Essentials of Anatomy and Physiology

skin and beneath the epithelial tissue of all the body
systems that have openings to the environment. Recall
that one function of white blood cells is to destroy
pathogens. How do pathogens enter the body? Many
do so through breaks in the skin. Bacteria and viruses
also enter with the air we breathe and the food we eat,
and some may get through the epithelial linings of the
respiratory and digestive tracts and cause tissue dam-
age. Areolar connective tissue with its mast cells and
many white blood cells is strategically placed to inter-
cept pathogens before they get to the blood and circu-
late throughout the body.

ADIPOSE TISSUE

The cells of adipose tissueare called adipocytesand
are specialized to store fat in microscopic droplets.
True fats are the chemical form of long-term energy
storage. Excess nutrients have calories that are not
wasted but are converted to fat to be stored for use
when food intake decreases. Any form of excess calo-
ries, whether in the form of fats, carbohydrates, or
amino acids from protein, may be changed to triglyc-

erides and stored. The amount of matrix in adipose

tissue is small and consists of tissue fluid and a few col-

lagen fibers (see Fig. 4–4).

Most fat is stored subcutaneously in the areolar

connective tissue between the dermis and the muscles.

This layer varies in thickness among individuals; the

more excess calories consumed, the thicker the layer.

As mentioned in Chapter 2, adipose tissue also cush-

ions organs such as the eyes and kidneys.

Recent research has discovered that adipose tissue

does much more than provide a cushion or store

energy. Adipose tissue is now considered an endocrine

tissue, because it produces at least one hormone.

Leptin is an appetite-suppressing hormone secreted

by adipocytes to signal the hypothalamus in the brain

that fat storage is sufficient (see also Chapter 17).

When leptin secretion diminishes, appetite increases.

Adipocytes secrete at least two chemicals that help

regulate the use of insulin in glucose and fat metabo-

lism. Adipose tissue is also involved in inflammation,

the body’s first response to injury, in that it produces

cytokines, chemicals that activate white blood cells.

Our adipose tissue is not simply an inert depository of

76 Tissues and Membranes

Fibrous tissueFibrous tissue

Collagen

fibers

Fibroblasts

Example: Trachea

(approximately 430X)

B

A

C

(Approximately 430X)

Haversian canal

Canaliculi

Matrix

Osteocytes

Bone

Chondrocytes

Matrix

Cartilage

Example: Tendons

(approximately 430X)

Figure 4–5. Connective tissues. (A) Fibrous. (B) Cartilage. (C) Bone.

QUESTION:What is the matrix of fibrous tissue, and of bone?