Sub-topic: Stages of human growth and development
Human life begins at conception, which occurs when a sperm cell unites with an egg from a female and forms a single cell called a zygote. After conception the growth and development of the human being proceeds as a result of cell division. There are two processes of cell division: –
- Mitosis – is the process of cell division in which a single body (somatic) cell divides into two exactly equal parts. Each of the parts has exactly the same 23 pairs of chromosomes as in the original cell.
- Meiosis – is the process of cell division that occurs during the production of sex or germ cells (ova and sperm). During this process each chromosome splits and separates so that the resulting ovum or sperm contains only 23 single chromosomes.
The period between conception and birth is referred to as the prenatal period. Human growth and development occurs most rapidly during this period. Prenatal development is divided into three stages: –
1. Germinal/pre-embryonic period (conception -14 days)
This period is characterized by rapid cell division and ends when the zygote implants itself in the walls of the uterus.
During this stage, cell differentiation occurs in which the cells of the blastocyst differentiate themselves into three layers. This is followed by organ genesis in which different organs of the body begin to form. Other developments during this period are the formation of the amniotic sac – which is filled by amniotic fluid. The placenta is also formed through which the embryo gets its nutrients during pregnancy. By the 8th week, the embryo has rudimentary eyes, ears, nose, mouth, liver, heart, and webbed fingers.
This period is very critical for development. This is because: The embryo is most susceptible to influences of teratogens – viruses, chemicals, drugs and radiation. If it is exposed to any of these, it is most likely that the embryo will have abnormalities of the organs of the body, which are in the process of being formed. Most miscarriages occur at this period. Embryo detaches from the wall and is expelled.
Foetal stage (9-40 weeks)
During this period, the major systems begin to function. There is also refining and improving of what is already formed. There is a major growth and development of the brain and the nervous system during the last three months of prenatal development. The 28th week marks the point of viability, which is the point at which survival outside the uterus may be possible. The baby is born approximately 266 days after conception or 280 days from the onset of the last period.
FACTORS AFFECTING PRENATAL DEVELOPMENT
Each year some babies are born with birth defects that range from gross anomalies that may lead to sudden death to minimal physical or mental defects. About 7% of children are born with a congenital defect. These defects may be caused by the following factors: –
- Abnormal genes and chromosomes.
- Harmful conditions in the womb.
- Abnormalities during the birth process.
Abnormal genes and chromosomes
Genetic abnormalities: – genetic abnormalities result from mutations – that is, a change in the chemical structure or arrangement of one or more genes that has the effect of producing a new phenotype. Some conditions that may result from this are: –
1. Cystic fibrosis – occurs in about 1 in 1000 births. The child lacks an enzyme that prevents mucus from obstructing the lungs and the digestive system. Many who inherit this problem die in childhood or adolescence.
2. Muscular dystrophy (MD) – it attacks the muscles. As the disease progresses, the individual starts showing slurred speech, becomes unable to walk, and may gradually lose most of the motor capabilities.
3. Phenylketonuria (PKU) – the child lacks an enzyme to digest foods that contain the amino acid phenylalanine (including milk). If not detected and the child placed on a diet of milk substitute, phenylpyruvic acid will accumulate in the body and attacks the developing nervous system. Long term effects of PKU are hyperactivity and severe mental retardation.
4. Tay-sachs disease is a degenerative disease of the nervous system that kills the victim by the 3rd birthday.
5. Hemophilia (bleeder’s disease) – the child lacks a substance that causes the blood to clot and can bleed to death if cut. Is rare in females.
6. Diabetes – an inherited condition in which the individual is unable to metabolize sugar properly because the body does not produce insulin. Can be controlled by taking insulin and restricting one’s diet.
Chromosomal abnormalities: – usually occur during the process of cell division after conception (meiosis and mitosis). May be caused by non-separation of chromosomes, deletion of chromosomes which may lead to loss of genes, and translocation of chromosomes i.e. deleting and attaching themselves to the wrong ones that is uneven segregation of chromosomes. Some conditions that they result include: –
Turner’s syndrome – XO – the person is female but is short, mentally retarded and sterile.
Klinefelter’s syndrome – XXY – a male with male external organs but with female body contours and also sterile.
Downes syndrome (mongolism) – the 21st chromosome does not separate so the zygote ends up having two no.21 chromosomes (trisomy 21). The child is mentally retarded and may have congenital eyes, ear, and heart defects; sloping forehead, protruding tongue, short stubby limbs, a slightly flattened nose, a distinctive fold to the eyelids that gives their eyes an oriental appearance.
N.B: The probability of having a child with Downes syndrome and Klinefelter’s syndrome increases dramatically if the mother is over 35 years. These defects can be detected in the first three months of pregnancy through a test called amniocentesis.
- Infancy (0-2 years)
- Early childhood (2-6 years)
Late childhood (6-12 years)
Adolescence (12-18 years)
Changes that take place in each of human growth and development
Challenges related to specific stages of human growth and development