Encyclopedia of Biology

extirpated species Aspecies no longer surviving in
regions that were once part of its range.

extracellular matrix(ECM) Material produced by
animal cells and secreted into the surrounding area,
serving as a glue to hold cells together in tissues. It is

composed of proteoglycans, polysaccharides, and pro-

teins. Plays a role in cell shape, growth, migration, and

differentiation.

extraembryonic membranes The YOLK SAC,

AMNION,CHORION,AND ALLANTOIS, four membranes

that support and nourish the developing embryo in rep-

tiles, birds, and mammals. The allantois performs gas

exchange and is a repository for the embryo’s nitroge-

nous waste. It is involved in the development of the uri-

nary bladder. The chorion is the outermost layer and

contributes to the formation of the placenta. The

amnion, the innermost layer, forms a fluid-filled sac

around the embryo to protect it from jarring. The yolk

sac surrounds the yolk and is the site of blood-cell for-

mation and germ-cell formation, which are the prede-

cessors of male and female gametes.

extrinsic asthma Asthma triggered by external

agents such as pollen or chemicals. Most cases of

extrinsic asthma have an allergic origin and are caused

by an IgE-mediated response to an inhaled allergen.

This is the type of asthma commonly diagnosed in

early life. Many patients with extrinsic asthma respond

to immunotherapy.

122 extirpated species

plasma membrane

smooth

endoplasmic reticulum

pinocytotic vesicle

nuclear envelope

chromatin

inside nucleus

nuclear pore

microfibrils

free ribosomes

mitochondrion

cytoplasm

lysosome

microtubules

ribosomes

rough

endoplasmic reticulum

2 centrioles

at right angles

Golgi vesicle

Golgi apparatus

microvilli

vacuole

nucleolus

Material produced by animal cells and secreted into the sur-
rounding area serves as a glue to hold cells together in tissues.