Human Physiology, 14th edition (2016)

16 Chapter 1

to the tissue. Cartilage is a type of supportive and protective tis-

sue commonly called “gristle.” It forms the precursor to many

bones that develop in the fetus and persists at the articular (joint)

surfaces on the bones at all movable joints in adults.

Bone is produced as concentric layers, or lamellae, of

calcified material laid around blood vessels. The bone-

forming cells, or osteoblasts, surrounded by their calcified

products, become trapped within cavities called lacunae.

The trapped cells, which are now called osteocytes, remain

Exocrine glands are also prominent in the reproductive sys-
tem. The female reproductive tract contains numerous mucus-
secreting exocrine glands. The male accessory sex organs—the
prostate and seminal vesicles —are exocrine glands that contribute
to semen. The testes and ovaries (the gonads) are both endocrine
and exocrine glands. They are endocrine because they secrete sex
steroid hormones into the blood; they are exocrine because they
release gametes (ova and sperm) into the reproductive tracts.

Connective Tissue

Connective tissue is characterized by large amounts of extra-
cellular material between the different types of connective tissue
cells. The extracellular material, called the connective tissue matrix,
varies in the four primary types of connective tissues: (1) connective
tissue proper; (2) cartilage; (3) bone; and (4) blood. Blood is classi-
fied as a type of connective tissue because about half its volume is
an extracellular fluid, the blood plasma (chapter 13, section 13.1).
Connective tissue proper, in which the matrix consists of
protein fibers and a proteinaceous, gel-like ground substance,
is divided into subtypes. In loose connective tissue (also called
areolar connective tissue ), protein fibers composed of col-
lagen (collagenous fibers) are scattered loosely in the ground
substance ( fig. 1.17 ), which provides space for the presence of
blood vessels, nerve fibers, and other structures (see the dermis
of the skin, shown in fig. 1.14 , as an example). Dense regular
connective tissues are those in which collagenous fibers are ori-
ented parallel to each other and densely packed in the extracel-
lular matrix, leaving little room for cells and ground substance
( fig. 1.18 ). Examples of dense regular connective tissues include
tendons (connecting bone to bone) and ligaments (connecting
bones together at joints). Dense irregular connective tissues,
forming tough capsules and sheaths around organs, contain
densely packed collagenous fibers arranged in various orienta-
tions that resist forces applied from different directions.
Adipose tissue is a specialized type of loose connective
tissue. In each adipose cell, or adipocyte, the cytoplasm is
stretched around a central globule of fat ( fig. 1.19 ). The syn-
thesis and breakdown of fat are accomplished by enzymes
within the cytoplasm of the adipocytes.
Cartilage consists of cells, called chondrocytes, surrounded
by a semisolid ground substance that imparts elastic properties

Figure 1.16 The structure of exocrine glands. Exocrine glands may be simple invaginations of epithelial membranes, or
they may be more complex derivatives.

Duct

Secretory portion

Simple tubular Simple acinar

Simple

branched acinar

Figure 1.17 Loose connective tissue. This illustration

shows the cells and protein fibers characteristic of connective

tissue proper. The ground substance is the extracellular background

material, against which the different protein fibers can be seen. The

macrophage is a phagocytic connective tissue cell, which can be

derived from monocytes (a type of white blood cell).

Mesenchymal

cell

Elastic fibers

Fibroblast

Collagen fibers

Reticular fibers

Blood vessel

Macrophage

Extracellular matrix

Protein

fibers (collagen)

Ground

substance

Adipocyte (fat cell)