Chapter 1 Clinical Vignette 3. Although most tissues of the body are affected to some extent, the heart is not a primary target tissue of ovarian steroid hormones. Review Questions 1. D 2. E 3. B 4. A mediastinal teratoma, which is likely to have arisen from an aberrant primordial germ cell that became lodged in the connective tissue near the heart. 5. In the female,…

After the eighth week of pregnancy, the period of organogenesis (embryonic period) is largely completed, and the fetal period begins. By the end of the embryonic period, almost all the organs are present in a grossly recognizable form. The external contours of the embryo show a very large head in proportion to the rest of the body and greater development of the cranial than of the…

This chapter follows the development of the heart from a simple tubular structure to the four-chambered organ that can assume the full burden of maintaining independent circulation at birth. Similarly, the pattern of blood vessels is traced from their first appearance to an integrated system that carries blood to all parts of the embryo and the placenta. (The early stages in the establishment of the heart…

The urogenital system arises from the intermediate mesoderm of the early embryo (see Figure 6.11 ). Several major themes underlie the development of urinary and genital structures from this common precursor. The first is the interconnectedness of urinary and genital development, in which early components of one system are taken over by another during its later development. A second is the recapitulation during human ontogeny of kidney…

The initial formation of the digestive system by the lateral folding of the endodermal germ layer into a tube is described in Chapter 6 . From its beginnings as a simple tubular gut, development of the digestive system proceeds on several levels, including molecular patterning, elongation and morphogenesis of the digestive tube itself, inductions and tissue interactions leading to the formation of the digestive glands, and…

Among the earliest vertebrates, the cranial region consisted of two principal components: (1) a chondrocranium, associated with the brain and the major sense organs (nose, eye, ear); and (2) a viscerocranium , a series of branchial (pharyngeal) arches associated with the oral region and the pharynx ( Figure 14.1A ). As vertebrates became more complex, the contributions of the neural crest to the head became much more…

The major sense organs arise in large measure from the thickened ectodermal placodes that appear lateral to the neural plate in the early embryo (see Figure 6.10 ). The following descriptions begin with the most cranial placodes and continue to the most caudal. The midline adeno hypophyseal placode , located just rostral to the anterior neural ridge (see Figure 6.10D ), becomes the primordium of Rathke’s pouch (the…

The neural crest, the existence of which has been recognized for more than a century, forms an exceptionally wide range of cell types and structures, including several types of nerves and glia, connective tissue, bones, and pigment cells. Its importance and prominence are such that the neural crest has often been called the fourth germ layer of the body. Not until adequate methods of marking neural…

Many fundamental developmental processes are involved in the formation of the nervous system. Some of these dominate certain stages of embryogenesis; others occur only at limited times and in restricted locations. The major processes are as follows: 1. induction , including primary induction of the nervous system by the underlying notochord and secondary inductions driven by neural tissues themselves 2. proliferation , first as a response…

Limbs are remarkable structures that are designed almost solely for mechanical functions: motion and force. These functions are achieved through the coordinated development of various tissue components. No single tissue in the limb takes shape without reference to the other tissues with which it is associated. The limb as a whole develops according to a master blueprint that reveals itself sequentially with each successive stage in…

The construction of the tissues of the body involves developmental phenomena at two levels of organization. One is the level of individual cells, in which the cells that make up a tissue undergo increasing specialization through a process called cytodifferentiation (see discussion of restriction, determination, and differentiation [p. 84]). At the next level of complexity, various cell types develop in concert to form specific tissues through…

Congenital malformations have attracted attention since the dawn of human history. When seen in humans or animals, malformations were often interpreted as omens of good or evil. Because of the great significance attached to congenital malformations, they were frequently represented in folk art as sculptures or paintings. As far back as the classical Greek period, people speculated that maternal impressions during pregnancy (e.g., being frightened by an…

One of the most characteristic features of human embryonic development is the intimate relationship between the embryo and the mother. The fertilized egg brings little with it except genetic material. To survive and grow during intrauterine life, the embryo must maintain an essentially parasitic relationship with the body of the mother for acquiring oxygen and nutrients and eliminating wastes. It must also avoid rejection as a…

After gastrulation is complete, the embryo proper consists of a flat, three-layered disk containing the ectodermal, mesodermal, and endodermal germ layers. Its cephalocaudal axis is defined by the location of the primitive streak. Because of the pattern of cellular migration through the primitive streak and the regression of the streak toward the caudal end of the embryo, a strong cephalocaudal gradient of maturity is established. This…

While it is implanting into the uterine wall, the embryo undergoes profound changes in its organization. Until the time of implantation, the blastocyst consists of the inner cell mass, from which the body of the embryo proper arises, and the outer trophoblast, which represents the future tissue interface between the embryo and mother. Both components of the blastocyst serve as the precursors of other tissues that…

The act of fertilization releases the ovulated egg from a depressed metabolism and prevents its ultimate disintegration within the female reproductive tract. Immediately after fertilization, the zygote undergoes a pronounced shift in metabolism and begins several days of cleavage. During this time, the embryo, still encased in its zona pellucida, is transported down the uterine tube and into the uterus. Approximately 6 days later, the embryo sheds…

The application of new techniques in molecular biology and genetics continues to revolutionize the understanding of the mechanisms underlying both normal and abnormal embryonic development. It is impossible to have a contemporary understanding of embryonic development without integrating fundamental molecular and morphological aspects of embryology. This chapter introduces the most important families of molecules known to direct embryonic development and some important genetic techniques that help…

Sexual reproduction is based upon the union of an egg and a sperm. This chapter explains how the egg and sperm cells come together in the female reproductive tract so that fertilization can occur. It then outlines the complex set of interactions involved in fertilization of the egg by a sperm. OVULATION AND EGG AND SPERM TRANSPORT Ovulation Toward the midpoint of the menstrual cycle, the…

Human pregnancy begins with the fusion of an egg and a sperm within the female reproductive tract, but extensive preparation precedes this event. First, both male and female sex cells must pass through a long series of changes ( gametogenesis ) that convert them genetically and phenotypically into mature gametes , which are capable of participating in the process of fertilization. Next, the gametes must be…