Chapter 1: An Introduction to Anatomy
Chapter 1: Introduction – Overview of Developmentby John F. Neas
The Science of Embryology
Human prenatal development is awesome and fascinating. The development of a human being starts at fertilization, or conception, as a single fertilized egg slightly larger than the period at the end of this sentence. Human development culminates about thirty-eight weeks later with a complex body of trillions of cells of approximately 200 different types organized into tissues, organs, and organ systems.
Embryology is the study of the sequential developmental events that occur during prenatal development as the various tissues, organs, and systems develop from conception to birth. Human embryology is the study of the human embryo and fetus. The descriptive science of human embryology is essentially developmental anatomy, but as in adult anatomy, known functional considerations have important significance. Development includes growth (an increase in the mass of tissue) and differentiation that increase complexity. Although early development, particularly that of the embryo, is the focus of embryology, development continues after birth. Thus, developmental biology is the study that also includes the sequence of events that occur during postnatal development from birth to formation of an adult organism capable of reproduction.
Descriptive embryology helps to understand the products of genetic and environmental interaction, namely the organs and systems. Chick embryos are often studied because of the easy access through the shell and their rapid development. Amphibian and echinoderm embryos also are often studied, and mice and pig embryos are extensively investigated as mammalian models. Genetic manipulation, induction of drugs, exposure to disease, radioactive tagging or dyeing of developing tissues, and x-ray treatments are some of the commonly performed experiments that provide information that can be applied to human development and birth defects. Experimental results from studies such as these using non-human embryos provide valuable insight into developmental processes. However, great caution, must be exercised in applying the data of comparative and experimental embryology to the interpretation of human development. Cellular and subcellular factors are being studied intensively in various species by the methods of molecular biology that integrates cell biology, biochemistry, and genetics.
Importance of Human Embryology
Embryology is inherently important in understanding how a single cell develops into an adult being. The science is also important because it contributes to an understanding of human anatomy (gross, microscopic, and neural), and it helps in the interpretation of congenital abnormalities, or anomalies.
Development involves gradual modification of anatomical structures during the period from fertilization to maturity. The developmental process involves the division of cells (cleavage), the formation of diverse specialized cell types through selective changes in genetic activity (differentiation), and reorganization of the structure and functional relationships of the cell types, tissues, organs, and body systems to produce or modify anatomical structures (morphogenesis). The factors that cause precise, sequential change from one cell or type of tissue to another are incompletely understood. The potential for change, however, is known to be programmed into the genetics of each cell and that under favorable environmental conditions these characteristics become expressed. The genome operates at several levels of organization to control development.
The attainment of form by a part or by the entire embryo involves the rearrangement of cells by a number of morphogenetic processes that include relative cell movement, cell adhesiveness, invagination, condensation, fusion, cell death, proliferation, and differential rates of growth. Programmed cell death under genetic control plays an important role in morphogenesis. Interdigital cell death, for example, contributes to the shaping of the limb and the separation of the digits from the hand and foot plates. Another example involves the establishment of the definitive number of neurons in the motor columns of the spinal cord and in the sensory ganglia. Important factors in the specification of cell fates (that is, for example, whether a cell becomes a muscle cell or neuron) include the inheritance of localized information in the cytoplasm of the oocyte, interactions between cells and substances called morphogens.
Induction is the stimulation of a competent tissue or area in the embryo to differentiate in a specific direction. The optic vesicle, for example, induces the overlying ectoderm to form the lens. An inductor is any tissue or agent that stimulates another tissue to differentiate. The primary inductor of the vertebrate axis is the organizer, a term used by Hans Spemann in the 1920s in his experiments on amphibian embryos in which he found that transplantation of a certain region (chordamesoderm) induced a secondary axis in the body. Much work continues to elucidate induction. It is thought that genes that contain homeoboxes (DNA sequences that encode amino acids) may participate in the regulation of the developmental potential of the organizer.
Development is a continuum usually separated into preembryonic, embryonic, and fetal periods characterized by specific anatomical changes. Knowledge about human development provides a foundation for understanding anatomical structures. There are "sensitive" stages of morphogenesis for each organ and system during which genetic or environmental conditions (e.g., maternal nutrition, smoking, or drug use) may affect the normal development of the baby. Many clinical problems are congenital in nature and have their roots during morphogenic development.
Terminology of Position and Direction in Embryology
The terms of position and direction in human anatomy are often unsuitable for the first few weeks of development. The terms that follow, however, are unambiguous and applicable to all vertebrate embryos. Figure A shows the sagittal and median planes. Figure B shows the transverse and coronal (frontal) planes. Transverse is used for planes or sections approximately at a right angle to the trunk. Because of the forward flexion of the head during prenatal development, however, coronal and transverse planes pass obliquely through the head as compared with their usage in the adult. Figure C shows that the rostral or caudal directions follow the longitudinal axis of the neural tube (or vertebral column/skull). Caudal means nearer the lower end of the body. Cephalic and cranial refer to the head-end and toward the skull, respectively. Dorsal and ventral refer to the back and front surfaces, respectively. The terms pre-axial and postaxial are used in relationship to the longitudinal axis through a limb. The thumb and great toe are on the preaxial side of the limbs. Figure D shows the greatest length (GL), exclusive of the flexed lower limbs. This, the most useful prenatal measurement, usually differs only slightly, if at all, from the crown-rump length (C-R). Additional segments to the heel (H) must be added to include the lower limbs to obtain the "standing height."
Terminology of Position and Direction in Embryology
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