Concept of Human growth and development:stages of growth- Pre-natal, natal, infant, childhood, adolescence, maturity, senescence.
Factors affecting growth and development genetic,environmental,biochemical,nutritional,cultural, and socio-economic.
Ageing and senescence. theories and observations-biological and chronological longevity. Human physique and somatotypes. Methodologies for growth studies.
Concept of human growth and development:
Laws of growth:
A.Orderliness:Growth follows a sigma shaped curve. There are specific periods in a childs life when the rate of growth accelerates decelerates or there is a steady build up of body tissues. The foetus grows fast in the first half of gestation. Therefore, the rate of growth is slowed down till the baby is born. In the early postnatal period the velocity of growth is high during the first few months. There is steady rate of growth during mid childhood. A second phase of accelerated growth happens during puberty. Growth decelerates for some time after that and then ceases altogether.
B.Growth gradient:order of growth in human beings is cephalocaudal and distal to proximal. During the foetal life growth of head antedates that of neck while arms grows before legs. Distal parts of body such as hands increase in size before upper arms. In the post natal life growth of head slows down but limbs continue to grow rapidly. At all ages the foot is nearer its adult status than the calf,and the calf nearer than the thigh. Girls are more advanced in maturity at all ages than boys without this affecting in any way the distal proximal gradient.
1.general body growth:rapid during foetal,1 to 2 years of postnatal life and also during puberty.
2.the brain growth:at birth the head size is 65 to 70 % of the adult size. It reaches 90% by age 2 years.
3.the growth of gonads:gonadal growth is dormant during childhood. It becomes conspicuous during pubescence.
4.Lymphoid growth:the growth of lymphoid tissue is most notable during mid childhood.
5.sun cutaneous fat:subcutaneous fat begins to be laid down in the foetus at about 34 weeks until about 9 months after which it decreases until age 7 years, when it begins to increase once again.
After illness or starvation which is a period characterized by slow growth, there has been found tendency in the younger subjects to bridge the deficit as soon as possible and catch up with the original growth curve. This is known as catch-up growth.
Prenatal development proceeds according to genetic instructions from a single cell to an extremely complex being. This development before birth is called gestation. It takes place in three stages: Germinal Embryonic and Fetal stage.
1.Germinal stage(Fertilization to 2 weeks):
Includes creation of zygote(the fertilized egg),cell division,attachment of the zygote to the uterine wall.
A group of Blastocysts(inner mass of cells) will eventually develops into the embryo.
Trophoblasts are the outer layer of the cell that later provide nutrition and support to the embryo.
Implantation is when the zygote attaches to the uterine wall.(takes place about 11-15 days after conception).
2.Embryonic period(2 to 8 weeks after conception):
Support system for cells forms, and organs appear.
Endoderm will develop the respiratory and the digestive systems.
Mesoderm becomes the circulatory system,bones,muscle,excretory system,and reproductive system.
Ectoderm becomes the nervous system,brain,sensory receptors and skin.
Amnion prenatal life support system that is a bag or envelop that contains a clear fluid in which the developing embryo floats.
Umbilical chord-life support system that contain two arteries and one vein and connect the baby to the placenta.
Placenta-life support system that consists of a disk shaped group of tissues in which small blood vessels from mother and offspring intertwine.
3.Fetal period:from 2 months after conception until birth(7months):
Fetus is about 3 inches long and weigh 3 ounces.
Fetus becomes very active.
Can usually tell if its a male or a female.
First trimester(first 3 months) second(second three months) third(third 3 months)
by the end of 6 months the fetus is about 14 inches long and weigh about 2 pounds. the eyes are now formed and hair begins to cover the head. The breathing movement starts to occur. By 8th month average fetus weigh 7 and a half pound and 20 inches long.
Postnatal growth starts from the time of birth upto adulthood or the time of maturity.
Infancy:immediately after birth the rate of growth increases. In case of weight the peak velocity is reached at two months after after birth. By building up the cytoplasm in the cells of the muscles and also incorporating salt in them, and by the formation of proteins the growth at infancy continues. The cervical and lumber curvatures of the spinal column appear as the baby begins to straighten the head and ties to sit up and to stand.
During infancy growth is very rapid. More than 50 percent of birth length and 200 percent of birth weight takes place during the first year of life. From this period the rate of growth decreases sharply. It has been found that growth during infancy is more rapid than during any period of postnatal life. Birth length represents approximately 1/4th of the total stature against 1/8th in the adult. Cranial capacity is about ½ of the adult.
Childhood:Both heredity and environment play their respective role in physical growth. Therefore, the hereditary growth potential of a child may be influenced by various environmental factors which include nutrition, state of health, socio-economic conditions.
Eruption of deciduous or milk teeth. The permanent dentition also shows its beginning. The rounded body form continues to persist. The head in relation to the trunk continues to predominate, but in lesser degree. During this period the growth is relatively more in width than in height.
During the period of middle childhood(7 to 10 years)permanent teeth, though not all, erupt. The rounded forms disappear. The size of head increases slightly. The linear growth of the body takes place rapidly. The thorax begins to become oval in transverse section. The waist line becomes definable. At about 7 and 8 years a nominal acceleration in the rate of growth occurs. It may be termed as mid growth spurt.
The late childhood phase of growth stage from the prepubertal period and continues up to the times of puberty. It occurs between 13 to 16 in boys and 12 and 15 years in girls. Sexual morphological differentiation normally begins during this phase. The rate of growth decreases during childhood but a notable increase in growth velocity known as the adolescent growth spurt, is observed which, however, is associated with the onset of sexual maturation.
Adolescence: from the time of puberty up to around 20 years. It causes many anatomical changes almost in all parts of the body. Sexual maturation takes place during this period.
The pre-puberal phases lasts for about two years. During this time the increase of weight is retarded. Height increases by the lengthening of the legs. The thorax becomes narrow; the trunk becomes short;legs and arms become long.
During puberal period sexual organs are matured. The body proportions are changed. Secondary sexual characters appears. In girls the first menstruation marks the beginning of ovarian activity. But in boys it is difficult to ascertain precisely the time of maturation of the testes. The most obvious secondary sexual characters in girls are the development of breasts,appearance of axilliary and pubic hairs. In boys also pubic hair,beard and moustache. Moreover the voices changes. Differentiation in the primary and secondary sexual characteristics mark the adolescent period.
Maturity or adulthood: the cessation of growth of height is regarded as a sign of maturity. Height ceases to grow when the long bones like tibia fibula etc lose their capacity to increase in length. Usually males attain adult height at about 21 years of age and females at 18 years of age.
Reproductive maturity. During adolescence reproductive maturity begins,but not completed. The average girl is not capable of reproduction until 14 years of age or older,though onset of puberty may take place earlier. Because after menarche there is a period of one to two years of adolescent sterility, when ovulation does not occur. On average adult males are heavier and taller than adult females.
Senescence: After the prime period of adulthood senescence
starts. The aging pattern shows great individual variation. During this period many molecular and cellular changes occur. Organismic changes are also there.
The process of growing old are scientifically studied in a special branch called gerontology.
Because of aging the tissues do not renew and as a result cells show senile involution. The memory declines. Aged persons need more time to learn and to react. Systolic blood pressure increases. The range of the accommodation of the eye lens decline. There is reduction of density of long bones and vertebrate and therefore height and sitting height decrease.
Factors affecting Growth and development:
Growth of an individual is affected by several factors, which may broadly be categorized as intrinsic or genetic and extrinsic or environmental factors.
Genetic materials:twin studies provide ample information on the influence of heredity on growth. It seems that the genetic materials operate throughout the entire period of growth. In skeletal maturity at different ages similarities are there in monozygotic twins. Heredity controls the time of eruption of teeth, both deciduous and permanent,and also the sequence in which teeth calcify.
Parent-offspring correlation in regard to height from birth to maturity for each age and sex has been reported.
Chromosomal abnormalities suggest genetical control on growth. Male individuals with klinefelter syndrome are on an average taller than normal males. On the other hand, phenotypically females with one X chromosomes are shorter on average than normal females.
All the genes are not active at the time of birth. Their activities depend upon physiological environment which varies with age. And therefore it can be termed age-limited.
Hormones:hormonal influence is another important factor in growth. Hormones are organic substances. These are synthesized in specific body tissue, commonly known as endocrine glands. Growth is regulated by the actions and interactions of hormones and other factors which influence growth. Regarded as growth promoting substance. The changes during adolescence are caused by hormones. Imbalance or disorder in secretion of hormone may lead to abnormal growth.
At least three hormone play vital role in regulating the orderly process of growth. These are thyroid,pituitary,and adrenal hormones.
Nutrition:an adequate supply of vitamins,minerals,clean water etc are necessary for the multiplication of cells and their enlargement in size. Nutritional deficiencies impair normal growth during childhood and delay the appearance of adolescent spurt.
State of health:illness major or minor may affect normal growth. It works through internal agencies,e.g.,genetical,endocrinal and physiological, or through environmental conditions. Chronic anemia may retard growth. Some infection may cause retardation of growth when certain organs are affected.
Psychological disturbance:the secretion of growth hormone is affected by the emotional and psychological environment of a person. Hence, physical growth is related to psychological factors, which are again associated with home environment, social environment and the like.
Rural-urban:several studies have revealed rural-urban variation in growth patterns due to different food habits,items of food,supply of food,medical care,sanitation,education and activities of people.
Socio cultural condition:here we are going to mention economic class,size of family,and cultural behaviour,though they are interrelated.
Children are grouped under different socio-economic classes on the basis of fathers occupation, e.g. farmer,day labourer,unskilled,semiskilled,trader,clerical,professional,manegerial,etc. Children of different socio-economic classes exhibit interclass difference on average body size. A study reported that height increases from farmer to day labourer and then to professional. In general the children of upper classes are taller and heavier.
Socio-economic condition is related to home conditions,nutrition,health care,education,etc.
Size of family exerts an indirect influence on the rate of growth,because this has relation to economic condition of the family. In large family with limited income a children do not get adequate care. As a result they suffer. Their growth is affected.
Cultural behaviour is another important factor,which has effect on growth. Breast feeding;care of child; consumption of rice,meat,milk,fish,etc.,or in other words food habit;cleanliness;sanitation;home environment and the like are parts of cultural behaviour.
Physique and somatotype:
the human body varies in innumerable ways. It is not possible to include all the variables to determine the physique of an individual. Therefore, one must choose some characters. Physique is again related to several factors,e.g. phisiology,susceptibility to disease,behaviour, etc. anthropometric measurements can help a lot describe physique. Several scholars have made attempts to ascertain varieties of physique and classify them. Here mentions of four classification will be made.
Viola’s types:he proposed a classification of biotype(somatotype or physique) based on a comprehensive system of anthropometric measurements. He took into account ten basic measurements. These are sternum length,upper abdominal height, lower abdominal height, length of arm, length of leg, upper abdominal breadth, lower abdominal breadth, abdominal depth, chest breath and chest depth. He also considered three compound measurements,namely stature,trunk height and total abdominal height. From the measurement he calculated four indices. These are:thoracic,upper abdominal,lower abdominal,total abdominal index. In respect of each of the indices the relative position of an individual was ascertained comparing the same with the standard value of the group belonging to the same age and sex, and accordingly the person was assigned to a class. He made four classes,longitype, brachitype,normotype and mixtype.
The personof Longitype have long limbs in relation to their trunk,a alrge thorax in relation to their abdomen, and large chest breadth in relation to their chest depth. The opposite picture is exhibited by the brachitypes. As the term implies, in case of mixed type persons,one index belongs one category,the other belongs to other category. Their indices do not agree among themselves.
His classification helps to understand the constitutional types.
Kretschmer’s types: he at first described three types: asthenic,athletic,pyknic.
Asthenic(Leptosomic) type can be described as follows:the bones are small and muscles are underdeveloped. They do not get weight inspite of having food in large amount. Their limbs are slender hands are bony with pointed fingers:body is long;abdomen is concave;face is thin and oval. Head is small;chest is flat;nose id pointed,etc. They are slightly anemic.
Athletic type:their limbs are strong and muscular;chest and shoulders are large in comparison to lips and legs;bony structure is strong;face is long;jaw is heavy and strong;cheek bones are projecting;abdomen is muscular;they are average to tall in height.
Pyknic type:they have tall stature,broad figure,thick and short neck. sturdy and stocky body,abdomen is flat;head and thorax are well developed; face is pentagonal in shape,nose is straight or concave:forhead is broad;clavicle and wrist are small;extremities have small muscles and are rounded.
His study is based on visual observation followed by measurements on the individuals. He observed some affinities between psychological characters of persons and their somatotypes.e.g.maniac depressive psychosis.
Pende’s type:he developed the concept of viola. He included psychic, physiological and neuro-endocrinological traits in his biotyping methods. Four types were defined by him a)sthenic slender type b)Asthenic slender or hyposthenic-hypotonic type,c)sthenic broad type d)Asthenic broad or hyposthenic type.
Sheldon’s type:he thought of five different approaches like anatomy and physical anthropology,psychology and psychiatry,clinical medicine and pathology. socioogy and other social sciences. However, the basis of his classification is embryonic germ layers. To him the degree of development of endoderm, mesoderm, and ectoderm determines the body type. These three components are possessed by every individual but in varying ratios. He called these components, endomorphy, mesomorphy, and ectomorphy.
Endomorphy means the relative predominance of soft roundedness in differnt parts of the body like head is round,abdomen is flat and large,arms and legs are weak and floppy,wrists and ankles are slenderly built.
Mesomorphy signifies the relative prodominance of bones, muscle and tissue like their head is massive and cubical,shoulders and chest are broad.
Ectomorphy is the indication of the relative predominance of linearity like face is thin,forehead is high,chin id receding,chest id thin and narrow.
Ageing and senescence:
the term aging simply implies growing older,and changes which are related to age regardless of when in the life span they occur. Senescence however implies changes which occur during the later years of life span when there is a functional decline.
Ageing process is an increased liability to die, an increasing loss of vigor, with increasing chronological age,with the passage of time.
Senescence and ageing are gradual processes. They are generally initiated at molecular level. The process is advanced much before the external symptoms appear.
1.the changes associated with ageing increase the probability of organisms death and hence these changes are deleterious.
2.Ageing decreases the animals capacity to cope with the environment.
3.the changes associated with ageing are cumulative. This implies that though death is sudden,the process that lead to death are gradual.
4.the processes associated with aging and senescence are inescapable. They are the fundamental and intrinsic characteristic of life.
Theories and observations:
many theories have been put forth to explain the causes of ageing.
theory of Hayflick limit.
1.human lung fibroblast divide and their division is limited.
2.the number of cell division is roughly related to the age and life span of the species.
3.the longer the life span of species the greater is the number of divisions.
4.if cells divide indefinetely, they are abnormal and/or cancerous.
5.senescence is due to the loss of cell functions that occur before cells reach their maximum devision point.
6.senescence sets in tissues that are more sensitive and then secondarily the other tissues of the body.
Theory of free radicalreaction.
1.free radicals of oxygen are produced in the cells which cause damage and death of a cell.
2.certain enzymatic reactions are responsible for generating super oxide radical.
3.in the presence of water this super oxide radical is converted into highly reactive hydroperoxyl radical(ho2)
4.this radical reacts primarily with unsaturated lipids present in the cells to form the pigment lipofusion
5.the free radicals are especially harmful when they react in the cell membrane.
6.this leads to cell senescence and cell death.
Theory of DNA error.
Errors in DNA replication process may result from time to time and the errors might also crop up as a result of mutation.
If the damage to dna is undetected by the dna repair system or damages are faster than the action of repair system,these errors accumulate.
The accumulated DNA errors impair cell functions and impede the synthesis of essential emzymes.
Thus cell senescence sets in and this leads to cell death.
The error of catastrophe theory.
Certain types of errors during the process of dna replication produce a great number of subsequent errors. Thus error in replication would be a error catastrophe.
A mutation in dna polymerase enzyme gene, which is responsible for dna replication would produce a new enzyme which produces further mistakes.
Such defective enzyme produced will fill the entire cell with a mosaic of defective proteins.
Theory of missing factors.
Ageing is not due to a malfunctioning of specific gene or genes but due to non functioning of genes responsible for maintaining adulthood.
The malfunctioning may be due to endogenic factors(mutation) and exogenic factors(nutrition)
thus the reproductive phase of individuals depletes certain factors which are responsible for maintaining adulthood.
These missing factors accelerate agening and process of senescence.
Extra cellular theory:
The efficiency of the immune system.(spleen,lymph nodes,bone marrow and thymus)decrease after attainment of adulthood.
The efficiency of thymus gland is especially significant. Its generation programs events that lead to ageing of the animal
immunity can also determine ageing by production of auto immune responses. the mutations where antibodies are produced against ones owns tissues-triggering a fast process of ageing.
About 40% of body protein is composed of collagen which is present in extra cellular spaces.
Collagen is made of three polypetide chains which forms a super helix
the three chains are supported by non covalent bonds
as age advances the three chains are further stabilized by covalent bonds
because of this change in the property of collagen,a healthy intracellular environment is not kept and the movements of product is thus obstructed
this leads to accumulation of toxins and undernourishment of cells and thus to cell senescence.
Programmed Ageing theories:
Bodies age according to a normal development pattern but built into every organism.
This program is only subject to minor modifications
this pattern of ageing is predetermined and inborn
this inborn mechanism of programmed ageing is supposed to be confined to chromosome 1 or chromosome 21.
Wear and Tear theory.
Body ages because of continuous use-ageing is the result od accumulated insults to the body.
As time passes,when cells grow older,they are less able to repair or replace damaged components.
Internal and external stressors aggravate the wearing down process.