All the organs of the body are made up of four basic tissues. In this lab you will look at various organs in order to become acquainted with the basic tissues of which they are made. The four basic tissues are (1) epithelia, (2) connective tissue, (3) muscle tissue and (4) nervous tissue. You will not see examples of every type of epithelium, connective tissue etc., but you should understand the essential features of each of the four basic tissues. (See Fundamentals of Histology in your binders for a synopsis).
For example, all epithelia are composed of cells that are tightly joined to one another to form a barrier to the outside. All surfaces in the body are lined by an epithelium. The appearance of epithelia is however quite variable: the simple columnar epithelium lining the lumen of the duodenum is very different in appearance from the stratified squamous epithelium which forms the epidermis of the skin.
Little detail can be seen at this magnification. The luminal surface of the duodenum has fingerlike projections called villi (sing. villus). Intestinal glands extend downward from the bases of the villi. (Intestinal glands are also called crypts of Lieberkuhn). A simple columnar epithelium lines the surface of the villi and continues downward to line the intestinal glands. The core of the villi is composed of a loose connective tissue called the lamina propria. The lamina propria also fills the spaces between the intestinal glands. (Very frequently, the intestinal glands appear as "circles" below the bases of the villi because their connection to the surface is not in the plane of sectioning). Underlying the lamina propria a very thin band of smooth muscle called the muscularis mucosae.
The epithelium, lamina propria and muscularis mucosae constitute the outermost layer of the intestinal wall, called the mucosa. Thus the mucosa is composed of an epithelium, a connective tissue and a muscle tissue.
The submucosa underlies the mucosa. The submucosa is a denser (thicker collagen fibres, and in greater abundance) and less cellular connective tissue than is the lamina propria. In the duodenum, much of the submucosa is occupied by the mucous-secreting Brunners glands. The Brunners glands stain pink with the PAS stain (due to the glycoproteins in the mucous). The submucosa is a moderately dense, irregular connective tissue. The Brunners glands are composed of a single layer of cells whose height and width are about equal: a simple, cuboidal epithelium.
Underlying the submucosa is the muscularis externa (sometimes just called muscularis) which is composed of an inner circular and outer longitudinal layer of smooth muscle. (The plane of sectioning sometimes makes these layers appear reversed).
The outermost layer of the duodenum is a layer of connective tissue which forms a "wrapping" around the organ. When this connective tissue wrapping blends in with the connective tissue of adjacent structures it is called an adventitia. When the surface of an organ faces the body cavity, it has a thin epithelium which covers the CT wrapping. In this case the wrapping is called a serosa. At this magnification, it would be impossible to distinguish between an adventitia and a serosa.
At this magnification, it is possible to see the cells making up the epithelium, and the loose connective tissue of the lamina propia which forms the core of the villus. The epithelium is made up of a single layer of tall cells, it is therefore called a simple columnar epithelium. The nuclei of the epithelial cells stain purple and are located near the base of the cells. Each cell has a single nucleus, but because of the thickness of the section and the oblique plane of sectioning in some areas (ie near the top of the villus on left), nuclei may appear as clusters. A pink-staining brush border can be identified at the luminal surface of the cells. The brush border consists of infoldings of the cell membrane called microvilli, which act to increase the absorptive surface area. The staining results from the glycoproteins that are associated with the brush border.
Certain cells of the epithelium also stain pink. These are the mucous-secreting goblet cells. The glycoproteins of the mucous react with the PAS stain. The nuclei of the goblet cells are also located basally (often not visible), and the apically-stored mucous widens the upper part of the cell, so that it is goblet-shaped.
Every epithelium sits on a basement membrane. In a simple epithelium, the base of every cell rests on that membrane. The basement membrane takes up the PAS stain to a slight extent. It is identifiable on the villus at right, and on the right-hand side of the villus at left.
The lamina propria makes up the core of the villus. It is composed of reticular fibers (much finer than collagen fibres, not easily seen without special stains), some ground substance (not identifiable) and numerous cells. The cells are not tightly apposed as they are in the epithelium. Often only their purple staining nuclei are identifiable. Some of the cells are fibroblasts, which secrete the noncellular components of connective tissues (fibres and ground substance). Many of the cells are lymphocytes. Some of the other cells that may be present are plasma cells, macrophages, and neutrophils. There is no need to identify any of the cells now.
The muscularis mucosae, a thin band of smooth muscle, separates the mucosa from the submucosa. Several intestinal glands are visible in the mucosa, the spaces between them are occupied by lamina propria. The epithelium of the intestinal glands (sectioned rather obliquely here) appears identical to and is continuous with that of the villi. Much of the submucosa of the duodenum is occupied by Brunners glands, hence its connective tissue (collagen fibres) is not that prominent. Brunners glands secrete mucous and therefore stain pink with PAS. Their mucous secretions coat the surface of the duodenum and protect it from the acidic chyme arriving from the stomach. The muscularis mucosae of the duodenum is often displaced or disrupted by Brunners glands. Brunners glands are an epithelium made up of simple cuboidal cells.
The submucosa is on the right and part of the inner circular layer of the muscularis externa is on the left. The collagen bundles of the submucosa appear more wavy than the spindle-shaped smooth muslce fibres running in parallel of the muscularis externa. In any unit area, more nuclei will be present in smooth muscle than in connective tissue, because each smooth muscle cell has a nucleus (located in the middle "fat" part of the spindle-shaped cell). The collagen fibres of the submucosa are produced by fibroblasts (whose purple nuclei can sometimes be seen). Some Brunners glands are visible in the submucosa.
Smooth muscle in cross section shows various more or less circular profiles of differing diameters, depending on whether the smooth muscle fibre was sectioned at its thin tapered ends or its thick middle section. The thick sections will often have a purple-staining spot in the centre: this is the nucleus of that fibre.
A nerve plexus, called Auerbachs plexus lies between the inner circular and outer longitudinal layers of the muscularis externa. If you scan along the boundary of the two muscle layers on your slides, you will see Auerbachs plexus in one or a few places. You may see both nerve fibres and cell bodies, or just one or the other. Nerve fibres often appear as small wavy bundles. The appearance of nerve cell bodies, as shown here, is quite distinct. The slightly staining nerve cell body has a large, round, very pale nucleus, and a small darkly staining nucleolus which looks like a dot. The ends of one nerve cell body are shown by asterisks, the nucleus is on the right of the cell, the nucleolus is in the middle of the nucleus.
Thick skin covers the palms and the soles of the feet. The two parts of the skin are evident, the epidermis, a stratified squamous epithelium, and the dermis, a dense, irregular connective tissue in which bundles of collagen are evident. Projections of dermis into the epidermis are called dermal papillae. These papilla form the papillary layer of the dermis. The collagen fibres of the papillary layer are finer than those of the underlying reticular layer.
The outer part of the epidermis is made up of keratinized cells. These cells are filled with keratin, have become anucleate and are extremely flattened In thick skin the keratinized layer is very thick. The cells in the non-keratinized layers of epidermis are polygonal in shape. The outer part of the non-keratinized layer of the epidermis contains keratohyalin granules. These granules will be converted to keratin as the epidermal cells move into the outer, cornified (keratinized) layer. Cells at the outer epidermal surface are constantly being shed and replaced by cells moving up from the basal layers. The ducts of several sweat glands can be seen emerging through the cornified layer of the epidermis.
The boundary between the keratinized and non-keratinized layers of epidermis is shown. The keratohyalin granules in the outer layers of non-keratinized cells can be seen. Ducts of sweat glands can be seen going through the keratinized layer. Part of a dermal papilla can be seen projecting into the base of the epidermis.
In this high magnification of a dermal papilla, you can see several sections of blood vessels containing red blood cells, and a Meissners corpuscle, an encapsulated nerve ending that functions as a touch receptor. Meissners corpuscles are typically found in dermal papillae just beneath the epidermis.
Scattered throughout the dermis you will see sweat glands. Sweat glands are simple, coiled tubular structures. Several profiles of both the sweat gland and the duct are shown. The sweat gland itself has columnar epithelium and stains more lightly than the ducts, which are lined by two layers of cuboidal cells and stain darker red. (The lumen of the duct is not always evident). Sometimes myoepithelial cells (indicated here with asterisks) can be seen at the base of the sweat glands (but not the ducts), these cells are contractile epithelial cells that squeeze the secretion toward the duct.
You will not find hair follicles in sections of thick skin because the palms and soles lack hair.
Nerves and blood vessels will be seen scattered throughout the dermis. The nerves will appear as encapsulated wavy bundles. In such sections, axons will not appear clearly distinguishable from the endoneurium (fine connective tissue) which envelops axons and their myelin sheath. The myelin sheath is removed during standard histological preparations. The nuclei that can be seen belong to Schwann cells (produce the myelin sheath) and to fibroblasts.
Cartilage is a specialized connective tissue, consisting of cells called chondrocytes in a turgid matrix. The matrix consists of both collagen fibres and ground substance. The collagen fibres are not visible. The ground substance has many hydrophilic macromolecules which give cartilage high turgor (water pressure) and enables cartilage to resist compression but remain flexible. The negatively charged sulfate groups in these macromolecules also stain with basic dyes and exhibit metachromasia. (Metachromasia occurs when basic dyes react with tissue components such that their normal color shifts from blue to red or purple. Thus occurs because the numerous polyanions cause the dye molecules to become close enough to form dimeric or polymeric aggregates whose absorption properties differ from individual non-aggregated dye molecules.)
You will note that the matrix stains more intensely around cell clusters in the middle of the cartilage. This occurs because there is a higher concentration of ground substance macromolecules around the cells. The more densely staining area around the cell clusters is called the territorial matrix.
Cartilage is enveloped by dense, regular connective tissue called the perichondrium. The collagen fibres of the perichondrium are secreted by fibroblasts. However, the fibroblasts on the inner surface of the perichondrium can change into chondroblasts. Chondroblasts begin to secrete cartilage, which contains a different kind of collagen as well as ground substance. When chondroblasts are completely surrounded by matrix they are called chondrocytes. Chondrocytes can continue to divide and to secrete matrix. Thus cartilage can grow from the inside through the action of chondrocytes (interstitial growth) and from the periphery (appositional growth).
Some glands, located in the submucosa can be seen at the bottom left of Image 11.
The chondrocytes can be seen sitting in spaces called lacunae. In life the chondrocytes fill the lacunae, but the cells shrink, or may even fall out, during histological preparation. The chondrocytes in the middle of the cartilage are larger and are often found in isogenous groups. The chondrocytes near the perichondrium at the periphery of the cartilage (at left) are smaller and more elongated.
Skeletal muscle is arranged in parallel fibres which show striations due to the arrangement of actin and myosin filaments within them. Unlike smooth muscle, each muscle fibre remains the same width throughout its length. Skeletal muscle fibres are multinucleate and the nuclei are located at the periphery of the fibres. Each individual muscle fibre is enveloped by a thin layer of connective tissue called the endomysium, bundles of muscle fibres are enveloped by a thicker layer of CT called the perimysium, aand the entire muscle (ie. the biceps) is enveloped by the epimysium. There is no perimysium or epimysium in this section, endomysium may appear as fine connective tissue between muscle fibres.
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