Lab 1 Basic Tissues
All the organs of the body are made up of four basic tissues. In this lab you will look
at various organs in order to become acquainted with the basic tissues of which they are
made. The four basic tissues are (1) epithelia, (2) connective tissue, (3) muscle tissue
and (4) nervous tissue. You will not see examples of every type of epithelium, connective
tissue etc., but you should understand the essential features of each of the four basic
tissues. (See Fundamentals of Histology in your binders for a synopsis).
For example, all epithelia are composed of cells that are tightly joined to one another
to form a barrier to the outside. All surfaces in the body are lined by an epithelium. The
appearance of epithelia is however quite variable: the simple columnar epithelium lining
the lumen of the duodenum is very different in appearance from the stratified squamous
epithelium which forms the epidermis of the skin.
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Little detail can be seen at this magnification. The luminal surface of the duodenum
has fingerlike projections called villi (sing. villus). Intestinal glands extend downward
from the bases of the villi. (Intestinal glands are also called crypts of Lieberkuhn). A simple
columnar epithelium lines the surface of the villi and continues downward to line the
intestinal glands. The core of the villi is composed of a loose connective tissue
called the lamina propria. The lamina propria also fills the spaces between the intestinal
 glands. (Very frequently, the intestinal
glands appear as "circles" below the bases of the villi because their connection
to the surface is not in the plane of sectioning). Underlying the lamina propria a very
thin band of smooth muscle called the muscularis mucosae.
The epithelium, lamina propria and muscularis mucosae constitute the outermost layer of
the intestinal wall, called the mucosa. Thus the mucosa is composed of an epithelium, a
connective tissue and a muscle tissue.
The submucosa underlies the mucosa. The submucosa is a denser (thicker collagen fibres,
and in greater abundance) and less cellular connective tissue than is the lamina propria.
In the duodenum, much of the submucosa is occupied by the mucous-secreting Brunner’s
glands. The Brunner’s glands stain pink with the PAS stain (due to the glycoproteins
in the mucous). The submucosa is a moderately dense, irregular connective tissue.
The Brunner’s glands are composed of a single layer of cells whose height and width
are about equal: a simple, cuboidal epithelium.
Underlying the submucosa is the muscularis externa (sometimes just called muscularis)
which is composed of an inner circular and outer longitudinal layer of smooth muscle. (The
plane of sectioning sometimes makes these layers appear reversed).
The outermost layer of the duodenum is a layer of connective tissue which
forms a "wrapping" around the organ. When this connective tissue wrapping blends
in with the connective tissue of adjacent structures it is called an adventitia. When the
surface of an organ faces the body cavity, it has a thin epithelium which covers the CT
wrapping. In this case the wrapping is called a serosa. At this magnification, it would be
impossible to distinguish between an adventitia and a serosa. |
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At this magnification, it is possible to see the cells making up the epithelium, and
the loose  connective tissue of the lamina
propia which forms the core of the villus. The epithelium is made up of a single layer of
tall cells, it is therefore called a simple columnar epithelium. The nuclei of the
epithelial cells stain purple and are located near the base of the cells. Each cell has a
single nucleus, but because of the thickness of the section and the oblique plane of
sectioning in some areas (ie near the top of the villus on left), nuclei may appear as
clusters. A pink-staining brush border can be identified at the luminal surface of the
cells. The brush border consists of infoldings of the cell membrane called microvilli,
which act to increase the absorptive surface area. The staining results from the
glycoproteins that are associated with the brush border.
Certain cells of the epithelium also stain pink. These are the mucous-secreting goblet
cells. The glycoproteins of the mucous react with the PAS stain. The nuclei of the goblet
cells are also located basally (often not visible), and the apically-stored mucous widens
the upper part of the cell, so that it is goblet-shaped.
Every epithelium sits on a basement membrane. In a simple epithelium, the base of every
cell rests on that membrane. The basement membrane takes up the PAS stain to a slight
extent. It is identifiable on the villus at right, and on the right-hand side of the
villus at left.
The lamina propria makes up the core of the villus. It is composed of
reticular fibers (much finer than collagen fibres, not easily seen without special
stains), some ground substance (not identifiable) and numerous cells. The cells are not
tightly apposed as they are in the epithelium. Often only their purple staining nuclei are
identifiable. Some of the cells are fibroblasts, which secrete the noncellular components
of connective tissues (fibres and ground substance). Many of the cells are lymphocytes.
Some of the other cells that may be present are plasma cells, macrophages, and
neutrophils. There is no need to identify any of the cells now. |
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The muscularis mucosae, a thin band of smooth muscle, separates the mucosa from the
submucosa. Several intestinal glands are visible in the mucosa, the spaces between them
are occupied by lamina propria. The epithelium of the intestinal glands (sectioned rather
obliquely here) appears identical to and is continuous with that of the villi. Much of the
submucosa of the duodenum is occupied by Brunner’s glands, hence its connective
tissue (collagen fibres) is not that prominent. Brunner’s glands secrete mucous and
therefore stain pink with PAS. Their mucous secretions coat the surface of the duodenum
and protect it from the acidic chyme arriving from the stomach. The muscularis mucosae of
the duodenum is often displaced or disrupted by Brunner’s glands. Brunner’s
glands are an epithelium made up of simple cuboidal cells. |
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The submucosa is on the right and part of the inner circular layer of the muscularis
externa is on the left. The collagen bundles of the submucosa appear more wavy than the
spindle-shaped smooth muslce fibres running in parallel of the muscularis externa. In any
unit area, more nuclei will be present in smooth muscle than in connective tissue, because
each smooth muscle cell has a nucleus (located in the middle "fat" part of the
spindle-shaped cell). The collagen fibres of the submucosa are produced by fibroblasts
(whose purple nuclei can sometimes be seen). Some Brunner’s glands are visible in the
submucosa.
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 Smooth muscle in cross section
shows various more or less circular profiles of differing diameters, depending on whether
the smooth muscle fibre was sectioned at its thin tapered ends or its thick middle
section. The thick sections will often have a purple-staining spot in the centre: this is
the nucleus of that fibre.
A nerve plexus, called Auerbach’s plexus lies between the inner circular and outer
longitudinal layers of the muscularis externa. If you scan along the boundary of the two
muscle layers on your slides, you will see Auerbach’s plexus in one or a few places.
You may see both nerve fibres and cell bodies, or just one or the other. Nerve fibres
often appear as small wavy bundles. The appearance of nerve cell bodies, as shown here, is
quite distinct. The slightly staining nerve cell body has a large, round, very pale
nucleus, and a small darkly staining nucleolus which looks like a dot. The ends of one
nerve cell body are shown by asterisks, the nucleus is on the right of the cell, the
nucleolus is in the middle of the nucleus. |
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Thick skin covers the palms and the soles of the feet. The two parts of the skin are
evident, the epidermis, a stratified squamous epithelium, and the dermis, a dense,
irregular connective tissue in which bundles of collagen are evident. Projections of
de rmis into the epidermis are called dermal
papillae. These papilla form the papillary layer of the dermis. The collagen fibres of the
papillary layer are finer than those of the underlying reticular layer.
The outer part of the epidermis is made up of keratinized cells. These cells are filled
with keratin, have become anucleate and are extremely flattened In thick skin the
keratinized layer is very thick. The cells in the non-keratinized layers of epidermis are
polygonal in shape. The outer part of the non-keratinized layer of the epidermis contains
keratohyalin granules. These granules will be converted to keratin as the epidermal cells
move into the outer, cornified (keratinized) layer. Cells at the outer epidermal surface
are constantly being shed and replaced by cells moving up from the basal layers. The ducts
of several sweat glands can be seen emerging through the cornified layer of the epidermis.
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The boundary between the keratinized and non-keratinized layers of epidermis is shown.
The keratohyalin granules in the outer layers of non-keratinized cells can be seen. Ducts
of sweat glands can be seen going through the keratinized layer. Part of a dermal papilla
can be seen projecting into the base of the epidermis.
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In this high magnification of a dermal papilla, you can see several sections of blood
vessels containing red blood cells, and a Meissner’s corpuscle, an encapsulated nerve
ending that functions as a touch receptor. Meissner’s corpuscles are typically found
in dermal papillae just beneath the epidermis. |
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Scattered throughout the dermis you will see sweat glands. Sweat glands are simple,
coiled tubular structures. Several profiles of both the sweat gland and the duct are
shown. The sweat gland itself has columnar epithelium and stains more lightly than the
ducts, which are lined by two layers of cuboidal cells and stain darker red. (The lumen of
the duct is not always evident). Sometimes myoepithelial cells (indicated here with
asterisks) can be seen at the base of the sweat glands (but not the ducts), these cells
are contractile epithelial cells that squeeze the secretion toward the duct.
You will not find hair follicles in sections of thick skin because the palms and soles
lack hair. |
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Nerves and blood vessels will be seen scattered throughout the dermis. The nerves will
appear as encapsulated wavy bundles. In such sections, axons will not appear clearly
distinguishable from the endoneurium (fine connective tissue) which envelops axons and
their myelin sheath. The myelin sheath is removed during standard histological
preparations. The nuclei that can be seen belong to Schwann cells (produce the myelin
sheath) and to fibroblasts. |
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 Cartilage is a
specialized connective tissue, consisting of cells called chondrocytes in a turgid matrix.
The matrix consists of both collagen fibres and ground substance. The collagen fibres are
not visible. The ground substance has many hydrophilic macromolecules which give cartilage
high turgor (water pressure) and enables cartilage to resist compression but remain
flexible. The negatively charged sulfate groups in these macromolecules also stain with
basic dyes and exhibit metachromasia. (Metachromasia occurs when basic dyes react with
tissue components such that their normal color shifts from blue to red or purple. Thus
occurs because the numerous polyanions cause the dye molecules to become close enough to
form dimeric or polymeric aggregates whose absorption properties differ from individual
non-aggregated dye molecules.)
You will note that the matrix stains more intensely around cell clusters in the middle
of the cartilage. This occurs because there is a higher concentration of ground substance
macromolecules around the cells. The more densely staining area around the cell clusters
is called the territorial matrix.
Cartilage is enveloped by dense, regular connective tissue called the perichondrium.
The collagen fibres of the perichondrium are secreted by fibroblasts. However, the
fibroblasts on the inner surface of the perichondrium can change into chondroblasts.
Chondroblasts begin to secrete cartilage, which contains a different kind of collagen as
well as ground substance. When chondroblasts are completely surrounded by matrix they are
called chondrocytes. Chondrocytes can continue to divide and to secrete matrix. Thus
cartilage can grow from the inside through the action of chondrocytes (interstitial
growth) and from the periphery (appositional growth).
Some glands, located in the submucosa can be seen at the bottom left of Image 11. |
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The chondrocytes can be seen sitting in spaces called lacunae. In life the chondrocytes
fill the lacunae, but the cells shrink, or may even fall out, during histological
preparation. The chondrocytes in the middle of the cartilage are larger and are often
found in isogenous groups. The chondrocytes near the perichondrium at the periphery of the
cartilage (at left) are smaller and more elongated. |
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Skeletal muscle is arranged in parallel fibres which show striations due to the
arrangement of actin and myosin filaments within them. Unlike smooth muscle, each muscle
fibre remains the same width throughout its length. Skeletal muscle fibres are
multinucleate and the nuclei are located at the periphery of the fibres. Each individual
muscle fibre is enveloped by a thin layer of connective tissue called the endomysium,
bundles of muscle fibres are enveloped by a thicker layer of CT called the perimysium,
aand the entire muscle (ie. the biceps) is enveloped by the epimysium. There is no
perimysium or epimysium in this section, endomysium may appear as fine connective tissue
between muscle fibres. |
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