27 Significance of age in growth studies

Dr. Meenal Dhall

CONTENT-

Introduction

1. Stages of human life cycle-

1.1. Changes in various stages of human life cycle

2. Growth Curve

2.1. Types of growth curves

2.1.1.    Genital Growth Curve

2.1.2.   General Growth Curve

2.1.3.   Neural Growth Curve

2.1.4.   Lymphoid Growth Curve

3. Significance of Age in growth studies

3.1. The Skin

3.2. The Skeletal System

3.3. The Muscular System

3.4. The Neurosensory System

3.5. The Gastrointestinal Tract

3.6. The Cardiopulmonary System

3.7. The Cerebrovascular System

3.8. The Urinary System

3.9. The Endocrine System

3.10. The Genital System

3.11. The Immune System

4. Methods for estimation of an individual’s age

4.1. Chronological Age

4.2. Skeletal age or Bone age

4.3. Secondary Sexual character

4.4. Dental age

4.5. Decimal Age

4.6. Shape or Morphological Age

Summary

Learning Objectives-

• Stages of human life.
• Types of growth curves and its significance.
• Significance of age in growth studies.
• Methods for analysing the age.

Introduction-

All organisms grown with time and the physiological and biological functions also changes to a great deal with time. In humans the individual passes through different stages and in every stage, the body undergoes various types of modification, development and even degradation after a certain period of time. Ageing is a term which is used to determine the process of becoming older or more mature. Maturation means being fully developed in terms of physical appearance, behaviour or thinking. The process of maturation is completed by various developmental processes. These changes lead to changes in body. Age is a very important factor to study such changes and how the changes differ from one population to another due to various intrinsic and extrinsic factors.

1. Stages of human life cycle-

The various developmental stages in the life of human life cycle are – Prenatal life (first trimester, second trimester and third trimester), Post natal life (neonatal period, infancy, childhood, juvenile, puberty and adolescence) and Adulthood (prime and transition, old age and senescence). During the different stages, the body undergoes various physiological and psychological changes. To study the changes, age plays a very important factor as with its help the changes between populations could be studied. Age is thus used as a factor which is considered while conducting growth studies.

1.1. Changes in various stages of human life cycle

Human being experiences various changes with growing age. These changes are dependent on various environmental and genetic factors, but in general the stages are universal in nature. Although, these changes are universal in nature, their onset can happen at different time and may lead to different physique, with slight differences when compared with one another. The most important changes are physical maturation with psychological and cognitive development. These changes are seen with age and thereby play a very significant role in studying the variation of changes in growth of the body.The various changes in different stages of human life cycle are –

2. Growth Curve-

These changes are of significant importance and due to specification of changes the various developmental stages are marked. These developments feature the growth of the body. The various changes in growth are described in a growth curve which was given by Richard Scammon in the year 1923 and it is also known as Systemic Growth Curve. This curve was made based on the postnatal growth by the analysis of weight of body, specific tissues and organs. According to Scammon, the growth of human body could be summarized in four curves, namely- genital curve, general curve, neural curve and lymphoid curve.

2.1. Types of growth curves-

There are four main types of growth curves which mark the development of human body. Different tissues fall under different growth curve, which determine their characteristic developmental pattern. The types of growth curves with their characteristic features are as follows.

2.1.1.    Genital Growth Curve- This type of growth curve is characterized by the growth of primary and secondary sexual characters. The primary sexual characters are development of testis, ovaries, penis, etc.; whereas the secondary sexual characters are development of breast, pubic and auxiliary hair, facial hair, growth of larynx, etc. This curve shows acceleration with the onset of puberty and it slightly varies in males and females. From birth till juvenile phase both male and female show slow and similar growth in sexual characteristics development.

2.1.2.    General Growth Curve- This curve describes the growth of the body as a whole. It includes growth of body in terms of height, weight, etc. It also denotes the growth of most system of body, muscle mass, respiratory system, heart, etc. It is generally an S-shaped or Sigmoid curve and shows a steep acceleration with the onset of puberty. Although height stops growing after the individual attains adulthood but other organs and systems continue to grow.

2.1.3.    Neural Growth Curve- This curve shows the growth of brain, nervous system and its associated structure. These types of tissues experience rapid growth during the early phase of post natal period. The neural tissues shows steady gain after 7 years of age with slight growth spurt during adolescent.

2.1.4.    Lymphoid Growth Curve- This type of growth curve describes the growth of lymph glands, thymus glands, tonsils, appendix and lymphoid patches of tissues in the intestine. These tissues help in the development of immune system, including the increase in resistance of infectious diseases. From infancy to end of juvenile phase, there is an intense growth of lymphoid system. The curve reaches its peak point in 11 or 13 years and at these ages, children have twice as much as lymphoid tissue as an adult.

3. Significance of Age in growth studies-

Age means the part or duration of a being which is between its beginning and any given time. It is countable and one of the stages of life at which some particular changes or development occurs, which distinguishes itself from any other stage of life. In growth related studies, age plays a very important factor as age is considered as all the physiological, psychological and cognitive development of an individual or a population could be best understood with aspect to age. Age is also a universal phenomenon and the changes that an individual undergoes, with increase in age is also universal with slight changes in the onset of the changes. The development and maturation of an individual could be studied on a dynamic basis with the help of age. With growth in age various changes are seen in various body systems. Age is an important factor as the various changes in growth is resulted from changes in body systems which could be studies with respect to age. These changes are studied keeping in mind the changes with time. These body systems are –

3.1. The skin:

With growing age, the skin flattens due to the loss of many tissues such as subcutaneous fat, skin cells, sebaceous glands, sweat glands, melanocytes and hair follicles. With age, lentigo occurs, the blood flow to the skin also decreases and nerve endings are lost or become less sensitive; which results in less effectiveness for protection against bacteria or an insulator as a thermal regulator and as a sensory receptor. These results in wrinkling, loss of elasticity, freedom of movement and expression are inhibited. The slowing of circulation results in slower healing. The loss of colour is also seen, as the hair becomes gray.

3.2. The Skeletal System:

The loss of bone matter is the primary change in the aging of the skeletal system. The basic cause of bone loss is the relative rates of production of osteoblasts (bone forming cells) and osteoclasts (bone dissolving cells) changes so that more bone matter is dissolved than is laid down. This loss is much greater in women than in men. Other factors that are seen in the skeletal system with growing age are loosened cartilage around the joints, depleted lubricating fluid in the joints, and hardened and contracted ligaments. These factors occur more in men than in women. There are significant effects of these changes on our health status because as the bones become brittle they become less supportive of our activities, resulting in less activity, which in turn results in poorer health. The excess bone that has been taken up tends to reside in the arteries and local blood vessels, causing decreased circulation.

3.3. The Muscular System:

Muscle cells are post-mitotic cells which are unable to replace themselves once they are formed, so all muscle cell loss is permanent. Muscular response gradually slows down with age even under the best conditions, although the loss of muscular capabilities is mostly the result of cell loss due to inactivity of the body. With loss in muscle cells, weakness and slowness or the muscles also increases. Exceptions are also seen in the muscles of the left ventricle of the heart, and the diaphragm as they do not lose muscle fibres with age, since they are continually active.

3.4. The Neurosensory System:

The nerve cells (neurons) are also post-mitotic fixed cells and with growing age, they lose dendrites, dendritic spines, and end branches; all of which are the intermediary parts necessary for the communication with one another. As the nervous system ages, signal conduction becomes slower, but much more so at the nerve synapses (nerve junctions) than within the nerve cells themselves. There is loss of taste buds, olfactory cells (sense of smell), nerve endings in the skin, and even brain cells. The hearing sensitivity degrades, especially in the higher ranges, as the ear ossicles harden. The vision changes as the lens of the eye becomes less flexible and yellows, and this results in the requirement of more light and glasses for close work.

3.5. The Gastrointestinal Tract:

The mouth, esophagus, stomach, small intestine, gallbladder, liver, pancreas, and large intestine all come together to form the Gastrointestinal tract. In the mouth, taste buds and teeth are lost after adulthood. Problems with the esophagus with growing can be dysphagia (difficulty swallowing), substernal pain, heartburn, belching, and general epigastric discomfort which is also seen after adulthood. Atrophic changes in the stomach, especially hypoacidity and achlorhydria, are common with growing age. Cell replacement is active in the small intestine, so few changes occur with aging; but obstructions are not uncommon. Gallbladder problems are most marked after age 65, rather than in middle age. Problems with the pancreas usually begin to develop about age 40 and they do tend to increase with age, especially if the Islets of Langerhans are damaged, or if they become over stressed by excess sugar consumption, which leads to diabetes development. The large intestine is most susceptible to disease with aging, but is also most amenable to preventive measures. Obstructions of the bowel – carcinogenic or otherwise, diverticular disease, hemorrhoids, and Gastrointestinal discomfort are common in the elderly, as is faecal incontinence.

3.6. The Cardiopulmonary System:

Heart disease is the most common cause of death in people with age 65 and above. The heart muscles reduce in size and the aorta loses some of its elasticity with growing age. Coronary artery disease increases as activity declines. Plaques accumulate on the interior of the arteries (atherosclerosis), and the arteries harden as they lose their elasticity (arteriosclerosis); both of these factors resulting in lessened blood flow. Hypertension (high blood pressure) also increases with age. The respiratory system also undergoes changes with age. The air sacs, airways, and tissues lose elasticity and become more rigid. These lead to changes in the body and other functions.

3.7. The Cerebrovascular System:

Atherosclerosis (plaque formation inside the arteries) and arteriosclerosis (hardening of the arteries) in the blood vessels that supply the brain is called cerebrovascular disease, and causes strokes; which is the main cause of death with growing age. Prior to the complete occlusion of the blood vessels, the brain is deprived of adequate blood flow resulting in less than optimal brain functioning, such as confusion, disorientation, and memory loss which is mostly seen in old age.

3.8. The Urinary System:

The bladder, urethra, urinary tract, prostate, and kidneys all show decremented incidence in most people with age, thus; urinary incontinence and urinary tract infections are the most common problems encountered. The capacity of the bladder reduces by half in the elderly; thereby the need to urinate is more frequently. From the age of 50 onwards, the prostate problems increase in frequency among men than in women. A decrease in the number of nephrons, the filtering module of the kidney, results in decrease efficiency of the kidneys, which results in increase in health problem with growing age.

3.9. The Endocrine System:

Endocrine system is one of the major systems of the most that functions the working of the hormones in our body. The pituitary, thyroid, and adrenal glands tend to function adequately throughout life, while size of the thyroid gland does decrease significantly with age. Sex hormones such as estrogens and testosterone are seen to increase in female and male respectively with the onset of puberty.

3.10. The Genital System:

The changes in the genital system tend to be non-problematic. Generally, responses slow gradually in both men and women. In men, the prostate may enlarge, and may create urinary problems. More changes occur in women than in men. In women the changes are more visible as there is development of the breast due to increase in estrogens. In men beard occurs change in voice and also development of muscles which is due to an increase in testosterone.

3.11. The Immune System:

The earliest deterioration in our bodies is seen in the thymus, which is the heart of the immune system. The thymus begins significant involution by the age of three years. Two types of white blood cells, B cells and T cells of the immune system malfunction and weaken with age. With growing age, B cells lose their vigour in attacking bacteria, viruses, and cancer cells; and the T cells lose their vigour in attacking cells foreign to the body, such as cancer cells and transplant cells. B cells and T cells also malfunction and attack normal healthy body cells which are seen mostly after adulthood and during old age.

4. Methods for estimation of an individual’s age –

Age of an individual could be determined by six methods and in these methods the physical changes of an individual is noted with time. There are even specifics calendars which are prepared to study the significant changes in the life of an individual with age. The various methods are as follows –

4.1. Chronological Age- This method of age estimation gives the individuals’ age in terms of years, months and days. It is mostly used to determine ones accurate age with respect to day, month and year. Let us take an example. If we need to find out an individual born on 2nd April 1990 on 10th January 2016, then one have to subtract the 1.1.2016 by 2.4.1990.

2016. 1. 10

– 1990. 4. 2

————–

    25.   7.   8

Thus, the age on the individual on 10th January 2016 would be 25 years, 7 months and 8days.

4.2. Skeletal age or Bone age – This method is determined by the intensity of ossification of bones. It is an indicator of physiological maturity and it is accessed with the help of radiography. The concept behind is that, each bone has a primary centre for ossification and it passes through various stages of enlargement, and shaping of ossified areas require times. In some cases more than one centre known as epiphyses is present where ossification begins independently without the help of the main centre and finally reaches a stage where it fuses with the main body of the bone. In this technique, i.e. radiography, the changes could be noticed on a radiograph which clearly distinguishes the area of ossification. The skeletal age is clearly defined from the number of centres developed or present along with the stages of development of each centre. In theory, any or all part of it could also be used to give the assessment of skeletal maturity but in practise hands and wrist are used. This is done as a radiograph of a hand could be easily done without any radiation being delivered to the other parts of the body and it requires a minute dose of X-ray and only the minimum X-ray equipments. Generally the left hand is used with the palm placed downward on the X-ray film and the X-ray tube being 75 cm. above the knuckle of the middle finger. The figure for the skeletal maturity is derived by comparing the given radiograph with the set of standards. The bone is passed through the sets of standard and given a particular score which is then compared with the range of scores of a standard group.

4.3. Secondary Sexual character – Development of sexual characters is an important phase in the life of any humans. With the end of juvenile phase, one enters the pubertal phase where there is an experience of growth hormones along with sex hormones. The development of reproductive system and secondary sexual characters occurs and adolescent spurt is observed which lead to linear growth of the body. Although the onset of spurt is different between males and females and in females it occurs 2 years earlier than that of males. Puberty last for few days or months and consecutively it is followed by adolescent phase. During this phase the secondary sexual characters develop. In females, oogenesis starts along with development of breast, auxiliary and pubic hair develops and the body becomes pear shaped as the hips broadens and more accumulation of fat occurs in the hip region. In males, there is change in voice with development of beard; chest, body, and pubic hair also develop with broader chest and more muscular development. In females the development occurs from the age of 10 years where as in males such features are observed after 12 years.

4.4. Dental age- Dental age is determined by counting the number of erupted teeth and then relating it to the standard figures. Another method to calculate dental age is to see the stages of calcification of teeth with the help of X-ray of the jaw. This method gives an proximate assumption of the age of the individual. The determination of age with the help of dentition is shown in the following table-

4.5. Decimal Age – In this method, the age of the individual is determined with the help of decimal age calendar. This method is useful in age grouping method where the age of the individual is denoted in terms of neutral, positive or negative factor. In such method, the value is considered from a decimal age calendar for the required date which is then subtracted by the age of the individual by considering the subjects birthday with the help of decimal age calendar. Let us take an example. If we need to find out an individual born on 2nd April 1990 on 10th January 2016, then one have to subtract the value of 1.1.2016 by 2.4.1990by using the decimal age calendar; and the value of 2nd April and 10th January according to decimal age calendar are 249 and 025 respectively. Thus, the age would be-

2016 .  025

– 1990 . 249

—————-

25  .  776

Thus, the age of the individual could be written in the following ways-

• 25+ (Since after decimal the value exceeds .5)
• 26- (Since after decimal the value is closer to the next value 26 with a difference of .3 which is less than .5)
• The age could be written as 25 if the value was 25.5 which is considered to be neutral.

4.6. Shape or Morphological Age – This is also known as Research age and is regarded as a research problem. It is extremely difficult to assign and is not merely done by calculation of height and weight. It involves a set of measurements on the individuals. In this method, a combination of body measurements are taken into account and all of the measurements changes with ages but are independent of final shape and size. It is not of much importance as it is a research problem as the individuals are characterized by various maturity levels.

Summary-

1. Age plays a very important factor in growth related studies. Age is considered as a dynamic factor which helps in the study of various developmental phases that leads to the maturation of an individual.
2. Age is used a parameter to determine the growth of a population or an individual.
3. Human beings undergo various phases of development and the most rapid growth of development is seen in the post natal phase of life.
4. The prenatal phase is divided into three trimesters and it lasts till the time of birth. During this phase rapid development occurs in length and weight of the body.
5. The post natal phase is divided in neonatal phase, infancy, childhood, juvenile, puberty and adolescent phase. Rapid growth spurt is experienced during this phase which leads to the psychological, physical and cognitive development with the influence of growth and sexual hormones.
6. Adulthood is the transition phase and by the end of this phase the body experiences degradation in its functional ability and structure.
7. The estimation of growth is done by various methods out of which cross sectional method is mostly used while conducting studies in field.
8. Growth rate is a feature which is widely dependent on intrinsic and extrinsic factors. It thereby brings about changes in growth pattern.
9. Age is a very important factor and the measurement of the age of the individual could be done with the help of chronological age, skeletal age, secondary sexual characters, dental age, decimal age and research age.
10. Chronological age and decimal age gives the accurate age of the individual, whereas the decimal and skeletal age gives the approximate age of the individual. The research age is not considered as an appropriate method and is used mostly as a research question.