Introduction to Tissue

Approximately 200 distinctly different types of cells compose the human body. These cells are arranged and organized into four basic tissues that, in turn, are assembled to form organs. When looking at tissue at a microscopic level, the ability to detect the presence and location of the four basic tissues allows identification of the organ at which you are looking. A basic knowledge of the general characteristics and cellular composition of these tissues is essential in histology, which is the study of tissues at the microscopic level. Tissues are groups of similar cells performing a common function. There are four categories of tissues:

Epithelial tissue Connective tissue

Nervous tissue

Muscle tissue

Epithelial Tissue

Epithelial tissue, or epithelium, has the following general characteristics:

Epithelium consists of closely packed, flat cells. There is little intercellular material.

The tissue is avascular, or without blood vessels. Nutrient and waste exchange occurs through neighboring connective tissues by diffusion.

The upper surface of epithelium is free, or exposed to the outside of the body or to an internal body cavity. The basal surface rests on connective tissue. A thin, extracellular layer called the basement membrane forms between the epithelial and connective tissue.

Cell division in epithelium occurs readily to replace damaged cells.

There are two kinds of epithelial tissues:

Covering and lining epithelium covers the outside surfaces of the body and lines internal organs.

Glandular epithelium secretes hormones or other products.

Epithelium that covers or lines

Epithelial tissues that cover or line surfaces are classified by cell shape and by the number of cell layers. The following terms are used to describe these features:

Cell shape:

Squamous cells are flat. The nucleus, located near the upper surface, gives these cells the appearance of a “fried egg.”

Cuboidal cells are cube or hexagon shaped with a central, circular nucleus. These cells produce secretions (sweat, for example) or absorb substances (digested food, for example).

Columnar cells are tall with an oval nucleus near the basement membrane. These thick cells serve to protect underlying tissues or may function to absorb substances. Some have microvilli, minute surface extensions, to increase surface area for absorbing substances, while others may have cilia that help move substances over their surface (such as mucus through the respiratory tract).

Transitional cells range from flat to tall cells that can extend or compress in response to body movement.

Number of cell layers:

Simple describes a single layer of cells. Stratified describes epithelium consisting of multiple layers.

Pseudostratified describes a single layer of cells of different sizes, giving the appearance of being multilayered.

Names of epithelial tissues include a description of both their shape and their number of cell layers. The presence of cilia may also be identified in their names. For example, simple squamous describes epithelium consisting of a single layer of flat cells. Pseudostratified columnar ciliated epithelium describes a single layer of tall, ciliated cells of more than one size. Stratified epithilieum is named after the shape of the outermost cell layer. Thus, stratified squamous epithelium has outermost layers of squamous cells, even though some inner layers consist of cuboidal or columnar cells. These and other epithelial tissues are illustrated in Figure 1 .

Figure 1 Types of epithelial tissues.

Glandular epithelium

Glandular epithelium forms two kinds of glands:

Endocrine glands secrete hormones directly into the bloodstream. For example, the thyroid gland secretes the hormone thyroxin into the bloodstream, where it is distributed throughout the body, stimulating an increase in the metabolic rate of body cells.

Exocrine glands secrete their substances into tubes, or ducts, which carry the secretions to the epithelial surface. Examples of secretions include sweat, saliva, milk, stomach acid, and digestive enzymes.

Exocrine glands are classified according to their structure (see Figure 2 ):

Unicellular or multicellular describes a single-celled gland or a gland made of many cells, respectively. A multicellular gland consists of a group of secretory cells and a duct through which the secretions pass as they exit the gland.

Figure 2 Exocrine glands can be classified as either simple or compound and according to tubular or alveolar structure.

Branched refers to branching arrangement of secretory cells in the gland.

Simple or compound refers to whether the duct of the gland (not the secretory portion) does not branch or branches, respectively.

Tubular describes a gland whose secretory cells form a tube, while alveolar (or acinar) describes secretory cells that form a bulblike sac.

Exocrine glands are also classified according to their function (see Figure 3 ):

In merocrine glands, secretions pass through the cell membranes of the secretory cells.

Figure 3 Exocrine glands can be classified according to their function.

In apocrine glands, a portion of the cell containing secretions is released as it separates from the rest of the cell.

In holocrine glands, entire secretory cells disintegrate and are released along with their contents.

Connective Tissue

A summary of the various kinds of connective tissues is given in Figure 1 and Table 1 . Some general characteristics of connective tissues follow.

Figure 1 General characteristics of connective tissues.

TABLE 1 Kinds of Connective Tissue

Tissue Type Cells Present Fibers Present Matrix Characteristics

Loose Connective Tissue:

areolar fibroblasts macrophages adipocytes mast cells plasma cells

collagen elastic reticular

loosely arranged fibers in gelatinous ground substance

adipose adipocytes reticular collagen

closely packed cells with a small amount of gelatinous ground substance; stores fat

reticular reticular cells reticular loosely arranged fibers in gelatinous ground substance

Dense Connective Tissue:

dense regular fibroblasts collagen (some elastic)

parallel-arranged bundles of fibers with few cells and little ground substance; great tensile strength

dense regular fibroblasts collagen (some elastic)

Irregularly arranged bundles of fibers with few cells and little ground substance; high tensile strength

Cartilage:

hyaline (gristle) chondrocytes collagen (some elastic)

limited ground substance; dense, semisolid matrix

fibrocartilage chondrocytes collagen (some elastic)

limited ground intermediate between hyaline cartilage and dense connective tissue

Tissue Type Cells Present Fibers Present Matrix Characteristics

elastic chondrocytes elastic limited ground substance; flexible but firm matrix

Bone (osseous tissue):

compact (dense) osteoblasts osteocytes

collagen rigid, calcified ground substance with (canal systems)

spongy (cancellous)

osteoblasts osteocytes

collagen rigid, calcified ground substance (no osteons)

Blood & Lymph (vascular tissue):

blood erythrocytes leukocytes thrombocytes

“fibers” are soluble proteins that form during clotting

“matrix” is liquid blood plasma

lymph leukocytes “fibers” are soluble liquid proteins that form during clotting

“matrix” is blood plasma

Nerve supply. Most connective tissues have a nerve supply (as does epithelial tissue).

Blood supply. There is a wide range of vascularity among connective tissues, although most are well vascularized (unlike epithelial tissues, which are all avascular).

Structure. Connective tissue consists of scattered cells immersed in an intercellular material called the matrix. The matrix consists of fibers and ground

substance. The kinds and amounts of fiber and ground substance determine the character of the matrix, which in turn defines the kind of connective tissue.

Cell types. Fundamental cell types, characteristics of each kind of connective tissue, are responsible for producing the matrix. Immature forms of these cells (whose names end in blast) secrete the fibers and ground substance of the matrix. Cells that have matured, or differentiated (whose names often end in cyte), function mostly to maintain the matrix.

o Fibroblasts are common in both loose and dense connective tissues.

o Adipocytes, or flat cells, occur in loose connective tissue.

o Reticular cells resemble fibroblasts, but have long, cellular processes (extensions). They occur in loose connective tissue.

o Chondroblasts and chondrocytes occur in cartilage.

o Osteoblasts and osteocytes occur in bone.

o Hemocytoblasts occur in the bone marrow and produce erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (blood platelets).

o In addition to the fundamental cell types, various leukocytes migrate from the bone marrow to connective tissues and provide various body defense activities.

o Macrophages engulf foreign and dead cells.

o Mast cells secrete histamine, which stimulates immune responses.

o Plasma cells produce antibodies.

Fibers. Matrix fibers are proteins that provide support for the connective tissue. There are three types:

o Collagen fibers, made of the protein collagen, are both tough and flexible.

o Elastic fibers, made of the protein elastin, are strong and stretchable.

o Reticular fibers, made of thin collagen fibers with a glycoprotein coating, branch frequently to form a netlike (reticulate) pattern.

Ground substance. Ground substance may be fluid, gel, or solid, and, except for blood, is secreted by the cells of the connective tissue.

o Cell adhesion proteins hold the connective tissue together.

o Proteoglycans provide the firmness of the ground substance. Hyaluronic sulfate and chondroitin sulfate are two examples.

Classification. There are five general categories of mature connective tissue:

o Loose connective tissue has abundant cells among few or loosely arranged fibers and a sparse to abundant gelatinous ground substance.

o Dense connective tissue has few cells among a dense network of fibers with little ground substance.

o Cartilage has cells distributed among fibers in a firm jellylike ground substance. Cartilage is tough, but flexible, avascular, and without nerves.

o Bone has cells distributed among abundant fibers in a solid ground substance containing minerals, mostly calcium phosphate. Bone is organized in units, called osteons (Haversian system). Each osteon consists of a central canal (Haversian canal), which contains blood vessels and nerves, surrounded by concentric rings (lamellae) of hard matrix and collagen fibers. Between the lamellae are cavities (lacunae) that contain bone cells (osteocytes). Canals (canaliculi) radiate from the central canal and allow nutrient and waste exchange with the osteocytes.

o Blood is composed of various blood cells and cell fragments (platelets) distributed in a fluid matrix called blood plasma.

Tissue origin. All mature connective tissues originate from embryonic connective tissue. There are two kinds of embryonic connective tissues:

o Mesenchyme is the origin of all mature connective tissues.

o Mucous connective tissue is a temporary tissue formed during embryonic development.

An epithelial membrane is a combination of epithelial and connective tissues working together to perform a specific function. As such, it acts as an organ. There are four principle types of epithelial membranes:

Serous membranes line interior organs and cavities. The serous membranes that line the heart, lungs, and abdominal cavities and organs are called the pericardium, pleura, and peritoneum, respectively.

Mucous membranes line body cavities that open to the outside of the body. These include the nasal cavity and the digestive, respiratory, and urogenital tracts.

Synovial membranes line the cavities at bone joints.

The cutaneous membrane is the skin.

Nervous Tissue

Nervous tissue consists of two kinds of nerve cells:

Neurons are the basic structural unit of the nervous system. Each cell consists of the following parts (see Figure 1 ).

Figure 1 A neuron is a basic structural unit of the nervous system containing a cell body, dendrites, and an axon.

The cell body contains the nucleus and other cellular organelles.

The dendrites are typically short, slender extensions of the cell body that receive stimuli.

The axon is typically a long, slender extension of the cell body that sends stimuli.

Neuroglia, or glial cells, provide support functions for the neurons, such as insulation or anchoring neurons to blood vessels.

Muscle Tissue

There are three kinds of muscle tissues (see Figure 1 ):

Skeletal muscle consists of long cylindrical cells that, under a microscope, appear striated with bands perpendicular to the length of the cell. The many nuclei in each cell (multinucleated cells) are located near the outside along the plasma

membrane. Skeletal muscle is attached to bones and causes movements of the body. Because it is under your conscious control, it is also called voluntary muscle.

Figure 1 Three kinds of muscle tissue exist: skeletal muscle, cardiac muscle, and smooth muscle.

Cardiac muscle, like skeletal muscle, is striated. However, cardiac muscle cells have single, centrally located nucleus, and the muscle fibers branch often. Where two cardiac muscle cells meet, they form an intercalated disc containing gap junctions, which bridge the two cells.

Smooth muscle consists of cells with a single, centrally located nucleus. The cells are elongated with tapered ends and do not appear striated. Smooth muscle lines the walls of blood vessels and certain organs such as the digestive and urogenital tracts, where it serves to advance the movement of substances. Smooth muscle is called involuntary muscle because it is not under direct conscious control.