EEB263 – Comparative Vertebrate Anatomy
Quiz 2 – Review
Life History
Introduction:
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Life history begins with fertilization, followed by embryonic development,
maturation and in some cases senescence, each stage being a prelude to the
next.
o Embryonic development or ontogeny: Fertilization to birth or
hatching. The egg is fertilized and divides into millions of cells, which
form the basic structural components of the individual.
o Maturation: Time from birth to the point of sexual maturity. Usually
involves a growth in size and acquisition of learned skills as well as
appearance of anatomical features that distinguish the reproductiveready adult.
§ Pre-reproductive individuals are called juveniles or immatures.
§ If the juvenile and adult are strikingly different in form, and
the change from one to the other is abrupt, then the
transformation is termed metamorphosis (i.e. tadpole to frog).
o Senescence, or aging: The loss of physical vigor and reproductive
ability. This is apparent in humans but rare in wild animals. In fact,
senescent animals usually provide an easy meal for ready predators.
Few examples of senescence are found in the wild (few species of
salmon, and social primates).
Early Embryology
Introduction:
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Early in embryonic development, the cells of the embryo (the product of a
fertilized egg, from the zygote until the fetal stage) become sorted into three
primary germ layers:
1. Ectoderm
2. Endoderm
3. Mesoderm
Each layer gives rise to specific regions that form body organs. Structures of
two species that pass through closely similar steps of embryonic development
can imply homology between these structures.
Close homology testifies to the phylogenetic relationship of both species.
The youngest stage of the embryo is the fertilized egg, or zygote, which
develops subsequently through the morula, blastula, gastrula, and
neurula stages. During these early stages, the embryonic area becomes
defined from the extraembryonic area that supports the embryo or delivers
nutrients but does not become a part of the embryo itself. The delineated
embryo first becomes organized into the three basic germ layers and then
passes through organogenesis (literally, “organ”-“formation”) during which
the germ layers differentiate into specific organs.
EEB263 – Comparative Vertebrate Anatomy
Quiz 2 – Review
Fertilization:
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The union of two mature sex cells, or gametes, constitutes fertilization. The
male gamete is the sperm and the female the ovum, or egg. Both of the
gametes carry genetic material from each parent, and both are haploid
(containing half the chromosomes of each parent). The sperm’s passage
through the outer layers of the ovum activates embryonic development.
Although an egg can be very large, as is a chicken egg, it is but a single cell
with a nucleus, cytoplasm, and cell membrane, or plasma membrane.
While still in the ovary, the ovum accumulates vitellogenin, a transport
form of yolk formed in the liver of the female and carried in her blood. Once
in the ovum, vitellogenin is transformed into yolk platelets consisting of
storage packets of nutrients that help support the growing needs of the
developing embryo.
The quantity of yolk that collects in the ovum is specific to each species, in
general there are 3 types:
1. Microlecithal: Slight amount
2. Mesolecithal: Moderate amount.
3. Macrolecithal: Enormous amount.
The distribution of yolk in the ovum can also be categorized into 2 types:
1. Isolecithal: Even distribution.
2. Telolecithal: Concentrated at one pole
When yolk and other constituents are unevenly arranged (telolecithal), the
ovum shows a polarity defined by a vegetal pole, where most yolk resides,
and an opposite animal pole, where the prominent haploid nucleus resides.
Gastrulation:
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Repeated mitotic division of the zygote occurs during cleavage. The embryo
experiences little or not growth in size, but the zygote is transformed from a
single cell into a solid mass of cells called the morula. Eventually the
multicelled and hollow blastula forms. The blastomeres are the cells
resulting from these early cleavage divisions of the ovum.
The first cleavage furrows appear at the animal pole and progress towards
the vegetal pole.
In microlecithal eggs of amiphioxus and placental mammals, cleavage is
holoblastic –mitotic furrows pass successfully through the entire zygote
from animal to vegetal pole. After the first few furrows pass through,
subsequence furrows perpendicular to these develop until a hollow ball of
cells form around an internal fluid-filled cavity. Structurally, the blastula is
the hollow ball around the internal blastocoel cavity.
In mesolecithal or microlecithal egg, cell division is impeded, mitotic
furrowing is slowed, only a portion of the cytoplasm is cleaved, and cleavage
is said to be meroblastic.
In extreme cases, such as in the eggs of many fishes, reptiles, birds and
monotremes, meroblastic cleavage becomes discodial because extensive yolk
material at the vegetal pole remains undivided by mitotic furrows and
cleavage is restricted to a cap of dividing cells at the animal pole.
EEB263 – Comparative Vertebrate Anatomy
Quiz 2 – Review
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In all chordate groups, cleavage converts a single celled zygote into a
multicelluar, hollow blastula. Variations in the cleavage process result from
characteristic differences in the amount of accumulated yolk reserves.
Gastrulation:
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Cells of the blastula undergo major rearrangements within the embryo to
reach the gastrula and neurula stages.
Gastrulation (literally “gut formation”) is the process by which the embryo
forms a distinct endodermal tube that constitutes the early gut. The space
enclosed within the gut is called the gastrocoel, or archenteron.
Neurulation (literally “nerve formation”) is the process of forming an
ectodermal tube, the neural tube. The tube is a precursor of the CNS and
encloses the neurocoel.
The two processes occur simultaneously in some species and include other
embryonic events with far-reaching consequences. During this time, the three
germ layers come to occupy their characteristic starting positions:
o Ectoderm: on the outside
o Endoderm: Lining the primitive gut
o Mesoderm: Between the two of them.
§ Sheets of mesoderm become tubular, and the resulting body
cavity enclosed within the mesoderm is the coelom.
Cleavage is characterized by cell division; gastrulation is characterized by major
rearrangements of cells. By the end of gastrulation, large populations of cells,
originally on the surface of the blastula, divide and spread toward the inside of the
embryo, a process that is much more than simple cell shuffling. Tissue-tissue
interactions established by this rearrangement are one of the major determinants of
organ formation.
Neurulation:
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Most common method of neurulation is primary neurulation wherein the
neural tube is formed through folding of the dorsal ectoderm.
o Specifically, the surface ectoderm thickens into a strip of tissue that
forms the neural plate along what is to be the dorsal and anteriorposterior side of the embryo.
o In tetrapods, sharks, lungfishes, and some protochordates, the
margins of the neural plate then grow upward into parallel ridges that
constitute the neural folds.
o The neural folds eventually meet and fuse at the midline, forming the
neural tube that encloses the neurocoel.
o The tube is destined to differentiate into the brain and spinal cord (the
CNS). Just before or just as the neural folds fuse, some cells within
these ectodermal folds separate out and establish a distinct population
of neural crest cells.
o These cells are organized initially into cords in the embryo’s trunk,
but in the head, they usually form into sheets. From their initial
position next to the forming neural tube, neural crest cells migrate out
EEB263 – Comparative Vertebrate Anatomy
Quiz 2 – Review
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along defined routes to contribute to various organs (these cells are
unique to vertebrates).
The endoderm is derived from cells moving inward from the outer surface of
the blastula. At first the endoderm forms the walls of a simple gut extending
from anterior to posterior within the embryo. But as development proceeds
outpocketings from the gut and its interactions with other germ layers
produce associated glands and their derivatives.
The mesoderm also is derived from cells entering from the outer surface of
the blastula. Mesodermal cells proliferate as they expand into a tissue sheet
around the insides of the body between the outer ectoderm and the inner
endoderm. Occasionally, instead of forming a sheet, mesodermal cells become
dispersed to produce a network of loosely connected cells called mesenchyme.
The notochord arises from the dorsal midline between lateral sheets of
mesoderm. Each lateral sheet of mesoderm becomes differentiated into 3
regions:
o Epimere (or paraxial mesoderm): Dorsal
o Mesomere (or intermediate mesoderm): Middle
o Hypomere (or lateral plate mesoderm): Ventral
The central cavity within the mesoderm is the paired primary or
embryonic coelom. Parts of the primary coelom often become enclosed in
the mesoderm, forming a myocoel within the epimere, a nephrocoel within
the mesomere, and simple coelom (body cavity) within the lateral plate
mesoderm.
Epimere:
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The epimere forms as a pair of cylindrical condensation adjacent to and
parallel with the notochord. It then becomes organized into connected
clusters of loosely whorled mesenchymal cells, termed somitomeres.
Beginning at about the neck and progressing posteriorly, spaces form
between somitomeres to delineate anatomically separate condensed clumps of
mesoderm, somites.
The somitomeres in the head remain connected, and may number 7 in
amniotes and teleosts, and 7 in amphibians and sharks.
They give rise to striated muscles of the face, jaws, and throat, with the
connective tissue component derived from the neural crest. The somites, in
series with the somitomeres, vary in number with species.
Somites in turn split into 3 separate mesodermal populations:
o Dermatome: Skin musculature
o Myotome: Body musculature
o Sclerotome: Vertebrae
Mesomere:
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The mesomere gives rise to portions of the kidney.
Hypomere:
EEB263 – Comparative Vertebrate Anatomy
Quiz 2 – Review
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As the coelom expands within the Hypomere, inner and outer mesodermal
sheets of cells are defined. The inner wall of the Hypomere is termed the
splanchnic mesoderm, and the outer wall the somatic mesoderm.
o These sheets of mesoderm come into association with the endoderm
and the ecoderm, with which they I nteract later to produce specific
organs.
o Collectively, the paired sheet of splanchnic mesoderm and the
adjacent sheet of endoderm form the splanchnopleure; the somatic
mesoderm and the adjacent ectoderm form the somatopleure.
Patterns of Gastrulation:
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Epiboly: Cells spread across the outer surface as a unit.
Involution: Cells turn inward and then spread over the internal surface.
Invagination: A wall of cells may indent or simply fold inward.
Delamination: Sheets of cells may split into parallel layers.
Ingression: Individual surface cells may migrate to the interior of the embryo.
Embryology Case Studies:
Amphioxus (Cephalochordata):
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Microlecithal eggs
Cleavage results in 2 blastomeres. Subsequent divisions of the blastomeres,
now less and less in synchrony with each other, yield the 32-celled blastula
surrounding the fluid-filled blastocoel.
Gastrulation in occurs by invagination of the vegetal wall.
As vegetal cells grow inward, they obliterate the blastocoel. Cells on the
inside next separate into endoderm and mesoderm (endomesoderm, or future
endoderm and mesoderm –unity at the moment).
The endomesoderm eventually moves up against the inside wall of the
ectoderm and forms the primitive gut. The gastrocoel communicates to the
exterior through the blastopore.
The embryo is consequently transformed during early gastrulation from the
single layer of blastomeres to a double layer of cell sheets consisting of the
ectoderm and the endomesoderm.
Delineation of the mesoderm occurs during neurulation in the embryo. A
series of paired outpocketings form and pinch off from the mesoderm. These
cavities merge to become the coelom.
As the paired mesodermal outpocketings take shape, the mesoderm at the
dorsal midline between them differentiates into the chordamesoderm. In
addition to giving rise to the notochord, the chordamesoderm stimulates the
differentiation of the overlying ectoderm into the CNS.
The epidermis lateral to the early neural plate detaches and moves across the
neural plate. Only after the two sides meet and form a continuous sheet of
epidermis does the neural tube below round up. The mesoderm then becomes
delineated into epimere, mesomere, and hypomere.