Human development: Encyclopedia II - Human development - Biological development

Human development - Biological development

Development begins with fertilization, the process by which the male gamete, the sperm cell, and the female gamete, the oocyte, fuse to give rise to a diploid cell, the zygote.

In medicine, pregnancy is defined as beginning when a fertilized zygote becomes implanted in a woman's uterus. This occurs when the zygote then becomes embedded into the endometrium (lining of the uterus) where it forms a placenta, for the purpose of receiving essential nutrients through the uterus wall. The umbilical cord in a newborn child signifies the remnants of implantation.

The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo.

Childbirth is the process in which the baby is born. It is considered by many to be the beginning of a person's life, where age is defined relative to this event in most cultures.

Human development - Physical stages

Terms for stages of age-related physical development include, with their approximate age ranges:

• Zygote, the point of Conception, fertilization
• Embryo; in the later stages also called fetus
• Birth

• Child
  • Infant (baby, newborn) (0-1.5)
  • Toddler (1.5-4)
  • Primary school age (also called prepubescence) (4-12)
    • Elementary school age (also called middle childhood) (4-8)
    • Preadolescence (preteen, or late childhood. The child in this and the previous phase are called schoolchild (schoolboy or schoolgirl), when still of primary school age.) (9-12)
• Adolescence and puberty (teenage) (13-19)
• Young adult (16-25)
• Adult (16-21 or older; exact minimum age may vary)
  • Early adulthood (20-39)
  • Middle age (40-59)
  • Advanced adult/Senior citizen (60+)
• Death (occurs at various ages, depending on person)

Also sometimes used are terms that specify one's age in decades, such as:

• Twenty something (20-29)
• Thirty something (30-39)
• Quadragenarian (40-49) (rarely used since 1980)
• Quinquagenarian (50-59)
• Sexagenarian (60-69)
• Septuagenarian (70-79)
• Octogenarian (80-89)
• Nonagenarian (90-99)
• Centenarian (100-109)
• Supercentenarian (110+)

Human development - Physical development milestones

• Ability to lift and control the orientation of the head
• Crawling begins
• Walking begins
• Speech begins
• Voice lowers in pitch (especially noticeable in boys)
• Pubic hair appears
• Genitals and reproductive organs mature
• Menses begin (females)
• Body hair and facial hair appears

Note: the Tanner stages can be used to approximately judge a child's age based on physical development.

Adapted from the Wikipedia article "Biological development", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki