Histology
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, threedimensional organs are
sectioned very thin (6 mm in thickness) they appear twodimensional. 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 illdefined.
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).
I. Epithelial Tissue
A. General Characteristics, Locations and Functions
1. General characteristics
a. Epithelial tissue is always on a free surface, next to a space rather than next to other structures.
Apical surface of a cell – exposed to the free surface. Basal surface of a cell – adheres to adjacent
underlying surface of connective tissue. Lateral surfaces of a cell – are intercellular membrane
junctions that adheres cells to each other.
b.The cells of epithelial tissue are closely packed with very little intercellular material between the cells.
The cells are held together at cell junctions by tight junctions, anchoring or adhering junctions -
desmosomes and hemidesmosomes or communicating junctions gap junctions.
Specialized Intercellular Membrane Junctions that occur between Cells
1. Tight Junctions The lateral cell membranes of adjacent cells towards the free (outer) surfaces fuse,
eliminating any extracellular space between the cells. The tight junction prevents passage of
substances into the body, except across the apical cell membrane.
2. Adherens Junctions – made of plaque, a dense layer of protein on the inside of the cell membrane.
Microfilaments extend from the plaque into the cell’s cytoplasm. Transmembrane glycoproteins
anchored in the plaque of one cell crosses the space between the membranes and connects with the
transmembrane glycoproteins of the adjacent cell to attach the cells. The adherens junctions extend
as a band that encircles the cell, as the adhesion belt. The adherens junctions are observed in
epithelial tissue to help epithelial surfaces to resist separation.
3. Desmosomes maintain a tight cell to cell adhesion between cells so cells do not pull apart from
one another. Desmosomes are spot adhesions, which help attach cells to each other. They are
composed of plaque and intermediate filaments and linked by tramsmembrane glycoproteins to the
adjacent cell. Hemidesmosomes are half a desmosome. They connect cells to the extracellular
material, so they anchor one kind of tissue to another kind of tissue in the body.
4. Gap Junction the cell membranes of adjacent cells are separated by a small extracellular space.
Small tubular channels (connexons) extend across the space and link the cytoplasm of adjacent
cells. At the gap junctions small molecules and ions can rapidly pass from cell to cell. The gap
junctions also play a role in transmission of electrical activity between cells as chemical and
electrical signals travel via the gap junctions. The gap junctions are observed in the cells of the
nervous system and in cardiac muscle.
c. Avascular no blood vessels.
d. Basement Membrane – All epithelial tissue rests on a basement membrane, a nonliving supporting
layer that attaches and separates the epithelium from the underlying connective tissue. The basement
membrane consists of 2 layers:
1. basal lamina closer to the epithelial cells is mainly made of collagen fibers secreted by the epithelial
cells. The basal lamina attaches the epithelial cells to the basement membrane. The basal lamina
functions as a filter to selective determine which molecules can diffuse from the underlying
connective tissue to enter the epithelium.
2. reticular lamina is closer to the connective tissue is mainly made of reticular fibers secreted by the
fibroblasts of the connective tissue. Attachment between the fibers of the two layers of the basement
membrane holds the two layers together. Hemidesomosomes attach the epithelial cells to the entire
basement membrane.
e. Some epithelial cells have the ability to undergo cell division to keep renewing the layer.
2. Locations
Epithelial tissue covers surfaces of the body as membranes outer surface of the body, outer surface of
organs, lining of tubes and cavities and also forms the glands of the body.
3. Functions
Since epithelium has a variety of locations, epithelial tissue has many functions. The cell shape and
number of cells in the layer reflect the functions.
a. Protection forms a barrier between the organism and the external environment - stratified
epithelium
b. Absorption, secretion and excretion simple cuboidal and simple columnar epithelium
c. Diffusion, filtration and osmosis simple squamous epithelium
B. Specialization of Epithelial Cell Surfaces
The free surface of many epithelial cells is highly modified for absorption and movement of substances
along the surface.
1. Microvilli function for absorption. Some epithelial cells have on their free surface small cytoplasmic
projections, the microvilli, which increases the surface area for absorption. The E/M resolved these
structures as cytoplasmic projections containing the microfilaments, actin. In the L/M, the microvilli
appear as the striated or brush border on the free surface of the epithelial cells.
2. Cilia function for moving substances along the surface. Some epithelial cells have on their free surface
cilia. The cilia are motile structures containing microtubules. The cilia rhythmically move to propel
material along the epithelial surface.
3. Smooth Surfaces function to reduce friction. Smooth surfaces are found lining the blood vessels and
lining the body cavities, these surfaces are non-wettable compared to rough surfaces that are wettable.
C. Classification of Epithelial Tissue
Epithelial tissues are classified by the number and arrangement of cell layers and shape of the cells at the
free surface.
1. Cell Layers
a. Simple epithelium a single layer of cells in contact with the basement membrane.
b.Stratified epithelium 2 or more layers of cells, only the deepest layer is in contact with the basement
membrane.
c. Pseudostratified epithelium (false) the tissue appears to consists, of several layers but it is a single cell
layer as all cells touch the basement membrane. It appears stratified because some cells are short and
do not reach the free surface, while the tall cells which reach the free surface have their nuclei at various
depths giving the layer it’s apparent stratified appearance.
2. Cell Shape
a. Squamous cell cell is flat and thin, with a central oval nucleus.
b. Cuboidal cell cell is equal in LxWxD, appears as a square in a section of a tissue with a centrally located
spherical nucleus.
c. Columnar cell cell is taller than wide and deep; appears as a rectangle in a section of a tissue with an oval
nucleus towards the base of the cell.
Epithelial tissue is named according to the number of cell layers and shape of the cell at the free surface.
3. General Classification of Epithelium as a Sheet or Membrane
Simple Epithelium
a. Simple squamous
1. Structure a single layer of squamous cells in a thin sheet resting on the basement membrane.
2.Location and 3. Functions Lines capillaries as a barrier separating blood from tissue fluid. Lines alveoli
or air sacs of the lungs, separating air from the tissue fluid. In these locations the simple squamous
functions for diffusion and osmosis. Simple squamous also forms the Bowman's capsule of the nephron
(kidney tubule), in this location it functions for filtration.
Endothelium simple squamous that lines all blood and lymphatic vessels and the heart.
Mesothelium. simple squamous that lines the thoracic, abdominal and pericardial (ventral) body cavities.
The mesothelial cells plus a thin layer of loose connective tissue below is the serous membrane. The
mesothelial cells secrete a clear watery fluid, serous fluid to keep the membrane moist. The serous fluid act
as a lubricant to prevent friction as the organs move over each other in the body cavities.
b.Simple cuboidal
1. Structure a single layer of cuboidal cells resting on the basement membrane.
2. Location makes up secretory part and ducts of glands, part of kidney tubule, outer layer of ovary
3. Functions secretion, absorption
c. Simple columnar
1. Structure a single layer of columnar cells resting on the basement membrane.
2. Location and 3. Functions nonciliated simple columnar epithelium functions for absorption and
secretion. The nonciliated columnar epithelial cells have on their free surface microvilli. Some of
the columnar cells are modified to secrete mucus, the goblet cell. The goblet cell can be considered to be
a unicellular gland. The secreted mucus lubricates the material in the digestive tract as it passes through.
The ciliated columnar epithelial cells have cilia on their free surface. These cells line the upper
respiratory tract (bronchi, nasal cavities), oviducts. The cilia move substances along the surface
to sweep passageways clear.
d. Pseudostratified columnar
1. Structure All cells contact the basement membrane, but short cells do not reach the free surface. The
nuclei of the taller cells are found at different levels in the cells, which gives the appearance of cell
stratification. On the free surface of the pseudostratified columnar epithelium are cilia. Scattered
among the ciliated columnar are goblet cells.
2. Location - lining of the trachea and primary bronchi.
3. Functions - secretion and sweeping passageways clean of dust and mucus. The mucus traps the dust
and debris and the cilia wavelike movement sweeps the mucus and trapped materials up to the mouth
for elimination. Mucous membrane lines internal passageways that lead to the outside. The mucus
secretions moisten and lubricate the membrane. The epithelial histology changes in different mucous
membranes.
Stratified Epithelium
e. Stratified squamous
1. Structure composed of 2 or more cell layers, the precise number of cells varies in different locations.
The basal cell layer rests on the basement membrane. The basal cells are columnar to cuboidal cells,
in the middle cell layers the cells become polygonal (more irregular and flatter). Toward and at the
free surface there are flat squamous cells. The basal cells undergo mitosis. These cells are pushed
toward the surface to replace older cells sloughed off by abrasion (desquamation).
2. Location found in those regions of the body subjected to friction and abrasion, such as the epidermis
of the skin, linings of the mouth, pharynx, esophagus, anus and vagina.
3. Function - Protection
a. Keratinized stratified squamous On the external surface of the body the stratified squamous
becomes keratinized to prevent drying out and to protect against abrasion. Keratin is a tough
waterproofing protein that resists friction and bacterial invasion.
b. Nonkeratinized stratified squamous Forms the lining of the upper and/or lower ends of organs
systems that open to the outside, such as the digestive, repiratory, urinary and reproductive systems.
This epithelium is kept moist by glands that secrete mucus, viscous fluid that moistens and
lubricates the free surface. These linings are considered to be mucous membranes.
f. Transitional epithelium
1. Structure -The shape of the surface cells and numbers of cell layers varies depending on whether the
urinary bladder is empty or full.
a. Empty bladder when bladder is empty, the epithelium is relaxed, the basal cells are
cuboidal to columnar and there are several cell layers. The surface cells are large rounded
domeshape cells.
b. Full bladder when bladder is full, the epithelium is stretched, the cells flatted out, the cell layer is 34 cells thick, surface cells become large flat and squamouslike cells.
2. Location -This stratified epithelial tissue lines the urinary bladder and ureters.
3. Function This tissue allows the organs to expand with little resistance. This tissue lessens the chance
of organ rupture and reduces discomfort as the organs become full.
4. Glandular Epithelium
A gland consists of epithelial cells that produce secretions.
a. Unicellular glands
Goblet cells mucus secreting
b. Multicellular glands
These glands are clusters of epithelial cells derived from the epithelial membrane and reside in the
connective tissue.
1. Exocrine glands connected by duct to the epithelial membrane. They produce specific secretions.
2. Endocrine glands ductless glands secrete into blood.
II. Connective Tissue
Connective tissue is the most abundant of the basic tissues. Connective tissue has many different forms and a variety of functions.
A. General Characteristics
Connective tissue is characterized by an abundance of intercellular substance. The cells are widely scattered.
Running through the intercellular substance are blood vessels (vascular tissue) and nerves. The intercellular
substance or matrix consists of ground substance and fibers. The cells and fibers are deposited in the ground
substance. The living connective tissue cells make and maintain the intercellular substance. The ground
substance has various degrees of consistency from fluid to solid.