18 Adipose Tissue

18.1 Introduction

In the overall balance sheet for energy in agriculture, relatively large amounts of feed energy are used when animals deposit fat in their bodies, but much of this fat is removed and wasted after the animal is slaughtered. About 6% of the live weight of a steer may be removed as fat in the abattoir, and an equal amount may be trimmed from the dressed carcass by the butcher.

Adipose tissue, the cells of which are shown above, only serves its proper function when an animal uses the energy and insulation provided by its adipose tissue to survive a period of inadequate feed intake or cold weather. Adipose tissue in meat, however, is not altogether undesirable and wasteful. It is desirable in moderation to give a "finished" appearance to a carcass and, without at least some subcutaneous fat, a carcass is judged to be unattractive by traditional standards.

Fat deposited within muscles (intramuscular adipose tissue) appears as a delicate pattern of wavy lines in the meat - hence its common name, marbling fat. It is traditionally thought marbling fat contributes to the juiciness of cooked meat because it melts away from between bundles of myofibres to make the meat more tender and more succulent. However, even though I agree with this intuitively, it is difficult to find much scientific evidence in support of this traditional view, except in poultry where subcutaneous and intermuscular fat baste the meat as it is being roasted. Steaks without marbling may have desirable palatability after rapid, high temperature cooking, but only if they have been properly conditioned or aged (21 days). Unscientific as it may be, there is a very sensible reluctance on the part of many butchers to allow completely lean beef onto the market as top quality meat, unless there is some redeeming factor such as extreme youth (heavy baby beef) or excellent conditioning (21 days or more).

Much of the characteristic flavour associated with different types of meat originates from carbonyl compounds concentrated in the adipose tissue. But the flavour of meat also may be modified by the animals' diet. Not all the fat in the carcass is macroscopically visible (visible by eye without a microscope) because many myofibres, particularly those in postural muscles, contain large numbers of microscopic fat droplets. The storage of fat droplets within myofibres is related to the overall metabolism of the animal. For example, the deposition of lipid in the postural muscles of cows reaches a peak at about one week after calving when lipid stores in the liver also are at their highest.

The ancestors of present-day farm animals once were used to feed populations of people, many of whom were manual workers who expended large amounts of energy in tasks that we now perform by machine. Fat contains more stored energy than lean, and the high fat content of meat once supplied much of the energy that people expended in their daily work. Now this extra energy is undesirable, and a reduction of the fat content of meat is a major goal in the continued improvement of meat animals.

Most of the meat eaten in the 19th century was derived from older animals than are marketed now, and most of these animals had already given long service in the production of milk or wool, or had been used to pull ploughs or wagons. The presence of marbling fat would have greatly improved the palatability of the tough and strong tasting meat derived from these mature animals. Also, large families were more common a hundred years ago, and large joints of meat cut from large carcasses were well suited to domestic requirements.

To understand how meat may be made leaner, we need to look at the origin, metabolism and proliferation of adipose cells.

18.2 Adipose cells

A mature adipose cell or adipocyte may have a diameter of about 0.1 mm and is filled with triglyceride. Thus, its nucleus and cytoplasm are restricted to a thin layer under the cell membrane.
When isolated from their surroundings, adipose cells are rounded in shape but, when packed together in adipose depots, they are compressed with flattened sides.
Pockets of very small adipose cells sometimes appear between normal-sized cells and this may bias measurements of mean cell size, depending on how well the small cells are detected.
Mature adipose cells have very little cytoplasm, which is why the water content of fat is so low. Think about this for a minute. When an animal makes 20 grams of meat protein, it adds them to 80 grams of water to make meat. But when it makes 20 grams of triglyceride, it can only add 1 gram of water to make fat. So this is why fat is an economic disaster: triglyceride not only has a high energy content but, even worse, it carries no water for free!
In many locations in the body, large numbers of adipose cells are grouped together to form adipose depots.
Adipose cells are kept in place by a meshwork of fine reticular fibres (Type III collagen fibres).
Large adipose depots usually are subdivided into layers or lobules by partitions or septa (singular is septum) of fibrous connective tissue.
In the layered subcutaneous fat of a pork carcass, the septa may follow the body contours and may create a weak boundary layer echo in the ultrasonic estimation of fat depth.
Adipose depots are well supplied by blood capillaries, which are readily visible if the animal is not properly bled out at slaughter.

18.3 Origin of adipose cells

Speculation concerning the origin of adipose cells has continued since the end of the 19th century, by which time most of the feasible possibilities had already been proposed. In certain locations in the body of a foetus, where adipose tissue will develop later in life, mesenchyme cells congregate in lobules resembling glandular tissue (in an embryo, the mesenchyme is a diffuse tissue composed of cells that may differentiate to form the connective tissues of the body, as well as blood and lymphatic vessels).
The mesenchyme cells in gland-like lobules begin to accumulate small droplets of triglyceride. The droplets coalesce as they are crowded together and, finally, they form a single large mass in the center of each cell, thus a multilocular (many lipid droplets) structure has been replaced by a unilocular one (one large lipid droplet). The cells are called gland-like because they have many features typical of gland cells but are not producing glandular secretions.
In other locations where adipose tissue is about to develop, the adipose cell precursors (sometimes called preadipocytes) may resemble fibroblasts (the cells producing connective tissue fibres). In this case, cells are spindle shaped with an oval nucleus. We must call them fibroblast-like cells because we cannot prove they form connective tissue fibres like true fibroblasts.

It is difficult to prove the gland-like and fibroblast-like cells giving rise to adipose cells are distinct types of cells with a rigid pattern of development. However, the possibility cells are rigidly programmed to develop exclusively into adipose cells is supported by the behaviour of transplanted cells. Cells transplanted from precursor adipose lobules to future low-fat regions will store triglyceride and become adipose cells, even in an inappropriate location. Similarly, precursor adipose cells from certain sites will continue their programmed development into adipose cells, even if they are removed from the body and cultured in the laboratory. However, the trigger to differentiation appears to be hormonal because it is influenced by insulin, thyroid hormones and insulin-like growth factor (which is called IGF-1 and is a hot topic for research right now). The trigger or inducer acts upon a committing gene for the differentiation pathway.

One source of adipose cells may be the recruitment of fibroblasts. Fibroblasts can increase in number by mitosis. When recruited to become adipose cells, they give up the shape and activities of a fibroblast, and pass through a multilocular stage of triglyceride accumulation, finally to become adipose cells resembling those derived from precursor cells. Adipose cells sometimes originate from the endothelial cells of the blood vascular system and there is evidence of this in pigs. Although extra adipose cells may be recruited when animals become obese, the extra cells are retained when an animal returns to its normal level of fatness.

18.4 Adipose tissue distribution

Adipose depots range in size from small groups of adipose cells located between muscle fasciculi (bundles of myofibres), to the vast numbers of adipose cells located subcutaneously and viscerally. It is important to distinguish between anatomical sites and systemic locations. Specific muscles or regions of the carcass are anatomical sites. The following are systemic locations.

Intermuscular (between muscles).
Intramuscular (within muscles).
Visceral (around the guts).
Subcutaneous (under the skin).

For example, fat from a specified anatomical site such as the shoulder may be separated into different systemic depots (subcutaneous, intermuscular and intramuscular). The distinction between anatomical sites and systemic locations is important commercially. For example, bovine intramuscular marbling fat sometimes first becomes noticeable in rump and loin muscles, where it adds to their value. In the same systemic location, but at a different anatomical site, such as the brisket (cut of meat including the sternum), marbling fat confers no economic advantage and is wasteful. In cattle, the relative growth of subcutaneous fat is similar in both the forequarter and the hindquarter. However, the relative growth of intermuscular fat is higher in the forequarter than in the hindquarter. There is no guarantee systemic fat depots are homogeneous, even at a single anatomical site. For example, the backfat seen on pork carcasses is subdivided into three layers differing in their composition and pattern of growth.

The systemic deposition of fat in a carcass influences commercial indices of carcass composition such as the dressing percentage. Intramuscular fat present in meat at the time of cooking is mostly retained within the meat. The total of omental fat, mesenteric fat, and kidney fat may constitute about 30% of the total fat in a beef steer.

The omentum is a thin connective tissue sack filled with fat and located over the intestines. The omental fat is often called caul fat.
The mesenteries are thin connective tissue membranes supporting the intestines.
Kidney (renal) fat is a knob of hard fat around the kidneys - it may be sold as suet fat.

DRESSING % = (CARCASS WEIGHT / LIVE WEIGHT) %

Thus, most of the fat deposited around the viscera is removed with the viscera at slaughter, and this reduces the dressing percentage. Fat deposited between or within carcass muscles increases the dressing percentage.

In cattle, it was traditionally maintained thought fat deposition followed three systemic phases.

In the first phase, fat was thought to be deposited around the viscera and kidneys, and within the caul and mesenteries.
In the second phase, fat was thought to be deposited subcutaneously and intermuscularly.
The third phase was thought to be the deposition of marbling fat within muscles.

But, with modern cattle, no simple chronological separation of the three phases of fat deposition is detectable, and the relative amounts of fat in the main systemic locations may remain constant. On a high energy ration, cattle may deposit subcutaneous fat at a greater rate than they deposit intermuscular fat. However, this difference does not appear when animals are on a low energy ration. Breeds of cattle differ in the way they develop their systemic fat depots. For example, Herefords may produce more subcutaneous fat and less perirenal and pelvic fat than Angus, Friesian and Charolais crossbred cattle. However, when adipose growth at different anatomical sites is examined, relative to total fat, only minor differences generally are found between breeds.

In pigs, subcutaneous fat grows at the same rate as total body fat, intermuscular fat grows more slowly, and visceral fat grows faster. The separation of phases in adipose deposition is complicated. While the experimenter usually works in terms of calendar days and weeks, the experimental animal is following its own physiological calendar. The animal's physiological calendar is based on events that mark the progress through its life cycle. For example, skeletal and reproductive development follow an orderly sequence of events, and different breeds may progress through this sequence at different rates. Measured in calendar weeks and months, early-maturing breeds deposit noticeable amounts of marbling fat before late-maturing breeds. Thus, the introduction of late-maturing breeds with a large adult size may be used to delay fat deposition and to enhance lean growth in cattle populations.

Sex hormones may produce large and economically important differences in the overall fatness of beef, mutton and pork carcasses. Provided that comparisons are made at equal fatness, however,

bulls, rams and boars generally produce leaner carcasses than steers, wethers and barrows, respectively,
steers and wethers generally produce leaner carcasses than cows and ewes, respectively,
in pigs the situation is reversed, and gilts generally produce leaner carcasses than barrows, and
the deposition of fat tends to occur at a lighter weight in heifers than in steers, and at a lighter weight in steers than in bulls.

In general, triglycerides located subcutaneously where they may be relatively cool in the live animal have a low melting point. Conversely, perirenal or suet fat in the beef carcass is brittle at room temperature since it comes from a warm place in the body and has a high melting point.

18.5 Reducing the fat content of meat

The genetic and nutritional factors regulating the amount and distribution of adipose tissue can be described in terms of cellular hyperplasia (an increase in cells numbers) and cellular hypertrophy (an increase in cell size). An early stimulus to research on these topics was provided by the hope of adipose cell numbers being genetically regulated, while adipose cell size might be nutritionally regulated. Had this been the case in meat animals, selection for animals with low numbers of adipose cells might have provided a method for enhancing the production of lean meat. It was also suggested that a low plane of nutrition early in life might reduce the numbers of fat cells, so animals would be less likely to deposit fat in the final stages of growth to market weight.

Once adipose cells start to store triglyceride, they are no longer capable of cell division, and insulin causes increased adiposity by increasing the size rather than the number of adipose cells. It has also been proposed adipose cell size may be involved in the regulation of feed intake and triglyceride deposition.

A basic cellular problem prevents reducing the number of adipose cells by a low level of nutrition early in life. Populations of outwardly identical adipose cells in adult animals are formed from an initial population of specific precursor cells which is variably supplemented by the recruitment of fibroblast-like cells. Thus, the apparently limitless possibilities for recruitment of extra adipose cells may allow compensatory growth to offset any reduction in the initial population of specific precursor cells.

However, some possibilities do remain. In double-muscled cattle, adipose cells are greatly reduced in number (cellular hypoplasia). Since double-muscling is a genetic condition, there could be a simple genetic mechanism regulating adipose cell numbers in these animals. Also, adipose cell number and size can be modified by the selective breeding of mice for either high or low postweaning growth rate. Regulation of adipose cell numbers is affected by thyroid hormones but not by growth hormone.

Passive immunization of meat animals with antibodies developed against adipose cell membranes offers a novel approach to the age-old problem of fat reduction in meat animals. Perirenal fat and subcutaneous fat may be reduced in lambs by this method without a detrimental effect on the efficiency of carcass production, using three daily intraperitoneal injections. The technique also works on pigs and has the added advantage of increasing muscle growth.

The possibility of repartitioning the flow of energy into parts of the body has been investigated with beta-agonists such as Clenbuterol. The fatty acids released by beta-adrenergic stimulation of adipose tissue appear to be diverted towards the provision of energy for protein synthesis. Thus, muscle growth is enhanced and adipose tissue growth is reduced. The effect may be enhanced by a simultaneous reduction in protein degradation and an increase in growth horomone.

Another approach to reducing adipose accumulation is the suppression of fatty acid synthesis by inhibition of gene expression, but how this might affect adipose cell numbers and size remains to be seen.

18.6 Fat in pigs

Newborn pigs have very little fat relative to other mammals.
Between birth and 4 weeks of age, the percentage of fat increases from about 1% up towards 18% of empty live weight.
Adipose tissue lobules in regions that will later deposit large amounts of fat contain few cells, relative to calves, and they are separated by areas of loose mesenchyme or undifferentiated tissue.
Triglyceride deposition in adipose cells first becomes microscopically detectable at about the third month of gestation.
After this time and for the remainder of gestation, the numbers of cells in each lobule show no great increase, although the numbers of lobules and the total numbers of adipose cells do increase.
In pigs, fibroblast-like cells and gland-like cells are not the only source of adipose cells, since perirenal adipose cells may originate from endothelial cells of the vascular system. Fat is deposited very rapidly around the kidneys, and perirenal fat has high enzyme activity, large adipose cells and a low connective fibre content.
The heritability of adipose deposition is quite high in pigs, typically about 50% for back fat thickness, which explains the rapid progress being made towards the production of very lean pork.
Commercial selection for decreased backfat thickness produces a real reduction in total carcass fatness, not simply a redistribution of adipose tissue. However, the backfat of pigs is divided into two or three layers (depending on the position of measurement) and the different layers exhibit some degree of independence in their depth changes in response to selection. The deepest layer may be most responsive.
Lean breeds of pigs may have a greater ability than fat breeds to mobilize their fat.

18.7 Fat in cattle

An increase in adipose cell numbers is mainly responsible for the postnatal accumulation carcass fat in cattle.
As in pigs, visceral adipose tissue in cattle develops before intramuscular adipose tissue.
Adipose precursor cells make their greatest contribution to cell numbers viscerally, while the recruitment of fibroblast-like cells is more important intramuscularly.
The increase in cell numbers (hyperplasia) is nearly complete in subcutaneous and perirenal depots by approximately 8 months. But, within muscles, progressive cellular recruitment, may continue for much longer, which is how the marbling of beef originates.
Also, the balance between adipose cell hypertrophy and hyperplasia might be influenced by animal nutrition. Steers fed hay may produce less total adipose tissue with larger cells than steers fed a high-energy ration. The animals on the high-energy ration may produce more total adipose tissue, but with smaller cells. The difference created by this nutritional treatment is simply one of a static adipose cell population versus an actively growing population. Mean cell size might be kept low by the formation of new cells in the active population.

Marbling fat in the rib-eye muscle (longissimus dorsi) is important in beef grading.

The subjective assessment of marbling fat is very important in the commercial evaluation of beef. Our basic problem is our customers want to buy beef that looks very lean - even completely devoid of marbling. But when we feed them steaks in a restaurant, they always prefer the ones that were moderately or highly marbled. In other words we have a real problem, because our customers have a taste preference for beef that they don't like the look of!

18.8 Fat in lambs

Lambs have little prenatal development of subcutaneous fat, although it develops rapidly immediately after birth.

18.9 Fat in poultry

Avian adipose cells differ from those of farm mammals since they have only a limited capacity for lipogenesis - forming new triglyceride within the cell. Thus, they rely mainly on the capture of circulating lipids synthesized in the liver or released by digestion in the gut.
There are marked differences in adipose cell numbers and diameters between layer-type and broiler-type fowl, with the lean broiler-type birds having fewer and smaller cells.
An early restriction of nutrient intake, although it inhibits adipose cell hypertrophy, has only a slight effect on adipose cell hyperplasia. Thus, adequate cell numbers for fat deposition are present in birds once they are returned to an ad libitum diet.
Growth of the abdominal fat pad in chickens is from a combination of adipose cell hyperplasia and hypertrophy, up to about 12 to 14 weeks, then it continues mainly by hypertrophy.

18.10 Steatosis

After cutting thousands of normal carcasses, it is always a surprise to find one radically different in its fat distribution. The accumulation of adipose cells is a common end result of a number of pathological conditions affecting skeletal muscle. In most of these conditions there is usually some evidence of myofibre regeneration. In meat animals, however, it is not uncommon to find myofibres replaced by adipose cells without any evidence of myofibre regeneration and with no decrease in overall muscle volume. The most appropriate name for this condition is muscular steatosis. Muscular steatosis is most frequent in cattle and pigs, but may also occur in sheep. Sometimes the occurrence of muscular steatosis is indicated before slaughter by an abnormal gait, but usually the condition is not found until a carcass is butchered. The dividing line between excessive marbling fat and muscular steatosis is sometimes difficult to establish, and it is often only the restriction of muscular steatosis to a single muscle or muscle group in an otherwise poorly marbled carcass making it conspicuous. Muscular steatosis sometimes occurs in conditions which suggest it has been caused by muscle damage or denervation. Strenuous muscle exertion may cause extensive muscle damage, particularly in those muscles used when an animal rears up on its hind legs. Steatosis can sometimes be a major problem when it affects a high proportion of pigs in a herd and, to the best of my knowledge, there is no scientific explanation of the problem.

18.11 Conclusion

Fat is a lot more complicated than it first looks, once we start looking at its cells under the microscope. The big problem we have before us in the meat industry is how to reduce the levels of fat in meat without leaving it dry and tasteless when we eat it. Fat contributes a lot of taste to meat, particularly flavours allowing us to recognize one species from another. Without it, we may end up with just a bland, general meaty taste.

Further information

Structure and Development of Meat Animals and Poultry. Pages 107-131.