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Australia: The Land Where Time Began |
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Fish to Tetrapod - Tissues and Skeletal
Structures
Clack1 points out that to understand the
relationships, physical and developmental, among the various skeletal
tissue types, as well as the features formed by them, an explanation of
the embryonic origins of these tissue types is necessary. She suggests
it is best to observe the characteristics demonstrating the phylogenetic
relationships between animals in their embryos, something of their
commonality of descent being shown by the developmental history of some
skeletal elements. She suggests more detail can be found in standard
vertebrate biology textbooks, such as the book by Kardong (2009), or
textbooks on developmental biology and embryology (e.g., Raff, 1996;
S.B. Carroll et a., 2004;
Wolpert et al., 2004). At a certain size the ball of dividing cells that
results from a fertilised ovum reach a point at which the surface to
volume ration come into effect, and it becomes necessary to change the
shape of the ball in order to keep all component cells close enough to
the surface of the ball to allow the continued exchange of nutrients and
wastes with the surrounding medium, which it does by invaginating. This
is achieved by a small hole appearing in the cell ball and the cells
surrounding the hole are swallowed into it, which results in a dimple,
then the developing hole, the blastopore, gets progressively deeper. The
cell ball eventually becomes hollow as the cells surrounding the hole
continue to move into it. Most of the cells have had their eventual
fates decided by this stage, with the ball already having a defined top
and bottom, front and back. The upper region of the hole’s margin, the
dorsal lip of the blastopore, is an important landmark. As the cells
move to the inside over this lip they are marked for particular
functions in the developing embryo, this entire process in the
development of the animal is called gastrulation. Clack1
suggests it is probably the most crucial moment in the life of the
animal; it is at this point that much of the animal’s developmental fate
is decided. Some form of this process is common to all multicellular
animals. The embryos of some animals, such as sea anemones,
get no further in the process of gastrulation than the 2 layer stage.
Most animals progress to the next stage, the triploblastic stage, in
which a third layer develops in the space between the other 2 layers.
The different parts of a developing embryo are formed by the 3 cell
“layers”, though Clack1 points out that “the third layer is
more than just a layer”: in vertebrates the ectoderm is the outer layer,
which forms the epidermis, the thin outer layer of skin, and a number of
other features. The endoderm is the inner layer that forms the gut
lining and other outgrowths from it, such as the liver. The mesoderm is
the middle layer of tissue, which forms most structures of the body,
including most of the bones, the muscles, and most internal organs, such
as the kidneys, blood vessels and glands. Neurulation is the next stage in embryonic
development, the stage at which some of the most basic features of all
vertebrates develop. At this stage 2 ridges of tissue gather up to form
into parallel crests, the blastopore at one end, which becomes the anus
in vertebrates and a small number of other related animal groups, such
as echinoderms. Ectodermal layer cells stream towards the ridges,
gathering up like waves, both waves eventually meet and merge along the
length of the animal, forming a loop at one end and petering out at the
other. As the 2 waves meet they enclose a tube beneath, the nerve cord,
the loop becoming the head, the swollen end of which becoming the
incipient brain. These ridges form within the ectodermal layer, making
the brain and nerve cord part of the outer surface, or ectoderm, of the
embryo that has become enclosed. The mesoderm forms a midline structure of its own
beneath the ectodermal nerve cord as the embryo grows. The notochord,
that is rodlike, forms here, is circular in cross section and extends
for the length of the embryo except for the anterior part of the
incipient head end. The middle layer of mesoderm begins to divide into
segments, beginning at the head end and working towards the rear end.
The segments, the somites, become the muscle blocks lying beside the
vertebral column and control its movements. Cartilage formed in the
mesoderm is the precursor of the bones of the internal limb skeleton,
the vertebral column (usually), parts of the skull and braincase, the
ribs, and some parts of the limb girdles, the cartilage eventually being
replaced by bones as the animal grows. The gill pouches appear just
behind the mouth of the growing embryo around the time that the limb
buds form. A population of cells, neural crest cells, that are
especially active in the head region, begin to migrate around the body,
behaviour that is rather unexpected, though most cells making up an
embryo essentially remain in their original location, following an
orderly pattern, responding. These
cells are formed just at the point where the embryonic ridges make
contact with each other, so are of ectodermal origin but they migrate
into regions of mesodermal or endodermal structures to reach their
destinations. When in their final destination they initiate the
development of a wide range of structures that have been found to be
unique to vertebrates. Clack1 suggests it was the development
of this tissue, that is highly active, that probably accounts for the
development of vertebrates as a group of animals that are diverse and
successful. Structures initiated by neural crest cells include jaws,
gill arches (including the hyomandibula), the trabecular cartilage of
the braincase, sensory capsules housing sense organs, the sensory
placodes forming the nasal, optical, ear-related sense organs, and
lateral lines, pigment cells, and in concert with mesoderm and normal
ectoderm, the teeth, scales, and dermal bones, as well as important
structures around the heart. According to Clack1 the difference in
origin between dermal and endochondral bone is important, as is the bone
formed by neural crest and bone formed by mesoderm (all dermal bones
originate in neural crest tissue, though not all bone derived from the
neural crest is dermal).
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |