Human Physiology, 14th edition (2016)

The Study of Body Function 19

Hair Sebaceous

gland

Sweat

pore

Stratum corneum

Stratum granulosum

Stratum spinosum

Stratum basale

Sweat gland

Arteriole

Venule

Sensory

nerve

Hair bulb

Adipose tissue

Hypodermis

Dermis

(Connective

tissue)

Epidermis

(Epithelial

tissue)

Arrector

pili muscle

(Muscle tissue)

Motor nerve

(Nerve tissue)

Figure 1.22 A diagram of the skin. The skin is an organ that contains all four types of primary tissues.

The zygote is totipotent —it can produce all of the differ-

ent specialized cells of the body. After a number of cell divi-

sion, when the embryo is at the stage when it implants into the

mother’s uterus, its cells are pluripotent —they can produce all

of the body cells except those that contribute to the placenta, and

so have been called embryonic stem cells. As development pro-

ceeds through successive cell divisions, the cells become increas-

ingly differentiated and lose the ability to form unrelated cell

types. However, genes are not lost during differentiation. This

was first demonstrated in the 1960s by a British scientist, who

found that he could produce a fully formed frog by transplanting

the nucleus from a differentiated frog intestinal cell into an egg

cell’s cytoplasm. In 2006 and later, Japanese scientists found that

they could transform differentiated fibroblasts into a pluripotent

state, similar to embryonic stem cells, by treating the fibroblasts

with a few specific regulatory molecules. The 2012 Nobel Prize

in Physiology or Medicine was awarded for these discoveries.

Because the specialized cells have a limited lifespan,

many organs retain small populations of cells that are less dif-

ferentiated and more able to divide to become the specialized

cell types within the organ. These less-differentiated cells are

known as adult stem cells. In the bone marrow, for example,

(derived from cholesterol under the influence of ultraviolet
light), which functions as a hormone.
The architecture of most organs is similar to that of the skin.
Most are covered by an epithelium that lies immediately over
a connective tissue layer. The connective tissue contains blood
vessels, nerve endings, scattered cells for fighting infection, and
possibly glandular tissue as well. If the organ is hollow—as with
the digestive tract or blood vessels—the lumen is also lined with
an epithelium overlying a connective tissue layer. The presence,
type, and distribution of muscle tissue and nervous tissue vary in
different organs.

Stem Cells

The different tissues of an organ are composed of cells that are
highly specialized, or differentiated. The process of differen-
tiation begins during embryonic development, when the fertil-
ized egg, or zygote, divides to produce three embryonic tissue
layers, or germ layers: ectoderm, mesoderm, and endoderm
(chapter 20; see fig. 20.45 a ). During the course of embryonic
and fetal development, the three germ layers give rise to the
four primary tissues and their subtypes.