- 1 -Applied Agricultural Science and TechnologyAnimal Growth and DevelopmentClass Notes KeyINTRODUCTIONGrowth and development have important implications for domestic animal production because theysignificantly influence the value of the animal being produced. A substantial proportion of agriculturalresearch focuses on how to make animal growth and development processes more efficient. Thisresearch involves several disciplines because animal growth and development are controlled by genesand hormones. Because growth and development are continuous and dynamic processes requiringintegration of numerous physiological functions, they are influenced by:! nutrition! efficiency of metabolism and respiration! hormonal regulation! immune responses! physiological status of the animal! diseases and parasites! maintenance of homeostasisAnimal growth and development can be separated into processes occurring before birth or hatching (prenatal)and those occurring after birth or hatching (post-natal). An animal originates from a single cell(ovum or egg), which is fertilized by the male spermatozoon (sperm). The resulting zygote thendevelops in an enclosed environment (either the uterus or an egg) for a certain time period known as thegestation or incubation period. In cattle, gestation is approximately 283 days; in sheep, approximately150 days; and in swine, about 112 days. The length of incubation of a chicken egg is 21 days.After they are born or hatched, young animals experience a period of rapid growth and developmentuntil they reach maturity. After an animal matures, some processes (for example, bone elongation) stopwhile others slow down (for example, muscle deposition). The maximum size of an animal isdetermined by its genetics, but nutrition and disease influence whether the animal reaches its geneticpotential for size. - 2 -PRE-NATAL GROWTH AND DEVELOPMENTPre-natal growth and development are broken down into two stages, embryogenesis and organogenesis.Embryogenesis extends from the union of female and male gametes to the emergence of the embryonicaxis and development of organ systems at the neurula stage. During embryogenesis, the zygote developsinto the morula, which becomes the blastula and then the gastrula.The zygote is a single cell that is repeatedly cleaved to form a multi-celled ball known as the morula.Cleavage is a process that involves mitotic division of the original cell into two cells, which then divideinto four cells and then into eight cells. Although the number of cells double at each stage of cleavage,individual cells do not grow or enlarge in size. So, the morula is the same size as the original zygote,even though it is made up of numerous cells, called blastomeres. Cleavage continues until the cells ofthe developing embryo are reduced to the size of cells in the adult animal. The cells of the morula arerearranged to form a hollow sphere filled with fluid. At this stage, the embryo is referred to as a blastulaand the fluid-filled space inside the sphere is called the blastocoel.The blastula undergoes a process known as gastrulation and becomes a gastrula. Up until this stage, celldivision has occurred but the blastomeres (cells) have not increased in size. The embryo is in thegastrula stage when cell growth occurs at the same time as cell division. The process of gastrulationinvolves extensive rearrangement of the blastomeres. The cells on one side of the blastula move inwardand form a two-layered embryo. The two layers formed are the ectoderm (outer layer) and theendoderm (inner layer). A third cell layer known as the mesoderm is formed between the ectoderm andthe endoderm. The cavity that forms within the gastrula is known as the primitive gut; it later developsinto the animal’s digestive system. All tissues and organs form from one of the three layers of cells inthe gastrula. After the germ layers are established, the cells rearrange and develop into tissues andorgans. During this phase, known as organogenesis, cells grow and differentiate.The process of organogenesis extends from the neurela stage to birth or hatching. The neurela stage isdistinguished by differentiation, which is when unspecialized embryonic cells change into specializedcells destined to form specific tissues or organs (refer to Table 1). - 3 -
Table 1. Organs and tissues that form from the three germ layers.
Ectoderm Mesoderm Endoderm
Nervous system including the
brain, spinal cord, and
nerves
Bones and muscle
Reproductive and excretory
systems
Lining of the digestive tract
Liver and pancreas
Lining of the mouth, nostrils,
and anus
Blood and blood vessels Lining of the trachea, bronchi,
and lungs
Epidermis of the skin, sweat
glands, hair, and nails
Inner layer (dermis) of the skin Thyroid, parathyroid, thymus,
and bladder
Differentiation starts at the upper surface of the gastrula. Cells of the ectoderm divide and form the
neural plate. Two raised edges or neural folds appear and gradually come together to form the neural
tube. A mass of cells called the neural crest is pinched off the top of the neural tube and then migrates
to other parts of the embryo to give rise to neural and other structures. Eventually, the front part of the
neural tube thickens and forms the brain; the remainder of the tube becomes the spinal cord.
In the first few weeks after conception, cells differentiate into organs and body structures. The embryo is
then referred to as a fetus and the body structures continue to grow and develop until birth. In horses, the
embryo is referred to as a fetus at about 40 days following conception, while in humans it takes
approximately 56 days to develop the fetus.
Body tissues and organs are formed in a specific sequence; the head is formed before the tail and the
spinal cord is formed before other organs. Some highly differentiated cells, such as brain and nerve
cells, cannot be replaced if they are destroyed after the original number is fixed during the fetal stage.
Thus, nerve cells that are seriously damaged thereafter are not replaced and usually remain permanently
damaged. Muscle cell numbers are also fixed during the fetal stage and can only increase in size, not in
number. Bone, and therefore skeletal size, can be increased to a degree by environmental conditions, but
not beyond the genetic potential of the animal.
POST-NATAL GROWTH
The period of post-natal growth extends from birth or hatching until death, and the length of this period
depends greatly on species. The average life span of a mouse is about 2 years, while humans and
elephants live to be well over 60 years of age. Sheep and cattle tend to live to be around 15 and 30 years
of age, respectively.
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Muscle, bone, and fat are the three main types of tissues that develop as an animal grows. The rate of
deposition depends on the age of the animal and the type of tissue being deposited.
Muscle fibers are formed from multiple cells called myoblasts. While the animal is still in the prenatal
stage, myoblasts fuse together to form a myotube, which develops into a muscle fiber. As a result, one
muscle fiber has multiple nuclei. Because no new fibers are formed after birth, postnatal growth of
muscle is characterized by increases in length and diameter. Muscle fibers are predominantly protein,
and therefore fiber size is determined by the rate of protein synthesis minus the rate of degradation. The
deoxyribonucleic acid (DNA) content of muscle cells also increases as the animal develops.
Bone tissue grows both before and after birth. A bone grows in length through the ossification or
hardening of the cartilage at each end. After the cartilage on the ends of a bone has completely hardened,
the bone stops growing. However, bones also have the capability of increasing in width and can repair
themselves if broken. Although individual bones reach a mature length and stop elongating, bone tissue
is constantly being deposited and resorbed.
Fat tissue is comprised of fat cells and connective tissue. Fat cells increase or decrease in size depending
on the nutritional status of the animal. Two types of fat tissue include white fat, which stores energy, and
brown fat, which maintains a constant body temperature. Fat is deposited in four different areas
throughout the body or carcass. Fat that is deposited in the abdominal cavity around the kidneys and
pelvic area is called intra-abdominal fat; it is usually the first fat deposited. Fat deposited just under the
skin is referred to as subcutaneous fat, or backfat, and is usually the largest amount of fat deposited. Fat
deposited between the muscles of animals is called intermuscular fat, while fat deposited within the
muscle is called intramuscular fat. The level of intramuscular fat deposited is referred to as the degree of
marbling and affects the quality and taste of meat. In the United States, an important factor effecting the
value of a beef carcass is its quality grade, which is determined by the degree of marbling in the carcass.
Therefore, manipulation of this process is very important in meat production systems. Intramuscular fat
is the last type of fat to be deposited, so animals with high degrees of marbling also have large amounts
of fat deposited in other areas of the carcass.
Muscle, bone, and fat are deposited differently throughout the animal’s life. Bone elongation stops after
the animal reaches a mature body size but bone tissue deposition and resorption continue until the
animal dies. The majority of muscle tissue develops between birth and maturity. Muscle growth then
slows down, but it is not physiologically halted as is bone growth. Fat deposition occurs mainly after the
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bulk of the muscle has been deposited. It is a common misconception that fat is only deposited in middle
aged or mature animals; a significant amount of fat is deposited in the young. It is only because protein
deposition declines markedly with age that fatteni
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