Animal Physiology-Growth

Question 1

Why have farm animals been well studied?

Answer 1

Due to their economic importance

Question 2

Mature size is when growth stops. In which order do the different tissue types develop?

Answer 2

Nervous tissue, bone, muscle and then fat.

Question 3

Describe the 3 stages of the sigmoidal weight/growth curve.

Answer 3

First stage is accelerating, usually pre-natal. Then constant phase then decelerating phase as animal reaches maturity

Question 4

What is the difference between the skeletal muscle growth prenatally and postnatally?

Answer 4

Prenatal muscle growth involves hyperplastic growth whereas postnatal muscle growth involves extensive hypertrophy (increase in size) of existing muscle fibres. There are exceptions (rats) however in most mammals there is no hyperplastic growth after birth

Question 5

What is myogenesis?

Answer 5

Myogenesis is the development of muscle fibres during the embryonic stage (pre-natal). Muscle fibres arise from individual embryonic muscle cells called myoblasts, which are mono-nucleated precursor cells.As the foetus develops myoblasts proliferate and fuse to other myoblasts creating elongated multinucleated myotubes, which synthesise muscle protein forming a muscle fibre

Question 6

Postnatally, hypertrophic growth of the skeletal muscle occurs. What sort of cells activate this kind of growth?

Answer 6

Satellite cells (discovered by Mauro) increase DNA accretion through proliferation followed by differentiation and fusion with existing muscle fibre content and can be used to replace damaged myofibres. IGF activates them from being dormant underneath the cell membrane

Question 7

In terms pigs’ final body weight, what is the difference between those who were born with a low or heavy birth weight?

Answer 7

The LW pigs took a lot longer to reach the same final body weight as the HW pigs. It has been found that the smaller piglets displayed more hypertrophic growth throughout their lives compared to the heavier piglets as they try to compensate for having lower fibre numbers at birth

Question 8

How do adipocyte tissues develop (adipogenesis)?

Answer 8

Adipoblasts multiply and differentiate to form pre-adipocyte cells which contain only a few lipid droplets and lipogenic enzymes. They then further develop into an adipocyte cell which is our fat cell containing a single large lipid droplet (90% of cell cover)- beige adipose tissue is thought to develop from these WAT by cold activation

Question 9

Does adipocyte tissue have clean cut prenatal hyperplastic growth and post natal hypertrophic growth?

Answer 9

No, these overlap and occur in both pre natal development and post natal. Hypertrophy occurs as lipid is deposited in the cell increasing the cell diameter and volume, therefore the rate of hypertrophy depends on the relative rates of esterification (trigylceride levels) and lipolysis, hypertrophy being absent if the two rates are equal. In turtles, We can see hyperplastic growth from 1-4 months and hypertrophy growth 3-7 months.

Question 10

What is the difference between WAT adipogenesis and BAT adipogenesis?

Answer 10

BAT pre-cursor cells are not the same as WAT, they are actually more similar to those of the myoblasts. These then develop into brown pre-adipocytes and then into the mature adipocytes then activated by cold activation. BAT has smaller lip droplets and no mitochondria

Question 11

Why should a pig be slaughtered at around 100 kg?

Answer 11

This gives a leaner carcass as both lipid and protein deposition increases up until then, after protein deposition starts to decrease but lipid still increases

Question 12

What is growth hormone controlled by?

Answer 12

Growth hormone (GH) is released form the anterior pituitary gland. Its release is stimulated by growth hormone releasing hormone (GHRH) and decreased by somatostatin (also known as growth hormone-inhibiting hormone) which are produced in the hypothalamus

Question 13

What are the direct effects GH has on lean growth (increasing it)?

Answer 13

Decrease in lipid accretion – more nutrients, such as glucose are available for lean growth
Decreases the uptake of glucose for lipid synthesis through decreased synthesis of the glucose transporter GLUT4
Decreases the synthesis of lipogenic enzymes
Acetyl-Co A carboxylase
Synthase
Decreases the stimulatory effects of insulin on glucose uptake and utilisation by adipocytes

Basically stopping lipid/ fat formation to allow lean growth

Question 14

What are the indirect effects GH has on lean growth (increasing it)?

Answer 14

Stimulating growth is mediated via insulin-like growth factor-1 (IGF-1). High blood levels of IGF-1 decrease the secretion of GH by direct negative feedback action on the pituitary and also by stimulating SS production by the hypothalamus

• IGF-1 increases the proliferation of chondrocytes to increase bone growth
• Increase proliferation of satellite cells

This is highlighted in studies where cows had free access to food showed high levels of IGF-1 whereas those with restricted access had low levels of IGF-1 indicating they would have less lean growth

Question 15

What is leptin?

Answer 15

Leptin is a protein hormone that plays a role in the control of feed intake, energy balance and body composition.Leptin is secreted from fat tissue (adipocytes) and has the ability to regulate feed intake so as fat deposition increases so does leptin concentration. Fat animals have more leptin and by negative feedback are told to reduce their energy intake.

Question 16

What is BUN and what can it be used to indicate?

Answer 16

Blood urea nitrogen (BUN) can be used to indicate how efficiently animals utilise protein in the diet for lean growth. If amino acids are consumed in excess, the extra amino acids undergo transamination and deamination and the nitrogen is converted into urea.High BUN is also associated with a low efficiency for amino acid deposition of lean meat and a high nitrogen excretion. So basically a very lean animals have low BUN and fat animals have high BUN (it is important to know what the animal is eating as high BUN can indicate that the animal is starving and therefore breaking down its own protein)

Question 17

What is creatinine? What can it indicate?

Answer 17

Creatinine is a waste product formed after dephosphorylation of phosphocreatinine to creatine in muscle

Creatinine is therefore released from the muscle in amounts proportional to muscle mass

Question 18

What are NEFAs? What can they indicate?

Answer 18

Lipid is stored as triacylglycerol which is hydrolysed (lipolysis) to Non- Esterified Fatty Acids (NEFAs) and glycerol. Serum NEFA can therefore be used as an indicator for rate of lipolysis

Question 19

What test can be used to measure the levels of metabolites such as leptin in samples?

Answer 19

Radioimmunoassay (not as used anymore), ELISA tests and Colorimetric assays

Question 20

What is the link between gender/sex and growth?

Answer 20

Males generally have

• Greater appetite
• Grow more rapidly
• Use their feed more efficiently for growth
• Have increased lean-to-fat ratios in the body

Question 21

What is the link between genotype and growth?

Answer 21

Certain breeds will be naturally leaner than other. All breeds have a maximum protein deposition maximum where protein deposition plateaus

Question 22

What is the link between nutrients and growth?

Answer 22

Too little or too much food intake/nutrients will reduce energy used for growth (too much means that energy will be used to break down the excess nutrients instead of growth)

Question 23

What is the link between geographical location and growth?

Answer 23

Depends on food availability, climate and population density

Question 24

What is the link between disease and growth?

Answer 24

Disease (even when sub-clinical) prevents an animal from growing to its full genetic potential and the reduced feed intake and ADG observed results in considerable economic loss. During an immune response, nutrients are diverted away from growth and redistributed towards tissues and cells involved in the immune response #

side note- Cholecystokinin (CCK) is secreted by the duodenum and makes you feel not hungry. There is a peak in this during disease and so this lowers food intake

Question 25

Do elephants exhibit determinate (like most mammals) or indeterminate growth?

Answer 25

It was thought that they showed indeterminate growth due to males’ body weight increasing up until death however it was found their height did not and so they show determinate growth

Question 26

What is ‘Compensatory growth’?

Answer 26

A phenomenon in which an animal accelerates its growth above normal rates after a period of suppressed growth, usually caused by feed restriction (also called ‘catch-up’ growth)

Seen in cattle, sheep and rats (and humans)

Question 27

How did a study in 1940 by McMeekan show compensatory growth to be real?

Answer 27

The classic study carried out by McMeekan (1940) demonstrated accelerated growth rates in pigs following removal of feed restriction. In the experiment feed intake was restricted from birth for 16 weeks

At the end of the 16 week period, restricted pigs were approximately half the body weight of control pigs. When the restricted pigs were fed ad libitum, accelerated growth rates were observed

Question 28

How is compensatory growth exploited by farmers?

Answer 28

It is cheaper to restrict the animals feed first then allow access to food as they will compensate for their growth with overall less food. This does not always happen though so some animals may just end up smaller than if they had no restriction. In addition, the compensation is due to increase in fat and organ weight rather than lean muscle

Question 29

Compensatory growth can be as a result of restricting feed intake or nutrient intake. Why may this be beneficial environmentally and economically?

Answer 29

Due to the high cost of protein sources, protein restriction is often used in compensatory growth feeding strategies, with major emphasis on lysine, the first limiting amino acid

Protein restriction is also of environmental interest since it reduces nitrogen output

Question 30

What occurs during compensatory growth of animals that have undergone nutrient limit as oppose to reduced feed intake?

Answer 30

They utilise the nutrients more efficiently so have more lean growth as oppose to fat as seen in animals that had undergone reduced feed intake

Question 31

The mechanisms behind compensatory growth are not well understood, however which three models have been proposed?

Answer 31

The sizo-stat theory

The growth plate theory

The peripheral control theory

Question 32

What is the sizo-stat theory of compensatory growth?

Answer 32

Was proposed by Tanner (1963) and suggests that compensatory growth may be controlled by a mechanism within the central nervous system which gives an indication of an animal’s actual body size and the size it ought to be (age-appropriate set-point). The animal can then adjust growth rate accordingly

Question 33

Which hormone is thought to play an important role in the sizo-stat theory of compensatory growth?

Answer 33

IGF-1. It is thought that the difference in the levels of circulating IGF-1 and the number of receptors in the body indicates what the animal is and what it should be

Question 34

what is the growth plate theory of compensatory growth?

What experiment supported this?

Answer 34

This theory suggests that the mechanism responsible for compensatory growth resides within the growth plates (end of bones that elongate)

Baron et al. (1994) administered dexamethasome directly into the proximal tibial growth plate of six week old rabbits, causing a reduction in tibial growth. After infusion ended, growth rate increased above that of the control side – showing compensatory growth. As compensatory growth occurred in a single growth plate, it cannot be explained by the sizo-stat theory as a neuroendocrine mechanism would have affected all growth plates

Question 35

What is the peripheral control theory of compensatory growth?

What experiment supported this?

Answer 35

This theory suggests that compensatory growth is controlled by the tissues where cell numbers (DNA) determine the extent of growth. It implies that if restriction takes place during hypertrophy, cell numbers will not be reduced and thus during realimentation, hypertrophic growth can take place at a faster rate resulting in compensatory growth; However if restriction takes place during hyperplastic growth, the number of cells will be reduced affecting the extent of compensatory growth

Winick and Noble (1966) restricted the energy intake of rats for 21 days at birth, weaning and at 65 days of age followed by ad libitum feeding. Those restricted from birth didn’t show compensatory growth whereas those restricted when older did

Question 36

Can compensatory growth be of a single organ or does it have to be whole body?

Answer 36

Single organs can exhibit compensatory growth. This was shown in a study showing that a mouse’s lung showed this growth after removal of the other lung

Question 37

Carcass dissection is a common way of measuring body composition. What different elements need to be measured?

Answer 37

Water
Fat
Protein
Inorganic constituents 
(Sacrifice a representative sample of animals, feed other animals on diet to be tested, measuring intake, sacrifice these animals and analyse the carcass, subtract starting weight of fat, protein from finishing weight)

Question 38

How do we measure the water component of a carcass dissection?

Answer 38

Water is dehydrated and subtracted from the original amount. Dehydration can be achieved in one of two ways:

• Freeze drying
• Oven drying

Question 39

How do we measure the fat component of a carcass dissection?

Answer 39

Measuring fat involves the extraction of lipids from the carcass

The soxhlet procedure is often used
Crude fat content is determined by extracting the fat from the sample using a solvent, then determining the weight of the fat recovered

Question 40

How do we measure the protein component of a carcass dissection?

Answer 40

The crude protein content of a sample (ground up carcass) is calculated from the nitrogen content. This is determined by a technique known as the Kjeldahl method (as it was devised by Kjeldahl over 100 years ago). In this method the sample is digested with sulphuric acid converting nitrogen to ammonia. Ammonia is released by adding sodium hydroxide to the digest and collected as standard acid. The quantity collected is determined by titration or colorimetric techniques

Question 41

How do we measure the inorganic component of a carcass dissection?

Answer 41

Ash represents the inorganic content of the carcass
This includes calcium and phosphorus (components of the bone). Ash is measured by heating the sample in an ash furnace at very high temperatures (550ºC)

Question 42

What are the advantages and disadvantages of carcass dissections?

Answer 42

Advantages
The most accurate way to measure body composition

Disadvantages
 Loss of animal (can be costly)
 Can only sample at one time point
 Chemical analysis can be costly
 Temptation to sample too few animals

Question 43

What is AUTOFOM?

Answer 43

Another way of looking at body composition; Most ultrasound systems work the same

• A high frequency sound is emitted and travels some distance through a medium until it hits a reflector that returns an echo to its source
• Time differences between sound emissions and the returning echoes and the intensity of the echo can be used to obtain information on the properties of the reflector
• The transmitted sound pulse is reflected in the carcass by transmission between different biological tissues (fat, muscle, bone)

Question 44

What are the advantages and disadvantages of AUTOFOM?

Answer 44

Advantages

Accurate
Fully automatic
1500 carcasses an hour
Vast amounts of information

Disadvantages

Only calibrated for use in slaughter weight pigs

Question 45

Why is visual assessment (just looking at the animal) good and bad?

Answer 45

Is a basic and straight forward method, study appearance of an animal and take measurements in specific places e.g. horses- measure withers, you can also take photographs and compare body dimensions

Advantage
Not invasive to the animal

Disadvantage
No indication of internal body composition

Question 46

What can be used with visual assessment to make it more accurate?

Answer 46

Handling- giving a score based on certain traits e.g back bones and hips visible/ covered in fat etc

Question 47

Ultrasound back fat measurement is very similar to AUTOFOM and any other ultrasound. What are the advantages and disadvantages?

Answer 47

Advantages
Portable
Cheap
Easy to use

Disadvantages
Doesn’t take into account the distribution of fat

Question 48

CT scans are a next step up in measuring body composition. How does it work?

Answer 48

Scans are a two-dimensional reconstruction of a cross section of the body and several CT scans allow 3-dimensional images. Information from the absorption of a low dosage X-ray is used to produce a greyscale picture of density with the dense tissue appearing lighter and the less dense tissues appearing dark

Question 49

What are the advantages and disadvantages of CT scanning?

Answer 49

Advantages
Can obtain very accurate results from CT scanning

Disadvantages
However it can prove to be very expensive especially if the animal needs sedating (extra cost of vet per animal)
Movement licence

Question 50

Bioelectrical Impedance Analysis (BIA) is a fast, non-invasive, relatively inexpensive method and does not require any training or a trained operator. How does it work?

Answer 50

In BIA, an alternating current is passed through the body and the resistance to that current is measured in order to measure body composition. Impedance is the measure of opposition to a flow of current. The technique is based on the principle that fat free mass conducts electricity and fat resists it

Question 51

What are the advantages and disadvantages of BIA?

Answer 51

Advantages
Fast
Cheap
No need for a trained operator

Disadvantages
Accuracy not yet known in animals (however research is currently being carried out)