Histology
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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).
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.
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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
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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.
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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 wave-like 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.
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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
dome-shape cells.
b. Full bladder- when bladder is full, the epithelium is stretched, the cells flatted out, the cell layer is
3-4 cells thick, surface cells become large flat and squamous-like 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.
B. Functions
Connective tissue forms a framework of the organs in which cells of the organ can be held in place. It
serves to attach one type of tissue to another. It serves to hold tissues and organs in place. It forms a rigid
support framework of the body. It protects and insulates organs. It serves as a storage site for excess adipose
tissue (fat) and blood. It serves as a site for blood cell production and destruction. It serves for transport of
material.
C. Classification
Due to structural diversity, connective tissue is classified into 3 major groups.
1. Loose (areolar, adipose, reticular) ———–––
Connective tissue proper (soft)
2. Dense - irregular and regular (parallel)—––––
White fibrous – irregular arranged – dermis
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regular arranged – tendons and ligments
Yellow fibrous – regular arranged – fenestrated (broken) membranes of the elastic arteries,
such as in the aorta
bundles of elastic fibers –
ligamenta nucha in the neck, helps support the neck,
ligamenta flava in the vertebral column, attach vertebrae to each other
3. Specialized (cartilage, bone, blood)
\ /
hard
D. Components of Connective Tissue Proper
1. Cells - There are several kinds of cells in connective tissue. The fibroblasts (fibrocyte) and macrophage
are most common in the connective tissue proper.
a. Fibroblast (fibrocyte) - from a surface view the cell is stellate shape with a central oval nucleus. From a
side view the cell is spindle-shape. These cells are up against the fibers. The fibroblasts (fibrocytes)
produce and maintain the ground substance and fibers of the connective tissue. Blast cells - retain
capacity for cell division and secrete ground substance and fibers. Cyte cells-just maintains the ground
substance and fibers.
b. Macrophages - irregular shaped cells, that are the phagocytes. They move through the loose
connective tissue by ameboid movement and phagocytose foreign matter in the connective tissue
proper. When not moving the macrophages are attached to the fibers.
c. Other cells - Plasma cells (B-lymphocytes), other WBC, and fat cells, Mast cells.
2. Intercellular Material or Matrix
The intercellular material or matrix consists of ground substance and fibers.
a. Ground substance - is amorphous and homogeneous. It has consistency that varies from fluid of the
blood to loose watery gel of connective tissue proper to semi-solid gel of cartilage to a solid gel of
bone. The main organic molecule found in the ground substance is hyaluronic acid. It has a slippery
jelly-like consistency. It is composed of protein and polysaccharide.
b. Fibers - There are 3 types of fibers.
1. Collagenous (white) fibers are tough, strong, thick, non-elastic fibers. The fibers are made of the
protein, collagen. The collagenous fibers have great tensile strength due to the overlapping
arrangement of parallel oriented collgen molecules. The collagenous fibers are relatively thick
fibers compared to elastic fibers and are branching in some types of connective tissue.
The collagenous fibers are white in color and sometimes referred to as white fibers.
__________
2. Elastic (yellow) fibers - thin homogenous fibers, which branch in some types of connective tissue.
The fibers are elastic, in that after being stretched or compressed the fibers return to their original
shape and length. These fibers are made of the protein, elastin. Elastic fibers are yellow in color
and sometimes called yellow fibers.
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3. Reticular fibers - short, thin, fine branching collagen fibers. The reticular fibers are fine bundles of
the collegen protein. They are non-elastic fibers. The reticular fibers are arranged in a tight
framework called a reticulum or stroma. The fibers form an internal framework of the organs in
which the cells of the organs can be trapped, supported and held in place. The reticular fibers are
part of the basement membrane. Reticular fibers are non-elastic and are composed of the
protein, collagen. The reticular fibers are similar to collagenous fibers, except the reticular fibers are
arranged in a net-like configuration.
Types of Connective Tissue
The connective tissues are classified according to the nature of the ground substance and types and organization
of the fibers in the ground substance.
E. Types of Connective Tissue Proper (soft connective tissue)
1. Loose (Areolar) connective tissue
Structure - The collagenous and elastic fibers are loosely and irregularly arranged (unorganized) with a
great deal of ground substance between the fibers. The cells of the loose connective tissue are of
many different types - fibroblasts and macrophages are the most common plus WBC such as
lymphocytes and mast cells. The ground substance is a loose watery gel.
b. Location - Loose connective tissue is the most widespread connective tissue of the body (superficial
fascia). The loose connective tissue attaches the dermis of the skin to skeletal muscle. It also is
beneath the epithelial tissue that lines organs. As a denser connective tissue it fills
spaces
between
organs to hold organs and blood vessels in place (deep fascia). It acts as a packing material.
c. Function - Loose connective tissue holds cells of tissues in an organ.
2. Adipose or fat tissue
Structure - The tissue is composed of groups of fat cells (adipocytes) surrounded by loose connective
tissue. Each adipose cell contains a large fat droplet of triglyceride in the center of the cell.
A thin ring of cytoplasm at the cell’s periphery surrounds fat droplet. The shape of the cell gives it
the name, the signet ring cell.
b. Location - Adipose tissue is abundant in the subcutaneous layer of the skin and surrounds organs of the
body.
c. Functions - Adipose tissue serves as a storage site for fat and acts as an energy reserve. In the
subcutaneous layer of the skin it has an insulating function against heat lost. As it surrounds the body
organs it protects these organs from injury as it act as a shock absorber.
3. Reticular tissue
a. Structure - consists of a network of reticular fibers, reticular cells (fibroblasts) and macrophages in a
loose watery gel ground substance.
b. Location - This tissue provides structural support for the cells of a tissue in an organ, such as the liver,
spleen, lymph nodes and skeletal muscle.
c. Function - Forms a framework to trap and hold cells of an organ in place.
4. Dense Connective tissue
Fibers are closely packed.
a. Dense Irregular Arranged Collagenous Connective tissue
1. Structure - Dense irregular arranged collagenous connective tissue contain the same components as
loose areolar connective tissue, but has fewer cells and more collagenous fibers. The fibers are thick,
closely interwoven and randomly oriented to form a compact tissue with little ground substance.
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2. Location - This tissue forms the dermis of the skin and capsules around organs, such as kidneys, organs
of the thoracic and abdominal cavities (liver, spleen), perichondrium, periosteum, and surrounding
joint cavities. The fascia organization of the connective tissue fibers resembles that of plywood.
All fibers in one layer run in the same direction. The fiber in the next layer runs in another direction.
3. Function - Protection and attachment.
b. Dense Regular Connective tissue
1. Dense Regular Arranged Collagenous Connective tissue
a. Structure - The collagenous fibers are tightly packed in parallel bundles. The fibers run in one
direction. The fibroblasts are few and are in parallel rows between the collagenous fiber bundles.
There is very little ground substance. The abundance of the collagenous fibers gives this tissue
great tensile strength and a white appearance. This tissue can be referred to as white fibrous
connective tissue.
b. Location - This connective tissue forms the tendons, (cord-like structures) that attach muscles to
bones, and ligaments that attach bones to bone. The aponeuroses are broad sheets that act as
tendons attaching muscle to some structures.
c. Function - attachment of structures to each other.
2. Dense Regular Arranged Elastic Connective tissue
a. Structure - This tissue contains elastic fibers regularly arranged. The elastic fibers are fused into a
sheet to form a membrane. Since these membranes are broken they are called fenstrated membranes.
b. Location - This type of tissue is found in the walls of the large arteries, trachea and vocal cords.
c. Function - The elastic fibers and membranes allow this tissue to be stretched and then-return to
its original length after the stretch.
F.
Blood
Blood is a connective tissue, since it has cells, plasma, the ground substance and fibers, the components of
connective tissue. The fibers, fibrin, form during clotting.
G. Hard Connective Tissue
1. Cartilage
a. Structure - Ground substance is a semi-solid gel in which collagenous fibers are embedded. The
ground substance contains hyaluronic acid and chondroitin sulfate, the latter molecule gives the
ground substance gel a firm consistency. The fibers and ground substance are produced and
secreted by the chondroblasts. When the chondroblasts become surrounded
by the matrix they reside in spaces, the lacunae. The mature cartilage cells are now
called chondrocytes. The chondrocytes are spherical cells. The cartilage surface is covered with a
fibrous connective tissue covering, the perichondrium. Within the perichondrium there are blood
vessels, undifferentiated cells and chondroblasts. It is from the perichondrium. that the cartilage
mainly grows. There are no blood vessels or nerves running through the ground substance of the
cartilage, thus the cartilage is avascular.
b. Growth of cartilage 1. Interstitial growth – Cartilage grows from within due to division of the
chondrocytes and deposition of martrix. Interstitial growth occurrs when the cartilage matrix is
pliable during childhood.
2. Appositional growth – Cartilage grows from the inner surface of the perichondrium as the
chrondroblasts lay down cartilage. The martix accumulates beneath the perichondrium causing
growth in cartilage from the outside of the cartilage.
There are 3 kinds of cartilage due to the kinds of fiber embedded in the ground substance.
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a. Hyaline cartilage
1. Structure - The collagenous fibers are dispersed throughout the ground substance, but the fibers are not
distinguishable in the ground substance. The hyaline cartilage matrix appears homogenous. Unstained the
matrix has a glassy, milky blue-white smooth appearance.
2. Location - This tissue is widely distributed in the body. It is found in the embryonic skeleton, the
costal cartilages at the ends of the ribs where the ribs attach to the sternum, nose, parts of the larynx,
the "C"-shaped rings of the trachea and rings of the bronchi and the articular cartilage on the ends of
bones, where bones meet to form joints.
3.
Function - Hyaline cartilage is found where support and flexibility are needed.
b. Elastic cartilage
1. Structure - Elastic cartilage contains elastic fibers in the ground substance.
2. Location - This tissue is found in the pinna of the external ear, larynx and epiglottis.
3. Function - Elastic cartilage is found where flexibility is needed.
c. Fibrocartilage
1.
Structure - Fibrocartilage contains dense irregular arranged collagenous fibers in a little ground
substance with scattered isolated groups of hyaline cartilage.
2. Location - Intervertebral disks, pubic symphysis, articular discs in certain joint cavities, such as the
knee joints
3. Function - Fibrocartilage has a weight bearing function.
2. Osseous (Bone) Tissue
Structure - The matrix of the osseous (bone) tissue is a solid, rigid mineralized gel with collagenous
fibers. Within the matrix, the inorganic salt crystals are deposited in the ground substance and among
the fibers. These crystals are hydroxyapatite crystals composed of Ca, OH and PO4. The cells that make
the osseous tissues are called osteoblasts. The osteoblasts when surrounded by the osseous tissue reside
in spaces, the lacunae and are now called osteocytes. The osteocytes are stellate shaped cells having
osteocyte processes. Radiating from the lacunae are small canals, called canaliculi, in which the
osteocyte processes reside. Osteoclasts are cells in the osseous tissue that dissolves bone. Osteoblasts
and osteoclasts working together constantly alter the density and the shape of bone in response to
stresses place upon the bone.
b. Location – Osseous (bone) tissue forms the adult skeleton.
c. Function - This tissue functions for support, movement, protection, mineral reservoir and houses sites of
blood cell formation. Triglyceride (fat) storage in the yellow marrow.
d. Growth of Bone – Appositional growth - Bone grows by apposition as the osteoblasts in the inner layer
of periosteum and from the endosteum lay down bone matrix. Thus bone grows from the outer surface
and from the inner surface.
There are 2 types of osseous (bone) tissue in the adult skeleton.
a.
Compact Osseous (Bone) tissue
Compact bone is the main type of osseous tissue. The basic unit of compact bone is the Haversian
system or osteon. Each Haversian system has central Haversian canal, which contains blood vessels and
nerves. Concentric rings or layer or lamellae of bone matrix surround the Haversian canal.
Between the lamellae are the lacunae, which contain the osteocytes. The osteocytes’ processes lie in the
canaliculi. It is through the canaliculi and the lacunae that the osteocytes are interconnected to each
other. This arrangement allows for diffusion of substances between the osteocytes.
Covering the outside of the bone is a layer of dense irregular arranged collagenous connective tissue, the
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periosteum. Sharpey's fibers attach the periosteum to the bone. In the periosteum there are blood vessels
and nerves that penetrate into the bone. Undifferentiated cells and osteoblasts allow the bone to grow in
thickness from the inner layer of the periosteum. Compact bone forms the outer part of an entire bone.
b. Cancelleous (Spongy) Bone tissue
The bone is in irregular meshlike arrangement of thin plates, called trabeculae. The cancelleous bone is
found toward the inner part of the bone extending into the marrow cavity under the compact bone.
Cancellous bone is covered by an endosteum. It consists of undifferentiated cells and osteoblasts, etc.,
which allows the bone to grow in thickness from the inside and osteoclasts which causes bone
resorption.
Appositional bone growth from the osteoblasts in the inner layer of the periosteum and from the
endosteum. Canelleous bone grows in thickness from the endoseteum surrounding its outer surfaces.
III. Muscle Tissue
The cells of muscle tissue are elongated, thin cells. Muscle tissue functions by contraction and relaxation
for movement, stabilizing body positions, regulating organ volume and generating heat. The characteristics
of muscle tissue that allow it to carry out its functions are:
1. irritability - responds to stimuli and conduct impulses;
2. contractility - shortens and thickens to generate a force to do work;
3. extensibility - can be stretched;
4. elasticity - returns to original shape after shortening or lengthening.
There are 3 types of muscle tissue.
A. Skeletal muscle
Skeletal muscle is voluntary muscle, contractions are controlled by conscious will of the individual.
Skeletal muscle requires nerve stimulation for contraction.
1. Location - Skeletal muscle is attached to the skeleton.
2. Function - Skeletal muscle causes movement of the bones at the joints and "locks" the joints for
maintaining posture.
3. Structure - Skeletal muscle cells are long cylindrical shaped cells, grouped together into masses called
muscles. Within the muscles, the cells are grouped into bundles (fascicles) with the cells parallel to
each other. Within each skeletal muscle cell there are longitudinal running microfilaments containing
the proteins, actin and myosin. These microfilaments, called myofilaments, are grouped into larger
units, called myofibrils. The myofibrils have alternating light (I band) and dark (A band) band
patterns crossing them due to the parallel and in register arrangement of the actin and myosin. This
banding pattern gives the skeletal muscle in longitudinal section its cross-striated appearance. Each
skeletal muscle cell is so large that it is multinucleated cell, with oval-shaped nuclei located just
under the cell membrane, i.e. peripheral located nuclei.
B.
Smooth muscle
Smooth muscle is involuntary muscle, contractions are not controlled by conscious will of the
individual. Smooth muscle does not require nerve stimulation for contraction.
1. Location - Smooth muscle cells are located in the walls of hollow internal organs, such as blood
vessels, digestive system, respiratory system and reproductive system.
2. Function - Smooth muscle controls movement of the contents within the lumens of hollow internal
organs by controlling the diameter of the lumen of the hollow organ and by regulating the volume
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of organs.
3. Structure - Smooth muscle cells are spindle-shaped cells which taper to points. The smooth
muscle cells are smaller than the skeletal muscle cells. There is a single oval shaped centrally
located nucleus in each cell. The smooth muscle cell contain actin and myosin myofilaments but
not in an orderly arrangement to give A and I band patterns, thus the muscle is called smooth.
The smooth muscle cells in the muscle are arranged in sheets with the cells offset and
overlapping each other.
C. Cardiac muscle
Cardiac muscle is involuntary muscle.
1. Location - Cardiac muscle forms the wall of the heart.
2. Function - Cardiac muscle on contraction develops the force or pressure to pump the blood through
the blood vascular system.
3. Structure - Cardiac muscle cells are short branching cylinders, organized into a branching network.
The cardiac muscle cells contain cross striated myofibrils similar to skeletal muscle cells, but the
striations are not as distinct. Each branch has 1 or 2 oval, centrally located nuclei. Where adjoining
cardiac muscle cells meet end to end a thickening of the cell membrane forms, the intercalated disk.
In the intercalated disk there are desmosomes for strength to prevent the cells from pulling apart
on contraction and relaxation and gap junctions. The intercalated disk is unique to cardiac muscle
and can only be seen in longitudinal section of cardiac muscle.
Importance of the gap junctions in the intercalated disks. The gap junctions allow for quick
conduction of impulses across the cells of the cardiac muscle. The gap junctions allow impulses to
reach all cardiac muscle cells at one time so that the entire heart contracts at once. Thus the cell
mass is acting as an entire functional unit, which is, called a syncytium. In the heart have a site that
initiates the impulse which spreads across the cardiac muscle cells through the gap junctions and
triggers a wave of contraction through the entire muscle mass at one time producing a great force of
contraction.
IV. Nerve Tissue
Nerve tissue is specialized for conduction of nerve impulses.
Nerve tissue is composed of 2 kinds of cells.
A. Nerve cells or neurons
The neurons are cells that receive and transmit the nerve impulses. The neuron is the structural and
functional unit of the nervous system. .
1. Function - The neurons conduct information in the form of the nerve impulse or action potential from
one part of the body to another.
2. Structure - The neuron is a long thin structure. It has three basic parts
a. Cell body, Perikaryon, Cyton - the area of the cell that contains the nucleus and other cellular
organelles.
b. Cytoplasmic Processes
1. Axon - A long thin cytoplasmic process that conducts the nerve impulse away from the cell
body.
2. Dendrites - Projecting from the opposite side of the cell body one or usually more shorter
processes. The dendrites conduct nerve impulses to the cell body.
B. Neuroglial Cells
Neuroglial cells are supporting and nutritive cells. They do not conduct impulses. Neuroglial means
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nerve "glue", as these cells hold the neurons together in the nerve tissue.
Neurilemmal cell is an example of a neuroglial cell found in the peripheral nerve.
Coverings of the axon
A visible fatty insulating sheath, the myelin sheath, covers most axons. The myelin sheath is
made by the neurilemmal cells. The myelin sheath is not continuous, but has gaps, the neurofibral
nodes (nodes of Ranvier). The neurofibral nodes maintain contact between the neuron and the
intercellular fluid, as the myelin sheath is an insulating material. This allows for rapid
rapid nerve impulse transmission. Each neurilemmal cell makes a small segment of myelin along the
length of the axon..
3. Location - Neurons and neuroglial cells are found in the nervous system.
A nerve is a group of neurons bound together by connective tissue.
V. Membranes
Membranes are flat sheets of pliable tissue that cover or line a part of the body. The combination of
epithelial tissue and underlying connective tissue makes up an epithelial membrane. The epithelial
membranes of the body are mucous membranes, serous membranes and cutaneous membrane (skin).
A synovial membrane which lines joint cavities contains connective tissue but has no epithelium.
A. Epithelial Membranes
1. Mucous Membranes
A mucous membrane (mucosa) lines body cavities that open to the outside. The digestive, respiratory,
reproductive and urinary tracts are lined by mucous membranes. These membranes contain a lining
layer of epithelium and an underlying layer of connective tissue.
The epithelial membrane of the layer is the primary body’s defense mechanism against pathogens. As
these organisms have difficulty in penetrating the layer. Tight junctions prevents materials from
leaking
between the cells on their lateral surfaces. Goblet cells and other cells of the membrane secrete mucus.
Mucus is a viscous fluid prevents the body cavities from drying out. Mucus traps debris in the
respiratory tract and also lubricates food as it passes through the GI tract. The epithelial layer secretes
enzymes for digestion and is the site for absorption of digested foods from the GI tract due to microvilli.
The epithelium of the mucous membrane varies in different parts of the body. For example, in the small
intestine the epithelium is nonciliated simple columnar; in the trachea the epithelium is
pseudostratified
ciliated columnar.
The connective tissue layer of the mucous membrane is composed of areolar connective tissue and is
called the lamina propria. The lamina propria supports the epithelium, binds the epithelium to the
underlying structures, protects the underlying structures and gives flexibility to the membrane. The
lamina propria contains blood vessels for exchange of materials from the lamina propria and the
epithelium.
2. Serous Membranes
A serous membrane (serosa) lines body cavities that do not open to the outside and covers organs
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that lies within the body cavities. Serous membranes consist of simple squalors epithelium (mesothelium)
below which is areolar connective tissue. The mesothelium secretes a watery (serous) fluid that permits
organs to move easily over each other and slide against the internal walls of the body cavities.
The serous membranes are composed of two layers: The layer covering the wall of the cavity is
the parietal layer; the layer covering and attaches the organs to wall of the cavity is the visceral layer.
The name of the serous membrane differs depending upon in which body cavity it is found. The pleura
lines the thoracic cavity and covers the lungs. The pericardium lines the pericardial cavity and covers
the
heart. The peritoneum lines the abdominal cavity and covers the abdominal organs.
3. Cutaneous Membrane
The cutaneous membrane (skin) covers the surface of the body. It has on its free surface the keratinized
stratified squamous epithelium (epidermis) which protects the underlying dermis of areolar and dense
irregular connective tissue.
4. Synovial Membranes
Synovial membranes are not epithelial membranes. The synovial membranes line freely movable joint
cavities. These cavities do not open to the outside. The synovial membranes is composed of a
discontinuous layer of synoviocytes which lines the cavity and deeper layer of areolar and adipose
connective tissue. The synoviocytes secrete synovial fluid which lubricates and nourishes the cartilage
covering the ends of the bones in the joint cavity. The synovial fluid also contains macrophages to
phgoctyose debris from the joint cavity.
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