Growth and Development

Growth is defined as a progressive increase in the size and weight of an animal due to the production and accretion of new biochemical units during a specific period of time. It is a fundamental process of all living organisms and is characterized by combination of increase in total cell size and number with protein deposition and differentiation.

Development is the process where the fertilized egg undergoes a series of diverse processes (cell division and differentiation) to form a fully functional new organism/animal. Due to the developmental process body shape and form are changed. It is regulated by the genetic makeup of the animal. Growth occurs in three distinct processes, viz., (1) Cell division (mitosis) where cell numbers increased, e.g., fertilized egg into daughter cells, formation of blood cells, hair follicles, and ectoderm cells. (2) Incorporation of substances taken from the environment leading to cell expansion or enlargement, e.g., proteins and minerals. (3) Cell enlargement and differentiation: increase in the size of a cell and changes in cell shape and form, e.g., skeletal tissue and nervous tissue.

When cell numbers increase (hyperplasia) their DNA con­tent also increases and most of the cells contain only one nucleus and each nucleus contains the same amount of DNA.

26.1.1 GrowthPatterninAnimals

Growth can be divided into two periods based on the occurrence,

1. Prenatal growth

2. Postnatal growth

26.1.2 Prenatal Growth

The period of growth between conception and birth is called prenatal growth. During this period, fertilized ovum fully transforms into a new organism with defined tissues, organs, and nervous system at birth.

The prenatal period is divided into three periods:

26.1.2.1 Pre-embryonic/Ovum Period

The period from fertilization to implantation is referred as pre-embryonic period or period of ovum. Figure 26.1 describes the various stages of pre-implantation embryonic development. Subsequently, a sequence of coordinated events takes place as follows:

• Fertilization

• Cleavage

• Formation of blastocyst

• Implantation

Fertilization is a process involving fusion of a sperm and ovum leading to the formation of zygote or embryo with a diploid number of chromosomes. After fertilization, the zygote undergoes differentiation and forms different kinds of cells, referred as cytogenesis or histogenesis.

Cleavage is the process by which the zygote undergoes repeated mitotic cell divisions and produces progressively smaller daughter cells known as blastomeres. Each blasto­mere will develop into specific tissues based on their location in the blastocyst. This event occurs when it passes down the oviduct and there will be an increase in total size. The zygote is divided into 16 or more blastomeres which then enter the uterus and form a compact ball of cells called as morula. Fluids accumulate between the cells and form a hollow, ball­shaped cellular structure called a blastocyst. Fluid-filled space is called the blastocoele or blastocystic cavity. Inner cells of the blastocyst constitute the inner cell mass that gives rise to the embryoblast which will become the proper embryo. Whereas, surrounding cells form an outer cell mass that gives trophoblast, which will become the fetal membranes. The trophoblast and part of the inner cell mass form the fetal part of the placenta, amnion, and chorion.

Fig. 26.1 Pre-implantation embryonic development.

Fertilized eggs undergo differentiation and form different type of cells and organs. Zygote becomes blastomere through repeated mitotic cell division then forming blastocyst; finally, embryo hatches and grows continuously in the uterus until implantation

Uterine glands are important for implantation and embryo survival because secretions from the uterine glands nourish the blastocyst.

Implantation: The embryo hatches out of the zona pellucida (zona hatching) and continues to grow, freely float­ing in the uterus until the implantation begins. Trophoblast cells start to pierce between the epithelial cells of the uterus.

26.1.2.2 Embryonic Period

This period is usually considered as the most important stage as major tissues, organs, and their system are developed during this phase. This period is characterized by rapid cell division, cell determination, and differentiation. Cell deter­mination is the process by which cells follow a specific pathway to form a particular tissue. Thus, it moves to a specific region of the embryo where they undergo further transformation and are differentiated into particular tissues or organs. Cell differentiation is defined as the transformation of unspecialized cells into a specific cell type. During this period embryo undergoes a series of successive changes for the ultimate formation of the fetus at birth.

After implantation, the embryo undergoes rapid elonga­tion and the trophoblast differentiates into cytotrophoblast (mitotically active single nucleus cells) and syncytiotrophoblast (a rapidly growing multinucleated mass). The growing embryo is surrounded by the amnion. The blastomeres of the blastula are rearranged and invaginated at one end resulting in the formation of a three­layered blastocyst called the gastrula. Three germ layers namely, ectoderm (outer), endoderm (inner), and mesoderm (middle) layers develop and give rise to the tissues and organs of the embryo.

The ectoderm give rise to the structures and organ such as skin, epithelium of the oral and nasal cavities, central nervous system, peripheral nervous system, mammary glands, pitui­tary gland, sweat glands, and teeth. Mesodermal layers give rise to muscle, bones, connective tissue, and the circulatory, urinary and genital systems. Endoderm becomes the gastro­intestinal tract, respiratory systems, and bladder.

26.1.2.3 Fetal Period

This period starts when cell proliferation and organ systems of the embryo complete its development in rudimentary state and ends at birth. In bovine fetal period starts from 9th week of pregnancy and ends at birth. This period is characterized by a period of fast growth and development, and maturation of all organs and systems. Initially, fetal length increases rapidly than its weight, in later stages length increases slowly but weight increases rapidly. During this period size of the fetus increased dramatically which is determined by various factors such as genetic factors, environmental factors, age of the mother, nutrition, and management. Muscle fiber number is fixed by the time of birth. During this period all the organs and systems are fully functional to support itself and for its survival after birth.

26.1.3 Postnatal Growth Period

The period of growth from birth to death is referred as postnatal growth. This period is categorized as young and

Fig. 26.2 Growth curve of farm animals. Sigmoid growth curve consists of rapid self-accelerating phase and slow retarding phase. Once animal reaches the point of inflection growth rate starts to slow down

adult period. Further young stage is classified as preweaning, weaning, and postweaning and adult period is divided into prepubertal, pubertal, and postpubertal period. Duration of this period greatly varies depending on the species.

Ex. mouse—2 years

Sheep/goat—15 years

Cattle—30 years

Elephants and humans—60 years

During this period, the growth of mainly muscle, bone, and fat tissues takes place. The rate of growth depends on the age of the animal, type of tissue, genetic makeup, and plane of nutrition.

26.1.4 Growth Curve

Initially, the animal grows slowly, then undergo a period of rapid growth followed by slow or stagnant growth when adult size is attained.

S-shape. Figure 26.2 describes the growth curve in farm animals.

The growth curve contains two phases:

26.1.4.1 Self-Accelerating Phase

During this phase, the growth rate accelerated to the maxi­mum until mature weight is reached. It is the steep slope of the curve. There are two forces acting on the growth rate of an organism which determine the shape of the growth curve.

26.1.4.1.1 Growth Accelerating Force

Growth accelerating force exists in all cells in the body and is due to the increase in the number of mitotic cells (hyperpla­sia), increase in size of the cells (hypertrophy), and inclusion of substances like proteins, vitamins, and minerals.

26.1.4.1.2 Growth Retarding or Decelerating Force Due to lack of space or food supply for tissue growth, the growth rate slowdown, and from this point onwards growth pattern shows stagnation or reduction. The growth retarding force is found in the surrounding cells’ environment. During the initial linear phase of growth, the two opposing forces are more or less in balance.

26.1.4.2 Self-Decelerating Phase/Retarding Phase

It is the decreasing slope of the curve. During the retarding phase, the rate of growth decreases and stops. It is the final phase of growth curve and starts when the animal attains maturity. There is an in-built mechanism that controls further growth by means of reducing feed intake. There is a regula­tion of dry matter intake and reduction in the body weight gain until the maintenance requirement is balanced due to the action of somatostatin.

A point at which the accelerating and decelerating phase meet, the growth rate starts to decrease. This point is known as the point of inflection. At this point, puberty occurs in all species and is also known as the point of pubertal inflection. This point specifies the time of maximal growth, pubertal age, starts increased mortality rate and point of reference to decide the age equivalents of various domestic animals. This point starts at 14 years in humans, 9-12 months in cattle, and

6- 7 months in sheep and goat.

26.1.4.3 Negative Growth Phase

In old age, all the parts of the body start to degenerate and this phase is known as negative growth phase.

Know More.................

• Growth decides the size and shape of the animals.

• Animal growth is a complex biological process determined by a variety of factors.

• Growth curve is sigmoid shape in all domestic animals.

• Marbling is the intramuscular fat deposition that completes when animals reach maturity.

26.2