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7/21/2019 Clase 8-Aves y MamIferos
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!"#$%%&''& )"*+%$,& -" $."# / *$*01"%
THIS FINAL CHAPTER ONTHE PROCESSES of early development extends our sur-
vey of vertebrate development to include theamniotesthosevertebrates whose
embryos form an amnio n, or water sac i.e., the reptiles, birds, and mam mals).
Birds and repti les follow a very s imilar pattern of developm ent Gil land and
Burke 2004; Coolen ct
al.
2008), and birds are considered by m oderntaxonomists
to be a reptilian clade. Cleavage in bird and reptile eggs, like that of the bony
fishes described in th e last chapter, ismeroblastic,with only a small portion of
the egg cytoplasm being used to make the cells of the embryo. In mamm als, how-
ever , holoblas t ic c leavage is modif ied to make a placenta, which enables the
embryo to develop inside another organism. Although methods of cleavage and
gastrulation differ among the vertebrate classes, certain underlying principles
are common to all vertebrates.
E RLYDEVELOPMENT IN IRDS
Cleavage
Ever since Aristotle first observed and recorded the details of its3-week-long
development, the domestic chicken Gallus gallus)has been a favorite organism
for embryological studies. It is accessible year-round and is easily raised. More-
over , a t any par t icular temperature, i ts developmental s tage can be accurately
predicted. Thus, large numbers of embryos can be obtained at the same stage.
Chick organ formation is accomplished by genes and cell movements similar to
those of mammalian o rgan formation, and the chick is one of the few organ-
isms whose embryos are amenable to both surgical and genetic manipula-
tions Stern 2005). Thus, the chick embryo has often served a s an inexpen-
sive surrogate forhuman embryos .
Fertilization of the chick egg occurs in the oviduct, before the albumen
and shell are secreted to cover it. Like the egg of the zebrafish, the chick egg
is
telolecithal,
with a sm all disc of
cytoplasmtheblastodiscsitting
atop a
large yolk. Like fish eg gs, theyolkyeggs of birds undergodiscoidalmerob-
lasticcleavage.Cleavage occursonlyin the blastodisc, which is about 2-3 mm
in diameterand is located at the animal pole of the egg. The first cleavage
furrow ap pears centrally in the blastodisc; other cleavages follow to create a
single-layeredblastoderm Figure 8.1). As in the fish embr yo, the cleavages
do not extend into the yolky cytoplasm , so the early-cleavage cells are con-
tinuou s with one another and with the yolk at their bases see Figure8.IE .
Thereafter, equatorial and vertical cleavages divide the blastoderm into a
My dear fellow ... life is infinitely
stranger than anything which the
mind ofman could invent. Weivould
not dare to conceive the things which
are really mere commonplaces of
existence.
A.CONANDOYLE 18 91)
Between the fifth and tenth days the
lump of stem cells differentiates into
the overall building plan of the
[mouse] embryo and its organs. It is a
bit like a lump ofiron turning into the
space shuttle. In fact it is the pro-
foundest wonder we can still imagine
and accept and at the same time so
usual that we havetoforceourselves
to wonder about the wondrousness of
this wonder.
MIROSLAV
HOLUB
1990)
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32 CHAPTER
STAGE
Zebrafish
anio rerio
Frog
Xenopus laevis
hicken
Gallus gallus
Mouse
Mus musculus
Early
cleavage
_y
Late
cleavage
Areapellucida
Extra
Areaopaca
e m b r y o n i c
\
ectoderm /
Epiblast
Primary Secondary
hypoblast hypoblast
Hypoblast
Early
gastrula
Epiboly
Epiboly
SMO
Late
gastrula
Epidermis
Prechordal plate Prechordal F orebrain Epidermis
mesoderm plii mesoderm \ Midbrain
SMO
Pharyngula
Forebrain
Hindbrain
Somite Midbrain
SMO
mesoderm
Prechordal
plate
mesoderm
Forebrain Epidermis Epiblast
Pre
somitic Gut
mesoderm endoderm
SMO
Somite Spinal
cord
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