2.2.Introduction to the developing human

Introduction to the developing human Human development is a continuous process that begins when an oocyte(ovum) from a female is fertilized by a sperm(spermatozoon) from a male.Cell division,cell migration,programmed cell death,differentiation,and growth,and cell rearrangement transform the fertilized oocyte,a highly specialized,totipotent cell,a zygote,into a multicellular human being.Although most developmental changes occur during the embryonic and fetal periods,important changes occur during later periods of development:infancy,childhood,adolescence,and early adulthood.Development does not stop at birth.Important changes,in addition to growth,occur after birth(e.g.,development of teeth and female breasts). Developmental periods: It is customary to divide human development into prenatal(before birth)and postnatal(after birth) periods.During the prenatal period,the most visible advances occur during the third to eighth weeks of embryonic development.During the fetal period differentiation and growth of tissue and organs occur.The rate of body growth increases during this period. Significance of embryology Literally,embryology means the study of embryos;however,the term generally refers to prenatal development of embryos and fetuses. Developmental anatomy is the field of embryology concerned with the changes that cells,tissues,organs,and the body as a whole undergo from a germ cell of each parent to the resulting adult.Prenatal development is more rapid than postnatal development and result in more striking changes 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. Embryology: Bridges the gap between prenatal development and obstetrics,perinatal medicine,pediatrics,and clinical anatomy. Develops knowledge concerning the beginning of human life and changes occurring during prenatal development. Is of practical value in helping to understand the causes of variations in human structure. Illuminates gross anatomy and explains how normal and abnormal relations develop. Knowledge that physicians have of normal development and of the causes of anomalies is necessary for giving the embryo and fetus the greatest possible chance of developing normally.Much of the modern practice of obstetrics involves applied embryology.Embryologic topic of special interest to obstetricians are ovulation,oocyte and sperm transport,fertilization,implantation,fetal-maternal relations,fetal circulation,critical periods of development, and causes of birth defects.In additon 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,e.g.,diaphragmatic hernia,,spina bifida,and congenital heart disease. Developmental anomalies cause most deaths during infancy.Knowledge of the development of structure and function is essential for understanding the physiologic changes that occur during the newborn period and for helping fetuses and babies in distress.Progress in surgery,especially in the fetal,perinatal,and pediatric age groups,has made knowledge of human development even more cinlically significant.Surgical treatment of the fetus is now possible.The understanding and correction of most congenital anomalies depend on knowledge of normal development and of the deviations that may occur.An understanding of common congenital anomalies and their causes also enables physicians,dentists,and other health care providers to explain the developmental basis of abnormalities,often dispelling parental guilt feelings. Molecular biology of human development: Rapid advances in the field of molecular biology have led to the application of sophisticated techniques(e.g.,recombinant DNA technology,chimeric models,transgenic mice,and stem cell manipulation).These techniques are now widely used in research laboratories to address such diverse problems as the genetic regulation of morphogenesis,the temporal and regional expression of specific genes,and how cells are committed to form the various parts of the embryo.For the first time,we are beginning to understand how,when,and where selected genes are activated and expressed in the embryo during normal and abnormal development The first mammal,Dolly the sheep, was cloned in 1997 by Ian Wilmut and his colleagues using the technique of somatic cell nuclear transfer.Since then,other animals have been successfully cloned from cultured differentiated adult cells.Interest in human cloning has generated considerable debate because of social,ethical,and legal implications.Moreover,there is concern that cloning may result in infants born with birth defects and serious diseases. Human embryonic stem cells are pluripotential,capable of self-renewal,and are able to differentiate into specialized cell types.The isolation and programmed culture of human embryonic stem cells hold great potential for the treatment of degenerative,malignancy,and genetic diseases. Descriptive terms in embryology: The English equivalents of the standard Latin forms of terms are given in some cases,e.g.,sperm(spermatozoon).Eponyms commonly used clinically appear in parentheses,such as uterine tube(Fallopian tube).In anatomy and embryology,several terms relating to position and direction are used,and reference is made to various planes of the body.All description of the adult are based on the assumption that the body is erect,with the upper limbs by the sides and the palms directedanteriorly.This is the anatomical position.The terms anterior or ventral and posterior or dorsal are used to describe the front or back of the body or limbs and the relations of structures within the body to one another.When describing embryos,the terms dorsal and ventral are used.Superior and inferior are used to indicate the ralative levels of different structures.For embryos,the terms cranial or rostral and caudal are used to denote relationships to the head and caudal eminence(tail),respectively.Distances from the source or attachment of a structure are designated as proximal or distal.In the lower limb,for example,the knee is proximal to the ankle and the ankle is distal to the knee. The median plane is an imaginary vertical plane of section that passes longitudinally through the body.Median sections divide the body into right and left halves.The terms lateral and medial refer to structures that are,respectively,father from or nearer to the median plane of the body.A sagittal plane is any vertical plane passing through the body that is paralled to the median plane. A transverse(axial) plane refers to any plane that is at right angles to both the median and coronal planes.A frontal(coronal) plane is any vertical plane that intesects the median plane at a right angle and divides the body into anterior or ventral and posterior or dorsal parts.