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Almost all multicellular organisms began existence as
single starting cells. The organisms whose beginnings were through
sexual reproduction virtually always began as zygotes, the
single-celled product of fertilization, the union of sperm
and egg cell. Most of that starting cell originated as the egg cell, including
machinery such as mitochondria; there also is strong evidence that
much of the early protein activity during the embryo stage (once the zygote
becomes 2 cells, it's an embryo) is using maternal messenger RNAs (a sort of
epigenetic
inheritance, as the alleles to produce it might actually not be present)
as working codes.
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Zygote, early embryos.
Epigenetic differences in parental contributions. |
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Several
mechanisms exist to prevent polyspermy, entrance by more than
one sperm nucleus into the egg cell. First, the sperm are using particular
molecules that attach to and open up an entrance for the sperm nucleus
and a centriole for the first cell division (but not sperm mitochondria,
which might not be compatible with egg cell mitochondria). In some
organisms, access may be restricted to a small part of the egg cell
surface. Once a
single sperm cell has successfully broken through, the egg cell membrane instantly changes its
chemistry to shut down any other sperm's entry system. If too many
sperm get inside, the cell will have entire extra sets of chromosomes (polyploidy)
and centrioles, and often will die from it. In some organisms, there
are so many egg cells that polyploid zygotes do just die as a response; in some organisms, the
extra sperm material is ejected from the cell; in others, it is broken
down inside the cell. Polyploidy in plant cells may not kill the
offspring, resulting in a brand new variant of plant that may still be
reproductively compatible with non-mutant relatives - several plant species
and crop varieties seem to have started that way.
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Gamete interactions.
Blocking
polyspermy.
Polyploidy can be bad and good.
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Except for the most ancient
groups, multicellular organisms consist of tissues, classes
of cells that do particular general jobs. Most organisms have epithelium,
a tissue used as, for one thing, an outer and inner
"skin." Another common tissue contributes to reproduction,
by being a site for mitosis and / or meiosis. Some tissue is
specialized, such as muscles in animals or photosynthetic tissue in
plants. Somehow, a homogeneous cluster of cells in an early embryo must go from
being all very similar to becoming various tissues, and then integrating
into structures and organs. This process is called differentiation,
which allows cells to access and express different combinations of the
genes that they all share. The genes that
are in use early to produce basic structure, tissues, and patterns are
generally called hox genes (sometimes homeotic genes, homeogenes
or homeobox genes) and seem to be some of the most conserved genes
around - a very similar allele is used to determine eye placement in a
human and a fruit fly, for example. It would make sense that such
critical-at-the-beginning codes, once they worked properly, would tend to
remain largely the same through tens of millions of generations, when you
think of the implications from changing them. |
Animal tissue
types (video).
How some hox genes work (blog post).
Hox genes have particular relative locations. |
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Plant
differentiation arises from its reproduction
tissue, called a meristem, which expresses different types
of genes as it grows. This makes sense, since the cell walls of
plants means that cells can't migrate and must be differentiated in
their places as the plant is built. Along with meristem, plant
tissues include dermal tissue, which forms coverings;
ground tissue, for support, storage, and photosynthesis;
vascular tissue, tubes for moving water, nutrients, and
photosynthetic products up and down the plant.
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Plant differentiation.
Plant tissues (terms slightly different). |
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Fungus seem to follow
comparable patterns, with more ability later to change the nature of
cells that already exist. Fungi are capable of generating new hyphae,
the fibrous structures that make up any fungus, from virtually any cell. A
fungus has somewhat differentiated areas involved in reaching out to
potential new areas (extension), growth in a nutrient-rich zone
(productive), aging into a
resources-consolidation form, or fruiting, producing spores for dispersal by
either sexual or asexual means (often when nutrient levels drop).
Mushrooms are a type of fruiting body. Spores are said to
germinate (same term used for plant seeds), and like
seeds the timing can be tied to environmental conditions where they land. |
Fungi growth zones
(terms can vary in biology).
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Animals follow developmental patterns one might expect with cells that can
change shapes
and migrate. There are two
basic approaches to development in the newer animal groups, depending upon
a very early difference. The protostomes follow a mosaic
pattern, where the differentiation fate of each cell is "set"
from a very early stage. The cells also divide unequally,
forming large and small cells which wrap around the initial ball of cells
in a pattern called spiral cleavage. The deuterostomes
follow a regulative pattern in which cells tend to have great
flexibility until very late in the developmental process - separated
clumps can even form entire organisms (so identical twins
are only possible in this group). Early cell divisions are equal
splits of the starting cells, so division lines radiate out from the
center of the ball - radial cleavage. There are other
differences between the groups that will be covered within the
discussion of animal development. |
Protostome - deuterostome differences.
On deuterostome origins (blog post). |
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Although the particular
details vary, animal embryos go from a single zygote cell through a series
of many divisions to a hollow ball (although not always actually a
spherical ball) of cells called a
blastula.
A dent on the surface becomes deeper in a process called
gastrulation,
until the ball is a
two-layered bowl. The opening of the bowl will
be an opening into the animal - in protostomes, the mouth, and in
deuterostomes, the anus. Each
layer
of the gastrula, called germ layers, will go
on to be different parts of the animal. The outer layer will be the
ectoderm, which gives rise to the surface features (skin and
related structures, including shells and external skeletons) and the nervous
system, which develops from a thickened surface plate and which
extends nerves into the other tissues during later development. The
inner layer, the endoderm, goes on to be digestive system
and related structures (including our lungs). Between the layers a
mesoderm
appears (as a mass in protostomes and an outgrowth of endoderm in
deuterostomes) and will eventually contribute to the rest of the internal
organs and structures, including muscles and internal skeletons. |
Zygote to blastula
(sped-up video).
Gastrulation.
The
layers.
Mesoderm
and endoderm in vertebrates. |
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As differentiation progresses, newly-different cells
express new receptors and secretions, communicating with one another and
controlling the roles and placement of the cells that follow. Limbs
grow out from the body in response to pockets of secreting cells, and
cells grow, change, and even die off to give form to the animal. The
embryo stage involves the establishment of a basic layout - when
everything important is at least in place, even if the structures are far
from functional, generally the embryo stage is over and the fetus
stage has begun. In some organisms, a functional but sexually
immature stage, the larva, may follow. But once the
form is in place, a growth period usually involves a
combination of cells getting bigger and cells multiplying. And
cells keep sending each other messages, often controlling distant
production through feedback, in which production is
turned on and off according to either the levels of the produced material
itself, or monitored levels of the effects produced by the material.
Feedback is often negative, where a rise in levels causes a
lowering of production rates. Rarer is positive feedback,
which tends to magnify a weak stimulus. This appears sometimes in
nervous systems, and also in some disease conditions. |
Embryo & fetal development in humans.
How homologous genes work to produce very different eyes.
How
vertebrate limbs develop. |
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Many organisms go through a
stage during their lifetimes called
metamorphosis.
This usually involves a significant change in form and often a shift in
niche; in fact, one of the useful aspects of many larval forms is
that young and old of the same species do not compete for the same
resources in
an ecosystem. Adult forms may also reflect difficulties in
accomplishing sexual
reproduction, as happens in corals and funguses, or the need to disperse
young, as happens with many insects that live mostly in fast-moving water. |
Metamorphosis in insects.
Metamorphosis
in amphibians. |
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