Histology

Question 1

epithelium

Answer 1

Epithelium constitutes a diverse group of tissues that cover all
body surfaces (e.g. epidermis of skin, intestinal epithelium,
respiratory epithelium), and line all body cavities and blood
vessels. An epithelium is made up of an uninterrupted layer of
cells that both forms a barrier and mediates exchange
between two compartments, usually the inside of the body and
the outside world.Most epithelia exist as barriers between the outside world and the
inside of the body. In fact there are only two types of epithelia that
do not face the outside world. These are endothelium, which lines
all blood vessels and the heart, and mesothelium, which lines the
pleural, pericardial and abdominopelvic cavities.
In addition to their function as barriers, epithelia conduct regulated
exchange between the compartments that they separate. This
includes: absorption e.g., of nutrients, fluids and electrolytes by
intestinal epithelial cells; secretion, e.g., of mucus by goblet cells,
digestive enzymes by pancreatic acinar cells, hormones by
endocrine gland cells; and excretion, e.g. of nitrogenous wastes
by kidney epithelial cellsMost epithelia exist as barriers between the outside world and the
inside of the body. In fact there are only two types of epithelia that
do not face the outside world. These are endothelium, which lines
all blood vessels and the heart, and mesothelium, which lines the
pleural, pericardial and abdominopelvic cavities.
In addition to their function as barriers, epithelia conduct regulated
exchange between the compartments that they separate. This
includes: absorption e.g., of nutrients, fluids and electrolytes by
intestinal epithelial cells; secretion, e.g., of mucus by goblet cells,
digestive enzymes by pancreatic acinar cells, hormones by
endocrine gland cells; and excretion, e.g. of nitrogenous wastes
by kidney epithelial cells.Epithelia form a discrete layer with high cell density and very
little extracellular matrix.Epithelia are avascular. Epithelial cells are dependent on the
capillaries present in underlying connective tissue to supply
their metabolic needs.
• Most glands in the body are formed from epithelium.
Glands may be unicellular (e.g. mucus-secreting goblet cells of
intestinal and respiratory epithelia) or multicellular (e.g.
salivary gland, thyroid gland). Exocrine glands (e.g. salivary
glands, sweat glands), secrete their products to the outside of
the body usually via ducts which are also formed by epithelial
cells. Endocrine glands (e.g. pituitary gland, thyroid gland)
secrete their products into connective tissue. Endocrine
secretions are picked up by capillaries in the connective tissue
investing the glands, and distributed via the vascular system to
the rest of the body

Question 2

apical

Answer 2

The apical surface of most epithelia faces the outside world. The
exceptions are endothelium, where the apical surface faces blood,
and mesothelium where the apical surface faces a body cavity.
The apical membrane of epithelial cells invariably forms some
microvilli. These are fingerlike projections of the plasma
membrane supported by a core of actin filaments. They function to
increase surface area, usually for absorption. In some epithelial
cells, e.g. intestinal epithelial cells which are very actively involved
in absorption, a dense, orderly array of microvilli called a brush
border, is present at the apical surface.
In addition to microvili, some epithelial cells also form cilia at the
apical surface. Cilia are motile surface projections with a core of
microtubule “doublets” arranged in a 9+2 array, called an axoneme.
Cilia bend because of the protein dynein, which, in the presence of
ATP causes movement of microtubules within the axoneme relative
to each other. Cilia are 7-10 um in length and beat in a
synchronous rhythm to move surface fluid in a constant direction. In
the respiratory system, the action of cilia moves mucus up to the
throat for excretion. In the female oviduct, ciliary action moves the
ovum toward the uterus.

Question 3

basal

Answer 3

By definition, the basal surface of epithelial cells faces the
connective tissue compartment and blood supply. The basal
surface sits on a basal lamina (=basement membrane), a secreted
layer of glycoproteins and other molecules that help attach the
epithelium to underlying connective tissue. Adherens junctions
Tissue Types 3
and hemidesmosomes formed on the basal plasma membrane
also contribute to attachment of the epithelium to the substrate.

Question 4

lateral

Answer 4

Lateral surfaces of epithelial cells face adjacent cells, and are
characterized by the presence of several different types of
intercellular junction. In most epithelia the apical-most of these is
the tight junction. The tight junction (aka zonula occludens) is a
continuous band around the apex of each cell that functions as a
seal preventing intercellular passage of fluid, ions and other
molecules across the epithelium. The tight junction is not
particularly strong mechanically, and is therefore closely associated
with a band-like adherens junction (zonula adherens) that attaches
adjacent cells to each other and, via linker proteins, to actin
filaments in the cytoskeleton. Lateral membranes also form
desmosomes (aka macula adherens), which are intercellular
attachment plaques that link to intermediate filaments in the
cytoskeleton.

Question 5

connective tissue

Answer 5

Capsules of organs and the supporting tissues within organs, as
well as fascia, tendons, ligaments, cartilage, bone, blood and fat
all fall into the category of connective tissue.
Bone, cartilage and blood are considered to be specialized
connective tissues. They are discussed in the Foundations Block
(blood), and in the Musculoskeletal Block (cartilage and bone).
All the other connective tissues fall into the broad category of
connective tissue proper, and we will examine these as part of
this topic.
Structurally, what all connective tissue subtypes have in common
is an abundance of extracellular molecules and a relatively low
density of cells compared to epithelium, muscle and nerve (though
there are some very important exceptions to this, e.g. lymphoid
tissue).
In addition to capsules and septa of solid organs as well as fascia,
tendon and ligaments, connective tissue proper includes dermis of
the skin and lamina propria of hollow organs. Although
sometimes considered as a special subcategory of connective
tissue, adipose tissue (fat) is also often included with connective
tissue proper and we will do that here.

Question 6

Functions of Connective Tissue

Answer 6

Mechanical support. This is especially true of dense connective
tissues such as the dermis of skin, and tendons, which connect
muscles to bones, and ligaments which connect bones to bones.
Metabolic support: Blood vessels including capillaries travel only
in connective tissues.
Defense: White blood cells, the main defensive cells of the body,
have their effects for the most part in connective tissues.
Inflammatory responses take place in connective tissue!

Question 7

extracellular matrix of connective tissue

Answer 7

Like epithelium, connective tissue is made up of cells and
extracellular matrix, but whereas in epithelium the cellular
component predominates, in connective tissue, cells are usually
relatively sparse and extracellular matrix is abundant.
The extracellular matrix of connective tissue is composed of
various kinds of fibers, and of non-fibrous molecules and the fluid
bound to them (together called ground substance). The relative
abundance of particular kinds of cells, fibers and ground
substance is what determines the varying properties of different
kinds of connective tissues.
The fibers and ground substance of the extracellular matrix are
secreted by fibroblastsThe fibers of connective tissue give it its tensile strength (collagen
fibers) and elasticity (elastic fibers).

Question 8

Which types of Collagen are most abundant in CT Proper?

Answer 8

Collagen is the most abundant fiber type in CT (and in fact is the
most abundant protein in the body). There are about 20 different
types of collagen, of which 2 (Type I and Type III) are prominent in
CT proper.

Question 9

Type I Collagen

Answer 9

Type I collagen is by far the most abundant and best studied of
the different kinds of collagen, and is usually the most
conspicuous component of CT in histological sections. Type I
collagen forms fibrils (visible in the EM) which combine to form
fibers (visible in the light microscope), and large bundles of fibers
(visible to the naked eye).

Question 10

Type III Collagen

Answer 10

Type III collagen fibers, often called reticular fibers, group in
small bundles (visible in LM only with special stains) that form a
loose three-dimensional network (reticulum) or scaffold that is the
main support of loose connective tissue, especially the lamina
propria of hollow organs. Reticular fibers also form the 3
dimensional scaffolding that underlies the structure of solid organs
such as lymph nodes, spleen and liver.
Cells secreting Type III Collagen are a subset of fibroblasts called
mesenchymal reticular cells or fibroblastic reticular cells.
Mutations in Type III collagen cause a kind of Ehlers Danlos
syndrome (Type IV), in which there are severe defects in the
structural stability of organs especially blood vessels and
intestines, which become prone to rupture.

Question 11

elastic fibers

Answer 11

Elastic fibers:
Elastic fibers confer elasticity on connective tissues. They are
synthesized by fibroblasts and smooth muscle cells as
tropoelastin, which is secreted and assembled extracellularly to
form fibers (e.g. in skin) and sheets (e.g. in the walls of arteries).
Assembly requires the glycoprotein fibrillin, which is incorporated
into the elastic fibers and sheets.
The elastin molecule is composed of short hydrophobic segments
that are cross-linked to each other. The tendency of the
hydrophobic segments to coil on themselves is what gives elastin
its elasticity.Elastic fibers:
Elastic fibers confer elasticity on connective tissues. They are
synthesized by fibroblasts and smooth muscle cells as
tropoelastin, which is secreted and assembled extracellularly to
form fibers (e.g. in skin) and sheets (e.g. in the walls of arteries).
Assembly requires the glycoprotein fibrillin, which is incorporated
into the elastic fibers and sheets.
The elastin molecule is composed of short hydrophobic segments
that are cross-linked to each other. The tendency of the
hydrophobic segments to coil on themselves is what gives elastin
its elasticity.

Question 12

ground substance

Answer 12

The cells and fibers of connective tissue are embedded in a gellike matrix or ground substance composed mainly of
proteoglycans and hyaluronan, and containing important
adhesive glycoproteins which mediate cell migration and regulate
cell differentiation.

Question 13

proteoglycans

Answer 13

Proteoglycans are like glycoproteins in that they are synthesized
in the RER and are composed of a core protein with sugar side
chains. They differ from glycoproteins in the length and
configuration of their sugar side chains: in glycoproteins the sugar
side chains are branched and tend to be short (oligosaccharides),
whereas in proteoglycans they are very long and unbranched.
These long polysaccharide chains are called
glycosaminoglycans (GAGs). They are highly negatively
charged and therefore repel each other, such that they stick out
from the protein core like the bristles on a bottle brush. The
negative charges of the GAGs attract cations which in turn attract
water, thus forming a highly hydrated gel which fills volume,
resists compression, and provides a space through which small
molecules and cells can travel.
There are a number of different kinds of GAGs, and proteoglycans
are often named after the particular GAG which forms their sidechains. Dermatan sulphate proteoglycan is present in skin,
aggrecan (containing chondroitin sulphate and keratan sulphate
GAGs) is present in cartilage and in developing heart and brain,
heparan sulphate proteoglycan is present in basal lamina.

Question 14

Hylauron

Answer 14

Hyaluronan is an unusual GAG in that it is the only one that exists
not linked to a core protein (i.e. as part of a proteoglycan) and not
synthesized in the RER (rather it is synthesized on the plasma
membrane of fibroblasts by enzymes secreted by the fibroblast).
Hyaluronan molecules are immense. They are composed of up to
25,000 repeating disaccharide units, each carrying negative
charge. As with other GAGs, hydration of hyaluronan forms a gellike substance.
Tissue Types 8
Hyaluronan is present in the cavities of joints, where it acts as a
lubricant, and in the vitreous of the eye, where it allows light
transmission. Hyaluronan binds aggrecan proteoglycan and forms
huge aggregates that fill vast molecular domains and provide
strong resistance to compression in cartilage as well as in
developing heart and brain.

Question 15

fibronectins

Answer 15

Fibronectins are adhesive glycoproteins that attach cells to
collagens. They bind to integrins, transmembrane proteins that
mediate attachment of cells to extracellular matrix. They are
essential for migration of macrophages and other immune cells
during inflammation and wound healing, and of many cell types
during embryogenesis.

Question 16

basal lamina

Answer 16

Epithelial cells secrete a molecular layer called the basal lamina
(basement membrane), which anchors the epithelium to the
substrate. Major components of basal lamina include laminin, an
adhesive glycoprotein which binds to integrins in the basal
membranes of the epithelial cells and to Type IV collagen in the
basal lamina. Type IV collagen forms a sheet-like network that
binds laminin to other components of the basal lamina and to
underlying extracellular matrix. It is essential for basal lamina
integrity.
(Note that Schwann cells in nervous tissue, skeletal muscle cells,
and adipocytes also secrete a basal lamina.)

Question 17

Cells of Connective Tissue Proper

Answer 17

1. mesenchymal
2. fibroblasts and fibrocytes
3. adipocytes
4. mast cells
5. plasma cells
6. WBC

Question 18

mesenchymal cells

Answer 18

derived from embryonic mesenchyme
• small inconspicuous cells usually situated alongside small blood
vessels
• serve as connective tissue stem cells

Question 19

fibroblasts and fibrocytes

Answer 19

derive from primitive mesenchymal cell;
• synthesize extracellular matrix molecules including collagen
fibers, elastic fibers, proteoglycans and glycosaminoglycans;
• a major player in wound healing and fibrosis (scarring): during
wound healing and during growth fibroblasts are capable of
locomotion and cell division
• during wound healing fibroblasts differentiate into myofibroblasts
which contract and help shrink the wound

Question 20

adipocytes

Answer 20

• derive from primitive mesenchymal cell;
Tissue Types 9
• very large cells each containing a single large lipid droplet which
displaces the nucleus and other organelles into a thin rim of
cytoplasm around the periphery.
• store and mobilize lipids based on energy needs of the body

Question 21

mast cells

Answer 21

• derived from stem cells in the bone marrow, travel to CT via the
blood
• reside alongside small blood vessels
• have abundant secretory granules containing histamine, heparin
and proteolytic enzymes
• degranulate in response to mechanical injury, some toxins,
allergens; degranulation in response to allergens is mediated by
IgE antibodies, for which mast cells have receptors
• function in initiation of inflammatory response: increase vascular
permeability (promoting edema), cause itching; histamine from
mast cells is an important mediator of allergic reactions including
anaphylaxis, a systemic allergic reaction which can cause death in
minutes due to bronchoconstriction and severe drop in blood
pressure.

Question 22

plasma cells

Answer 22

• differentiate from antigen-stimulated B lymphocytes
• reside in connective tissue, usually near the site where antigen
encounter by the parent B cell occurred
• secrete antibodies (up to 2000 molecules/second)
• occur in increased numbers during chronic inflammation

Question 23

WBC

Answer 23

White blood cells, including neutrophils, eosinophils,
monocytes (macrophages in tissues; aka histiocytes), and
lymphocytes originate in bone marrow, circulate in blood, and
function in tissues.
Obviously, for this cell migration to happen there has to be a
mechanism for wbcs to leave blood vessels and enter tissues.
They do this through the walls of post-capillary venules. Postcapillary venules are the leakiest part of the blood vascular system
(even more leaky than most capillaries). In response to
chemotactic factors (inflammatory mediators) produced in tissues,
gaps form between endothelial cells lining post-capillary venules.
In addition, post-capillary venule endothelial cells express surface
receptors that bind to white blood cells and promote their transfer
across the endothelium and into connective tissue.Neutrophils and eosinophils enter tissues and survive for hours or
days (depending on their level of activity) before they die there
and become part of the debris of inflammation.
Monocytes enter tissues and become macrophages; as
macrophages they live for several months (or less if they are
called into action), then die without returning to the blood
circulation. A small subset of tissue macrophages travel from the
tissues via lymphatic capillaries and larger lymphatic to lymph
nodes, where they present antigen then die, but they don’t reenter
the blood.
For these white blood cells, travel through the body is from their
site of origin in the bone marrow, into the blood vascular system
where the cells circulate until they are stimulated by chemotactic
factors to exit the blood. WBC’s exit the blood at post-capillary
venules to enter tissues, where they function and die.
Lymphocytes also follow this route from blood circulation to
connective tissues, but unlike other white cells, lymphocytes recirculate between the tissues and the blood. The re-circulation of
lymphocytes will be discussed later with immune system.

Question 24

3 Types of Muscle Tissue

Answer 24

1. skeletal
2. cardiac
3. smooth

Question 25

skeletal muscle

Answer 25

Skeletal muscle provides rapid, powerful contractions, under
voluntary control.
A skeletal muscle is a gross-anatomical structure – an organ – that
has an origin and insertion, innervation, and blood supply, and is
surrounded by a dense, collagenous sheath of connective tissue.
Most muscles are composed of multiple fascicles that run parallel
to each other along the length of the muscle.
Large blood vessels and nerves penetrate the outer sheath of the
muscle, and then branch within the connective tissue between
fascicles and around individual fibers.

Question 26

smooth muscle

Answer 26

Smooth muscle provides relatively weak, slow contractions, not
under voluntary control.
Smooth muscle is found in the walls of hollow organs – the
gastrointestinal tract, portions of the reproductive and urinary
tracts, in the walls of blood vessels, in respiratory passages.
Smooth muscle strengthens the walls of hollow organs. In the gut,
its contraction drives peristalsis. In blood vessels and bronchi, its
contraction regulates blood pressure and flow and air flow,
respectively. Smooth muscle is also present in skin, where it
causes erection of hairs.
Smooth muscle normally contains few blood vessels, although it is
always bordered by connective tissue that contains more blood
vessels, and nutrients from those vessels can reach smooth
muscle cells by diffusion. Smooth muscle cells’ metabolic needs
are less than those of skeletal or cardiac muscle, so they can
tolerate this more limited blood supply.

Question 27

cardiac muscle

Answer 27

Cardiac muscle provides regular, powerful contractions, not under
voluntary control.
Cardiac muscle is found only in the heart and the portion of the
pulmonary veins where they join the heart. Cardiac muscle
constitutes the bulk of the heart.

Question 28

neural tissue

Answer 28

The nervous system is composed of two major structural
subdivisions, the Central Nervous System (CNS), consisting of the
brain and spinal cord, and the Peripheral Nervous System (PNS).
The PNS includes peripheral nerves and localized clusters of
neuron cell bodies that are called ganglia.

Question 29

ganglia

Answer 29

Any collection of nerve cell bodies outside the central nervous
system is called a ganglion. There are three main types of
ganglia in the PNS: Dorsal root ganglia, situated on the spinal
nerves where they enter the spinal column, contain the cell bodies
of sensory nerves carrying sensory stimuli from the periphery into
the spinal cord. Sympathetic ganglia, containing cell bodies of
neurons that make up the sympathetic arm of the autonomic
nervous system are situated along the length of the spinal
column in the sympathetic chain of ganglia. Parasympathetic
Tissue Types 15
ganglia, on the other hand, are mainly situated within the organs
that they innervate.
Parasympathetic ganglia are easiest to find in organs of the
digestive tube, since they are consistently present within the
myenteric plexus, which is located between the longitudinal and
circular layers of smooth muscle that make up the muscularis
propria (aka muscularis externa). The neurons of the ganglion
innervate the surrounding smooth muscle and maintain the
peristaltic activity of the alimentary canal.
Like all peripheral nervous tissue, dorsal root ganglia, sympathetic
ganglia and parasympathetic ganglia contain numerous Schwann
cells, which support the neurons’ function.

Question 30

peripheral nerve tissue

Answer 30

The extension of a neuron that it uses to transmit signals to other
neurons or other target cells is called an axon. Many axons can
bundle together to form a small nerve or nerve fascicle, and
several fascicles can bundle to form a nerve.
Nerve fascicles are surrounded by a specialized layer of
connective tissue called the perineurium. This is composed of
modified fibroblasts that form a continuous sheath sealed cell to
cell by tight junctions. It is an essential structure for allowing
maintenance of an appropriate ionic environment within the nerve
fascicle.
Peripheral nerves range in size from nearly an inch in diameter to
fine, nearly invisible threads. Some are sensory, others are motor,
and some are mixed function. They can contain myelinated axons,
unmyelinated axons, or a mixture of the two.

Question 31

perineurium

Answer 31

Surrounds nerve fascicles,composed of
modified fibroblasts that form a continuous sheath sealed cell to
cell by tight junctions. It is an essential structure for allowing
maintenance of an appropriate ionic environment within the nerve
fascicle.

Question 32

Schwann Cells

Answer 32

Schwann cells in peripheral nerves
Peripheral nerves contain many nuclei, and nearly all of them
belong to Schwann cells, which reside within the nerve fascicle in
close association with neuronal cell processes. Each Schwann
cell surrounds a small segment of a neuronal cell process, and
there are many Schwann cells present along each neuron, such
that the neuron travels in a continuous sheath of surrounding
Schwann cells. This is the case for both myelinated and
unmyelinated nerve fibers.
In myelinated nerve fibers, Schwann cells form the myelin
sheath. They do this by wrapping multiple times around the
neuron like a jelly roll, forming many layers of plasma membrane -
collectively called myelin - that provide electrical insulation for the
segment of neuron covered by the sheath. The narrow gap
Tissue Types 16
between myelin segments is called a “node of Ranvier.” Nodes of
Ranvier have a critical function in conduction of action potentials
that will be discussed in the Nervous System block. They also
have a distinctive appearance in the microscope that can help you
identify peripheral nerve in tissue sections.
In the case of unmyelinated nerves, the function of Schwann cells
is thought to be related to maintaining the proper environment
surrounding the axon rather than increasing conduction velocity.

Question 33

squamous

Answer 33

epithelial cell, simple or stratified
simpleLining of lung
alveoli, some
kidney tubules;
Endothelium (lining
of blood vessels)
Mesothelium (lining
of body cavities)
stratifiedEpidermis of skin
(stratified
squamoous
keratinized);
Lining of oral
cavity, esophagus,
anal canal, cervix,
vagina, distal
urethra (stratified
squamous nonkeratinized)

Question 34

cuboidal

Answer 34

epithelial cell, simple or stratified
simpleLining of kidney
tubules; many
ducts of salivary
glands and
pancreas
stratified:Lining of sweat
gland ducts

Question 35

columnar

Answer 35

epithelial cell, simple or stratified
simple:Lining of GI tract,
bronchioles in lung,
oviduct, uterus in
female reproductive
tract
stratified: Lining of some
salivary gland ducts

Question 36

pseudostratified epithelium

Answer 36

epithelial cell looks stratified because cell nuclei
are at different levels and not all cells reach the cell surface. It is
considered a simple epithelium, however, because all cells touch
the basal lamina.

Question 37

transitional epithelium

Answer 37

epithelial cell (aka urothelium) is a special kind of
stratified epithelium that is capable of extensive stretching. The
epithelium varies in thickness depending on the degree of stretch.
Membrane reservoirs in umbrella cells (aka dome cells) at the
luminal surface of the epithelium contribute to the ability of the
epithelium to stretch.