Human development (biology)
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Fertilization occurs when the sperm successfully enters the ovum's membrane. The genetical material of the sperm and egg that combine to form a single cell, called a zygote, and the germinal stage of prenatal development commences. The germinal stage refers to the time from fertilization, through the development of the early embryo, up until implantation. The germinal stage is over at about 10 days of gestation.
The zygote contains a full complement of genetic material and develops into the embryo. Prior to implantation, the embryo remains in a protein shell, the zona pellucida, and undergoes a series of cell divisions. A week after fertilization the embryo still has not grown in size, but hatches from its protein shell and adheres to the lining of the mother's uterus. This induces a decidual reaction, wherein the uterine cells proliferate and surround the embryo thus causing it to become embedded within the uterine tissue. The embryo, meanwhile, proliferates and develops both into embryonic and extra-embryonic tissue, the latter forming the fetal membranes and the placenta. In humans, the embryo is referred to as a fetus in the later stages of prenatal development. The transition from embryo to fetus is arbitrarily defined as occurring 8 weeks after fertilization. In comparison to the embryo, the fetus has more recognizable external features, and a set of progressively developing internal organs. A nearly identical process occurs in other species, especially among.
There are no universal definitions for terms of age-related physical development stages, but following are some approximate age ranges:
- Prenatal (sperm fertilizes egg - birth)
- Embryo - fertilization - 8 weeks after fertilization)
- Fetus, (10th week of pregnancy - birth)
- Neonate (newborn) (0 – 30 days)
- Infant (baby) (0 month - 12 months)
- Toddler (1 – 3 years)
- Play age (4–5 years)
- Primary school age (middle childhood also called prepubescence) (4-12)
- Adolescence and puberty (13 – 19 years)
- Adulthood (20+ years)
- Death (occurs at various ages, depending on person)
- Decomposition (breakdown of the body after death)
Also sometimes used are terms that specify one's age in numbers, such as:
- Young child (0-9)
- Pre-teenager (10-12)
- Teenager (13-19)
- Twentysomething (20-29)
- Thirtysomething (30-39)
- Fortysomething (40-49) (formerly also Quadragenarian, rarely used since 1980)
- Quinquagenarian (50-59)
- Sexagenarian (60-69)
- Septuagenarian (70-79)
- Octogenarian (80-89)
- Nonagenarian (90-99)
- Centenarian (100-109)
- Supercentenarian (110+)
The Tanner stages can be used to approximately judge a child's age based on physical development.
- Child development
- Developmental biology
- Life-history theory
- Mammalian embryogenesis
- Sherk, Stephanie Dionne. "http://www.healthline.com/galecontent/prenatal-development". Gale Encyclopedia of Children's Health, 2006. Gale. Retrieved 6 October 2013.
- "germinal stage". Mosby's Medical Dictionary, 8th edition. Elsevier. Retrieved 6 October 2013.
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