20
U N I T 1
Cell and Tissue Function
tissues is called
cell differentiation
, a process that is
controlled by mechanisms that switch genes on and off
(see Chapter 4).
All of the approximately 200 different cells of the
body can be classified into the four basic or primary tis-
sue types: epithelial, connective, muscle, and nervous
(Table 1-1). These basic tissue types are often described
by their embryonic origin. The embryo is essentially a
three-layered tubular structure (Fig. 1-13). The outer
layer of the tube is called the
ectoderm
; the middle layer,
the
mesoderm
; and the inner layer, the
endoderm
. All of
the mature tissue types originate from these three cel-
lular layers. Epithelium has its origin in all three embry-
onic layers, connective tissue and muscle develop mainly
from the mesoderm, and nervous tissue develops from
the ectoderm.
Epithelial Tissue
Epithelial tissue forms sheets that cover the body’s outer
surface, line the internal surfaces, and form glandular tis-
sue. Underneath all types of epithelial tissue is a fibrous
extracellular layer, called the
basement membrane
,
which serves to attach the epithelial cells to adjacent
connective tissue and may serve other functions, such
as providing a barrier against cancer cell invasion and
contributing to the filtration function of the glomerulus.
The cells that make up epithelium have three general
characteristics: (1) they have three distinct surfaces: a
free surface or apical surface, a lateral surface, and a
basal surface; (2) they are closely apposed and joined
by cell-to-cell adhesion molecules, which form special-
ized cell junctions; and (3) their basal surface is attached
TABLE 1-1
Classification of TissueTypes
TissueType
Location
Epithelial Tissue
Covering and lining of body surfaces
Simple epithelium
Squamous
Lining of blood vessels, body cavities, alveoli of lungs
Cuboidal
Collecting tubules of kidney; covering of ovaries
Columnar
Lining of intestine and gallbladder
Stratified epithelium
Squamous keratinized
Skin
Squamous nonkeratinized
Mucous membranes of mouth, esophagus, and vagina
Cuboidal
Ducts of sweat glands
Columnar
Large ducts of salivary and mammary glands; also found in conjunctiva
Transitional
Bladder, ureters, renal pelvis
Pseudostratified
Tracheal and respiratory passages
Glandular
Endocrine
Pituitary gland, thyroid gland, adrenal and other glands
Exocrine
Sweat glands and glands in gastrointestinal tract
Neuroepithelium
Olfactory mucosa, retina, tongue
Reproductive epithelium
Seminiferous tubules of testis; cortical portion of ovary
ConnectiveTissue
Embryonic connective tissue
Mesenchymal
Embryonic mesoderm
Mucous
Umbilical cord (Wharton jelly)
Adult connective tissue
Loose or areolar
Subcutaneous areas
Dense regular
Tendons and ligaments
Dense irregular
Dermis of skin
Adipose
Fat pads, subcutaneous layers
Reticular
Framework of lymphoid organs, bone marrow, liver
Specialized connective tissue
Bone
Long bones, flat bones
Cartilage
Tracheal rings, external ear, articular surfaces
Hematopoietic
Blood cells, myeloid tissue (bone marrow)
MuscleTissue
Skeletal
Skeletal muscles
Cardiac
Heart muscles
Smooth
Gastrointestinal tract, blood vessels, bronchi, bladder, and others
NervousTissue
Neurons
Central and peripheral neurons and nerve fibers
Supporting cells
Glial and ependymal cells in central nervous system; Schwann and satellite cells
in peripheral nervous system
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text continued from page 17
)
