histology - important notes

8/7/2019 Histology - Important Notes

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Hyaline Cartilage, H&E

Stained

Hyaline cartilage consists of small cells with blue staining nuclei

embedded in an amorphous matrix. Note how the cells are sepa-

rated from each other. The matrix does not stain very well, and in

this case appears mostly a pale blue colour.

Lymph Node H&E

Stained

This image shows part of a lymph node. Most of the cells are

small immature lymphocytes. Their nuclei stain dark blue but

their cytoplasm is very small and can not be rersolved in this pho-

tograph.

Skeletal Muscle, H&EStained

This image shows skeletal muscle fibres cut in longitudinal sec-

tion. The contractile proteins stain pink whereas the nuclei (oval

in shape) stain blue and lie alongside the fibres. The cells are long

and thin (fusiform) in shape.

Spinal Cord - Silver

Stained

Spinal cord stained with a silver stain showing a large motor neu-

rone with a pale nucleus and 3 long processes. Parts of other cells

and processes can be seen surrounding it.


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Cilia

This series of pictures are of cilia. They illustrate the arrangement

of the micro-tubules into 9 peripheral doublets and a pair of cen-

tral mictotubules. These microtubules are composed mainly of

tubulin links together with dynein. These two proteins interact to

cause bending of the cilia.

Columnar Epithelium of

the Intestine

This micrograph shows a row of enterocyte from the lining of the

small intestine. These cells are columar in shape and thier uper

surface (apical surface) shows a regular arrangement of finger-

like processes called microvilli. These increase the surface area of

the cell, which is important for the process of absorbtion.

Endothelial CellHigh magnification view of a capillary. Its wall is formed by a

flattened (squamous) endothelial cell. This cells can be seen to

contain many small pinocytotic vesicles. These transport fluid

from one side of the cell to the other.

Thyroid Gland, H&E

Stained

This slide shows part of the thyroid gland. The cells are arranged

as hollow spheres (follicles) in the centre of which is some amor-

phous pink-staining colloid. This colloid contains inert (storage)

forms of the hormones (e.g. throxine) that are eventually secreted

by the gland. Note the cells of the folliclesare cuboidal in shape .


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Intestinal Cell Showing

Microvilli

This micrograph shows part of an gut enterocyte. The finger-like

processes are microvilli on the apical surface of the cell. These

greatly increase the surface area of the cells and facilitate the ab-

sorbtion of food-stuffs. Each of these microvilli contains a core of

contractile filaments. The rest of the cells can be seen to contain

many mitochondria..

Liver cell

This micrograph shows part of a liver cell. The rough endoplas-

mic reticulum is studdied with small dark-staining ribosomes. The

smooth endoplasmic reticulum appears as small circular vesiclesin the bottom right-hand corner. Some mitochondria can be sen in

the top left hand corner

Mesenteric NerveThis image shows fine unmyelinated nerves from the gut (there

are no gut epithelial cells present). The nerves contain very many

small membrane vesicles that transport neuro-tranmitter sub-

stances to the nerve endings (not evident). In between the axons

are some fibres of collagen that can be seen to be finely banded.

Myelinate Nerve

This image shows a transverse section through a myelinated nerve

with some skeletel muscle below. It is part of a nerve that supplies

the muscle. Within the nerve can be seen a number of dark-

staining rings. These dark rings are myelin sheaths that surround

the pale-staining nerve axons. The nuceli are those of Schwann

cells that manufacture the myelin sheath. No nerve cell nuclei can

be seen in this micrograph. At the bottom of the picture is someskeletal muscle


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Nerve in Transverse Sec-

tion

This image shows several myelinated axons and unmyelinated

axons cut in transverse section. The nuclei visible in the section

are Schwann cell nuclei. Note how each spiral of myelin has a

single axon at its core. In the case of the unmyelinated nerves

however, several (2 to 6) axons can be seen embedded within the

cytoplasm of the Schwann cells. Note that both myelinated and

unmyelinated nerves have Schwann cells associated with them.

Nerve in Transverse Sec-

tion

This image shows several myelinated axons and unmyelinated

axons cut in transverse section. The nuclei visible in the section

are Schwann cell nuclei. Note how each spiral of myelin has a

single axon at its core. In the case of the unmyelinated nerves

however, several (2 to 6) axons can be seen embedded within the

cytoplasm of the Schwann cells. Note that both myelinated and

unmyelinated nerves have Schwann cells associated with them.

Secretory CellThis high magnification image shows part of a secretory cell. In

the middle is a Golgi apparatus. This organelle is always well

developed in secretory cells. There are also a number of dark

staining secretory granules. The pale staining granule-like struc-

ture next to the Golgi apparatus is a condensing vacuole. This

appears damaged but would normally mature into a fully formedsecretion granule. Some rough endoplasmic reticulum and some

small mitochnodria can also be seen in this picture. The surface of

the cell (top right hand corner) is elaborately folded.

Skeletal Muscle in Longi-

tudinal Section

This microgrpah show skeletal (voluntary) muscle cut in longitu-

dinal section. It shows alternating dark (A) and light (I) bands in

register. In the middle of the I band is a dark Z line.


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Skeletal Muscle in Trans-

verse Section

This microgrpah shows skeletal muscle myofibrils cut in trans-

verse section. The polygonal shapes are myofibrils and within

each it is possible to see thick and/or thin filaments. In a few

places it is possible to see thick filaments surrounded by six thin

filaments. The membrane vesicles between the myofibrils are part

of the sacroplamsic reticulum

Tubules and sarcoplasmic

reticulum

This micrograph of skeletal muscle shows 3 sarcomeres. The ones

on either side are sectioned thruogh the myofibrils whereas the

one sandwiched in the middle has been slices to reveal the t tu-

bules and sacroplasmic reticulum that surrounds each myofibril.

Bladder H&E stainedThe image shows a fold of the lining of the bladder. The epithe-

lium lining the bladder is a specialised (urine-proof) epithelium. It

appears to composed of many layers of cells but as the bladder

fills with urine the epithelium is stretched and becomes thinner.

However, the surface is always covered by a surface layer of

multi-nucleate cells sometimes referred to as umbrella cells that

are abel to resist the toxic deleterious effects of urine.

Bronchus H&E stained

This image shows a pseudo-stratifide epithelium lining the bron-

chus of the lung. In places the epithelium appears to be composed

of more than one layer of cells but when the brochus expands the

surface streches out to form a single layer of cells.


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Gall Bladder Epithelium,

H&E stained

This image shows a thin finger-like fold of the lining of the gall

bladder. On the surface is a simple columnar epithelium consisit-

ing of a single row of identical cells with oval shaped nuceli and

pink cytoplasm..

Intestinal Villi, H&E

stained

This picture shows parts of 5 intestinal villi. The surfaces of each

of these is covered by a simple columnar epithelium. This epithe-

lium appears to have 2 main cell types, dark staining enterocytes

(the majority) and pale staining goblet cells. Inside the villi (villus

core) is a mixture of blood vessels, lymphatics and connective

tissues.

Nasal Septum stainedwith Alcian Blue and

H&E

This picture show a bony septum from the nose. On each side

(surface) is a simple cilated epithelium. The blue sataining cells

are gobelt cells that are interspersed with the cilated cells. The cila

are very fine and can only be seen in places.

Non-hairy Skin, H&E

stained

This image shows a purple staining stratified squamous epithlium

with layers of pink-staining keratinised squames on top. The cells

of the deeper purple staining layers are living cells whereas the

surface squames that are full of keratin are dead. No nuclei are

present in the surface squames.


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Oesophagus, H&E

stained

This image shows a fold in the lining of the oresophagus. On each

surafe is a statified squamous epithelium. The deeper layers of

cells of this compound epithlium are round and purple staining

but the surface layers are flattened and palely stained. All the cells

of this epithelium are living and contain nuclei.

PAS Stained Intestinal

Villi

This image shows parts of 2 inmtestinal villi. The tissue has been

stained by the PAS procedure (periodic acid Schiff)and with

haematoxylin. The PAS procedure demonstrates the distribution

of carbohydrates containing hexose sugars. Here the mucin con-

taining goblet cells, the brush border of the enterocytes

(absorbitive gut cells)and the basement membranes of all the tis-sues stain dark pink (magenta). The tissue has been counter

stained with haematoxylin so the cell nuclei all stain blue.

Small Intesine H&Estained

This image shows mainly smooth muscle from the wall of the

small intestine. The outside surface of the intestine is covered in

many places by a simple squamous epithelium (serosa). This se-

rosa pervents the intestinses from sticking to one another and al-

lows them to slide smoothly over each other. The serosal layer is

composed of flattened cells with oval nuclei and here forms the

uppermost layer of cells.

Sublingual Salivary

Gland , H&E stained

This image shows a duct inside the sublingual salivary gland. The

duct lies in the middle of the picture and is lined by a cuboidal

epithelium. The duct is branched and the cells have a clear pink-

staining cytoplasm and a round blue-staining nucleus. Surround-

ing the duct are many pale-staining mucus producing cells


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Aorta stained with elastic

stain

This picture shows mainly the medial layer of the aorta. The dark

brown lines are concentric sheets of elastic tissue that occupy the

medial layer.

Aorta stained with H&E

This picture shows mainly the medial layer of the aorta. The aorta

is an elastic artery and the medial layer consists mainly of concen-

tric sheets of elastic tissue. These sheets are perforated to permitthe diffusion of oxygen and nutrients and the migration of cells

though them.

Fatty connective tissuestained with H&E

This picture shows of white fat permeated by capillary blood ve-

sels. The triglyceride contained in the fat cells as a single large

droplet is extracted duringg tissue processing, leaving only a hol-

low shell. The fat cells appear as large open circles with the occa-

sional blue staining nucleus. The small capillaries can be seen to

contain brightly stained red blood cells.

Liver stained with a silver

stain

This picture shows part of the liver. The black strands are fibres of

reticulin (type III collagen) that form a delicate framework be-

tween the liver cells (poorly stained) to which the liver cells are

loosely attached. Similar reticular meshworks are present in many

parenchymatous organs including liver, lung, spleen and lymph

nodes


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Oesophagus stained with

H&E

This picture shows the lamina propria of the oesophagus. This

layer is composed of loose connective tissue. Small blood vessels

and fine strands of collagen or elastic tissue can beseen. The clear

spaces between the fibres isoccupied by a highly hydrated jelly-

like substance rich in glycosaminoglycans (GAGS). The epithe-

lium at the top and left hand side of the picture is a stratified

squamous, non-keratinising epithelium.

Penis stained with H&E

This picture shows part of a transverse section through the penis.

The broad dark pink staining band in the centre of the picture is

part of the capsule that surrounds one of the erectile compart-ments of the shaft of the penis. It is a good example of dense regu-

larr connective tissue and is composed mainly of collagen. The

inelastic properties of collagen means that when the blood vessels

become engorged with blood the penis becomes erect.

Skin stained with H&EThe slide shows the dermis of the skin of the scalp. This is a good

example of dense irregular connective tissue. Braod strands of

dark pink staining collagen fibres are clearly evident.

Tendon

Plastic section stained with H&E showing part of a tendon. This is

an example of dense regular connective tissue. Extracellular fibres

of type I collagen are aligned parallel with one another to form a

strong inelastic tendon. The fibroblasts that synthesis the tropocol-

lagen subunits from which the tendon is constructed are sand-

wiched between the fibres. Their elongated nuclei can be clearly

seen


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Heart stained with H&E

This picture shows cardiac muscle cells with some pale staining

connective tissue is between. The cells appear rectangular in

shape and have a prominent nucleus at their centre. The cells are

joined end to end by intercalated discs (pale staining) to form fi-

bres. Note how the fibres branch.

Intestine stained with

H&E

Picture showing parts of the two layers of smooth muscle that

make up the muscularis externa of the intestine. The inner

(circular) layer appears in the bottom right hand corner. The

smooth muscle cells are cut longitudinally and can be seen to have

cigar-shaped nuclei. The outer (longitudinal) layer runs diagonally

across the picture and can be seen to contain a small arteriole andvenule. The muscle cells of this layer are cut transversely and

appear circular in outline. Those cells sectioned through their

equator show round nuclear profiles.

Teased skeletal muscle fi-bres stained with iron

haematoxylin

This picture shows a number of whole skeletal muscle fibres

stained with iron haematoxylin. The banding (striations)of the

fibres can be clearly seen

Tongue stained with

H&E

This picture shows bundles of skeletal (voluntary) muscle cut in

transverse section. The polygonal shaped pink blocks are muscle

fibres with blue staining nuclei located at their periphery. These

fibres are grouped together into fasciculi.


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Tongue stained with

H&E (2)

This picture shows skeletal (voluntary) muscle fibres sectioned

longitudinally. The muscle appears as separate fibres each of

which is composed of numerous microfibrils. The nuclei lie at the

periphery of the fibres. Each fibre is a syncitium derived from

many fused myoblasts.

Dorsal root ganglion

stained with H&E

Dorsal root ganglion showing numerous mauve staining nerve cell

bodies. The small cells surrounding the nerve cell bodies are satel-

lite cells. A tract of pink staining nerve axons pass obliquely

across the centre of the picture.

Dorsal root ganglionstained with H&E (2)

Dorsal root gandglion showing large pseudo-unipolar nerve cell

bodies and small dark blue staining satellite cells. At the bottom

of the picture is a tract of pink staining axons.

Myelinated and unmyeli-

nated axons

An electron microscope image showing several myelinated and

unmyelinated axons. The dark staining myelin sheaths each sur-

round a single large axon. Many smaller unmelinated axons can

be seen partially embedded in Schwann cell cytoplasm. A large

Schwann cell nucleus is present lower right (A)


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Nerve stained with H&E

Large peripheral nerve bundle showing obliquely sectioned axons

contained surrounded by perineurium (connective tissue).

Neuromuscular junction

Electron microscope image of a neuro-muscular junction. A nerve

ending containing several dark staining mitochondria and numer-

ous small neurosecretory vesicles lies mid left. To the right is themuscle fibril with which it synapses.

Peripheral nerve stainedwith osmium tetroxide

Part of a nerve bundle surrounded by pink staining perineurium.

Numerous brown staining myelinated axons can be seen inside the

nerve bundle, together with fine strands of connective tissue

(endoneurium).

Peripheral nerve stained

with osmium textroxide

Peripheral nerve stained with osmium tetroxide. Axons cut longi-

tudinally can be seen passing from side to side. Several of these

axons show constrictions known as nodes of Ranvier.


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Small nerve stained with

H&E

Small nerve surrounded by connective tissue. Individual myeli-

nated axons can be seen within the nerve bundle. The blue stain-

ing nuclei within the nerve bundle are those of fibroblasts

(flattened, endoneurium) or of Schwann cells (rounded). The bun-

dle is enclosed by a connective tissue sheath of perineurium.

Small nerve stained with

H&E

Small nerve surrounded by connective tissue. Individual myeli-

nated axons can be seen within the nerve bundle. The blue stain-

ing nuclei within the nerve bundle are those of fibroblasts

(flattened, endoneurium) or of Schwann cells (rounded). The bun-

dle is enclosed by a connective tissue sheath of perineurium. .

Respiratory Epithelium

This type of epithelium lines the nose, nasopharynx, larynx. tra-

chea, bonchi and bronchioles.

It consists of a simple or pseudo-stratified columnar ciliated epi-

thelium punctuated by goblet cells.

The underlying connective tissue often contains a rich plexus of

thin-walled blood vessels and additional sero-mucous (mixed)

glands that secrete via ducts adding to the protective mucous layer

that covers the surface.

This mucous prevents dehydration of the epithelium and traps

particulate matter that is eventually expelled from the system by

the beating of the cilia.

Cilia beat towards the throat and contaminated mucous is swal-

lowed or expectorated.


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Nose

Except at its opening (hairy skin), the nose is lined by a simple

ciliated respiratory epithelium.

Three bony projections extend from the lateral walls of nose

(concha) and these too are covered by a respiratory epithelium.

A similar epithelium lines the nasal sinuses that connect with thenasal cavity.

A rich plexus of thin-walled arterioles and venules lie deep to

the epithelium (swell bodies). These are the source of many nose

bleeds. They help to warm and humidify the inspired air.

Olfactory Epithelium

A small area of epithelium in the roof of the nose, below the

cribiform plate, is adapted for the detection of smells.

Unmyelinated olfactory nerves pass through holes in the cribi-

form plate to connect to the olfactory bulb of the brian.

The columnar pseudo-stratified epithelium of the olfactory mu-

cosa consists of basal and sustentacular(supporting) cells. em-

bedded in this are bipolar neurones whose dendritic processes

reach as far as the apical surface.

Deep to the epithelium are serous glands whose ducts open on to

the epithelial surface. The watery secretion they produce acts as

a solvent for odorous substances. They also irrigate the surface

and help to refresh the epithelium.

Larynx

The larynx is lined by a respiratory epithelium except over the

vocal folds.

The vocal folds are covered by a stratified squamous epithelium

that can withstand the vibrations of the folds.

The folds contain the free upper margin of a cone of elastic tis-sue (conus elasticus). This cone is tensioned by the voluntary

(skeletal) muscle fibres of the vocalis muscle.

Further manipulation of the folds is undertaken by the extrinsic

muscles of the larynx including the crico-thyroid muscle illus-

trated here.


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Trachea

The trachea is lined by a pseudo-stratified cilated epithelium

(respiratory epithelium).

It is held open 12-15 incomplete ('C' shaped) rings of hyaline

cartilage.

The ends of the 'C' lie posteriorly, towards the oesophagus and

are joined by smooth muscle (trachealis muscle).

Deep to the epithelium are numerous sero-mucous (mixed)

glands and lymph nodules.

Bronchus

Bronchi are lined by a pseudo-stratified cilated epithelium

(respiratory epithelium).

They are held open by an external framework of hyaline carti-

lage. This often appears as isolated islands of cartilage in thin

section.

Smooth muscle underlies the epithelium

Lymph nodules lie in the underlying connective tissue.

Branches of the vascular tree - pulmonary arteries and veins -

usually lie close by.Part of the wall of a bronchus stained with H&E. To the left is

part of the supporting plate of hyaline cartilage and the lumen isto the right. The bronchus is lined by a pseudo-stratified ciliated

epithelium (respiratory epithelium).

Lung tissue stained with H&E. This picture shows part of the

wall of a large pulmonary vein. The wall contains a large num-

ber of pink staining elastic fibres.

Pulmonary Vessels

Large pulmonary blood vessels accompany the bronchi. These

have longitudinally running elastic fibres in their walls.

Large pulmonary veins often have a clearly defined medial layer

consisting of muscle and elastic fibres and a well marked inter-nal elastic lamina.

Smaller pulmonary veins that accompany the brochioles are dif-

ficult to distinguish from their equivalent arteries since they of-

ten have similar amounts of smooth muscle in their walls.


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Picture of lung tissue stained with H&E showing a large bron-

chiole. Unlike bronchi, bronchioles have no cartilage in theirwalls and can be constricted by the smooth muscle in their

walls.

Bronchiole

Bronchioles divide repeatedly and are typically less than 1

mm in diameter.

They are lined by a simple ciliated epithelium (respiratory

epithelium).

They are not supported by hyaline cartilage but display aprominent band of smooth muscle.

Lymph nodules lie in the underlying connective tissue.

Branches of the vascular tree - pulmonary veins and arteries -

usually lie close by.

Picture of lung tissue showing a terminal bronchiole opening

into an alveolar duct. The terminal bronchiole is the last part ofthe bronchial tree. Beyond this point gasseous exchange takes

place.

Terminal and Respiratory Bronchioles

The conducting system of the lungs (bronchial tree) ends with

small diameterterminal bronchioles.

In these bronchioles the lining is a simple cuboidal epithelium

that is only sparsely ciliated but includes Clara cells whose

precise role remains unresolved.

A prominent sphincter-like ring of smooth muscle surrounds

the airway at this point.

Next come respiratory bronchioles. These are lined by a

similar epithelium, but are usually larger in diameter than

terminal bronchioles. They mark the beginning of the respira-

tory portion of the lung.

Alveolus

Alveoli occupy most of the volume of the lung.

These thin walled sacs have capillaries embedded in their

walls.

The walls also contain fibroblasts that produce a framework

of reticulin (collagen III) and large amounts of elastic tissue.

Adjoining alveoli that arise from the same terminal bronchi-

ole are interconnected by pores that ensure equal inflation.

High magnification picture of lung alveoli stained with H&E. The walls of the alveolar sacsare composed of flattened capillary endothelial cells and Type I pneumocytes sandwiched

together to form a thin air-blood barrier normally no more than 0.6 microns thick.


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Air-Blood Barrier

Air that is drawn into the alveoli comes in close contact with blood circulating in the capillaries.

The two are separated by the air-blood barrier that is only 0.2 and 0.6 micrometres thick.

The barrier consists of flattened Type I pneumocyte on the alveolar side and flattened capillary endothelial cell on the capillary

side.

The two cells are fused to an intervening basement membrane.

The surface of the Type I pneumocytes is covered by a thin layer oflipid-rich surfactant.

Type II Pneumocytes

Type II pneumocytes are globular cells that are interspersed with

the flattened Type I pneumocytes of the alveolar wall.

They are normally less numerous than the Type I cells.

Evidence suggests that they are stem cells from which Type I

pneumocytes arise.

They synthesise, store and secrete surfactant, a phospholipid-based secretion that spreads out over the whole lining of the al-

veolus. It reduces surface tension within the alveolus, facilitating

expansion and collapse of the air sacs during respiration. It also

helps to prevent desiccation of the air sacs.

Alveolar Macrophages

Macrophages roam freely within the alveoli phagocytosing

particulate material and cell debris.

These differentiate from monocytes that migrate from the blood

stream.

Other resident macrophages remain within the substance of the

lung and engulf material that penetrates the alveolar wall.

High magnification picture of lung alveoli. Alveolar macrophages containing black injested dust particles can be clearly seen.


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Blood Vessels

The walls of arteries and veins have 3 histological layers, an in-

tima, media and adventitia.

The innermost layer is the intima; a layer of endothelial cells rest-

ing on loose connective tissue.

The medial layer contains smooth muscle, elastic and collagen

fibres in variable proportions.

The outermost layer of adventitia contains densely packed colla-

gen and elastic fibres.

Usually the wall of an artery is much thicker than that of the ac-

companying vein.

Lymph Vessels

Small lymphatics are similar in structure to capillaries.

Larger ones have valves and a wall composed mainly of connec-

tive tissue.

The lumen is devoid of blood but contains plasma and some white

blood cells, mainly lymphocytes.

It must be appreciated however that most lymphocytes migratearound the body via blood vessels and not via lymphatics.

Elastic Arteries

Slide M76 is of an aorta. The aorta and great vessels close to the

heart are elastic arteries.

The medial layer of these vessels is thick and consists of alternat-

ing layers of smooth muscle cells and perforated sheets of elastic

tissue.

Elastic recoil of these vessels helps to smooth out the surge in

blood coming from the heart and to drive blood around the coro-

nary circulation.

The walls of these vessels may have a blood supply (small ves-

sels) of their own, the so-called vasa vasorum.


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Muscular Arteries

Slide M17 contains good examples of arteries and veins. In mus-cular arteries the medial layer is well defined and composed

mainly of smooth muscle interspersed with fine strands of elastic

tissue.

The media is separated from the intima by a "wavy" internal

elastic lamina - a perforated sheet of elastic tissue.

The profile (outline) of muscular arteries is more regular and the

lumen smaller than that of the accompanying vein.

The adventitia consists of thick strands of collagen and elastic

fibres and is separated from the media by an external elastic

lamina.Wall of a large muscular artery with its lumen to the

right. Note the internal and external elastic laminae (blue)

either side of the medial layer. A small nerve lies mid-left.

Arterioles

Arterioles are found within the substance of organs and tissues.

They are arbitrarily defined as small diameter arterial vessels

with fewer than 3 layers of smooth muscle in their medial layer.

They often lack an internal elastic lamina. This allows them to

be completely closed off.

They are highly responsive to vasoactive stimuli allowing sig-

nificant regulation of peripheral resistance.

The adventitial layer is often poorly developed.

Capillaries

Capillaries are small diameter vessels formed from endothelial

cells together with thier underlying basement membrane.

Contractile pericytes often lie alongside but do not form a com-

plete layer.

Most capillaries are continuous (closed) so that solutes must be

transported across them.

However, capillaries of the gut, kidney glomerulus and endo-

crine glands are fenestrated (open). This allows solutes to pass

freely though macroscopic pores.

Sinusoids are thin walled, large diameter vessels with a fenes-

trated endothelium. These have a poorly developed basement

membrane but are supported by surrounding tissue. They com-

monly occur in the liver and spleen.


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Endothelial Cells

Endothelial cells are highly specialized cells that release a variety of vasoactive agents.

They actively transport molecules across their cytoplasm.

They influence the tone of the surrounding layers of muscle.

They modulate blood coagulation.

When activated they produce adhesion molecules that can influence lymphocyte and

neutrophil migration.

Venules

Small diameter post capillary vessels have an almost complete

layer of pericytes or 1 or 2 layers of smooth muscle (muscular

venules).

Larger venules have a surrounding adventitial layer.

Veins

Examine slide M17. The medial layer of a vein is thinner than that

of the accompanying artery.

The medial layer is composed of a mixture of smooth muscle and

collagen fibres. Its boundaries with the intima and adventitia are

often poorly defined.

The profile (outline) of veins is irregular and the lumen is larger

than that of the accompanying artery.

Veins

Many veins contain flap-like valves composed of collagen fibres

covered by a layer of endothelium.

Valves are absent from the largest veins such as the jugular veins

and the vena cavae.

Large veins have a thick wall with a distinct intima separated from

the media by an internal elastic lamina.

In large veins the medial layer is composed of longitudinally run-

ning bundles of smooth muscle surrounded by collagen fibres.


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Myocardium

Slide M95. This is composed of branching chains of discrete

cardiac myocytes joined end to end by complex junctions

called intercalated discs.

Like voluntary (skeletal muscle) these cells contain

myofibrils with repeat sarcomeres that give rise to visible

striations.

Unlike voluntary muscle,the cells remain separate and contain

a single centrally place nucleus.

Unlike voluntary muscle the fibres branch giving a

characteristic appearance.Cardiac muscle fibres showing striations and dark staining

intercalated discs.

Cardiac myocytes showing branched fibres and pale stainingintercalated discs. Note the centrally placed nuclei.

Myocytes

The myocytes of the atria are normally smaller than those ofthe ventricles due to differences in resistance and workload.

Atrial myocytes, especially those of the right atrium contain

small secretion granules close to their nucleus. These contain

atrial natruretic hormone that is released when the cells are

stretched excessively.

This hormone increases the excretion of water and sodium

and potassium ions by the kidney. It also inhibits renin secre-

tion causing a lowering of blood pressure.

Intercalated Discs.

Individual myocytes are joined together by intercalated discs.

These complex junctions contain desmosomes and adherent

junctions that bind the cells together.

They also contain gap junctions that allow the cells to become

electrically coupled.

Diagram of an intercalated disc. These join cardiac myocytes

together and include desmosomes, gap junctions and adherentjunctions. These provide physical linkage and electrical

coupling between the myocytes


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Sino-atrial and Atrio-ventricular nodes

These nodes are composed of very small, irregularly ar-ranged myocytes adapted for impusle conduction rather

than contraction. They are surrounded by numerous blood

vessels and nerve plexuses.

The SA node is located on the medial side of the superiorvena cava at its junction with the right atrium.

The AV node is located at the base of the inter-atrial sep-

tum, anterior to the opening of the cardiac sinus.

There are no clearly defined anatomical fibres linking the

SA and AV nodes although large conducting fibres

(Purkinje fibres) arise from the right and left bundle

branches.

Diagram of the conducting system of the heart. The SA node

is located in the wall of the right atrium, adjacent to the supe-rior vena cava. This is linked by poorly defined pathways to

the AV node from which fast impulses are relayed to the apex

of the heart via the right and left bundle branches.

Purkinje Fibres

These are large modified muscle fibres lying mostly just

beneath the endocardium in the interventricular septum.

The cells have large vacuoles but have few myofibrils.

Thus appear pale in H&E stained sections. They containstores of glycogen that allows them to be easily visual-

ised with the PAS staining procedure.

Myocardium showing large pale-staining Purkinje Fibres.

Photograph showing the atrial surface of the tricuspid valve. The pin marks the posterior cusp.

Valves

All 4 valves have a similar histological structure.

They consist of thick collagen fibres with occasional

strands of elastic tissue. Both surfaces are covered by a

layer of endothelial cells.

The free margins of the atrio-ventricular and mitral valves

are connected to papillary muscles by fibrous chordae

tendinae.

Damage frequently take the form of excessive collagen

(scar) deposition or calcification. This results in reduced

flexibility and can lead to stenosis or incompetence.


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Gut Tube

The gastro-intestinal tract has 4 layers

an innermost mucosa

an underlying submucosa

an external muscle coat (muscularis externa)

a serosa

The mucosa has 3 components, a lining epithelium, a layer of connec-tive tissue (lamina propria) and a ring of smooth muscle (muscularis

mucosa).

The submucosa is composed of loose connective tissue. Glands and lymphoid tissue are often contained within it. Many of the blood

vessels and nerves that supply the gut run it this layer.

The muscularis externa usually consists of 2 layers of smooth muscle whose peristaltic contractions help to propel foodstuffs through

the alimentary tract.

A serosal layer consisting of a simple squamous epithelium separates the gut tube from the peritoneal cavity.

Oesophagus

This is lined by a thick stratified squamous non-keratinized epi-

thelium.

The submucosa contains many sero-mucous glands that help to

lubricate the oesophagus.

At the distal end of the oesophagus the submucosal layer containsmany large, thin-walled veins. These may become the site of oe-

sophageal varicosities.

At the proximal end of the oesophagus the outermost layer of

muscle (muscularis externa) is composed mainly of skeletal mus-

cle but at the distal end it is composed of smooth muscle.

Stomach

The stomach can be divided into 3 regions

1. cardiac region

2. body

3. pyloric regionEach of these regions has a distinct histological appearance to its

mucosal layer.

The mucosa is thrown into macroscopic longitudinal folds known

as rugae.

The muscular coat (muscularis externa) which is 3 layers thick

has an extra oblique layer of fibres closest to the mucosa thathelps to churn up the food within the stomach.


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Gastric Mucosa

The stomach is lined by a simple columnar epithelium. The tallcolumnar cells produce a special acid-resistant mucin that pro-

tects the lining of the stomach.

The surface epithelium also has deep invaginations (gastric pits)

at the bottoms of which are the openings of the gastic glands.These glands occupy the lamina propria.

Several tall straight or branched glands open into each gastric

pit. The glands contain different types of secretory cell.

Low magnification picture of the stomach. The mucosa (top of

picture) is thrown into folds (rugae). Below this are closely

packed gastric glands.

Acid Producing Parietal (Oxyntic) Cells

These cells are found in the gastric glands in the body of the

stomach. They occur mostly in the upper parts of the glands,

close to the surface epithelium.

The cells are globular in shape and stain bright pink with H&E.

The surface of these cells is deeply invaginated to form intracel-

lular canaliculi and the cells contain many mitochondria.

The cells are rich in the enzyme carbonic anhydrase. They se-

crete hydrochloric acid. They are also believed to be the source

of intrinsic factor essential for the absorption of vitamin B12

High magnifaction picture of gastric glands from the body of the

stomach. This slide has been specially stained and the parietal

cells appear rounded and dark pink. The chief cells appear pale.

Regional Variations and Endocrine Cells

The glands of the cardiac region and pylorus do not normally have acid-producing nor chief cells. Instead their cells produce

mainly mucus (mucous neck cells).

The glands of the pylorus do, however, contain cells that produce gastrin and a bombesin-like peptide.

The glands of the stomach also contain cells that produce serotonin, somatostatin and vasoactive intestinal peptide (VIP). Thesepale staining cells are scattered throughout all of the glands of the stomach.


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hief (Zymogenic) Cells

The cells are pyramidal in shape and normally occupy the

deeper parts of the glands.

Part of their cytoplasm stains blue with H&E and the cells can

be seen to contain secretion granules.

They synthesise and secrete pepsinogen (pepsin) and lipases.

Like the acid producing cells, these enzyme-secreting cells arefound mostly in the glands in the body of the stomach and not

in the glands of the cardiac or pyloric regions.

Villi

The small intestine is thrown into macroscopic circular foldsknown as plicae circulares. These increase the surface area

available for absorption.

The surface area is further augmented by finger-like projec-

tions of the mucosa known as villi.

Villi are covered by a simple columnar epithelium with 2 main

cell types, enterocytes and goblet cells.

Cells of the villi are short lived and are replaced by cells from

tubular glands (crypts of Lieberkuhn) that lie between the villi.

Each villus contains a small arteriole, a thin walled venule and

a blind-ended lymphatic (lacteal).

The venules and lacteals carry absorbed nutrients to the liver.


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Duodenum

The duodenum has a relatively smooth lining with few plicae circulares.

The villi are broad leaf-like structures and the epithelium has relatively few goblet

cells.

The crypts contain stem cells for the replacement of villus cells and Paneth cells

that secrete lysozyme, an enzyme important for the breakdown of bacterial cellwalls.

The submucosa of the duodenum is filled by a mucous secreting Brunner's gland.

Its alkaline secretion helps to neutralise the chyme emerging from the stomach.

Jejunum

The jejunum has close packed plicae circulares and the villi are

long and narrow.

Many goblet cells are present within the epithelium.

The submucosa is normally broad and composed of loose con-

nective tissue.

Lymph nodules are present in the lamina propria but not nor-

mally in the submucosa.

Jejunal villi stained with H&E. The villi are narrower Finger-like) than those of the duodenum and there are more pale-

staining goblet cells in the epithelium.

Ileum

Here the plicae circularis are not as closely packed as in the

jejunum and the villi are not as tall.

Goblet cell numbers increase towards the distal end.

Large patches of lymphoid tissue (Peyer's patches) are pre-

sent.

These occupy not only the lamina propria but also penetrate

the muscularis mucosa to occupy the submucosa.

Low magnification view of the ileum. Note the large, round, blue-staining lymph nodules within the submucosa.


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Vermiform Appendix

The vermiform appendix is lined by a simple columnar epi-thelium with goblet cells but with no villi and only a few ru-

dimentary crypts.

The lamina propria and submucosa are filled with lymphoid

tissue.

The amount of lymphoid tissue present is large in children but

declines with age.

Low magnification picture of the appendix. Note that the surface epithelium is smooth and has no villi and that there are very few

crypts. The lamina propria and submucosa are filled with lymphoid material.

Colon and Rectum

There is very little macroscopic folding of the lining of the

large bowl.

Furthermore, there are no villi.

The epithelium consists of close packed intestinal crypts oc-

cupied mainly by goblet cells.

The muscularis external consists of a thickened inner circularlayer of smooth muscle and an outer layer that is drawn up

into 3 longitudinal bands known as taeniae coli.

Low magnification picture of the colon. The mucosa has numerous crypts but no villi. The crypts have large numbers of pale-

staining goblet cells.

Anus

The anal canal in lined mainly by a stratified squamouskeratinizing epithelium that arises abruptly from the simple

columnar epithelium of the rectum.

The submucosa contains fat and a prominent (internal) plexusof veins that may give rise to anal varicosities.

The smooth muscle of the mucularis externa is thickened to

form the internal anal sphincter and is surrounded by striated

muscle of the external anal sphincter.

Low magification view of the anal canal. The epithelium is a stratfied squamous one. Deep to this is pale staining fatty tissue and

muscle of the anal sphincter.


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Overall Organisation

The liver is divided into 4 macroscopic lobes (left, right,

caudate, quadrate). Within these, the cells are organised into

polygonal lobules separated from each other by delicate

connective tissue septa.

Each lobule consists of cords of tightly joined hepatocytes

that radiate from the centre.

Between the cords are thin walled sinusoidal blood vessels

that bathe the hepatocytes in mixture of arterial and venous

blood.

The sinusoids are fed with blood by the terminal branches of

the hepatic artery and hepatic portal vein that are distributed

around the periphery of each lobule.

Blood drains from each lobule via the central vein that coa-

lesces with others to form the hepatic veins.

Low magnification picture of a liver lobule showing cords of

hepatocytes (liver cells) converging on a central vein.

Picture of liver showing a portal triad. These are located

around the periphery of each lobule and contain three smallvessels, an arteriole (branch of hepatic artery), a venule, here

containing red blood cells (branch of the hepatic portal vein)

and a intrahepatic bile duct lined by a cuboidal epithelium

Biliary Canaliculi and Ducts

A network of bile canaliculi (singular. canaliculus) run be-

tween the hepatocytes, in the plane of the cords.

These are sealed channels between adjacent hepatocytes

created by the presence of occluding junctions on either

side.

The hepatocytes secrete bile (bilirbubin and bile acids) into

these channels (canaliculi).

The canaliculi coalesce and on leaving the lobule (at its pe-

riphery) drain into small intra-hepatic bile ducts that are

lined by a simple cuboidal epithelium.

These can be seen at the 'corners' of the lobules along with

terminal branches of the hepatic artery and hepatic portal

vein where the form a "portal triad".

Liver Sinusoids

The sinusoids of the liver are thin walled fenestrated vessels.

These vessels sit on a delicate meshwork of reticulin (collagen

III fibres) and are separated from the cords of hepatocytes by the

space of Disse, from which blood cells, but not plasma are nor-

mally excluded.

Included within the lining endothelium are specialised macro-

phages known as Kupffer cells.

One of their functions is the production of bilirubin that is takenup and excreted by the hepatocytes.


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Low magnification picture of the gall bladder. The lining is a

simple columnar epithelium that is thrown into folds (not villi).

Gall Bladder and Biliary Tree

The biliary tree is lined throughout by a cuboidal epithelium. Inthe smaller vessels it is a single layered (simple) epithelium but

towards the duodenal papilla it often becomes stratified.

The gall bladder is lined by a simple columnar epithelium cells

of which are all of the same type.

These cells have a poorly developed brush border (microvilli).

They are adapted for the re-absorption of water and salts and

thereby serve to concentrate the bile.

The epithelium, which sits on a submucosa of connective tis-

sue, is thrown into folds. Outside of this is a smooth muscle

coat.

This contracts, particularly in response to CCK and causes bile

to be expelled into the common bile duct. Concentrated and

compacted bile products (gall stones) may be present in the

lumen of the gall bladder.

Exocrine Pancreas

The exocrine pancreas constitutes over 90% of the whole gland

and is responsible for the synthesis and secretion of a wide

range of digestive enzymes.

These are discharged into the 2nd

part of the duodenum. Theendocrine pancreas that produces insulin, glucagon and other

hormones is examined elsewhere.

The exocrine gland is a wholly serous gland whose cells each

produce a mixture of enzymes that are packaged and released

together.

The fluid component of the secretion is mainly produced by the

duct cells, including the proximal cells of the duct that arise at

the centre of each acinus (centro-acinar cells).

Low magnification picture of the pancreas. The exocrine cells

are arranged into circular (spherical) clumps known as acini. Thenuclei and cell cytoplasm at the periphery of each acinus stains

dark blue but the secretion granules at the centre of each acinus

stain bright pink. Blue staining nuclei can also be seen at the

centres of the acini to the left of the picture. These are the nuclei

of the bicarbonate-producing centro-acinar cells. Note also the

small duct at the top of the picture.

Secretory Acinus

The pyramidal cells are arranged into spherical clumps each with

their apex pointing to the centre of the acinus.

The cells have a large centrally placed nucleus and the cytoplasm

is strongly polarised.

The base of the cells (periphery of the acinus) is occupied by

rough endoplasmic reticulum whereas the apex of the cells (centre

of acinus) contains large strongly staining zymogen granules.

Stimulation with CCK promotes the fusion of the granules with the

apical membrane and the release of their contents by exocytosis.

Stimulation with secretin, on the other hand, promotes the flow of

abundant fluid, released mainly by the ducts.


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Mouth

This is lined mostly by a stratified squamous non-keratinizing

epithelium but keratinized areas are common at sites of abrasion.

It sits on a connective tissue layer (submucosa) containing colla-

gen and elastic fibres. The deeper layers often contain skeletal

muscle fibres that help to change the shape of the oral cavity.

Many small blood vessels lie in the submucosa and fluid from

these helps to keep the epithelium moist.

There is an abrupt transition to stratified squamous keratinising

epithelium at the margin of the lips.Low magnification picture of the lip showing the junction be-

tween skin - stratified squamous keratinizing epithelium to theleft (outside of lip) and the lining of the mouth - stratified

squamous non-keratizing epithelium to the right. Deep to the

epithelia are connective tissue, blood vessels and bundles of

muscle fibres.

Tongue

The tongue is covered by a stratified squamous epithelium thatremains non-keratinized on its ventral surface but is heavily

keratinized on its dorsal surface.

Inside, it is composed of coarse bundles of skeletal muscle fibresthat run in many different directions. Some of these insert onto

the jaw but others attach to the fibrous connective tissue under-

lying the mucosa (intrinsic fibres).

Many mixed sero-mucous salivary glands are embedded withinthe substance of the tongue. So too are lymph nodules, particu-

larly in the posterior 1/3rd of the tongue (e.g. lingual tonsils).

Picture of the lower surface of the tongue showing a sgtratifiedsquamous epithelium. Below this is connective tissue and glan-

dular tissue from the sublingual salivary glands.

High magnification picture of part of the surface of the tongue

showing small pale-staining taste buds embedded in the surface

epithelium.

Papillae

The dorsum of the tongue is thrown into complex folds known as

papillae.

Tall, pointed filiform papillae are the most common and cover the

whole of the anterior 2/3rd of the tongue, producing a rasp-like

surface.

Less numerous mushroom shaped fungiform papillae are found at

the tip and sides of the tongue.

Pale-staining taste buds are embedded in the epithelium on their

undersides of these papillae. Here they are protected from the oral

cavity.

A V-shaped row of dome-shaped circumvallate papillae separatethe anterior 2/3rd and the posterior 1/3rd on the tongue's surface.

These too bear taste buds.


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Parotid Salivary Glands

These wholly serous glands lie anterior to the ear on the side ofthe face and enter the mouth via ducts adjacent to the 1st upper

molar teeth.

They are penetrated but not innervated by branches of the facial

nerve and have a large lymph node contained within their cap-

sule.

The secretory cells produce mainly digestive enzymes and flu-

ids.

The secretory cells are organised into spherical clumps called

acini (singular, acinus) that show all the characteristics of pro-

tein secreting cells.

The cells are pyramidal in shape with a spherical nucleus. The

basal cytoplasm is filled with rough endoplasmic reticulum and

prominent secretion granules lie at the apex of the cell.

The acini drain into ducts lined by a simple cuboidal epithelium

that may become stratified at its distal end.

Part of the parotid salivary gland. This is a wholly serous

gland. The secretory acini are all of the same kind. This picture

shows a number of circular clumps of secretory cells. The se-

cretion granules stored in the cells stain dark pink.

Sublingual Salivary Glands

These lie in the floor of the mouth, beneath the tongue and dis-charge by a series of separate ducts either side of the frenulum of

the tongue.

These branched tubular-acinar glands are almost wholly mucous

secreting. The swollen cells are filled with pale staining mucusgranules and the oval nucleus is squashed to the base of the cell.

Secretion from these glands drain into ducts lined by a simple or

stratified cuboidal epithelium.Part of sublingual salivary gland. This is a wholly mucous se-

creting gland and the picture shows pale staining secretory

cells and a pinker staining duct with a simple cuboidal epithe-

lium to the right. Note that the secretory cells have characteris-

tic flattened nuclei that are pushed to the base of the cells. Submandibular Glands

These well-defined globular glands lie subcutaneously, below

the mandible, either side of the tongue.

They discharge by a pair of ducts on to the frenulum of thetongue.

This branched tubulo-acinar gland is often infiltrated by fat cells.

The enzyme secreting serous cells form acini or lie at the closed

ends of the tubules where they form characteristic crescent-

shaped "demilunes".

The pale-staining mucous secreting cells line the tubular por-

tions of the glands and drain into a prominent system of ducts

lined by a simple or stratified cuboidal epithelium.

Like the parotid, this gland is a major source of epidermal

growth factor that promotes the growth of the epithelium of the

tract.

Picture of the submandibular salivary gland. This is a mixed, sero-

mucous gland. The lower half of the picture shows mostly serousacini but at the top of the picture some pale-staining tubular mu-

cous glands (middle and right) and a duct (left) can be seen.


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histology - important notes

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