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Histology is the study of tissues. The organs of the body consist of four primary tissues: epithelial, connective,
muscle, and nervous. Tissues perform specialized functions that enable the organs of the body to carry out
specific tasks.
A tissue is made up of cells similar to one another in both form and function and the intercellular material in
which the cells reside. The intercellular material (ICM) or matrix contains intercellular fluid or ground
substance of various consistency and fibers.
Tissue = Cells + Intercellular Material (Matrix) consists of
ground substance + fibers
To understand the tissues' functions the organization, shape and locations of the tissues must be recognized.
Procedures
A. General Guidelines for Studying Tissues
1. Be familiar with how tissues within an organ are organized.
2. Know the orientation of the tissue slice on the slide. In preparing specimens, three-dimensional organs are
sectioned very thin (6 m in thickness) they appear two-dimensional. Cross section (c.s.) and longitudinal
section (l.s.) is included on the slide label to explain how the tissue was prepared.
3. Under the microscope, a tissue is made up of cells, fibers and ground substance. Not every feature of a
tissue is unmistakable. Elongated cells are often confused with fibers. Nuclei may be interpreted as whole
cells, when the cytoplasm or the cell membrane is ill-defined.
4. Examine the slide first under low power, then with high power. Under low power a panoramic view of the
specimen is seen. When you have recognized the anatomical relationships among the different tissues in
the organ, choose a particular region and examine under high power.
5. You must know the function(s) and at least 1 or 2 locations of each tissue. When you understand tissue
structure and location, you appreciate its function. The function of tissues is related to their shape,
arrangement and location.
6. Label diagrams, fill out function and location tables (refer to textbook).
B. Epithelial Tissue
General Characteristics of Epithelial Tissue
Structure - The epithelial tissue is highly cellular. The cells are closely packed with very little intercellular
substance between the cells. Specialized intercellular junctions hold the cells together.
There is an absence of blood vessels in epithelial tissue.
All epithelial tissues are characterized by a free apical surface and an attached basal surface.
The basal surface of epithelial tissue rests on a nonliving basement membrane and the underlying layer of
connective tissue.
Function – discussed in lecture.
Location - Epithelial tissue as a membrane covers the outer surfaces of the body and all organs in the body
cavities and lines hollow organs, tubes and cavities. Epithelial tissue also forms glands.
Regeneration – Some epithelial cells undergo mitosis to maintain the layer.
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As a membrane the types of epithelial tissue are classified according to shape and arrangement of the cells.
Shape
Squamous cells are flat and thin (scale-like) with a central oval horizontal positioned nucleus.
Cuboidal cells are cube shaped with a large central spherical nucleus. In two-dimensions, they appear almost
square.
Columnar cells are taller than they are wide and appear rectangular or column-shaped with a large oval
longitudinal nucleus at the base of the cell.
Transitional Epithelium contains cells whose shape varies from large rounded columnar cells to large squamous
cells. The shape of these cells change in this tissue as the organ becomes distended.
Cell Arrangement
Epithelial tissue consists of:
1. Simple epithelium - a single layer of cells.
2. Stratified epithelium - two or more layers of cells.
3. Pseudostratified epithelium - the tissue appears to be stratified, but is composed of a single layer with all
cells touching the basement membrane.
C. Connective Tissue
Connective tissue is the most abundant primary tissue in the body. It forms the supportive framework of the
organs and of the body. Connective tissues are characterized by an abundance of intercellular substance or
matrix. The matrix can be of various consistencies. It can be fluid (blood), loose watery gel (areolar connective
tissue), semi-rigid gel (cartilage), or rigid gel (bone). The ground substance is responsible for the consistency of
the intercellular substance. Fibers of various types are deposited within the ground substance.
Components of Connective Tissue
Cells + Intercellular Substance (matrix) consists of
ground substance + fibers
The cells and the fibers are embedded in the ground substance. The living connective tissue cells produce the
ground substance and the fibers.
Classification of Connective Tissue
Connective tissues are classified into 3 major groups.
Connective Tissue Proper (most abundant)
1. Loose connective tissue - fibers are loose and irregularly arranged - areolar, adipose, reticular
2. Dense connective tissue - fibers are dense and irregularly or regularly (parallel) arranged
white fibrous - irregular arranged- dermis
- regular arranged- tendon, ligament
yellow fibrous - regular arranged- fenestrated membranes of the elastic arteries, such as the aorta,
ligamenta nucha, a sheet-like ligament that attaches the cervical vertebrae to the skull, and
ligamenta flava, connect vertebrae above and below
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3. Specialized Connective Tissue
Hard connective tissue - cartilage, bone
blood
Characteristics of Connective Tissue
The cells of connective tissue are loosely packed and widely separated. Within the ground substance there are
blood vessels and nerves.
Histology of Areolar Connective Tissue
The predominant cell is the fibroblast. This cell is a multipolar cell from surface view with a centrally placed
nucleus and spindle shaped on side view. It produces the ground substance and the fibers. The ground substance
has a consistency of a loose watery gel. Embedded in the ground substance are the fibers, which are loose and
irregularly arranged.
The fibers are of 3 types:
a. Collagenous fibers (white fibers) - these fibers are tough, non-elastic fibers which have great tensile
strength. They are made up of the protein, collagen. The collagenous fibers are thick and
non-homogenous. They contain bundles of tiny collagen fibrils in parallel arrangement.
b. Elastic fibers (yellow fibers) - these fibers are can be stretched and when stretch is released return to
their original length. They are made up of the protein, elastin. The elastic fibers are thin, homogenous
fibers.
c. Reticular fibers - these fibers are similar to collagenous fibers in composition, the collagen fibrils are
arranged in a netlike configuration.
D. Muscle Tissue
Muscle tissue consists of cells specialized for contraction and relaxation. During contraction, a muscle cell
or fiber shortens and condenses to create movement. Skeletal muscles as organs are usually grouped in
opposing pairs, so while one muscle contracts, the other muscle relaxes. The elastic properties of connective
tissue around the muscle tissue allows the skeletal muscle tissue to return to its original shape after it’s
action is over.
Muscle tissue exists in 3 forms:
1. Skeletal muscle is named because it is attached to the skeleton. It is voluntary muscle as it contracts
through conscious effort. The other two forms are involuntary. Skeletal muscle is prominently cross
striated with dark A bands and light I bands.
2. Smooth muscle is located in layers or sheets within the walls of hollow organs or tubes surrounding the
lumen of the organs. Smooth muscle is non-striated.
3. Cardiac muscle is unique to the heart. Its cells form a branching pattern. The cells have faint cross
striations. Intercalated disks, thickenings on the cell, separate cardiac muscle cells from each other
where cell-to-cell contact is made. Intercalated disks do not line up one under the other, since
cardiac muscle cells' branches are not all the same size.
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Skeletal Muscle Cells
Structure: Each cell is a long cylindrical cell, multinucleated with oval nuclei peripherally located (just under the
cell membrane or sarcolemma). Within the cytoplasm (sarcoplasm) the myofibrils have prominent cross
striations, alternating dark A bands and light I bands. The myofibrils are aligned to give the muscle cell (fiber)
and the entire muscle a striped appearance. The cells are lined up one under each other, i.e. the cells are in
“register”.
Smooth Muscle Cells
Structure: Each cell is a short spindle-shaped cell with a single, oval nucleus centrally located. The cells are
non-striated. The cells overlap each other, as the thin tapering ends of one cell are next to the thick center region
of another cell. The smooth muscle cells are located in layers or sheets within the walls of hollow organs,
surrounding the lumen of the organs. There are usually two layers of smooth muscle that run at right angles to
each other. The circular smooth muscle in the wall of the organ going around the lumen controls the diameter of
the organ’s lumen. The longitudinal smooth muscle in the wall of the organ going along the length of the lumen
controls the propulsion of substances along the lumen.
Cardiac Muscle Cells
Structure: Each cell is a short branching cylinder, with branches not the same length. Each branch has an oval
nucleus centrally located. The cells have faint cross striations (A, I bands) compared the skeletal muscle cells.
At the ends of the branches, the cell membranes thicken to form the intercalated disks. The intercalated disks do
not line up one under the other, since the cardiac muscle cell’s branches are not all the same length, thus the
intercalated disks appear to be in a stepwise arrangement. Within the intercalated disk are gap junctions that
allow direct cytoplasmic contact between adjacent cells. In this way the cardiac muscle acts as a unit pumping
and propelling blood from the heart into the blood vessels.
E. Nervous Tissue
Nervous tissue is specialized for:
1. Irritability – perceiving stimuli-changes in the environment;
2. Conducting of nerve impulses;
3. Processing information so as to make appropriate responses to stimuli.
Nervous tissue is composed of:
1. Neurons or nerve cells exist in various forms, all contain:
a. a cell body (cyton, perkaryon) in which the nucleus is located;
b. cytoplasmic processes
1. axon an elongated process that extends from one side of the cell body,
2. dendrites project from the opposite side of the cell body. The dendrites can be one or more short
processes or one long process.
Neurons may or may not possess myelin, a fatty insulating material that surrounds the axon. When present, the
myelin sheath is interrupted by breaks in the myelin, the neurofibral nodes or nodes of Ranvier. These nodes
maintain continuity between the neuron and the intercellular fluid and allows for rapid conduction of the nerve
impulse. A type of neuroglial cell, the neurolemmal cells (Schwann cells), produces the myelin sheath.
2. Neuroglia cells that support, protect and help nourish the neurons.
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Neurons can receive stimuli and send information about these stimuli in the form of nerve impulses from one
part of the body to another.
A nerve is composed of many neurons and neurolemmal cells bounded together by connective tissue.
Neuron
Structure: Each cell has a cell body from which cell processes project, one long axon and one to several
(usually) shorter dendrites. The cell body in its cytoplasm (neuroplasm) has a large spherical nucleus and a
prominent nucleolus.
Myelinated Axon of the Neuron
Structure: Surrounding the axon (axis cylinder) is a fatty insulating material, the myelin sheath. The
neurolemmal (Schwann) cells that surround segments of the axon produces the myelin sheath. The neurolemmal
cells end-to-end form the neurolemmal sheath. Where the neurolemmal cells meet end to end there are
discontinuities in the myelin sheath. These are the neurofibril nodes or nodes of Ranvier.
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