Development of human beings

Development is the process where someone or something grows or changes and becomes more advanced.Human development is a continuous process that does not stop at birth. It continues after birth for increase in the size of the body, eruption of teeth, etc. Development before birth is called prenatal development, and that after birth is called postnatal development. Each period is further subdivided into several stages.
Prenatal Development There are three stages in prenatal development. They are:
 1. Preimplantation/pre-embryonic period
 2. Embryonic period 3. Fetal period.
Preimplantation/Pre-embryonic Period- It extends from fusion of male and female gametes to form single-celled zygote to formation of primitive germ layers of developing organism. It includes 1st and 2nd weeks of intrauterine development. The following morphogenetic events take place during this period.
 1. Fertilization: Fusion of male and female gametes resulting in the formation of zygote.
 2. Cleavage: A series of mitotic divisions of zygote resulting in the formation of morula.
 3. Transportation of cleaving zygote, i.e. morula along the fallopian tube toward the uterus.
 4. Blastocyst: Structural and functional specialization and reorganization of cells (blastomeres) of cleaving zygote that becomes blastocyst.
 5. Implantation: Process of attachment of blastocyst to the uterine endometrium is called implantation
6. Specialization of primordial embryonic tissue: It involves specialization of blastomeres to form embryonic structures (embryoblast) and supportive/nutritive structures (trophoblast).
 7. Differentiation of embryoblast—to form the primitive two layered (bilaminar) germ disc having ectoderm and endodermal 
8. Differentiation of trophoblast into cytotrophoblast and syncytiotrophoblast.
Embryonic Period It extends from 3rd week of intrauterine life to 8th week of intrauterine life. The following morphogenetic events take place during this period 
1. Trilaminar germ disc differentiation: Formation of three layered germ disc with the appearance of mesoderm in between ectoderm and endoderm.
 2. Early organogenesis: Formation of primordia of various organs like lungs, heart, liver, etc.
 3. Formation of extraembryonic supportive organs and membranes: Placenta, umbilical cord, amnion, allantois.
Fetal Period It extends from 9th week to 9th month. This period includes the following:
 1. Growth of fetus in all dimensions
 2. Specialization of various body structures. Postnatal Period of Development It extends from birth of an individual to adulthood. The various stages in postnatal development are as follows: 
1. Neonatal period: It extends from birth to 28 days after birth. These first 4 weeks are critical in the life of the newborn/neonate as various systems especially respiratory and cardiovascular have to make adjustments with the external/extrauterine environment. Neonatology: The branch of medicine that takes care of neonates is called neonatology. Perinatology: It is the branch of medicine that takes care of the fetus and newborn from 28th week of intrauterine life to 6th day of extrauterine life. 
2. Infancy: It extends from 1 month to 1 year and the newborn during this period is called infant. 
3. Childhood: It extends from 2nd year to 12th year of age and an individual is called a child. It is the period of rapid growth and development. This age is also called pediatric age. Pediatrics and pediatrician: The medical branch that deals with infants and children is called pediatrics. The specialist who treats them is known as pediatrician.
4. Puberty: It extends from 12 years to 16 years. There will be rapid physical growth and development of secondary sex characters and it depends on the interaction of sex hormones and growth hormones.
 5. Adolescence: It extends from 17 years to 20 years. During this period, there will be rapid physical growth and sexual maturation. The reproductive ability is established. 
6. Adulthood: It extends from 21 years to 40 years.
 7. Middle age: It extends from 40 years to 60 years. 8. Old age: It extends from more than 60 years to death. 

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