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Introduction to Human Development
Human development is a continuous process that begins when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male to form a single-celled zygote ( Fig. 1.1 ). Cell division, cell migration, programmed cell death (apoptosis), differentiation, growth, and cell rearrangement transform the fertilized oocyte, a highly specialized, totipotent cell, the zygote, into a multicellular human being. Most developmental changes occur during the embryonic and fetal periods; however, important changes occur during later periods of development: the neonatal period (first 4 weeks), infancy (first year), childhood (2 years to puberty), and adolescence (11 to 19 years).
Early stages of development. Development of an ovarian follicle containing an oocyte, ovulation, and the phases of the menstrual cycle are illustrated. Human development begins at fertilization, approximately 14 days after the onset of the last normal menstrual period. Cleavage of the zygote in the uterine tube, implantation of the blastocyst in the endometrium (lining) of the uterus, and early development of the embryo are also shown. The alternative term for the umbilical vesicle is the yolk sac; this is an inappropriate term because the human vesicle does not contain yolk.
Developmental Periods
It is customary to divide human development into prenatal (before birth) and postnatal (after birth) periods. The development of a human from a zygote to birth is divided into two main periods, embryonic and fetal . The main changes that occur prenatally are illustrated in the Timetable of Human Prenatal Development (see Fig. 1.1 ). Examination of the timetable reveals that the most visible advances occur during the third to eighth weeks—the embryonic period. During the fetal period, differentiation and growth of tissues and organs occur, and the rate of body growth increases.
Stages of Embryonic Development
Early development is described in stages because of the variable period it takes for embryos to develop certain morphologic characteristics. Stage 1 begins at fertilization, and embryonic development ends at stage 23, which occurs on day 56 (see Fig. 1.1 ). A trimester is a period of 3 months, one third of the 9-month period of gestation. The most critical stages of development occur during the first trimester (13 weeks), when embryonic and early fetal development is occurring.
Postnatal Period
This is the period occurring after birth. Explanations of frequently used postnatal developmental terms and periods follow.
Infancy
Infancy is the period of extrauterine life, roughly the first year after birth. An infant age 1 month or younger is called a neonate (newborn) . The transition from intrauterine to extrauterine existence requires many critical changes, especially in the cardiovascular and respiratory systems. If neonates survive the first crucial hours after birth, their chances of living are usually good. The body grows rapidly during infancy; total length increases by approximately one half, and weight is usually tripled. By 1 year of age, most infants have six to eight teeth.
Childhood
This is the period between infancy and puberty. The primary (deciduous) teeth continue to appear and are later replaced by the secondary (permanent) teeth. During early childhood, there is active ossification (formation of bone), but as the child becomes older, the rate of body growth slows down. Just before puberty, however, growth accelerates—the prepubertal growth spurt.
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