reticulum and the Golgi apparatus, that are inside a
eukaryotic cell; divides the cytoplasm into compart-
mentswith various functions, with the compartments
related via direct physical contact or by the use of
membranous vesicles.
endometrium The lining (mucous membrane) of the
uterus and cervix. The endometrium becomes thicker
as the menstrual cycle advances in preparation for a
fertilized egg. If fertilization does not occur, the
endometrium is shed with the menstrual flow. It con-
sists of the stratum functionale, a thick part of
endometrium that is lost during menstruation, and the
stratum basal, a layer retained during menstruation
that serves as a stem source for regeneration of the
upper stratum functionale.
Acancerous growth in the endometrium is called
endometrial cancer, and an overgrowth in the
endometrium, called endometrial hyperplasia, can
cause abnormal menstrual bleeding and become pre-
cancerous.
endoparasite Any parasitic organism that lives and
feeds from inside its host.
See alsoECTOPARASITE.
endoplasmic reticulum (ER) An extensive convo-
luted membranous network in the cytoplasm of
eukaryotic cells containing two types. The first is a
rough endoplasmic reticulum, so called because it
appears to be rough due to its surface being covered
with ribosomes, that functions to help cells process
proteins in sacs called cisternae. The second type of
ER is smooth ER and helps cells to process fats. ER
generally contains enzymes to break down both pro-
teins and fats.
See alsoCELL.
ENDOR See ELECTRON-NUCLEAR DOUBLE RESO-
NANCE.
endorphin (endogenous morphine or opiod) A
class of endogenous (made in the body) hormones
produced in the brain and anterior pituitary that are
chemically similar to opiate drugs (such as morphine)
and are released to cope with acute stress and to deal
with pain.endoskeleton An internal skeleton.
See alsoSKELETON.endosperm A nutrient, food-storage tissue, formed
from double fertilization (sperm cell fuses to two polar
nuclei) in the seeds of angiosperms, which nourishes
the developing embryo.endospore A thick-coated, environmentally resis-
tant protective seedlike cell produced within a bacte-
rial cell that is exposed to harsh conditions. In
mycology, it is the term for spores formed on the
inside of a sporangium; a spore produced within a
spherule.endosymbiotic theory A theory on the evolution of
eukaryotic cells. Originally mitochondria and chloro-
plasts were free-living self-replicating cells that devel-
oped a symbiotic relationship with prokaryote cells and
eventually lost their independence.endothelium The simple thin layer of endothelial
cells that lines blood and lymph vessels. It plays a num-
ber of roles, including acting as a selective barrier for
molecules and cells between the blood and surrounding
tissues, and secreting and modifying several veinous
signaling molecules. The endothelium also helps to
make up the blood-brain barrier between the central
nervous system and the rest of the body; summons and
captures white blood cells (leukocytes) to the site of
infections; regulates coagulation of the blood at trauma
sites; controls contraction and relaxation of veins; and
regulates the growth of the veinous muscular cells,
among others.
It is also the term used for the innermost layer of
the eye’s cornea, one cell layer thick (5–10 microns or
0.005–0.01 millimeters), that provides hydration bal-
ance to maintain the cornea’s transparency.114 endometrium