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).
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.
Puberty
Puberty is the period when humans become functionally capable of procreation (reproduction). In females , the first signs of puberty may be after age 8; in males, puberty commonly begins at age 9.
Adulthood
Attainment of full growth and maturity is generally reached between the ages of 18 and 21 years. Ossification and growth are virtually completed during early adulthood (21 to 25 years). Brain development continues into early adulthood, including changes in gray-matter volume.
Significance of Embryology
Clinically oriented embryology refers to the study of embryos; the term generally means the prenatal development of embryos, fetuses, and neonates (infants aged 1 month and younger). Developmental anatomy refers to the structural changes of a human from fertilization to adulthood; it includes embryology, fetology, and postnatal development. Teratology is the division of embryology and pathology that deals with abnormal development (birth defects). This branch of embryology is concerned with various genetic and/or environmental factors that disturb normal development and produce birth defects (see Chapter 20 ).
Clinically oriented embryology:
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Bridges the gap between prenatal development and obstetrics, perinatal medicine, pediatrics, and clinical anatomy
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Develops knowledge concerning the beginnings of life and the changes occurring during prenatal development
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Builds an understanding of the causes of variations in human structure
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Illuminates clinically oriented anatomy and explains how normal and abnormal relations develop
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Supports the research and application of stem cells for the treatment of certain chronic diseases
The knowledge that physicians have of normal development and the causes of birth defects is necessary for giving the embryo and fetus the best possible chance of developing normally. Much of the modern practice of obstetrics involves applied embryology . Embryologic topics of special interest to obstetricians are oocyte and sperm transport, ovulation, fertilization, implantation, fetal-maternal relations, fetal circulation, critical periods of development, and causes of birth defects.
In addition to caring for the mother, physicians guard the health of the embryo and fetus. The significance of embryology is readily apparent to pediatricians because some of their patients have birth defects resulting from maldevelopment, such as diaphragmatic hernia, spina bifida cystica, and congenital heart disease.
Birth defects cause most deaths during infancy . Knowledge of the development of structure and function is essential for understanding the physiologic changes that occur during the neonatal period (first 4 weeks) and for helping fetuses and neonates in distress. Progress in surgery, especially in the fetal, perinatal, and pediatric age groups, has made knowledge of human development even more clinically significant. Surgical treatment of fetuses is now possible in some situations. The understanding and correction of most defects depend on knowledge of normal development and the deviations that may occur. An understanding of common congenital birth defects and their causes also enables physicians, nurses, and other health-care providers to explain the developmental basis of birth defects, often dispelling parental feelings of guilt.
Health-care professionals who are aware of common birth defects and their embryologic basis approach unusual situations with confidence rather than surprise. For example, when it is realized that the renal artery represents only one of several vessels originally supplying the embryonic kidney, the frequent variations in the number and arrangement of renal vessels are understandable and not unexpected.
Historical Gleanings
If I have seen further, it is by standing on the shoulders of giants. —Sir Isaac Newton, English mathematician, 1643–1727
This statement, made more than 300 years ago, emphasizes that each new study of a problem rests on a base of knowledge established by earlier investigators. The theories of every age offer explanations based on the knowledge and experience of investigators of the period. Although we should not consider them final, we should appreciate rather than scorn their ideas. People have always been interested in knowing how they developed and were born and why some embryos and fetuses develop abnormally. Ancient people developed many answers to the reasons for these birth defects.
Ancient Views of Human Embryology
Egyptians of the Old Kingdom, approximately 3000 BC, knew of methods for incubating birds’ eggs. Akhnaton (Amenophis IV) praised the sun god Aton as the creator of the germ in a woman, maker of the seed in man, and giver of life to the son in the body of his mother. The ancient Egyptians believed that the soul entered the infant at birth through the placenta.
A brief Sanskrit treatise on ancient Indian embryology is thought to have been written in 1416 BC. This scripture of the Hindus, called Garbha Upanishad , describes ancient views concerning the embryo. It states:
From the conjugation of blood and semen [seed], the embryo comes into existence. During the period favorable for conception, after the sexual intercourse, [it] becomes a Kalada [one-day-old embryo]. After remaining seven nights, it becomes a vesicle. After a fortnight it becomes a spherical mass. After a month it becomes a firm mass. After two months the head is formed. After three months the limb regions appear.
Greek scholars made many important contributions to the science of embryology. The first recorded embryologic studies are in the books of Hippocrates of Cos , the famous Greek physician (circa 460–377 BC), who is regarded as the father of medicine . To understand how the human embryo develops, he recommended:
Take twenty or more eggs and let them be incubated by two or more hens. Then each day from the second to that of hatching, remove an egg, break it, and examine it. You will find exactly as I say, for the nature of the bird can be likened to that of man.
Aristotle of Stagira (circa 384–322 BC), a Greek philosopher and scientist, wrote a treatise on embryology in which he described the development of the chick and other embryos. Aristotle promoted the idea that the embryo developed from a formless mass, which he described as a “less fully concocted seed with a nutritive soul and all bodily parts.” This embryo, he thought, arose from menstrual blood after activation by male semen.
Claudius Galen (circa 130–201 AD), a Greek physician and medical scientist in Rome, wrote a book, On the Formation of the Foetus , in which he described the development and nutrition of fetuses and the structures that we now call the allantois, amnion, and placenta.
The Talmud contains references to the formation of the embryo. The Jewish physician Samuel-el-Yehudi, who lived during the second century AD, described six stages in the formation of the embryo, from a “formless, rolled-up thing” to a “child whose months have been completed.” Talmud scholars believed that the bones and tendons, the nails, the marrow in the head, and the white of the eyes were derived from the father, “who sows the white,” but the skin, flesh, blood, and hair were derived from the mother, “who sows the red.” These views were according to the teachings of both Aristotle and Galen.
Embryology in the Middle Ages
The growth of science was slow during the medieval period, but a few high points of embryologic investigation undertaken during this time are known to us. It is cited in the Quran (seventh century AD), the holy book of Islam, that human beings are produced from a mixture of secretions from the male and female. Several references are made to the creation of a human being from a nutfa (“small drop”). Reference is made to the leech-like appearance of the early embryo. Later the embryo is said to resemble a “chewed substance.”
Constantinus Africanus of Salerno (circa 1020–1087 AD) wrote a concise treatise entitled De Humana Natura . Africanus described the composition and sequential development of the embryo in relation to the planets and each month during pregnancy, a concept unknown in antiquity. Medieval scholars hardly deviated from the theory of Aristotle , which stated that the embryo was derived from menstrual blood and semen. Because of a lack of knowledge, drawings of the fetus in the uterus often showed a fully developed infant frolicking in the womb ( Fig. 1.2 ).
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