4/6/2009

1

Animal growth and development – 1st review

Animal Science 123Animal Growth & Development

R. D. SainzLecture 09

Growth curves Growth curves

300

400

500

wt,

kg

0

100

200

0 5 10 15 20 25 30 35

Age, months

Bod

y w

Allometric growthAllometric growth

60

80

100

, kg

Muscle

Fat

0

20

40

0 50 100 150 200 250 300

Carcass wt, kg

Wt,

Bone

4/6/2009

2

SummarySummary

• Growth cannot be simply an increase in size, but must instead be accompanied by development

• Development includes increasing complexity, d h i f d f tiand changes in form and function

• Allometric growth means that different body components grow at different rates and times

SizeSize

• Weight*

• Length

• Height

CompositionCompositionThe main chemical components of the body

are water, fat, protein and ash:

Lean animal5%

Fat animal3%

60%15%

20%

45%

37%

15%

Water

Fat

Protein

Ash

4/6/2009

3

Measuring body compositionMeasuring body composition

• Direct– Kill, grind, sub-sample and analyze

• Indirect methods (in vivo)– Non-invasive (varying degrees)– Non-destructive

External measurements:– Weight– Height

Measuring body compositionMeasuring body composition

– Body mass index = W/H2 (kg/m2)– Subcutaneous fat» Skinfold thickness» Ultrasound» CAT scans

• Example: assume lean and fat animals, both weighing 100 kg;– Inject 10 mL of D2O– Allow to equilibrate– Sample body fluid

• Body water (L) = Dose (mL) / Concentration (mL/L)

• Body water % = 100 x body water (L) / body wt (kg)y y ( ) y ( g)

• Then: use previously determined ratios of water:protein:ash, e.g. 12:4:1, so that

• Body protein % = body water % ÷ 3

• Body fat % = 100 – [body water % * (12+4+1)/12]

4/6/2009

4

More methodologiesMore methodologies• Cellular

– Cell culture– Primary vs cell lines

• Hormonal– Bioassays– Radioimmunoassays (RIA)– Enzyme-linked immunosorbent assays

(ELISA)– Ligand binding studies (receptors)

General principles• Endocrine: hormone is produced in a specific gland,

secreted into circulation, and exerts specific actions on remote target cells;SOURCE REMOTE FROM TARGET

• Paracrine: hormone is produced by some tissue cells, a ac e o o e s p oduced by so e ssue ce s,and diffuse through the extracellular space to exert specific actions on nearby target cells;SOURCE NEAR TARGET

• Autocrine: hormone is produced by cells and exerts its actions on the same cells;SOURCE = TARGET

From: LeRoith et al., 2001

4/6/2009

5

• Indirect effects of GH are mediated by IGF-I

• Most of the IGF-I in circulation is produced by the liver, in response to GH stimulation

Growth hormone & IGF-I

y , p• Responsiveness to GH is regulated by

nutrition (insulin) and also by sex steroids• Whole-body growth is sensitive to GH and

IGF-I, therefore to GH and responsiveness (receptor number, affinities) to GH

3% of BW

3% of BW

3% of BW

3% of BW

1.8% of BW

3% of BW

3% of BW

1% of BW

3% of BW3% of BW(10 days)

From: Breier, BH, Bass, JJ, Butler, JH and Gluckman, PD. 1986. The somatotrophic axis in young steers: influence of nutritional status on pulsatilerelease of growth hormone and circulating concentrations of insulin-like growth factor 1. J. Endocr. 111, 209-215.

Hormone action involves several steps1. hormone binds to specific receptors,2. binding alters the conformation of the receptor,3. post-receptor signaling events

a) membrane receptors – for non-permeable hormones; peptides, catecholamines • 2nd messengers (including cAMP, Ca2+, Tyr-PO4, etc.)

mediate actions• receptor cycling (membrane ↔ cytosol)

b) cytosolic receptors – for membrane-permeable hormones; lipid-soluble, steroids internalization of the hormone-receptor complex• transfer into nucleus, produce direct effects on gene

expression

4/6/2009

6

Insulin receptor: Tyrosine Insulin receptor: Tyrosine kinase activationkinase activation

From: Saltiel, 2003 NEJM 349:26

Insulin receptor: Tyrosine Insulin receptor: Tyrosine kinase activationkinase activation

From: Saltiel, 2003 NEJM 349:26

From: Herrington & Carter-Su, 2001. TIEM 12:252.

4/6/2009

7

From: Herrington & Carter-Su, 2001. TIEM 12:252.

From: Herrington & Carter-Su, 2001. TIEM 12:252.

From: LeRoith & Nissley, 2005. J. Clin. Invest. 115:233.

4/6/2009

8

Two 11 week old male mice. The larger mouse (44.2 g) to the left is a high-growth, C57BL/6J-hghg, homozygous for a deletion in mouse chromosome 10 that includes the hg gene. This deletion is now known to affect expression of the SOCs-2 gene. The smaller mouse (24.9 g) to the right is a normal C57BL/6J ofthe right is a normal C57BL/6J of the same age.

Functions of boneFunctions of bone• Linear growth – also sets maximum size

(morphostasis)• Framework/support• Force transmission/movement• Mineral depotp• Haemopoiesis

– production of blood cells (in red marrow), including erythrocytes, B- and T-lymphocyte precursors;

– also osteogenic cells

DefinitionsDefinitionsChondroblast: immature cartilage cell (but already

committed to differentiate)Chondrocyte: mature cartilage cell; secretes extracellular

matrix proteins & components that surround cell

Perichondrium: cell layers (relatively undifferentiated, fibroblast like) that surround cartilage (likefibroblast-like) that surround cartilage (like a sock)

Osteoblast: immature bone cell, committed to differentiate

Osteocyte: same as osteoblast, but now surrounded by mineralized matrix (∴less fluid transfer)

4/6/2009

9

DefinitionsDefinitions

Osteoid: bone matrix, same as cartilage but type I collagen instead of type II; framework that can be impregnated by mineral crystals

Periosteum: relatively undifferentiated (fibroblast-like) cell layers that surround bone

DefinitionsDefinitionsPhysis: growth plate; also known as the

epiphyseal growth plate; region in which longitudinal growth of long bones occurs

Epiphysis: ends of long bonesDiaphysis: portion of long bone in between proximal

and distal metaphysesM t h i f t f ti f til i tMetaphysis: zone of transformation of cartilage into

bone, between the physis and diaphysis; area of bone resorption, ossification and calcification

From: Frandson, R. D. 1986. Anatomy and Physiology of Farm Animals. Lea & Febiger, Philadephia.

4/6/2009

10

Bone FormationBone Formation

• All bones form from a cartilage model, which first becomes ossified, and finally calcified.

• Ossification: production of osteoid (bone p (matrix) by osteoblasts and osteocytes

• Calcification: mineralization of the osteoid by deposition of hydroxyapatite crystals

Two types of developmentTwo types of development

• Endochondral: most common, occurring in long bones; osteogenesis takes place within a cartilage model (anlage) that forms in the developing limb budforms in the developing limb bud

• Intramembranous: osteogenesis occurs within a membrane, no cartilage model; flat bones (e.g., skull, pelvis, scapula)

Big picture of developmentBig picture of development

1. During embryonic development, shortly after the appearance of the notochord, clusters of differentiated cells appear in pairs along the dorsal streak; these clumps are called somites;

2. Within each somite, cells begin differentiating into distinct types, destined to become myogenic (muscle-forming) or chondrogenic (bone-forming)

4/6/2009

11

From: Gerrard, DE & Grant, AL 2003. Principles of Animal Growth & Development. Kendall/Hunt Pub. Co., Dubuque, IA

From: Frandson, RD. 1986. Anatomy and Physiology of Farm Animals. Lea & Febiger, Philadelphia.

4/6/2009

12

Three zones are present in the growth Three zones are present in the growth plateplate

• The resting zone contains inactive chondroblasts that can be recruited to divide;

• The proliferating zone contains actively dividing chondroblasts, forming columns of cells by clonal expansion;

• The hypertrophic (maturing) zone contains live chondrocytes that secrete matrix which is being calcified.

• Interval between closure of 1st and last growth plates– ≈ 5-6 years (human)– 3 years (rat)– >3.5 years (sheep)

• Generation time for GP cell division2 3 d ( t) 20 d (h )– ≈ 2.3 d (rat) vs. 20 d (human)

• SO:– no general rules about proximal vs. distal;– no single event causes GP closure (local regulation +

systemic factors)

From: Ham, AW & Cormack, DH. 1987. Ham’s Histology. Lippincott, Philadelphia.

4/6/2009

13

Radial growth and remodelingRadial growth and remodeling

1. As the GP grows, the shaft elongates due to growth of cartilage and its replacement by bone;

2. The epiphysis enlarges (in all directions p p y g (outwardly) through growth of cartilage and its replacement by bone;

3. The shaft is remodeled being resorbed (by osteoclasts) and added to (by osteocytes);

50

75

100

ngth

, % o

f birt

h

Length

Bone is an early-maturing tissue, and is relatively resistant to nutritional or other

0

25

0 1 2 3Trimester

Wt o

r len

Weight restriction, therefore, body length (crown-rump) is often used as an indicator of fetal age.

Effects of tensionEffects of tension

1. The periosteum is attached to the bone (firmly) at the epiphyses and (loosely) at the diaphysis;

2. As the bone elongates, the periosteum is stretched, creating tension on the periosteum;

4/6/2009

14

Effects of tensionEffects of tension3. The periosteum responds to the increased

tension by ↑ing its rate of cell proliferation;

4. This proceeds until it overshoots bone length, decreasing the tension on the bone;

5 Th th l t d t th d d5. The growth plate responds to the reducedtension by ↑ing its rate of proliferation and ossification, which elongates the bone and starts the entire cycle over again.

• There is an optimal tension for bone growth and maintenance; this is related to the presence of an electrical current (e.g., fracture repair, electric blankets, high-voltage power lines)

Effects of tensionEffects of tension

• If periosteal attachments are severed, growth plate proliferation is very rapid;

• Similarly, if one GP is damaged or closes prematurely, the other GP may compensate by more rapid proliferation.

• First major regulator identified: growth hormone (1920’s-30’s)

• Hypophysectomy (hypox) – removal of anterior pituitary

PostPost--natal growthnatal growth

– No further bone elongation

– Narrower GPs – no proliferative & hypertrophic zones

• GH replacement– GP widens, growth is near-normal

4/6/2009

15

Sex steroidsSex steroids

• Mainly estrogen and testosterone

• Can stimulate bone growth (e.g., puberty) or stop itstop it

• Lack of estrogen is involved in the post-menopausal onset of osteoporosis

Estrogen Estrogen • ERα and ERβ both present in GP• Inhibit cartilage growth and enhance maturation

of the GP;• Low levels stimulate osteoblasts indirectly (via ↑

IGF-I)

• High levels (e.g., at puberty) inhibit growth:↓ cartilage growth

↑maturation of the GP

• Inhibit bone resorption by osteoclasts

Growth factor Osteoblast OsteoclastDNA Collagen resorption

Insulin-like growth factor-I (IGF-I) + + -IGF-II + +Prostaglandins + + +Epidermal growth factor (EGF) + - +Transforming growth factor-α (TGFα) +TGF-β1 + - +Platelet-derived growth factor (PDGF) + + +Fibroblast growth factor (FGF α, β) ++ -Interleukin-1 (IL-1 α, β) + + +Tumor necrosis factor (TNF) -Interferon-γ - - -Bone morphogenetic protein (BMP) + +

4/6/2009

16

Regulation of calcium and phosphorusRegulation of calcium and phosphorus

• All tissues/cells require precise concentrations of Ca2+ for proper function

• Circulating Ca2+ exchanges with tissue pools and the storage pool in bone

• 99% of body calcium (ca. 1 kg) is in the mineralized bone

From: Murray, RK, Mayes, PA, Granner, DK, and Rodwell, VW. 1990. Harper’s Biochemistry. Appleton & Lange, San Mateo, CA.

Parathyroid hormone (PTH)Parathyroid hormone (PTH)• Principal role = maintenance of plasma calcium

concentration• Released by the parathyroid gland in response to low

plasma [Ca2+]; undergoes further processing in the liver• Raises plasma Ca2+ by stimulation of:

– bone resorption by osteoclastsy– renal tubule reabsorption of calcium– 1-25-(OH)2D3 production → increased intestinal Ca2+ absorption,

bone resorption

• Lowers plasma P by stimulation of renal excretion, thus preventing harmful CaPO4 precipitates in tissues

• PTH may be anabolic or catabolic, depending upon its interactions with other hormones and health/disease state

4/6/2009

17

Vitamin D (calcitriol)Vitamin D (calcitriol)• Nutrient and hormone• Maintains normal plasma calcium

↑ resorption of bone by osteoclasts↑ intestinal absorption of Ca and P↑ l t ti f PO↑ renal retention of PO4

• Direct effects on osteoblasts↑collagen synthesis↓ osteocalcin synthesis↑ IGF-I synthesis? osteoblast proliferation

CalcitoninCalcitonin

• Produced in the C cells of the thyroid gland

• Released in response to elevated plasma [Ca2+]

• Increases renal Ca excretionIncreases renal Ca excretion

• Decreases bone resorption (direct effects on osteoclasts)