blastodisc. it is essentially an elongated
blastopore
I am chicken, hear me Roar!
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13Development of the Chick (cont)
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ickdev.htm
14Extraembryonic Membranes of the Chick
- eggs develop in water. in the amniote egg, this
water is held in a set of membranes formed from
the tissues of the embryo, known as the
extraembryotic membrane. - the endoderm is derived from the cells of the
hypoblast, and the mesoderm and ectoderm are from
cells in the epiblast. - yolk sac- membrane of mesoderm and endoderm
surrounding the yolk. nutritive function - amniotic cavity- the amniotic folds of ectoderm
and mesoderm fused together surrounding the
embryo - fusion of the amniotic folds creates two
membranes, each made up of a layer of ectoderm
and mesoderm and separated by the extraembryotic
coelom. the inner membrane is the amnion and the
outer membrane is the chorion. - allantois- posterior pouch formed from the
primitive hindgut and composed of endoderm and
mesoderm. first function is excretion. - chorioallantoic membrane- formed by the fusion of
allantoic wall with the chorion. acts as an
efficient respiratory membrane for the embryo
during its later development
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16Organogenesis The Formation of Organ Systems
- organogenesis is the later stages of development
after cleavage and gastrulation. - it begins with the inductive interaction between
ectoderm and underlying chordamesoderm. each of
the 3 primary tissues formed during gastrulation
then starts to undergo growth, differentiation,
and morphogenesis. - ectoderm neutral tube stretches out along the
dorsal surface growing and becoming longer and
thinner as the embryo gets bigger. differentiate
into cartiledge and give rise to the hollow
vertebral column - secondary induction- inducing tissue is the
result of the primary induction of dorsal
ectoderm by dorsal mesoderm. - mesoderm differentiates into sclerotome cells
(skeletal elements), dermatome cells (skin), and
myotome cells (musculature). - endoderm differentiates into tissues of the
respiratory and digestive tracts and a number of
related organs. - morphogenesis the processes include
increase/decrease in rates of cell growth and
division, changes in cell adhesion with neighbor
cells, deposition of extracellular materials,
changes in cell shape by extension/contraction. - pattern formation- developmental process that
gives rise to structural differences of the body.
17Organogenesis (cont)
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