Somf Background Concepts

FIG. 3» Photomicrographs oi histologic sections of the basic tissues. (A) Epithelium skill epidermis, cutrtilcoui surface it top; . Maturation occurs by proliferation of basal cells (arrows) and their migration to the surface to eventually become nonliving keratin. H Full cross section of skin: K, keratin: epidermis-, and d. dermis. [C) Connective tissue (boner. Vertebral trabecule (arrow) surrounded by bone marrow. (D) Muscle tissue (bean muscic), with cross-stria! sons family apparent. 'Si Nerve, All stained with hematoxylin and eosm <H&t:: A, >' }5S; B. '<142: C, > 37 i; D and E, '<349.

coilagens (types 1, II, Ml and V), have periodic cross-stmtions. Type I, the most abundant, is present in most connective tissues. Type 0 collagen is a major component of hyaline cartilage. The fibrillar collagens provide a major component of tissue strength. Most of the remaining collagen types are nonfibrillar, the most common of which is type IV, a major constituent of all basement membranes. Elastin fibers confer an elastic flexibility to tissues.

In the amorphous matrix, glycosaminoglycans (GAGs), with the exception of hyaluronic acid, are found covalently bound to proteins (as proteoglycans). Proteoglycans serve as major structural elements of the extracellular matrix; some proteoglycans are bound to plasma membranes and appear to be involved in adhesiveness and receptor binding. A set of large noncollagenous glycoproteins is important in binding cells to the extracellular matrix, including fibroncctins (the best understood of the non collagenous glycoproteins), la mini ns. chondronecrin, and osteonectins. Fibroncctins, found almost ubiquitously in the extracellular matrix, are synthesized by many different cell types. The circulating form, plasma fibronectin, is produced mainly by hepatocytes. Fibro-nectin's adhesive character makes it a crucial component of blood clots and of pathways followed by migrating cells. Thus, fibronectin-rich pathways guide and promote the migration of many kinds of cells during embryonic development and wound healing.

Basement membranes provide mechanical support for resident cells, serve as semipermeable barriers between tissue compartments, and act as regulators of cellular attachment, migration, and differentiation. 7'hey consist of a discrete zone of amorphous, noncollageneous glycoprotein matrix (including laminin), proteoglycans, and type IV collagen.

Basic Tissues

The basic tissues play specific functional roles and have distinctive microscopic appearances. They have their origins in embryologica! development, a stage of which is the formation of a tube with three layers in its wall: (1) an outer layer of ectoderm, (2) an inner layer of endoderm, and (3) a middle layer of mesoderm (Fig. 2). The basic tissues in animals and humans have over a hundred distinctly different types of cells, which are separated into four groups: epithelium, connective tissue, muscle tissue, and nerve tissue (Table I and Figs. 2 and 3).

Epithelium covers the internal and external body surfaces. It provides a protective barrier (e.g., skin epidermis), and an absorptive surface (e.g., gut lining), and generates internal and external secretions (e.g., endocrine and sweat glands, respectively). Epithelium derives mostly from ectoderm and endoderm, but also from mesoderm.

Structurally heterogeneous and complex epithelia accommodate diverse functions. An epithelial surface can be (1) a protective dry, cutaneous membrane (as in skin); (2) a moist, mucous membrane, lubricated by glandular secretions (digestive and respiratory tracts); (3) a moist, serous membrane, lubricated by serous fluid that derives from blood plasma (peritoneum, pleura, pericardium), lined by meso-thelium; and (4) the inner lining of the circulatory system, lubricated by blood or lymph, called endothelium. Epithelial cells play fundamental roles in the directional movement of ions, water, and macro molecules between biological compartments, including absorption, secretion, and exchange. Therefore the architectural and functional organization of epithelial cells includes structurally, biochemically, and physiologically distinct plasma membrane domains that

Solid Organs Histology Stroma Parenchyma
FIG. 6. Organization of compact organs. (Reproduced by permission from M. boryseiiko and T. Beringer, Functional Histology, 3rd ed. Copyright © Liide. Broivn, and Co.)

connective tissue and blood vessels support and provide nourishment to the epithelium. There are two basic patterns: tubular (or hollow) organs and compact (or solid) organs.

The blood vessels and the digestive, urinary-genital, and respiratory tracts have similar architectures in that each is composed of layers of tissue arranged in a specific sequence. For example, each has an inner coat consisting of an internal lining of epithelium, a middle coat consisting of layers of muscle (usually smooth muscle) and connective tissue, and an external coat consisting of connective tissue and often covered by epithelium. Specific variations reflect organ-specific functional requirements (Fig. 4).

The inner epithelial surface of a tubular organ can be either (1) a protective dry membrane (e.g., skin epidermis); (2) a moist, mucous membrane, lubricated by glandular secretions (e.g., digestive and respiratory tracts); (3) a moist, serous membrane, lubricated by serous fluid that derives from blood plasma, lined by mesothelmm (e.g., peritoneum, pleura, pericardium); or (4) rhe inner lining of the circulatory system, lubricated by blood or lymph, whose surface is composed of endothelium. Thus, blood vessels, considered tubular organs, have an intima (primarily endothelium), an epithelium, a media (primarily smooth muscle and elastm), and an adventitia (primarily collagen). The outside epithelial lining of an organ suspended in a body cavity is called a serosa. In contrast, the outer coat of an organ that blends into surrounding structures is called the adventitia.

The blood supply of an organ comes from its outer aspect. In tubular organs, large vessels penetrate the outer coat, perpendicular to it, and give off branches that run parallel to the tissue layers (Fig. 5). These vessels divide yet again to give off penetrating branches that course through the muscular layer, and branch again in the connective tissue parallel to the layers. The small blood vessels have ¡unctions (anastomoses) with one another in the connective tissue. These junctions may provide collateral pathways that can allow blood to bypass obstructions. Compact, solid organs have an extensive connective tissue framework, surrounded by a dense, connective tissue capsule (Fig. 6). Such organs have a hi jus or area of thicker connective tissue where blood vessels and other conduits (e.g., bronchi in the lungs) enter the organ. From rhe hilus, strands of connective tissue extend into the organ and may divide it into lobules. The remainder of rhe organ has a delicate structural framework, including supporting cells, extracellular matrix, and vasculature (essentially the "maintenance" or "service core'"), which constitutes the stroma.

The dominant cells in specialized tissues comprise the parenchyma (e.g., thy roglobulin-producing epithelial cells in the thyroid, or cardiac muscle cells in the heart). Parenchyma occurs in masses (e.g., endocrine glands), cords (e.g., liver), or tubules (e.g., kidney). Parenchymal cells can be arranged uniformly in an organ, or they may be segregated into a subcapsular region (cortex) and a deeper region (medulla), each performing a distinct functional role. In compact organs, the blood supply enters the hilus and then branches repeatedly to small arteries and ultimately capillaries in the parenchyma. In both tubular and compact organs, veins and nerves generally follow the course of the arteries.

Parenchymal cells are generally less resistant than stroma

TABLE 2 Regenerative Capacity of Cells Following Injury

Category

Norma] rate of replication

Response to stimulus/injury

Examples

Renewing/ labile

High

Modest increase

Skin, intestinal mucosa, bone marrow

Expanding/ stable

Low

Marked increase

Endothelium, fibroblasts, liver cells

Static/

permanent

None

No replication; replacement by scar.

Heart muscle cells, nerves

to chemical, physical, or ischemic (i.e., low blood flow) injury. Moreover, when an organ is injured, the underlying stroma must be present to permit orderly replacement of parenchymal cells, for cells capable of regeneration.

Cell Regeneration

The turnover of cell population generally is strictly regulated. The rates of proliferation of various populations are frequently divided into three categories: (1) renewing (also called labile) cells have continuous turnover, with proliferation balancing cell loss that accrues by death or physiological depletion; (2) expanding (also called stable) cells, normally having a low rate of death and replication, retain the capacity to divide following stimulation, and (3) static (also called permanent) cells have no normal proliferation, and have lost their capacity to divide. The relative replicative and regenerative capacity of various cells following injury is summarized in Table 2.

In renewing (labile) cell populations (e.g., skin, intestinal epithelium, bone marrow), cells that are less differentiated than the cells they are meant to replace (often called stem cells) proliferate to form "daughter" cells that can become differentiated. A particular stem cell produces many daughter cells, and, in some cases, several different kinds of cells can arise from a common multipotential ancestor cell (e.g., bone marrow multipotential cells lead to several different types of blood cells). In epithelia, renewing cells are at the base of the tissue layer, away from the surface; differentiation and maturation occur as the cells move toward the surface (Fig. 3A).

In expanding (stable) populations, cells can increase their rate of replication in response to suitable stimuli. Stable cell populations include glandular epithelial cells, liver, fibroblasts, smooth muscle cells, osteoblasts and vascular endothelial cells. In contrast, permanent (static) cells have virtually no normal mitotic capacity and, in general, cannot be induced to regenerate. In labile or stable populations, cells that die are generally replaced by new ones of the same kind. However, since more specialized (i.e., permanent) cells are not easily regenerated, injury to such cells is repaired by the formation of granulation tissue, which evolves gradually into a fibrotic connective tissue patch called a scar. The inability to regenerate certain tissue types results in a deficit that in certain cases can have clinical ramifications, since the function of the damaged tissue is irretrievably lost. For example, an area of dead heart muscle cells cannot be replaced by viable ones; the necrotic (dead) area is repaired by scar tissue, which not only has no contractile potential, but the remainder of the heart muscle must assume the workload of the lost tissue.

Cell Communication

Integrated systems coordinate and regulate the growth, differentiation, and metabolism of diverse cells and tissues so that the various activities of the body are coherent. Communication and regulation among cells can be local through direct intercellular connections or chemicals, or long range by extracellular products, such as endocrine hormones and soluble peptide mediators.

An important mechanism of communication (short or long range) is through chemical signals. There are three major types of chemical signaling: (1) Chemical mediators are secreted by many cells; some are taken up or destroyed rapidly, and therefore only act on cells in the immediate environment, other can act long range. (2) Hormones, produced and secreted by specialized and grouped endocrine cells, travel through the bloodstream to influence distant target cells. (3) Neurotransmitters are very short-range chemical mediators which act at specialized junctions between nerve cells (called synapses) or between nerve and muscle cells (called neuromuscular junctions).

Most chemical signals influence specific target cells that have receptors for the signal molecules, either by altering the properties or rates of synthesis of existing proteins, by initiating the synthesis of new proteins, or by acting to accomplish an immediate function, such as secretion, electrical depolarization, or contraction. Thus, target cells are adapted in two ways: (1) they have a distinct set of receptors capable of responding to a complementary set of chemical signals, and (2) they are programmed to respond to each signal in a characteristic way. The distinction is made among effects which are due to (1) a response to a cell's own secreted products (autocrine stimulation), (2) a response to secreted products of another cell in the vicinity (paracrine stimulation), or (3) a response to secretory products originating at a distance and travelling to the target cell(s) via the circulation (endocrine stimulation).

Factors in the extracellular medium control a cell's response through receptors. However, many hormones and other extracellular chemical signals are not soluble in lipids and therefore cannot diffuse across the cell membrane to interact with intracellular receptors. Therefore, a receptor protein on the surface of the target cell, or in its nucleus or cytosol, has a binding site with high affinity for a signaling substance that initiates a process called signal transduction. Processing of the resulting information activates an enzyme that generates a short-lived increase of intracellular signalling compound, termed a "second messenger," Second messenger molecules control the functions of an enormous variety of intracellular proteins by altering their activity (e.g., enzymes). Some receptors, however,

TABLE 3 Techniques for Studying Cells and Tissues"

Technique

Gross examination

Light microscopy (LM)

Transmission electron microscopy (TEM)

Scanning electron microscopy

(SEM) Enzyme histochemistry lmmunohistochemistry

In situ hybridization

Microbiologic cultures

Morphometric studies (at gross, LM or TEM levels)

Chemical, biochemical, and spectroscopic analysis

Energy-dispersive X-ray analysis (EDXA)

Autoradiography (at LM or TEM levels)

Purpose

Overall specimen configuration; many diseases and processes can be diagnosed at this level

Study overall microscopic tissue architecture and cellular structure: special stains for collagen, mucin, elastin, organisms, etc. are available.

Study ultrastructure (fine structure) and identify cells and their organelles and environment

Study topography and structure of surfaces

Demonstrate the presence and location of enzymes in gross or microscopic tissue sections

Identify and locate specific molecules, usually proteins, for which a specific antibody is available

Localizes specific DNA or RNA in tissues to assess tissue identity or recognize a cell gene product

Diagnose the presence of infectious organisms

Quantitate the amounts, configuration, and distribution of specific structures

Assess concentration of molecular or elemental constituents

Perform site-specific elemental analysis on surfaces

Locate the distribution of radioactive material in sections

"Modified by permission from F. J. Schoen, Interventional and Surgical Cardiovascular Pathology: Clinical Correlations and Basic Principles, Saunders, 1989.

do not use second messengers, but rather directly modify the activity of cytoplasmic proteins by regulating their phosphorylation and dephosphorylation. The appropriate target cells respond with great selectivity to light, transmembrane potential, carbohydrates, small amines, large proteins, and lipids. Thus, the plasma membrane translates extracellular signals into a form intelligible to the limited, conserved system of intracellular controls.

Each type of receptor includes both a binding site that recognizes its own special ligand, and a signalling domain that determines the intracellular control pathway along which its information will be sent. Closely related receptors can exert a wide variety of regulatory effects. Diverse extracellular influences thereby feed information into cells through relatively few but ubiqitous signaling pathways and intracellular messengers that control an enormous variety of responses. Normal cell growth is controlled by the opposing effects of growth stimulators and growth inhibitors (growth factors). Some of these are polypeptides present in serum or produced by cells locally, which stimulate or retard cell growth.

Pathology

As a speciality of medicine, pathology includes not only the diagnosis of disease but also study of the mechanisms by which abnormalities at the various levels of microscopic and submi-croscopic structure lead to the manifestations of macroscopic disease, thereby permitting scientifically based treatment. Disease is usually caused by environmental influences (deleterious physical or chemical stimuli), intrinsic genetic or other defects, or exaggeration of normal physiologic processes in individual cells. Cells may (1) be injured or die, (2) become hypo/hyperactive, or (3) become hyperactive with bizarre growth patterns (cancer). Lethal cell injury (cell death) is the permanent cessation of the life functions of a cell and has two forms: necrosis and apoptosis.

Necrosis comprises cell death with consequent loss of function. The body treats necrotic tissue as a foreign body and attempts to remove it by an inflammatory response. In some instances, the dead tissue is replaced by tissue of the same type by regeneration. If this cannot be accomplished, necrotic tissue is replaced by unspecialized connective tissue (fibrous repair by granulation tissue) to form a scar. In contrast, apoptosis (programmed cell death) is the chief normal mechanism of the body for eliminating unwanted cell populations. In contrast to necrosis, apoptosis induces neither an inflammatory nor a connective tissue response.

techniques for analysis of cells and tissues

There are a number of techniques available to observe cells directly in the living state in culture systems, which can be extremely useful in investigating the structure and functions of isolated cell types. Cells in culture (in vitro) can often continue to perform some of the normal functions they do in the body (in vivo). Through measurement of changes in secreted products under different conditions, for example, culture methods can be used to study how cells respond to certain stimuli. However, since cells in culture do not have the usual intercellular organizational environment, alterations from normal physiological function can be present.

Techniques commonly used to study the structure of either normal or abnormal tissues, and the purpose of each mode of analysis, are summarized in Table 3. The most widely used technique for examining tissues is light microscopy, which is described in the following paragraphs. Details of other useful procedures are available (Schoen, 1989).

Light Microscopy

The conventional light microscopy technique involves obtaining the tissue sample, followed by fixation, paraffin embed-

Iron

Calcium phosphates 'or calcium] Fibrin

Iron

Calcium phosphates 'or calcium] Fibrin

Inflammatory cell types

Movat

Gram

Merhenamme silver or periodic acid Schiff (PAS)

Prussian blue von Kossa (or alif.arin red)

Lend turn or phosphotungsnc acid hematoxylin (FT AH)

Congo red

Esterases (e.g., chloroacetate esterase for neutrophils, nonspecific esterase for macrophages)

'Reproduced by permission from F. J. Schoen, Interventional and Surgical Cardiovascular Pathology: Clinical Correlations and Basic Principles, Saunders. 1989.

Fixation

To preserve the structural relationships among cells, their environment, and subcellular structures in tissues, it is necessary to cross-link and preserve (i.e., fix) the tissue in a permanent state. Fixative solutions prevent degradation of the tissue when it is separated from its source of oxygen and nutrition (i.e., autolysis) by coagulating (i.e., cross-linking, denaturing, and precipitating) proteins. This prevents cellular hydro lytic enzymes, which are released when cells die, from degrading tissue components and spoiling tissues for microscopic analysis. Fixation also immobilizes fats and carbohydrates, reduces or

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