Netter's Atlas of Human Embryology E-Book (eBook)

CHAPTER 1

AN OVERVIEW OF DEVELOPMENTAL EVENTS, PROCESSES, AND ABNORMALITIES

PRIMORDIUM

The zygote is the beginning of human development.

OVERVIEW

Prenatal development can be divided into a period of cell division (weeks 1 and 2 after fertilization), an embryonic period (weeks 2 through 8), and a fetal period (weeks 9 through 38). In the first 2 weeks after fertilization, a blastocyst develops and sinks into the mucosal lining of the uterus during implantation. It consists of a two-layered embryonic disc of cells and three membranes that are external to it (trophoblast/chorion, amnion, and yolk sac). Most of the organ systems develop in the main embryonic period through week 8, and the embryo assumes a human appearance. The fetal period occupies the last 7 months. It is a period of growth and elaboration of organs that are already present. Three categories of genes (maternal, segmentation, and homeotic) establish patterns and tissue fates in the embryo, and dynamic interactions between cells characterize the differentiation and development of organs. Abnormal development can be classified by the cause (e.g., genetic versus environmental), by the nature of the effect on a structure or tissue, by the relationship between defects, and by their severity.

TIMELINE

FIGURE 1.1 THE FIRST AND SECOND WEEKS

Cell division and the elaboration of structures that will be outside the embryo (extraembryonic) characterize the first 2 weeks. The morula, a ball of cells, becomes hollow to form a blastocyst that develops into a placenta and membranes that will surround the future embryo. The embryo is first identifiable as a mass of cells within the blastocyst late in the first week. By the end of week 2, the embryo will be a disc two cell layers thick. The conceptus (all of the intraembryonic and extraembryonic products of fertilization) takes most of week 1 to travel down the uterine tubes to the uterine cavity. In week 2, the blastocyst sinks within the endometrial wall of the uterus (implantation).

FIGURE 1.2 THE EARLY EMBRYONIC PERIOD

The embryonic period (weeks 3 through 8) begins with gastrulation in the bilaminar disc and ends with an embryo that looks very human. The embryonic disc folds into a cylinder to establish the basic characteristics of the vertebrate body plan, and the primordia of all the organ systems develop. It is a very dynamic period of differentiation, development, and morphological change. The cardiovascular system is the first organ system to function (day 21/22) as the embryo becomes too large for diffusion to address the metabolic needs of the embryonic tissues.

FIGURE 1.3 THE LATE EMBRYONIC PERIOD

In the second half of the embryonic period, the human appearance of the embryo emerges. The neuropores have closed, the segmentation of the somites is no longer visible, and the pharyngeal arches are blending into a human-looking head. The upper and lower extremities are extending from the body, and fingers and toes develop. Eyes, ears, and a nose are visible, and the embryonic tail disappears with relative growth of the trunk.

FIGURE 1.4 THE FETAL PERIOD

The theme of the 7-month fetal period is the growth and elaboration of structures already present. Movement of the fetus within the amniotic fluid is a crucial part of the process. The fluid is maternal tissue fluid that crosses the chorion and amnion. It is increasingly supplemented by fetal urine, which is more similar to blood plasma than urine because metabolic waste products in the blood are eliminated in the placenta. The fetus swallows up to 400 mL of amniotic fluid each day for the normal development of oral and facial structures and to provide a favorable environment for the development of the epithelia lining the airway and gastrointestinal tract. The fluid is absorbed into fetal tissues via the latter.

FIGURE 1.5 SAMPLES OF EPITHELIA AND CONNECTIVE TISSUE

Histology is the microscopic study of cells, tissues, and organs. Every tissue in the body is classified as nerve, muscle, epithelium, or connective tissue. Epithelia line body surfaces and have cells in tight contact with each other. Epithelia are classified as simple (one cell layer thick) or stratified and according to the shape of the cells on the surface (e.g., squamous [flat], cuboidal, columnar). Connective tissue cells are dispersed in some type of extracellular matrix. Dense connective tissue is dense with fibers and contains a higher ratio of matrix to fibroblasts, the cells that secrete and maintain the matrix. Loose connective tissue has relatively more cells than dense connective tissue and a greater variety of fibers, cells, and matrix molecules.

FIGURE 1.6 SKIN AND EMBRYONIC CONNECTIVE TISSUE

The epidermis of skin is a stratified, squamous epithelium with a protective, keratinized layer of dead cells on the surface. The dermis is dense, irregular connective tissue where the collagen fibers are arranged in “irregular” bundles. The fascia below the skin (subcutaneous) is loose connective tissue with a high fat content. The epidermis develops from the surface ectoderm of the embryo; the connective tissue layers are derived from loose, undifferentiated embryonic connective tissue called mesenchyme (demonstrated in B, the wall of the yolk sac). Mesenchyme is a very cellular connective tissue with stellate-shaped cells.

FIGURE 1.7 INDUCTION

Induction is the interaction between two separate histological tissues or primordia in the embryo that results in morphological differentiation. One tissue usually induces the other, but one or both can participate in subsequent organogenesis. The signal varies. It may be a molecule, an extracellular matrix secreted by one of the tissues and encountered by the other, or it may require direct cellular contact between the two embryonic rudiments. Some inductions (e.g., neural tube formation) are nonspecific. A variety of factors can cause the response; the inducing tissue plays no unique role.

FIGURE 1.8 APOPTOSIS

Apoptosis is programmed cell death, an extremely important process of normal development. It is initiated in mitochondria in response to a variety of stimuli. Cytochrome c and other molecules are released into the cytoplasm, triggering a cascade of reactions involving a number of cystein proteases called caspases. The result is the condensation of chromatin in the nucleus and the degradation of DNA. There may also be caspase-independent mechanisms for apoptosis that act in very early development.

FIGURE 1.9 GENETIC DETERMINATION OF EMBRYONIC AXES AND SEGMENTS

The establishment of a bilaterally symmetrical, segmented body plan with craniocaudal and dorsoventral axes is a hallmark of chordate (and vertebrate) development. These features are the result of three gene categories: maternal effect, segmentation, and homeotic genes. Their products are mostly transcription factors that regulate the expression of other genes. Many of these genes contain a 183 base pair homeobox, a phylogenetically conservative segment whose product is the DNA-binding component of the transcription factor. These three gene groups act in sequence in a complex cascade involving regulatory gene interactions within each group, from one group to the next, and with structural genes.

FIGURE 1.10 SEGMENTATION AND SEGMENT FATES

Segmentation is expressed throughout the embryo in the formation of cranial and spinal nerves, the vertebral column and ribs, early muscle development, and patterns of blood vessel formation. The pharyngeal arches of mesoderm in the embryonic head are the most externally visible segments. Segmentation genes of the Hox gene family (and others) play a major role in arch development, and they extend their effects to the cranial somites and segments of the hindbrain (rhombomeres). Homeotic genes are required to determine the fate of the segments. Examples shown in part B include the development of ear ossicles, hyoid bone, cartilages of the larynx, etc., from mesoderm in each pharyngeal arch.

FIGURE 1.11 CELL ADHESION AND CELL MIGRATION

Most events in embryogenesis involve the association, disassociation, and migration of cells. The interrelated processes involve dynamic changes in the molecules expressed in cell membranes. Cell adhesion molecules (CAMs) cause cells to aggregate. Their inactivation is a requirement of the initiation of cell migration, but control of the migration pathway is very complex. Trails of connective tissue fibers often help guide cells, a process termed contact guidance. Chemical signals may attract cells, and an inhibitory effect of cells bordering the path may also play a role. The deposition of hyaluronic acid, a connective tissue protein that binds water, creates a favorable environment for cell migration.

FIGURE 1.12 CELL DIFFERENTIATION AND CELL FATES

Cells in the first few days before the embryo develops are totipotent. Each is capable of forming a normal embryo or developing into any of the more than 200 cell types in the body. Cells in the blastocyst, including the early embryo, are...