Thyroid and parathyroid glands develop as epithelial outgrowths from the floor and walls of the pharynx; they then lose their attachments from these sites of original outgrowth. As an epithelial outgrowth of the pharyngeal wall, the thymus grows into the mediastinum and also loses its original connection. Figure 4.5 summarizes the derivatives of the three germ layers.
Recognition of tissues is based on the presence of specific components within cells and on specific cellular relationships.
Keeping these few basic facts and concepts about the fundamental four tissues in mind can facilitate the task of examining and interpreting histologic slide material. The first goal is to recognize aggregates of cells as tissues and determine the special characteristics that they present. Are the cells present at a surface? Are they in contact with their neighbors, or are they separated by definable intervening material? Do they belong to a group with special properties such as muscle or nerve?
The structure and the function of each fundamental tissue are examined in subsequent chapters. In focusing on a single specific tissue we are, in a sense, artificially separating the constituent tissues of organs. However, this separation is necessary to understand and appreciate the histology of the various organs of the body and the means by which they operate as functional units and integrated systems.
chapter 4 Tissues: Concept and Classific ation  I DE NTI FYI N G TISSU ES 103 FIGURE 4.5  Derivatives of the three germ layers. Schematic drawing illustrates the derivatives of the three germ layers: ectoderm, endoderm, and mesoderm. GI, gastrointestinal. (Based on Moore KL, Persaud TVN. The Developing Human, Clinically Oriented Embryology. Philadelphia: WB Saunders, 1998.) cranial and sensory ganglia and nerves adrenal medulla melanocytes pharyngeal arch cartilages head mesenchyme and connective tissue Schwann cells odontoblasts surface ectoderm epidermis, hair, nails, cutaneous, and mammary glands anterior pituitary gland enamel of theeth internal ear corneal epithelium and lens of eye neuroectoderm (neural tube) central nervous system retina pineal body posterior pituitary gland head mesoderm cranium (skull) connective tissue of head dentin paraxial mesoderm skeletal muscle of trunk and limbs except cranium muscles of head dermis of skin connective tissue intermediate mesoderm urogenital system including gonads, ducts and accessory glands lateral mesoderm connective tissue and muscle of viscera serous membranes of pleura, pericardium, and peritoneum blood and lymph cells cardiovascular and lymphatic systems spleen adrenal cortex endoderm respiratory tract (trachea, bronchi, lungs) GI tract (pharynx, esophagus, stomach, small and large intestines) urinary bladder and urachus thyroid gland tympanic cavity auditory tube tonsils parathyroid glands liver pancreas epithelial lining of: epithelial parts of: It is of clinical interest that, under certain conditions, abnor-mal differentiation may occur. Most of the tumors derive from the cells that originate from a single germ cell layer. However, if the tumor cells arise from the pluripotential stem cells, their mass may contain cells that differentiate and re-semble cells originating from all three germ layers. The result is formation of a tumor that contains a variety of mature tis-sues arranged in an unorganized fashion. Such masses are referred to as teratomas. Since pluripotential stem cells are primarily encountered in gonads, teratomas almost al-ways occur in the gonads. In the ovary, these tumors usually develop into solid masses that contain characteristics of the 
FOLDER 4.1 Clinical Correlation: Ovarian Teratomas