Embryology Lecture

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5 Dec 2010
EEB263 Comparative Vertebrate Anatomy
Quiz 2 Review
Life History
Life history begins with fertilization, followed by embryonic development,
maturation and in some cases senescence, each stage being a prelude to the
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 reproductive-
ready 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
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.
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EEB263 Comparative Vertebrate Anatomy
Quiz 2 Review
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 sperms 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.
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.
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