Integumentary System
In a developing embryo, skin cells approach as they divide and join initially at
sites where a few cadherin proteins bind. As time passes and incoming signals
indicate that the skin is where it should be, more cadherins join at the sites of
the original ones until the cells are strongly, but F
exibly, attached.
he skin is an amazing organ. It provides strength and flex-
ibility, is waterproof, and covers our bodies in one smooth
sheath. But investigating exactly what lies behind these prop-
erties has been hampered by limited technology—until now.
Preparing skin cells for visualization using a light micro-
scope strips away important proteins. Preparing cells for imaging with an
electron microscope requires harsh chemical treatment or coating with
metal. As a result, these standard forms of microscopy do not provide three-
dimensional close-ups of the junctions between cells as they are in the body.
A technique called cryo-electron tomography images skin cells in their natu-
ral state. Skin samples taken from a healthy man’s arm were F
ash-frozen and
then probed from various angles with a special electron microscope. Zeroing
in on the desmosomes (see ±
gure 5.1), and especially the cadherin proteins
that link the cells, revealed the secret of skin’s strength and F
Cadherins emanate straight out from the cell membranes of squamous
epithelium, yet maintain the ability to move about 20 degrees in any direc-
tion. This F
exibility is essential for movements associated with growth and
development that move and stretch the skin. Cadherin proteins also have a
right-left orientation. They link to each other with alternating symmetry, a
little like children holding hands, but alternating the direction in which each
child faces. The strength of the skin comes from the fact that each cadherin
protein binds not only to its neighboring cells in one plane, but to juxtaposed
cells too—similar to sheets of stamps glued together. An individual cadherin
is not very adhesive, but there is strength in numbers. When cadherins are
aligned at the surface of a skin cell facing others on all sides, the combined
integrity is formidable.
Although researchers have observed only static views of the skin using
cryo-electron tomography, they hypothesize how this organ might develop.
Cryo-electron tomography provides three-dimensional reconstructions of the
junction between two skin cells (40,000×). The cells touch at the tan area. The
nucleus is blue, set o²
by the light blue nuclear envelope. Nuclear pores are red.
The purple structures are mitochondria, the green are microtubules, and the
steel blue in the center is endoplasmic reticulum.
Two or more types of tissues grouped together and perform-
ing specialized functions constitute an
The skin, the
largest organ in the body by weight, and its various accessory
structures make up the
Skin is a strong yet fl
exible covering of our bodies.
The skin is composed of several types of tissues
(f g. 6.1)
. It
is one of the more versatile organs of the body and is vital
in maintaining homeostasis. A protective covering, the skin
prevents many harmful substances, as well as microorgan-
isms, from entering the body. Skin also retards water loss by
diffusion from deeper tissues and helps regulate body tem-
perature. It houses sensory receptors; synthesizes various
chemicals, including vitamin D; contains immune system
cells; and excretes small quantities of waste.
Dense irregular
Adipose tissue
An organ, such as the skin, is composed of several types
of tissues (30×).
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