1 Definition and concepts of growth, differentiation, maturation and development

Dr. Deepali Verma

Contents of the unit

1. Introduction
2. Growth
3. Development and differentiation
4. Maturation
5. Principles of Growth
6. Pattern of human growth
7. Canalization: Catch-up and catch-down growth
8. Why study these phenomena?
9. Summary

Learning objectives

By the end of this unit, you will be able to

• Differentiate the term- growth, maturation and development.
• Understand the basic principles of human growth and development
• Review the distinct patterns of human growth
• Understand the concept of catch-down and catch-up growth.
• Comprehend the significance of growth studies

Introduction

We, as anthropologist have always been interested in the variation of human across socio-cultural and geographic dimensions. These variations often demonstrate the adaptive responses to the induced environment which involve the basic pattern of growth and development during different phases of lifespan- infancy, childhood, adolescence and adult. Thus, it is important to understand these basic biological processes leading to the disparity among human being. The module would explain the basic biological processes which transform single fertilized egg into a mature multi-cellular organism, the basic principles and pattern of growth, and the concept associated with deviation from the normal growth. The last section would review the significance of understanding these concepts in relation to the human population.

1. Growth

The British Medical dictionary (1961) defines growth as „the progressive developments of a living organisms or part of an organism from its earliest stage to maturity including the attendant increase in size.” It may be defined as quantitative increase in size of the body as whole or size attained by specific part of the body. The increase in size is a result of assimilation of nutrients into the protoplasm and includes both cell multiplication and expansion of cell cytoplasm. Thus the cellular process responsible for the growth can be summarized as (Malina et al. 2004):

• Hyperplasia: an increase in cell number It is a function of cell division which involves the replication of DNA and subsequent migration of the replicated chromosomes into functional and identical cells.
• Hypertrophy: an increase in cell size The increase in cell size involves an increase in functional units within the cell, particularly protein and substrates, as is especially evident in the muscular hypertrophy that occurs during growth.
• Accretion: an increase in intercellular substances The intercellular substances are both organic and inorganic, and they often function to bind the cells in complex networks, as collagen fibers provide matrix for the adipocytes of adipose tissue.

These processes occur during growth, but predominance of one or another process varies with age and all the tissue involved.

2. Development

Development refers to the progressional change, either quantitative or qualitative, that leads from an undifferentiated or immature state to a highly organized, specialized and mature state (Bogin, 2002). Growth and maturation occurs simultaneously with development but latter denotes a broader concept within two distinct contexts- biological and behavioural (Malina et al. 2004; Berk 2008):

In biological context, it refers to the processes of differentiation and specialization of pluripotent embryonic stem cell into different cell types, tissues, organs, and functional units. Differentiation mainly occurs early in prenatal life when tissues and organ systems are being formed. Onset of function in a particular tissue characterizes attainment of complete differentiation. The course is regulated by activation or repression of sets of gene interacting with hormones and nutrition.

In behavioural context, it relates to the development of competence in a variety of interrelated domains:

• i) Social, cognitive and emotional development which refers to the acquisition and refinement of behaviours depending on the basis of culture in which a child is born and reared. It includes understanding and expression of emotions, perception about others, self-awareness, abstract thinking, learning and reasoning.
• ii) Motor skills development relates to the acquisition of enhanced proficiency for a specific motor task. These varies from postural control (ability to hold head up, sit and stand independently) to the coordination of movements (hands working in coordination with the eyes)

3. Maturation

Maturation is often described as the process of becoming mature, or progression towards the mature biological state. Maturation level varies with the biological system considered-whether endocrine, reproductive, skeletal, digestive or immunological. These variations may be designated to differences in the timing and tempo of maturation. Timing refers to when a specific maturational event occurs eg. age at appearance of pubic hair in boys and girls or age at maximum growth during adolescent growth spurt while tempo refers to the rate at which

maturation progresses. Maturation of the nervous and endocrine systems is a major factor in sexual, skeletal and somatic maturation during late childhood and adolescence.

Individuals end up as adults with a fully ossified skeleton, but they reach this stage at different times and attain different adult heights. This provides fundamental distinction between growth and maturation. Growth focuses on size attained at a given point in time, whereas the latter focuses on progress (rate) in attaining adult size and maturity. However, both processes must be viewed as dynamic cellular processes targeting at achievement of mature adult state from conception state. The endpoint of maturation is the attainment “functionally mature individual” who is able to successfully procreate and raise offspring (Cameron 2012).

Growth, maturation and development occur in a time framework. They are measured at a single point of time or several points over time. The children of similar chronological age may differ by several years in their levels of biological maturity. For example, among girls aged 12 years, some are sexually mature while others will mature in next several years.

4. Principles of growth and development

4.1. Cephalocaudal and proximodistal development

Human development during pre- and postnatal period is not a random process. In fact, it proceeds in a systematic direction as a function of pre-programmed genetic mechanisms. Both, physical and motor development follows two distinct patterns- cephalocaudal and proximodistal.

The cephalocaudal refers to the development proceeding from head to toe. In an embryo, the head is approximately half the total body length. However by the time of birth, the proportion of the head is reduced to approximately one third of the total body length and becomes further reduced as the limbs and trunk grow increasingly faster. The infant shows voluntary control of the head and shoulders before control of lower limbs. Proximodistal development proceeds from the centre of the body to the periphery. For instance, infants acquire control over the muscle of the neck and trunk before coordination of the hands and fingers.

Figure1b: Cephalocaudal development from conception to adulthood 4. 2 Development Proceeds from General to Specific

Initially the infant‟s movements are very generalized, undirected and reflexive such as waving arms or move whole body. But later in response to physical and motor development, they become capable of making specific responses or creep toward an object. The infant grasp an object with the whole hand before using only the thumb and forefinger due to development in large muscle movements to more refined (smaller) muscle movements

4.3. Development is a continuous process

Development is continuous predictable process which proceeds from conception to the death. It includes the physical, motor and behavioural development in early life which leads the child to maturity. Changes continues even after maturity has been attained, progressing to senescence and death. It follows a sequential pattern where initial phase of growth and development lays foundation for the next phase. As a child develops, new skills acquired become the basis for further achievement and mastery of skills. For instance, there is a predictable sequence of developments that occur before walking. The infant hold its head up, learn to sit without support, stand by holding on to furniture and then eventually walk alone. Children learn to hold a pencil and crayon before they write or draw.

4.4 Development Proceeds at Different Rates

Although each child follows a general pattern and sequence of growth and development, the rates at which individual child reach various developmental stages will be different. There are interspersed periods of great intensity versus plateau with marked individual differences due to age and sex of children. From birth to early adulthood, both stature and weight shows a rapid gain in infancy and early childhood, steady gain during middle childhood, rapid gain during adolescent phase and slow increase until growth ceases (plateau) with the attainment of adult stature. Body weight however continues to increase into adult life. During early adolescence phase, girls are taller and heavier than boys due to earlier growth spurt. But as the boys attains adolescent growth spurt, they catch-up and eventually suppress girls in body size. Despite this general pattern each child is unique and varies in body dimensions. The rates of development across different domains also may not be uniform within an individual child. Some children will walk at ten months while others walk a few months older at eighteen months of age. A child‟s intellectual development may progress faster than his emotional or social development. Thus, there is no validity in comparing one child‟s progress with or against another child.

4.5 Development is an interrelated process-genetic, environmental and behavioural

The gene determines the potential of a child to achieve optimal development but is influenced by the socio-cultural and physical environment. Maturation, referring to the sequential development of cognitive and motor skills over time, interacts with the child‟s experiences (learning) in a given environment leading to overall development. A baby cannot feed itself until he or she can sit up and develop the ability to grasp the things between the fingers and thumb. The speech development of a child is affected if the child has difficulties in hearing clearly or if no one talks directly to him or her. Thus, the development of children to their potential requires support and guidance from others in their lives. A child who does not receive love and attention may fail to grow and develop. Abstract thinking and social behaviour during the process of maturation develops according to the peer and family environment they share.

5. Patterns of human growth

5.1 Growth curves

Measurements of an individual taken at specific intervals, when plotted against time provide a graph of progress or total amount of growth achieved. The pattern of growth observed via curve is function of the frequency of data acquisition. For instance, if a child is measured only at birth and at the age of 18years, then the graph plotted by joining up these data points would indicate growth to be a linear process. Thus, the more frequently data is collected (daily, weekly, monthly or yearly) the more we understand the actual pattern of growth. The graph in which the measurements are plotted against the time it was taken is called distance curve. It represents the amount of growth achieved at a given moment or at successive time periods. For example, height gained for a given age reflects how far the child has progressed towards adulthood. The growth is not a linear process as we do not gain the same amount of height each year (Cameron 2012). It is apparent that changes in rate of growth are associated with each stage of development. The curve representing the variation in rate of growth with time is known as velocity curve. These measurements are ordinarily plotted on a graph with chronological age on horizontal axis to derive growth curves. Sir D‟Arcy Wentworth Thompson, a famous British natural scientist and mathematical biologist in his book entitled “On growth and form (1942)”, wrote that the distance curve, “showed a continuous succession of varying magnitudes”, while the curve of the rate of change of height with time, “shows a succession of varying velocities”.

Figure 2: Growth in height of De Montbeillard‟s son from birth to 18 years, 1759-77 (Source: Tanner, 1962)

De Montbeillard measured the height of his son about every 6 months from his birth in 1759 until he was 18 years of age in 1777. Richard E. Scammon from Department of Anatomy and the Institute of Child Welfare at the University of Minnesota plotted the graph from these measurements and published his results in 1927 in the American Journal of Physical Anthropology under the title of “The first seriatim study of human growth”.

The non-human organisms share the basic sigmoid curve (s-shaped) of growth characterized by single peak. The initial period of maximum growth rate or acceleration is followed by period of deceleration in growth rate. The usual sigmoid curve is modified in case of human and is marked by two sigmoid curves, one in early life –the infant phase and the other in adolescent phase. They are separated by period of relatively slow growth rate during childhood.

Figure 3: Human growth curve

Thus, the growth curve has four distinct phases corresponding to relatively rapid growth in infancy, steady growth in childhood, rapid growth during adolescence and very slow growth as the individual approaches adulthood. A rapid growth during childhood known as mid- growth spurt is also observed but is not a universal phenomenon.

The above curve represents the general growth pattern for height, weight and some major organ systems (respiratory, digestive and urinary) of an individual, but different organs have their own pattern of growth. Richard Scammon (1930) proposed that the growth of different tissues and systems could be summarized in four patterns (Figure 4):

General curve

The general curve describes the growth of the body as whole and most of the system of body including skeleton, respiratory, digestive, urinary and circulatory (heart and blood vessels). The sigmoid or S-shaped curve represents four phases of growth (Malina et al. 2004):

• Rapid growth in infancy and early childhood
• steady but constant growth during middle childhood
• rapid growth during the adolescence
• slow and eventual cessation of growth after adolescence which continues into third decade of life

Neural curve

It characterizes the growth of brain, nervous system and associated structures such as eyes, upper face and parts of skull. These tissues experience rapid growth in early postnatal life so that, about 95% of the total increment in the size is attained by the age of 7 years. After 7 years the neural tissue show steady gain with slight growth spurt during adolescence (Malina et al. 2004).

Reproductive Curve

The reproductive curve illustrate the growth pattern of primary (such as ovaries, uterus etc in females and testes, seminal vesicles etc in male) and secondary sexual characteristics (such as breast in females, larynx in male, pubic hair and others). These tissues show slight growth in infancy followed by latent period during childhood, and rapid growth and maturation during adolescence (Malina et al. 2004)

Lymphoid curve

The lymphatic system which includes thymus, tonsils and spleen and lymph nodes acts as a circulatory system for tissue fluid and develop immunological capacities. The lymphoid curve demonstrates a rapid growth until the early adolescent years. Later, it declines perhaps as a result of the activities of sex hormones during puberty (Cameron 2012)

6. Canalization: Catch up and catch down growth

Human growth and development is a complex phenomenon regulated by both, genetic and environmental factors. At conception, an individual obtain a genetic blueprint which determine the potential for achieving adult size and shape. In an unconstraint or neutral environment, individual would exhibit a pattern of growth that is more or less parallel to the genetically predetermined trajectory. This phenomenon was described as „canalization‟ or „homeorrhesis‟ by British geneticist C.H. Waddington (1957). However, none of us has lived or been brought up in a completely unconstrained environment. During the prenatal and postnatal life we are exposed to an array of adverse environmental factors (such as diseases, malnutrition etc) which influence the growth velocity. It either slows down known as catch-down growth or in a more severe case would cease (Cameron 2012). The amount of deviation from the predetermined pathway depends on the frequency, duration and severity of exposure to the environmental factors. Slight exposure for short duration will cause minor change while considerable exposure for long duration in the velocity. The extent of influence is also determined by age and gender of the individual (Tanner 1978; Cameron 2012).

However, even after being pushed off the trajectories, there exists an ability to stabilize and return to a predetermined growth curve when conditions improve. During such recovery phase, initial growth velocity isabove that of normally expected for children of his age or even of his skeletal maturity and decline as the child recovers. This rapid increase in growth velocity following a short term period of starvation or illness was termed as catch-up growth by Prader et al. (1963). The “catch-up” phenomenon also describes the relatively rapid postnatal growth observed in infants born with low birth weight due to intrauterine growth restriction.

Catch-up growth is usually considered in terms of weight or height, but it is important to consider the type of tissue deposited in the body. For example, several prospective studies have observed the progressive deposition of more body fat during childhood and adulthood after a period of catch-up growth during infancy. This leads to differences in body composition relative to infants not exhibiting catch-up growth (Reilly et al. 2005, McCarthy et al. 2007).

Catch up growth may completely restore the scenario to normal or it may be insufficient to do so. If it is incomplete, the individual does not achieve the same adult height that would have been achieved when there was no growth impairment. This depends on the extent to which conditions are optimised during recovery.

6.1 Patterns of catch-up growth

There are distinguished temporal pattern of catch-up growth following pre or postnatal growth restriction as described by James Tanner (1978):

Type A pattern is characterized by transient increase in growth velocity until the deficit is eliminated, followed by normal velocity when normal growth curve is achieved. Such patterns are common in infancy and childhood when a child recovers from starvation or illness. Type B pattern shows growth at low velocity but for a longer duration, which may extend beyond the usual age with a delay in the onset of puberty. Often, catch up growth appears to be the blend of above patterns where increased growth velocity is coupled with delayed and prolonged period of growth during favourable environment.

6.2 Critical and sensitive period

In a human life span, there are phases of development when environmental exposures have long term biological and behavioural consequences than at other times. These consequences could eventually influence the health potential of an individual in a favourable or unfavourable direction. The period during which certain kinds of environmental stimuli are necessary for normal development and the absence of these stimuli could produce permanent irreversible consequences, are known as critical period. While the period when organisms are susceptible to certain kinds of stimuli in their environments, but the absence of those stimuli does not always produce irreversible consequences is known as sensitive period. The development may hinder during this period but it is possible to overcome the earlier deficits when experience favourable environment in later life. In other words, the concept of sensitive period recognizes the plasticity of human development.

7. Why study these phenomena?

7.1 Human variability

An array of environmental factors (such as nutrition, disease, physical activity and others) interacts with the genetic potential of an individual during the years of growth and maturation. This results into a wide range of inter- and intra population variation through differential rate of growth of particular parts of the body relative to others. Understanding these biological variation in terms of origin and distribution among different populations underlie the significance of conceptual knowledge of these phenomena.

7.2 Health status

The assessment of growth and maturity at periodic interval monitors the health status of a child and identifies deviation from normal growth if any. When compared with the healthy children of same age and sex, it indicates the overall health and nutritional circumstances in a community. This approach is often used in the context of nutritional status and general health surveys.

Barker‟s concept of foetal origins of adult disease specifies that the early development environment influences the risk factors associated with chronic disease, which manifest later in adulthood. For instance, a low birth weight baby accompanied by weight gain during childhood is at risk of developing hypertension, coronary heart disease or diabetes when become adult. Thus, detection of poor growth in early life would contribute significantly to the prevention of disease.

7.3 Progress

The measurement of children at specific intervals of time provides an estimate for the rate of growth and maturation. It estimate the level of biological maturity for the child‟s chronological age- whether early (advanced), late (delayed) or average (appropriate). A child who grows 5cm over a period of one year has a growth rate of 5cm/year. The measurements over time on the same individual trace the constancy in the growth indicators from childhood through adolescence to adulthood. Several studies have predicted the tendency of overweight adolescent to become overweight adult by measuring or observing similar individual over period of time. However several factors may influence the prediction which includes interval between measurements, age at first observation, timing and tempo of adolescent spurt, significant environmental change and measurement variability (Malina et al. 2004).

7.4 Physical activity

Regular participation in physical activity is presumably necessary for optimal growth and maturation of a child. However to evaluate and recognize the potential effect of physical activity, an understanding of normal process of growth and maturation is important. Also, the performance of an individual is influenced by the growth and maturity status. Among boys of the same age, those who are biologically mature tend to be taller, heavier and stronger than boys who have not matured. Muscular strength is related to body size and such provide an advantage to athletes in variety of sports such as gymnastics, divers etc.

7.5 Evolution

Over a period of time, human have evolved morphologically and physiologically to adapt to the surrounding environment. The evolution of morphological characters is the consequence of alternations in the inherited pattern of human growth and development. Thus, the study of growth is crucial in elucidating the mechanisms of evolution.

8. Summary

• An individual in its lifespan not only grow biologically but also behaviourally with proficiency in motor skills.
• In constraint environment, growth curve deviates from the genetic potential of an individual. The probability of resuming to the normal growth curve depends upon the intensity and duration of insult.
• Despite a general growth curve an individual follow, different body parts grow at different rate and timing.
• Thus, understanding the basic biological phenomena–growth, maturation and development, is important to evaluate the biological variability, health status and performance of an individual or a community.