Clase 8-Aves y MamIferos

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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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Clase 8-Aves y MamIferos

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