4. Tissues: Concept and Classification

components of nervous tissue

neurons and glial cells

Ectodermal Derivatives

The ectoderm is the outermost of the three germ layers. The derivatives of the ectoderm may be divided into two major classes: surface ectoderm and neuroectoderm.

loose connective tissue

areolar, adipose, reticular

Examples of specialized connective tissues include

bone, cartilage, and blood.

dense connective tissue

dense regular, dense irregular, elastic

types of muscle tissue

skeletal, cardiac, smooth

CONNECTIVE TISSUE

Connective tissue is characterized on the basis of its extracellular matrix. Unlike epithelial cells, connective tissue cells are conspicuously separated from one another. The intervening spaces are occupied by material produced by the cells. This extracellular material is called the extracellular matrix. The nature of the cells and matrix varies according to the function of the tissue. Thus, the classification of connective tissue takes into account not only the cells but also the composition and organization of the extracellular matrix. Embryonic connective tissue derives from the mesoderm, the middle embryonic germ layer, and is present in the embryo and within the umbilical fold. It gives rise to various connective tissues in the body. A type of connective tissue found in close association with most epithelia is loose connective tissue (Fig. 4.2a). In fact, it is the connective tissue that most epithelia rest on. The extracellular matrix of loose connective tissue contains loosely arranged collagen fibers and numerous cells. Some of these cells, the fibroblasts, form and maintain the extracellular matrix. However, most of the cells are migrants from the vascular system and have roles associated with the immune system. In contrast, where only strength is required, collagen fibers are more numerous and densely packed. Also, the cells are relatively sparse and limited to the fiber-forming cell, the fibroblast (Fig. 4.2b). This type of connective tissue is described as dense connective tissue. Examples of specialized connective tissues include bone, cartilage, and blood. These connective tissues are characterized by the specialized nature of their extracellular matrix. For instance, bone has a matrix that is mineralized by calcium and phosphate molecules that are associated with collagen fibers. Cartilage possesses a matrix that contains a large amount of water bound to hyaluronan aggregates. Blood consists of cells and an extracellular matrix in the form of a protein-rich fluid called plasma that circulates throughout the body. Again, in all of these tissues, it is the extracellular material that characterizes the tissue, not the cells

Endodermal Derivatives

Endodermal Derivatives Endoderm is the innermost layer of the three germ layers. In the early embryo, it forms the wall of the primitive gut and gives rise to epithelial portions or linings of the organs arising from the primitive gut tube. Derivatives of the endoderm include: • alimentary canal epithelium (excluding the epithe- lium of the oral cavity and lower part of the anal canal, which are of ectodermal origin); • extramural digestive gland epithelium (e.g., the liver, pancreas, and gallbladder); • lining epithelium of the urinary bladder and most of the urethra; • respiratory system epithelium; • thyroid, parathyroid, and thymus gland epithelial components; • parenchyma of the tonsils; and • lining epithelium of the tympanic cavity and auditory (Eustachian) tubes. 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.

Epithelium

Epithelium is characterized by close cell apposition and presence at a free surface.

EPITHELIUM

Epithelium is characterized by close cell apposition and presence at a free surface. Epithelial cells, whether arranged in a single layer or in multiple layers, are always contiguous with one another. In addition, they are usually joined by specialized cell-to-cell junctions that create a barrier between the free surface and the adjacent connective tissue. The intercellular space between epithelial cells is minimal and devoid of any structure except where junctional attachments are present. Free surfaces are characteristic of the exterior of the body, the outer surface of many internal organs, and the lining of the body cavities, tubes, and ducts, both those that ultimately communicate with the exterior of the body and those that are enclosed. The enclosed body cavities and tubes include the pleural, pericardial, and peritoneal cavities as well as the cardiovascular system. All of these are lined by epithelium. Classifications of epithelium are usually based on the shape of the cells and the number of cell layers rather than on function. Cell shapes include squamous (flattened), cuboidal, and columnar. Layers are described as simple (single layer) or stratified (multiple layers). Figure 4.1 shows epithelia from three sites. Two of them (see Fig. 4.1a and b) are simple epithelia (i.e., one cell layer) that line a free surface that is exposed to the lumen of the structure. The major distinction between these two simple epithelia is the shape of the cells: cuboidal (see Fig. 4.1a) versus columnar (see Fig. 4.1b). The third example (see Fig. 4.1c) is a stratified squamous epithelium that contains multiple layers of cells. Only the top layer of squamous cells is in contact with the lumen; the other cells are connected with each other by specialized cell-to-cell anchoring junctions or to the underlying connective tissue (lower dark-stained bottom layer) by specialized cell-to- extracellular matrix anchoring junctions.

HISTOGENESIS OF TISSUES

In the early developing embryo during the gastrulation phase, a trilaminar embryo (trilaminar germ disc) is being formed. The three germ layers include the ectoderm, mesoderm, and endoderm, which give rise to all the tissues and organs.

Ovarian Teratomas

It is of clinical interest that, under certain conditions, abnormal differentiation may occur. Most of the tumors derived 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 resemble cells originating from all three germ layers. The result is the formation of a tumor that contains a variety of mature tissues arranged in an unorganized fashion. Such masses are referred to as teratomas. Since pluripotential stem cells are primarily encountered in gonads, teratomas almost always occur in the gonads. In the ovary, these tumors usually develop into solid masses that contain characteristics of the mature basic tissues. Although the tissues fail to form functional structures, frequently organ-like structures may be seen (i.e., teeth, hair, epidermis, bowel segments, and so forth). Teratomas may also develop in the testis, but they are rare. Moreover, ovarian teratomas are usually benign, whereas teratomas in the testis are composed of less differentiated tissues that usually lead to malignancy. An example of a solid-mass ovarian teratoma containing fully differentiated tissue is shown in the center micrograph of Figure F4.1.1. The low power reveals the lack of organized structures but does not allow identification of the specific tissues present. However, with higher magnification, as shown in the insets (a-f), mature differentiated tissues are evident. This tumor represents a mature teratoma of the ovary often called a demoid cyst. This benign tumor has a normal female karyotype 46XX; based on genetic studies, these tissues are thought to arise through parthenogenic oocyte development. Mature teratomas are common ovarian tumors in childhood and in early reproductive age. The example given in Figure F4.1.1 shows that one can readily identify tissue characteristics, even in an unorganized structure. Again, the important point is the ability to recognize the aggregates of cells and to determine the special characteristics that they exhibit.

Mesodermal Derivatives

Mesoderm is the middle of the three primary germ layers of an embryo. It gives rise to: • connective tissue, including embryonic connective tissue (mesenchyme), connective tissue proper (loose and dense connective tissue), and specialized connective tissues (cartilage, bone, adipose tissue, blood, and hemopoietic tissue, and lymphatic tissue); • striated muscles and smooth muscles; • heart, blood vessels, and lymphatic vessels, including their endothelial lining; • spleen; • kidneys and the gonads (ovaries and testes) with genital ducts and their derivatives (ureters, uterine tubes, uterus, ductus deferens); • mesothelium, the epithelium lining the pericardial, pleural, and peritoneal cavities; and • the adrenal cortex.

MUSCLE TISSUE

Muscle tissue is categorized on the basis of a functional property, the ability of its cells to contract. Muscle cells are characterized by large amounts of the contractile proteins actin and myosin in their cytoplasm and by their particular cellular arrangement in the tissue. To function efficiently to effect movement, most muscle cells are aggregated into distinct bundles that are easily distinguished from the surrounding tissue. Muscle cells are typically elongated and oriented with their long axes in the same direction (Fig. 4.3). The arrangement of nuclei is also consistent with the parallel orientation of muscle cells. Although the shape and arrangement of cells in specific muscle types (e.g., smooth muscle, skeletal muscle, and cardiac muscle) are quite different, all muscle types share a common characteristic. The bulk of the cytoplasm consists of the contractile proteins actin and myosin, which form thin and thick myofilaments, respectively. Skeletal muscle (see Fig. 4.3a) and cardiac muscle (see Fig. 4.3b) cells exhibit cross-striations that are produced largely by the specific arrangement of myofilaments. Smooth muscle cells (see Fig. 4.3c) do not exhibit cross-striations because the myofilaments do not achieve the same degree of order in their arrangement. Contractile proteins actin and myosin are ubiquitous in all cells, but only in muscle cells are they present in such large amounts and organized in such highly ordered arrays that their contractile activity can produce movement in an entire organ or organism.

NERVE TISSUE

Nerve tissue consists of nerve cells (neurons) and associated supporting cells of several types. Although all cells exhibit electrical properties, nerve cells, or neurons are highly specialized to transmit electrical impulses from one site in the body to another; they are also specialized to integrate those impulses. Nerve cells receive and process information from the external and internal environment and may have specific sensory receptors and sensory organs to accomplish this function. Neurons are characterized by two different types of processes through which they interact with other nerve cells and with cells of epithelial and muscle. A single, long axon (sometimes longer than a meter) carries impulses away from the cell body, which contains the neuron's nucleus. Multiple dendrites receive impulses and carry them toward the cell body. (In histologic sections, it is usually impossible to differentiate axons and dendrites because they have the same structural appearance.) The axon terminates at a neuronal junction called a synapse at which electrical impulses are transferred from one cell to the next by secretion of neuromediators. These chemical substances are released at synapses to generate electrical impulses in the adjacent communicating neuron. In the central nervous system (CNS), which comprises the brain and spinal cord, the supporting cells are called neuroglial cells. In the peripheral nervous system (PNS), which comprises the nerves in all other parts of the body, the supporting cells are called Schwann ( neurilemmal) cells and satellite cells. Supporting cells are responsible for several important functions. They separate neurons from one another, produce the myelin sheath that insulates and speeds conduction in certain types of neurons, provide active phagocytosis to remove cellular debris, and contribute to the blood-brain barrier in the CNS. In an ordinary hematoxylin and eosin (H&E)-stained section, nerve tissue may be observed in the form of a nerve, which consists of varying numbers of neuronal processes along with their supporting cells (Fig. 4.4a). Nerves are most commonly seen in longitudinal or cross-sections in loose connective tissue. Nerve cell bodies in the PNS, including the autonomic nervous system (ANS), are seen in aggregations called ganglia, where they are surrounded by satellite cells (Fig. 4.4b). Neurons and supporting cells are derived from neuroectoderm, which forms the neural tube in the embryo. Neuroectoderm originates by the invagination of an epithelial layer, the dorsal ectoderm of the embryo. Some nervous system cells, such as ependymal cells and cells of the choroid plexus in the CNS, retain the absorptive and secretory functions characteristic of epithelial cells.

IDENTIFYING TISSUES

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 the 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.

OVERVIEW OF TISSUES

Tissues are aggregates or groups of cells organized to perform one or more specific functions. At the light microscope level, the cells and extracellular components of the various organs of the body exhibit a recognizable and often distinctive pattern of organization. This organized arrangement reflects the cooperative effort of cells performing a particular function. Therefore, an organized aggregation of cells that function in a collective manner is called a tissue [Fr., tissu, woven; L., texo, to weave]. Although it is frequently said that the cell is the basic functional unit of the body, it is really the tissues, through the collaborative efforts of their individual cells, that are responsible for maintaining body functions. Cells within tissues are connected to each other by specialized anchoring junctions (cell-to-cell attachments, page 98). Cells also sense their surrounding extracellular environment and communicate with each other by specialized intercellular junctions (gap junctions, page 98); facilitating this collaborative effort allows the cells to operate as a functional unit. Other mechanisms that permit the cells of a given tissue to function in a unified manner include specific membrane receptors that generate responses to various stimuli (i.e., hormonal, neural, or mechanical). Despite their disparate structure and physiologic properties, all organs are made up of only four basic tissue types. The tissue concept provides a basis for understanding and recognizing the many cell types within the body and how they interrelate. Despite the variations in general appearance, structural organization, and physiologic properties of the various body organs, the tissues that compose them are classified into four basic types. • Epithelium (epithelial tissue) covers body surfaces, lines body cavities, and forms glands. • Connective tissue underlies or supports the other three basic tissues, both structurally and functionally. • Muscle tissue is made up of contractile cells and is responsible for movement. • Nerve tissue receives, transmits, and integrates information from outside and inside the body to control the activities of the body. Each basic tissue is defined by a set of general morphologic characteristics or functional properties. Each type may be further subdivided according to specific characteristics of its various cell populations and any special extracellular substances that may be present. In classifying the basic tissues, two different definitional parameters are used. The basis for the definition of epithelium and connective tissue is primarily morphologic; for muscle and nerve tissue, it is primarily functional. Moreover, the same parameters exist in designating the tissue subclasses. For example, whereas muscle tissue itself is defined by its function, it is subclassified into smooth and striated categories: a purely morphologic distinction, not a functional one. Another kind of contractile tissue, myoepithelium, functions as muscle tissue but is typically designated epithelium because of its location. For these reasons, tissue classification cannot be reduced to a simple formula. Rather, students are advised to learn the features or characteristics of the different cell aggregations that define the four basic tissues and their subclasses.

Tissue

are aggregates or groups of cells organized to perform one or more specific functions

Despite their disparate structure and physiologic properties, all organs are made up of only four basic tissue types.

• Epithelium (epithelial tissue) covers body surfaces, lines body cavities, and forms glands. • Connective tissue underlies or supports the other three basic tissues, both structurally and functionally. • Muscle tissue is made up of contractile cells and is responsible for movement. • Nerve tissue receives, transmits, and integrates information from outside and inside the body to control the activities of the body.

Neuroectoderm gives rise to:

• the neural tube and its derivatives, including components of the CNS, ependyma (epithelium lining the cavities of the brain and spinal cord), pineal body, posterior lobe of the pituitary gland (neurohypophysis), and the sensory epithelium of the eye, ear, and nose; • the neural crest and its derivatives, including components of the PNS (cranial, spinal, and autonomic ganglia; peripheral nerves; and Schwann cells); glial cells (oligodendrocytes and astrocytes); chromaffin (medullary) cells of the adrenal gland; enteroendocrine (APUD) cells of the diffuse neuroendocrine system; melanoblasts, the precursors of melanocytes; the mesenchyme of the head and its derivatives (such as pharyngeal arches that contain muscles, connective tissue, nerves, and vessels); odontoblasts; and corneal and vascular endothelium.

Surface ectoderm gives rise to

•epidermis and its derivatives (hair, nails, sweat glands, sebaceous glands, and the parenchyma and ducts of the mammary glands), • cornea and lens epithelia of the eye, • enamel organ and enamel of the teeth, • components of the internal ear, • adenohypophysis (anterior lobe of the pituitary gland), and • mucosa of the oral cavity and lower part of the anal canal.