166
UNIT ONE
5.2
FROM SCIENCE TO TECHNOLOGY
Tissue Engineering: Replacement Bladders and Hearts
I
f an appliance part is damaged or fails, replac-
ing it is simple. Not so for the human body.
Donor organs and tissues for transplant are
in short supply, so in the future spare parts may
come from tissue engineering. In this technology,
a patient’s cells, extracellular matrix, and other
biochemicals are grown with a synthetic scaf-
fold to form an implant. The cells come from the
patient, so the immune system does not reject
them. Tissue engineering has provided skin, car-
tilage, and blood vessels.
Building a Better Bladder
Each year in the United States, about 10,000 people
need their urinary bladders repaired or replaced.
Typically a urologic surgeon replaces part of the
bladder with part of the large intestine. However,
the function of the intestine is to absorb, and the
function of the bladder is to hold waste. Tissue
engineering is providing a better replacement
bladder. The natural organ is balloonlike, with
smooth muscle on the outside and lining tissue
(urothelium) and connective tissue on the inside.
Researchers created replacements for part
of the bladder of seven children and teens who
have spina biF
da, a birth defect in which the mal-
functioning bladder can harm the kidneys. Each
patient donated a postage-stamp size sample of
bladder tissue that consisted of about a million
cells. ±rom the samples, the researchers separated
two types of progenitor cells—for smooth muscle
and urothelium—and let them divide in culture in
a speciF
c “cocktail” of growth factors. Within seven
weeks the million cells had divided to yield 1.5
billion cells. The cells were then seeded onto syn-
thetic, three-dimensional domes. After con²
uent
layers of cells formed, the domes were surgically
attached to the lower portions of the patients’
bladders, after removing the upper portions. The
scaffolds degenerated over time, leaving new
bladders built from the patients’ own cells.
A Healed Heart
A heart is a considerably more complex organ
than a bladder, essentially a muscular sac. The
heart’s architecture is difficult to reproduce, so
researchers used a different approach called
“decellularization.” They took hearts from dead
rats, removed the cells, and seeded the remain-
ing extracellular matrix with progenitor cells
taken from the hearts of newborn rats. Over the
ensuing days, the cells divided and di³
erentiated,
occupying the nooks and crannies of the hearts’
“skeletons” to rebuild the organ. On the eighth
day, the hearts beat! To replace failing human
hearts, one day it may be possible to decellular-
ize hearts from cadavers and seed them with pro-
genitor cells from patients.
8. Stratif
ed columnar epithelium
a. The top layer o± cells in this tissue contains
elongated columns. Cube-shaped cells make up the
bottom layers.
b. It is in part o± the male urethra and ductus
de±erens, and parts o± the pharynx.
c. This tissue ±unctions in protection and secretion.
9. Transitional epithelium
a. This tissue is specialized to become distended.
b. It lines the urinary bladder, ureters, and superior
urethra.
c. It helps prevent the contents o± the urinary
passageways ±rom di±±using out.
10. Glandular epithelium
a. Glandular epithelium is composed o± cells
specialized to secrete substances.
b. A gland consists o± one or more cells.
(1) Exocrine glands secrete into ducts.
(2) Endocrine glands secrete into tissue fl
uid or
blood.
c. Exocrine glands are classif
ed according to the
organization o± their cells.
(1) Simple glands have ducts that do not branch
be±ore reaching the secretory portion.
(2) Compound glands have ducts that branch
repeatedly be±ore the secretory portion.
(3) Tubular glands consist o± simple epithelium-
lined tubes.
(4) Alveolar glands consist o± saclike dilations
connected to the sur±ace by narrowed ducts.
b. It carries on secretion and absorption in the
kidneys and various glands.
4. Simple columnar epithelium
a. This tissue is composed o± elongated cells whose
nuclei are near the basement membrane.
b. It lines the uterus and digestive tract, where it
±unctions in protection, secretion, and absorption.
c. Absorbing cells o±ten possess microvilli.
d. This tissue usually contains goblet cells that
secrete mucus.
5. Pseudostratif
ed columnar epithelium
a. This tissue appears stratif
ed because the nuclei
are at two or more levels.
b. Its cells may have cilia that move mucus over the
sur±ace o± the tissue.
c. It lines tubes o± the respiratory system.
6. Stratif
ed squamous epithelium
a. This tissue is composed o± many layers o± cells; the
top layers are fl
attened.
b. It protects underlying cells ±rom harm±ul
environmental e±±ects.
c. It is the outer layer o± the skin and lines the oral
cavity, esophagus, vagina, and anal canal.
7. Stratif
ed cuboidal epithelium
a. This tissue is composed o± two or three layers o±
cube-shaped cells.
b. It lines the larger ducts o± the mammary glands,
sweat glands, salivary glands, and pancreas.
c. It ±unctions in protection.
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