Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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0021 Awide range of human studies have been conducted,

using enriched stable isotopes of nutrient minerals,

including determination of the absorption and metab-

olism of iron, zinc, calcium, copper, selenium, and

molybdenum. An example of the type of information

that can be obtained is provided by a study of iron

absorption from different weaning foods, and the

effects of vitamin C on iron absorption (Figure 4).

These measurements, conducted using enriched stable

isotopes of

Fe and

Fe, demonstrated a doubling of

iron absorption when a drink containing 50 mg of

vitamin C was administered with the food.

0022 A recent development, especially useful for con-

ducting very long-term studies of the effect of diet

on metabolism, is to administer extremely low levels

of long-lived radioisotopes, e.g.,

Ca, that are then

measured by the ultrasensitive, highly specific tech-

nique of accelerator mass spectrometry. Because the

activity of the radioisotope is extremely low (typically

<10

6

of annual background radiation dose), it is

safe for human use. Accelerator mass spectrometry

is required to determine these radioisotopes because

the activity is too low to measure by counting decay

particles.

Vitamins

0023 Mass spectrometry has always had considerable,

though until recently unrealized, potential in the

study of vitamins and their metabolism. The newer

LC/MS techniques of APCI and ESI are particularly

useful, as many vitamins are too thermally labile

or unstable for conventional (EI and CI) mass-

spectrometric measurement. The current range of

mass-spectrometric techniques can be used to deter-

mine all vitamins in foods, biological tissues and

fluids. Mass spectrometry is not generally a cost-

effective technique for determining vitamins, as alter-

native, cheaper techniques are usually available.

However, mass spectrometry can be used as a ‘gold

standard’ method to validate alternative techniques

and is also the method of choice for measuring iso-

tope ratios in human studies of vitamin absorption

and metabolism.

0024Vitamins D

and D

and their major metabolites

have been studied extensively by GC/MS of volatile

derivatives; these studies include quantitative deter-

mination by isotope dilution. More recently, ESI and

particle-beam LC/MS and LC/MS/MS have been used

in the qualitative and quantitative measurement of

vitamin D and vitamin D analogs.

0025Recent trends include studies of the metabolism

of isotopically labeled folates and carotenoids by

GC/MS and LC/MS. There is evidence that

folates are associated with a reduced risk of chronic

disease (particularly cardiovascular disease) and with

prevention of neural-tube defects. Improved mass-

spectrometric methods determining these B-group

vitamins are desirable, and there is evidence that ESI

LC/MS fulfills this requirement, without the necessity

for preparing volatile derivatives (a necessity in

GC/MS studies). Figure 5 shows the negative ion ESI

spectrum of 5-methyltetrahydrofolic acid at low and

high cone voltage.

0026Carotenoid metabolism is also undergoing study by

LC/MS techniques because of epidemiological evi-

dence of their putative role in cancer prevention.

Although ESI LC/MS has shown promise in prelimin-

ary mass-spectrometric studies, APCI LC/MS now

appears to be the method of choice because of its

robustness and tolerance of a wider range of organic

solvents.

Trace Components of Foods

Food Additives

0027Food additives comprise a wide variety of compounds

that are generally monitored by techniques other than

mass spectrometry; however, occasional quantitative

applications do occur, exemplified by GC/MS deter-

mination of antioxidants in stored products.

Biologically Active Naturally Occurring

Nonnutrients

0028Biologically active (bioactive) nonnutrients in foods en-

compass natural substances that may have deleterious

or beneficial effects. This wide-ranging category of

food components has not been studied as much as

Milupa

garden

vegetables

Geometric mean %

incorporation of isotope dose into

red blood cells of 9 month old infants

Weetabix Wholemeal

bread

Baked

beans

−vit.C

+vit.C

∗

fig0004 Figure 4 Bioavailability (percentage incorporation into red

blood cells, measured by TIMS mass spectrometry) of iron in

selected weaning foods given to 9-month-old infants, with and

without a vitamin C drink. From Fairweather-Tait SJ, Fox T, Wharf

SG and Eagles J (1995) The bioavailability of iron in different

weaning foods and the enhancing effect of a fruit drink containing

ascorbic acid. Pediatric Research 37: 389 with permission.

3754 MASS SPECTROMETRY/Applications

anthropogenic compounds (e.g., pesticide or veterin-

ary drug residues). However, this situation is now

changing for a variety of reasons. These include the

amounts present in the diet (e.g., natural pesticides in

foods may be found at concentrations 10 000 times

higher than synthetic pesticide residues) and the po-

tential health benefits or toxicity of these substances.

Because such a wide range of bioactive natural sub-

stances are found in foods, it is only possible here to

discuss mass-spectrometric analysis of a small range

of these compounds.

0029 Glucosinolates (Figure 6) and their biologically

active breakdown products are found in many plant

foods. More than 100 different types of glucosinolate

have been isolated from plants. Although some glu-

cosinolates may have toxic (e.g., goitrogenic) proper-

ties, they are known to be potent inducers of Phase II

enzymes that protect against carcinogens and other

potentially toxic electrophiles.

0030 The structural variability is in the aglycone, R, that

may comprise linear or branched alkyl and alkenyl

side-chains, alcohols, methylthioalkyl, methylsulfinyl,

aralkyl or heterocyclic groups. GC/MS is the most

useful mass-spectrometric technique for analyzing

volatile glucosinolate breakdown products. Negative

ion ESI LC/MS is now the method of choice for

determining intact glucosinolates and positive ion

APCI LC/MS for analyzing the more thermally labile

breakdown products (e.g., sulforaphane). ESI LC/MS

and MS/MS are also useful for conducting metabolic

studies, for example, by detecting and determining

glutathione conjugates of isothiocyanate breakdown

products of glucosinolates. Figure 7 shows a positive

ion ESI mass spectrum of the glutathione conjugate of

allyl isothiocyanate, a metabolite of the glucosinolate

sinigrin, under low and high cone voltage and MS/MS

conditions. These data are useful in developing

qualitative and quantitative methods for analyzing

glucosinolate metabolites in vivo and in vitro.

0031Phenolics are a distinctive feature of all plant

tissues and are of interest because they can affect

OSO

−

fig0006Figure 6 General structure of the glucosinolates.

100

120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480

m/z

104.2

121.2

128.1

150.1

164.1

115.1

127.0

145.6 164.2

186.2 206.7

228.7

197.6 219.7 235.6 255.2 281.2 307.1 329.1 352.7 374.2 393.2 411.3 440.0 480.0

341.1293.2

189.1

206.6

221.1

233.1

255.2

269.2

284.1

297.1

311.1

340.1

354.1

371.1

396.1

414.1

440.0

480.0

458.1

422.0

329.1

Cone voltage = 50

Cone voltage = 20

fig0005 Figure 5 Negative ion ESI spectrum of 5-methyl tetrahydrofolic acid at low (upper trace) and high (lower trace) cone voltages.

Fragment ions (e.g., m/z 329 and 164) are more intense, and the doubly charged ion at 228.7 is considerably reduced at the higher cone

voltage (copyright Institute of Food Research, redrawn with permission).

MASS SPECTROMETRY/Applications 3755

the palatability, taste, nutritional value, and particu-

larly the health properties of foods. For example, the

flavonoids and isoflavonoids occur very widely in

plants, and there is great interest in their role as

protective factors in the diet. More recently, the

modern methods of LC/MS analysis APCI and elec-

trospray have been applied to the detection and quan-

tification of flavonoids and isoflavonoids. Although

early mass-spectrometric techniques for analyzing

these molecules focused on GC/MS methods (after

appropriate derivatization), the newer LC/MS

methods are now coming to the fore. For example,

isoflavones and their conjugates have been deter-

mined in soy foods (including soy beans and soy

flour) by positive and negative ion APCI LC/MS.

These methods also offer great potential for conduct-

ing metabolic studies.

0032 Mycotoxic secondary metabolites produced by

Aspergillus (aflatoxins) and Fusarium (tricothecenes,

etc.) food-spoilage molds comprise two of the most

notorious groups of natural toxicants. Many different

mass-spectrometric techniques, including EI, positive

and negative ion CI, GC/MS, LC/MS, supercritical

fluid MS, and MS/MS have been devised to monitor

the levels of mycotoxins in foods, body fluids, and

tissues. The most recent developments include ESI

LC/MS determination of aflatoxins down to low

picogram levels. Additional selectivity (at the cost of

loss of sensitivity) is provided by MS/MS selected

reaction monitoring.

Anthropogenic Toxicants

0033Many examples of mass-spectrometric methods fo

determining toxic or potentially deleterious

anthropogenic compounds in foods can be found in

the scientific literature. The range of compounds

analyzed is wide and includes dioxins, polyaromatic

hydrocarbons, pesticide and veterinary drug residues,

80 100 120 140

145.0

179.0

202.9

244.1

261.0

278.1

332.1

MS/MS product ion spectrum of m/z 407

collision energy = 12 eV

407.1

220.9

160 180 200 220 240 260 280 300 320 340 360 380 400 420

m/z

100

84.1

93.1

99.1

116.0

130.1

130.0

153.1

145.1

162.0

175.0

179.0

170.0

199.1

187.0

215.0

221.0

227.1

233.1

245.1

244.1

261.1

278.1

298.4

290.1

324.3

308.1

318.2

308.1

332.1

355.1

350.1

377.1

364.1

389.1

407.2

429.1

Cone voltage = 28

Cone voltage = 60

fig0007 Figure 7 Positive ion ESI spectrum at low cone voltage (upper trace), high cone voltage (middle trace), and the MS/MS product ion

spectrum of the [M þH]

ion (m/z 407) of the glutathione conjugate of allyl isothiocyanate, a metabolite of the breakdown products of

the glucosinolate sinigrin (copyright Institute of Food Research, redrawn with permission).

3756 MASS SPECTROMETRY/Applications

plasticizers from packaging materials, and environ-

mental contaminants. GC/MS and GC/MS/MS tech-

niques are used widely for determining these

compounds down to p.p.b. or even p.p.t. levels.

0034 Isotope dilution mass spectrometry is an accurate

and sensitive technique for determining toxic trace

elements in food matrices. Lead, cadmium, and

thallium have been analyzed down to very low levels

by TIMS and ICP-MS. The latter technique is particu-

larly useful for simultaneous measurement of a wide

range of elements. Because the toxicity of an element

can be highly dependent on its chemical form (for

example, organomercury compounds are more toxic

than inorganic mercury), ICP-MS is also useful in the

speciation of toxic minerals in foodstuffs by combin-

ation with HPLC or size-exclusion chromatography.

See also: Amino Acids: Determination; Carbohydrates:

Determination; Fatty Acids: Analysis; Mass

Spectrometry: Principles and Instrumentation; Peptides;

Protein: Determination and Characterization; Sensory

Evaluation: Texture; Aroma; Taste; Vitamins:

Determination

Further Reading

Gilbert J (ed.) (1987) Applications of Mass Spectrometry in

Food Science. Amsterdam: Elsevier.

Mellon FA and Sandstro

m B (1996) Stable Isotopes in

Human Nutrition: Inorganic Nutrient Metabolism.

London: Academic Press.

Mellon FA, Self R and Startin JR (2000) Mass Spectrometry

of Natural Substances in Food. London: Royal Society

of Chemistry.

Mayonnaise See Dressings and Mayonnaise: The Products and Their Manufacture; Chemistry of the

Products

MEAT

Contents

Sources

Structure

Slaughter

Preservation

Eating Quality

Sausages and Comminuted Products

Analysis

Nutritional Value

Hygiene

Extracts

Sources

A J Kempster, Formerly of Meat and Livestock

Commission, Milton Keynes, UK

This article is reproduced from Encyclopaedia of Food Science,

Meat Species

0001 Meat is defined as the flesh of animals used as food.

In practice this definition is restricted to a few dozen

of the 3000 mammalian species, but it is often

widened to include, as well as the flesh, organs such

as liver and kidney, brains, and other edible tissues. A

major part of the meat consumed in the world is

derived from pigs and cattle (Table 1). Mutton and

lamb and goat meat also make a significant contribu-

tion. Poultry meat is important and growing rapidly,

at the rate of 4% per annum. Refer to individual

sources.

0002In addition, substantial quantities of horse meat,

buffalo meat, and venison are consumed; and various

other mammalian species are eaten in different parts

of the world according to their availability or because

of local custom. Thus, for example, the seal and polar

bear are important in the diet of Eskimos, and a wide

MEAT/Sources 3757

range of species in that of tribes of Central Africa: the

kangaroo is eaten by aborigines and the whale is

eaten in Norway and Japan. (See Marine Foods:

Marine Mammals as Meat Sources.)

0003 The vast majority of meat derives from domesti-

cated animals. Domestication of cattle followed the

establishment of settled agriculture about 5000 bc.

Domesticated hump-backed cattle (Bos indicus

Zebu) existed in Mesopotamia by 4500 bc. Major

developments of breeds have taken place, with

important differences in characteristics between

parts of the world.

0004 Pigs were not domesticated before the permanent

settlements of Neolithic agriculture. They are des-

cendants of wild pigs, of which the European repre-

sentative is Sus scrofa and the eastern Asiatic

representative is S. vittatus. There is definite evidence

for their domesticity by about 2500 bc in Europe.

0005 Domestic sheep belong to the group Ovis aries and

appear to have originated in eastern Asia. By 3000 bc

several breeds of domestic sheep were well estab-

lished in Mesopotamia. The domestic goat derives

from the wild goat, Capra aegagrus, and was prob-

ably the first ruminant to be domesticated, some time

before 7000 bc in Iran.

0006 Several other bovine species and bison have been

domesticated and make a small contribution, to-

gether with camels, llamas and alpacas, rabbits, rein-

deer, and some rodents.

0007 In order to classify meat further, consideration has

to be given to the carcass from which it comes. This

has a major impact on the size and composition of

prepared cuts. The next sections will, therefore, be

concerned with carcass composition and evaluation.

Selection for Meat Production

0008 Over the centuries, the domesticated species have been

selected to produce carcasses with high meat content,

using visual appearance and tactile assessments.

Historically, carcass evaluation has been dominated

by considerations of size and shape. Carcass shape has

been considered very important, its influence stem-

ming from the esthetic appeal of certain types of

stock. For example, in the last century, the traditional

British beef breeds, Hereford, Aberdeen Angus, and

beef shorthorn, were all altered in shape to fit a human

idea of what a ‘good’ beef animal should look like. In

making this change to blocky, rectangular animals,

breed producers and meat traders have come to be-

lieve that these shapes are associated with more meat

and better-quality meat. More recently, selection has

focused on producing larger, leaner animals to meet

consumer demand. This has been particularly success-

ful with the pig in producing a dramatic increase in

leanness in the European and North American pig

populations.

0009Detailed compositional studies of carcass compos-

ition, involving tissue separation, are a relatively new

phenomenon. About 70 years ago, Hammond and

coworkers at Cambridge University began to examine

the influence of stage of growth and level of feeding

on proportions of lean (muscle), fat, and bone in the

carcass. Some 40 years ago, Butterfield in Australia

demonstrated that differences in shape between

breeds are not reflected in important differences in

the way total muscle weight (of a particular species) is

distributed between different joints of the carcass.

0010Today, the principal trend in the western world is to

reduce the proportion of fat in the carcass because of

concern about the effects of high fat consumption,

especially of saturated fat on health. However, there

is a conflict here with the need to maintain sufficient

fat in the meat to insure good eating quality, and a

minimum fatness level is often required.

Carcass Structure and Composition

0011The type of meat from a carcass is influenced by the

carcass from which it comes, in particular its (1)

weight, (2) proportions of the main tissues (muscle,

fat, and bone), (3) distribution of these tissues

through the carcass, (4) muscle thickness, (5) chem-

ical composition, and (6) meat quality.

Weight

0012The weight and size of a carcass have a major influ-

ence, not only on the quantity of the various tissues,

but also on the size of the muscles exposed on cutting

and of the individual joints prepared from it. This is

of importance particularly in relation to a retailer’s

ability to provide cuts of suitable size for customer

requirements. Over generations, the meat industries

in different countries and the different regions of the

same country have become accustomed to handle

tbl0001 Table 1 World meat production (1989)

Million

head

Million

tonnes

Change in weight

produced over

1980 (%)

Beef and veal 237 49.0 þ12

Buffalo meat 11 1.5 þ68

Mutton and lamb 443 6.5 þ13

Goat meat 204 2.4 þ31

Pig meat 882 67.2 þ30

Horse meat NA 0.5 4

Poultry meat NA 37.8 þ43

Reproduced from Food and Agriculture Organization of the United Nations

(1990) ProductionYearbook, vol. 43. Rome: FAO, with permission.

NA, information not available.

3758 MEAT/Sources

certain weight ranges of carcasses; abattoir practices

and cutting methods have been developed accord-

ingly. Most wholesalers state desired weight ranges

in buying schedules for their producers and apply

discounts to carcasses falling outside these ranges.

For example, in the UK pork has traditionally been

produced from pigs of relatively light carcass weights;

in the USA, lamb has been produced from relatively

heavy carcasses.

Proportions of the Tissues

0013 Among carcasses of similar weight, the percentage

formed by each tissue varies considerably depending

on breed type and growth rate. The proportion of

lean meat in the carcass is of major importance

since this is the principal determinant of meat yield

and commercial value. Certainly, among western

consumers, leanness is the criterion by which most

consumers judge quality and value for money. Taken

as a generalized ideal, the best carcasses should have

a maximum lean content, an optimum level of

fatness and minimum bone. The meat from the

carcass should follow the same ideal but with

most bone removed before sale. There are many dif-

ferent commercial techniques for assessing carcass

composition.

Distribution of Tissues

0014 The distribution of tissues through the carcass is po-

tentially important because there are wide differences

in the quality and retail value of meat from different

regions of the carcass, largely influenced by the

degree of tenderness and the type of cooking normally

required. For example, beef fillet steak (the psoas

muscle) is worth nearly three times as much as

stewing steak at present in the UK. However, muscle

weight distribution is a fairly constant characteristic

and there is little variation to exploit commercially.

On the other hand, the distribution of fat between

different depots in the carcass varies considerably.

The general distribution of fat is important because

it influences the overall efficiency of meat production:

fat in the body cavity or excess fat trimmed during

retail preparation is of little commercial value in

comparison with that sold as part of retail cuts. The

position of fat in the carcass is also important because

subcutaneous fat can be trimmed more easily than

intermuscular fat and is, therefore, preferable in car-

casses containing fat in excess of consumer require-

ments. Indeed, excess intermuscular fat cannot be

trimmed from some joints, especially from lamb car-

casses, without mutilating them. The evenness of fat

distribution is also important because wedges or

bulges of fat in some joints can lead to excessive

trimming or devaluation. Intramuscular fat (that

within the muscles) is important in insuring that the

meat is juicy and tender.

Muscle Thickness

0015While this characteristic appears to vary considerably

from carcass to carcass, much of this variation can be

attributed to weight and fat content variations.

Among carcasses of similar weight and fatness,

‘blockier’ carcasses will tend to have thicker muscles,

but this remaining variation has little impact on retail

realization values. Retailers tend to favor carcasses

with good meat thickness, however, as this is thought

to be associated with a higher yield of saleable meat

and to improve the appearance of joints. There may

also be advantages in terms of tenderness and reduced

weight losses in the preparation and cooking of cuts,

but the commercial significance of these advantages is

not clear.

Carcass Classification

0016The above characteristics influence the commercial

value of carcasses and national and international

carcass classification schemes have been developed

to describe carcasses. These have been used for

grading purposes (encouraging the production of

meat most suited to consumer requirements) and to

facilitate trade at a distance. Techniques range from

simple visual assessments of fat cover and shape to

much more sophisticated objective measurement

devices.

Carcass Anatomical Structure and

Different Cuts of Meat

0017The carcass is composed of hard and soft tissues.

Bones, and to some degree cartilage, form the hard

tissues, while muscle, fat, and connective tissue form

the soft tissues. Important differences exist between

the main meat-producing species in size and bone

structure which influence the types of cut produced.

0018The largest component of the carcass is muscle.

Excluding the head, there are over 100 different

muscles, which can be grouped in a variety of ways

and in different degrees of detail. The distribution and

structure of muscles have a major impact on the cuts

of meat produced. Approaches to commercial cutting

differ markedly between countries. In many coun-

tries, and especially the English-speaking world, par-

ticular joints are made by cutting across muscles, so

joints as sold will consist of a mixture of muscles

differing markedly in expected eating quality. In

other countries, notably France and Belgium,

commercial cutting is more anatomically based, the

muscles are trimmed of fat and often individually

MEAT/Sources 3759

separated, allowing cuts as sold to consist of more

homogeneous groupings of muscles.

0019 In commercial practice, muscle or lean is usually

sold with some adhering fat. This mixture of lean and

fat is referred to as ‘saleable meat’ or ‘trimmed

deboned cuts,’ although, depending on the species,

cut, and country, saleable meat may contain some

bone. For example, lamb cuts in the UK are com-

monly sold bone-in. In developed countries, there is

a general trend for meat to be sold deboned as pre-

pared retail cuts or in more processed forms.

0020 The ratio of lean to fat in the meat sold depends on

the fat content of the carcass from which the cuts are

derived, the nature of the cut, and the preferences of

retailers and their customers. When customers self-

select their purchases from retail display cabinets,

there is a worldwide tendency for those cuts with

high lean-to-fat ratios to be preferred, other things

being equal; but there is still much variation in the fat

content of the carcasses that butchers select and in the

amount of fat they trim in the preparation of retail

cuts. Consumer attitude studies show that many con-

sumers in developed countries do not like fat and that

the preference for lean meat and products is growing.

Fat requires extra labor for trimming and, in any case,

fat is extremely expensive to produce. There can be

little doubt that market forces will filter back to

producers and economics will favor the production

of leaner-meat animals.

0021 In less-developed countries where meat con-

sumption is low, all parts of the animal are often

considered to be of equal value (certainly as far as

boneless meat is concerned), and the degree of pro-

cessing is relatively low. There is also a greater toler-

ance for fat.

See also: Marine Foods: Marine Mammals as Meat

Sources

Further Reading

Butterfield RM (1963) Relative growth of the musculature

of the ox. In: Tribe DE (ed.) Carcase Composition and

Appraisal of Meat Animals. CSIRO: Melbourne.

Food and Agriculture Organization of the United

Nations (1990) Production Yearbook 1989, vol. 43.

Rome: FAO.

Kempster AJ (1990) Marketing procedures to change

carcase composition. In: Wood JD and Fisher AV (eds)

Reducing Fat in Meat Animals, pp. 437–458. London:

Elsevier Applied Science.

Kempster AJ, Cuthbertson A and Harrington G (1982)

Carcase Evaluation in Livestock Breeding, Production

and Marketing. Granada: London.

Mason IL (ed.) (1984) Evolution of Domesticated Animals.

Longman: London.

Sissons S and Grossman JD (1975) The Anatomy of the

Domestic Animals. London: WB Saunders.

Woodward J and Wheelock V (1990) Consumer attitudes to

fat in meat. In: Wood JD and Fisher AV (eds) Reducing

Fat in Meat Animals, pp. 66–100. London: Elsevier

Applied Science.

Structure

C R Calkins and K M Killinger, University of

Nebraska Lincoln, Lincoln, NE, USA

Introduction

0001Meat is the flesh of animals used for food. The pri-

mary components of meat include muscle, fat, and

connective tissue. Of these, muscle is the major food

component. The unique composition and structure of

muscle allow it to perform a variety of physiological

functions and give rise to the characteristic properties

of meat.

Composition

0002Moisture (water), protein, and lipid (fats) are the

primary constituents of muscle. Their proportions

can be quite variable, as are their functions. Add-

itional trace minerals, carbohydrates, and inorganic

compounds are located within the tissue, but contrib-

ute little to the gross structure.

0003Water makes up 68–80% of muscle weight; much

of this is located within the individual muscle cell.

There it functions as the liquid medium (sarcoplasm),

as a coolant, and as a transporter of nutrients and

waste products. Water-soluble compounds essential

for life are distributed throughout the sarcoplasm.

0004Proteins contribute 15–22% of muscle weight. The

numerous and varied types of protein provide critical

features to muscle. Proteins are frequently classified

on the basis of solubility. Sarcoplasmic proteins are

water-soluble. Myofibrillar proteins are salt-soluble

and contribute to the contractile machinery (located

within the myofibril). In contrast, stromal proteins

create a fibrous, structural element within the tissue

and are relatively insoluble. Another nomenclature

reflects the specific function of proteins. Under this

system, proteins are classified as contractile, regula-

tory, or cytoskeletal (Table 1).

0005Lipid content is the most variable of all meat con-

stituents, making up 0.5–20% (or more) of the weight.

Lipids are located within cell membranes, adipose

3760 MEAT/Structure

cells, or the sarcoplasm of muscle cells. Lipids are a

source of energy in living muscle and also serve a vital

role in cell membrane structure.

Muscle Structure

0006 Contraction must be translated from the cellular level

to gross muscle movement. This is accomplished

through the integration of skeletal muscle contraction

with a connective tissue harness that traverses the

entire muscle down to the individual muscle cells

(Figure 1). The connective tissue harness is primarily

composed of the protein collagen. The epimysium,

the outermost connective tissue layer, surrounds the

individual muscles themselves. Individual muscles

contain large groups of muscle cells (called bundles)

which are surrounded by the perimysium, the middle

connective tissue layer. Finally, the individual muscle

cells or myofibers are surrounded by the endomy-

sium, the innermost layer of connective tissue. Myo-

fibers, which are usually several centimeters in length,

rarely transverse the entire length of a muscle.

0007Muscle cells contain the organelles typically asso-

ciated with eukaryotic cells; however, some muscle

cell structures merit attention. The prefix ‘sarco’ is

derived from the Greek word for flesh and is often

used to denote certain muscle cell structures, includ-

ing the sarcolemma (cell membrane), sarcoplasm

tbl0001 Table 1 Location and function of muscle proteins

Protein Locationin sarcomere Proposed function

Major contractile proteins

Myosin Thick filaments Contraction

Actin Thin filaments Contraction

Regulatory proteins

Tropomyosin Thin filaments Regulates contraction

Troponin Thin filaments Regulates contraction

a-Actinin Z-line Anchors thin filaments from opposing sarcomeres in the Z-line

Cytoskeletal proteins

Titin Extends from M-line to Z-line Protects sarcomere from overstretch

C-protein Thick filaments Inhibits skeletal muscle myosin adenosine triphosphatase

(ATPase)

H-protein Thick filaments Inhibits skeletal muscle myosin ATPase

F-protein Thick filaments Binds to thick filaments, inhibited by C-protein

I-protein Thick filaments Inhibits Mg

2þ

-ATPase activity of actomyosin in the absence of

2þ

Nebulin Thin filaments Controls thin filament length

Tropomodulin Thin filaments Blocks depolymerization of actin filaments

b-Actinin/CapZ Thin filaments Anchors thin filaments in Z-line

M-protein M-line Forms the transverse portion of the M-line in fast-twitch fibers

Creatine kinase M-line Binds to M-protein

Myomesin M-line Anchors titin in the M-line and binds myosin

Skelemin M-line Attaches M-lines of adjacent myofibrils

Cypher Z-line Binds at Z-line

Telethonin/T-cap Z-line Binds titin in Z-line

Desmin Z-line periphery Transversely links myofibrils at Z-line

Filamin Z-line periphery Transversely links myofibrils at Z-line

Vimentin Z-line periphery Transversely links myofibrils at Z-line

Synemin Z-line periphery Links desmin to Z-line, modulates assembly/disassembly of

intermediate filaments

Dystrophin Near the sarcolemma Stabilizes sarcolemma and binds actin

Utrophin Neuromuscular junction Immobilizes acetylcholine receptor clusters

Dystrophin-associated proteins Sarcolemma Family of proteins involved in anchorage of dystrophin in the

sarcolemma

Vinculin Near the sarcolemma Attachment of actin to the sarcolemma

Talin Near the sarcolemma Attachment of actin to the sarcolemma

Paxillin Near the sarcolemma Adapter molecule for signal transduction

Tensin Near the sarcolemma Anchors actin and maintains tension

Integrin Sarcolemma Family of mechanoreceptors, anchors some cytoskeletal proteins

in the sarcolemma

Spectrin Near the sarcolemma Binds to actin and stabilizes the sarcolemma

Ankyrin Near the sarcolemma Attachment of spectrin to the sarcolemma

Some proteins listed have several functions, while some functions have not been fully characterized.

Adapted from Pearson AM and Young RB (1989) Meat and Muscle Biochemistry. San Diego, CA: Academic Press.

MEAT/Structure 3761

(cytoplasm), and sarcoplasmic reticulum (endoplas-

mic reticulum).

0008 The structure of the sarcolemma is unique in that

it has two sturdy outer layers, the reticular lamina

and the basal lamina, in addition to the lipid

bilayer (Figure 2). The sarcolemma also has invagin-

ations which reach deep into the muscle cell. These

invaginations are known as the transverse tubules or

t-tubules. In mammalian, striated skeletal muscle, the

t-tubules are located at the junction of the dark and

light bands, thus each sarcomere is associated with two

t-tubules. The terminal cisternae of the sarcoplasmic

reticulum is connected to the t-tubules via protein

bridges called ryanodine receptors (Figure 3). The

sarcoplasmic reticulum regulates the dynamic pool

of calcium within the muscle cell.

0009Skeletal muscles are unique because they are multi-

nucleated, with the nuclei typically located between

the sarcolemma and the underlying myofibrils. Nuclei

contain the genetic material that regulates protein

synthesis within the muscle. Numerous mitochondria

are also found in muscle cells and are located just

beneath the sarcolemma as well as between myo-

fibrils. Mitochondria, the powerhouse of the cell,

synthesize energy compounds necessary for cellular

functions. Muscle cells that perform more aerobic

metabolism tend to contain more mitochondria. The

sarcoplasm and the myofibrils make up most of the

muscle cell volume.

0010When viewed under a light microscope, skeletal

muscle exhibits a characteristic pattern of alternating

dark and light bands that are parallel to the long axis

of the myofiber. This pattern gives rise to the term

‘striated, skeletal muscle.’ The dark bands, called A-

bands, are dense, birefringent, and anisotropic, which

means they do not allow the passage of light. The

light bands, called I-bands, are less dense and are

isotropic (allow the passage of light). The I-bands

are bisected by a dark line perpendicular to the long

axis of the myofiber, which is called the Z-line.

0011Within a myofiber, the aggregation of contractile

proteins into long filaments creates myofibrils

(Figure 2), which contain sarcomeres, the functional

contractile unit of the muscle cell. Each sarcomere is

approximately 2–2.5 mm and spans from Z-line to

Z-line (Figure 4). The sarcomere is composed of a

series of overlapping protein filaments. The thick

myofilaments, located within the A-band, contain

myosin as the predominant protein. The thin myo-

filaments, predominantly located within the I-bands,

are composed mostly of actin. When a myofiber is at

rest, the thin and thick myofilaments overlap slightly

in the sarcomere. The H-zone spans the central por-

tion of the sarcomere where the thin and thick myo-

filaments do not overlap. Within the H-zone, the

pseudo-H-zone lies over the portion of the thick

myofilament which does not contain the globular

heads of myosin. Additional structural and regulatory

proteins are also found in the center of the A-band

in the centrally located M-line, which serves to main-

tain the three-dimensional alignment of the thick

myofilaments.

Muscle cell or muscle fiber

Muscle cell nuclei

Endomysium: connective tissue

around a muscle cell

Perimysium: connective tissue around

a bundle of muscle fibers

Epimysium: connective tissue around

a muscle

fig0001 Figure 1 Connective tissue organization of muscle. Courtesy of

Dr JE Novakofski, University of Illinois Urbana-Champaign.

Myofibril

Muscle fiber nuclei

Sarcolemma

Transverse tubules

Satellite cell

Basal lamina

Endomysial reticular fibers

Capillary endothelial cells

Endomysial collagen

and elastin fibers

Fibroblast

fig0002 Figure 2 Organization of a muscle cell. Courtesy of Dr JE

Novakofski, University of Illinois Urbana-Champaign.

3762 MEAT/Structure

0012 Myofibril cross-sections have different appear-

ances depending on the location within a sarcomere

(Figure 4). In the I-band, the thin myofilaments

(smaller in diameter) form off-set rows, with every

other row being aligned. In the H-zone, the thick

myofilaments (larger in diameter) form a similar

pattern of off-set rows, with every other row being

aligned. Within the area where the A- and I-bands

overlap, the thin myofilaments surround the thick

myofilaments. Each thick myofilament has access to

six thin myofilaments, and each thin myofilament is

accessible to three thick myofilaments.

Function

0013 The proteins associated with the M-line are believed

to provide support to the thick myofilaments and to

help maintain the structural integrity of the sarco-

mere. The proteins in the Z-line interior are believed

to help anchor the thin myofilaments, while the pro-

teins at the periphery of the Z-line are believed to play

a role in attaching adjacent myofibrils.

0014 The costameres are rib-like structures encircling

the myofiber. They overlie the I-bands on each side

of the Z-line (Figure 5). The costamere structure

is comprised of several proteins (like vinculin,

dystrophin, and spectrin), some of which are closely

associated with the sarcolemma and others which

are filamentous strands that appear to associate

with the Z-line. Some of the proteins found in costa-

meres also appear to connect the M-line to the sarco-

lemma, although this connection has not been fully

characterized. Costameres are integral in the attach-

ment of the sarcolemma to underlying myofibrils. In

addition, they help transmit the force of contraction

to the extracellular matrix and laterally along the

myofiber.

0015The thin myofilament is predominantly composed

of actin. Actin monomers, called globular actin or G-

actin, are almost spherical in shape and polymerize to

form two strands of actin that wrap around each

other in a helical fashion to form the bulk of the

thin myofilament. The strands of polymerized actin

are called filamentous actin or F-actin. Each G-actin

molecule contains a myosin-binding site that is crit-

ical for contraction to occur. A rod-shaped protein,

tropomyosin, winds around the strands of actin and

blocks the myosin-binding site on the actin molecules

in muscle at rest. Troponin, a second regulatory pro-

tein, is located at regular intervals along the thin

myofilament. Troponin is composed of 3 subunits.

Troponin T (TnT) binds to tropomyosin and to

One sarcomere

I Band A-band Triad Z-line

Myofilaments

Myofibrils Terminal

cisternae

Transverse

tubule

Fenestrated

collar

Longitudinal tubule

Mitochondria

Sarcolemma

fig0003 Figure 3 Diagram of t-tubules and sarcoplasmic reticulum in longitudinal view of skeletal muscle. From Pearson AM and Young RB

(1989) Meat and Muscle Biochemistry. San Diego, CA: Academic Press, with permission.

MEAT/Structure 3763