Nucleus What you should have learned from this exercise:Understanding of the following terms: euchromatinheterochromatinWhat functional difference underlies the different appearance of euchromatin and heterochromatin. (euchromatin stains LIGHT; heterochromatin stains DARK)The general function of the nucleolus. That nuclei vary in appearance from one cell type to another. This will come in handy for identifying specific cell types. That nuclei may appear in different locations in different types of cells. This will come in handy for identifying specific cell types. That not all cells have a single nucleus, and that some cells, perhaps surprisingly, have no nucleus at all (RBCs for example). This variation often sheds light on the function of specific cell types.
Cytoplasm What you should have learned from this exercise:Understanding of the following terms: Nissl substance (large aggregations of RER ribosomes)Rough endoplasmic reticulum acidophilic (basic (+) molecules that stain with acidic dyes such as Eosin)The cellular component rendered visible by the mucicarmine stain. (mucicarmine stains mucigen/mucus the color red)The cellular component rendered visible by osmic acid. (osmic acids stains lipids the color black)The reason why lipid does not appear in sections stained with Hematoxylin and Eosin. (lipid is lost during routine histological processing for H&E staining, so lipid droplets appear empty—unstained)That different cell types may contain different specialized inclusions within their cytoplasm. These inclusions usually relate to the cell's specific function(s).
Simple squamous epithelium note the flattened and elongated cells and nuclei of the cells that line its inner surface
Simple columnar nuclei at basal end Gall bladder gastric lumenal surface and pit mucus seen at apical surface (darker stained)...will be secreted and form protective layer nuclei at basal surface (lightly stained) Gall bladder Stomach
Pseudostratified epididymis Many of these cells have apical sterocilia (long microvilli). There are ciliated columnar cells, basal rounded cells, and goblet cells (that appear at apical surface; lightly stained in this image)
Stratified squamous The surface cells are squamous, or flattened (arrows); this is the basis for the "squamous" classification of this epithelium, even though cells in the underlying layers are more rounded. Note that this epithelium is keratinized (50X, *) (50X). Note the absence of blood vessels in the epithelium (all epithelia are avascular).
Keratinized vs. Nonkeratinized Stratified Squamous Note that the epidermal (outside the mouth) surface is keratinized (20X). Note that the oral mucosal (inside the mouth) surface is non-keratinized (40X). Observe the transition zone (schematic; 20X) between these two surfaces (note: this is NOT a "transitional" epithelium, just a region where one type of epithelium changes into another). The transition zone corresponds to the red (vermillion) area of the lip. Non-keratinized Keratinized
Stratified cuboidal two cells layers that form the (bi)stratified cuboidal epithelium (2X; 40X) of eccrine sweat gland ducts. rare
Stratified columnar WILL NEVER BE ON THE EXAM!!! Too difficult to image… DO NOT CHOOSE THIS IF IT’S AN ANSWER!
Transitional Note that the shapes of the surface cells differ in these two cases; rounded in the relaxed state and flattened in the distended state. Compare the apparent number of cell layers in relaxed and distended states. Can be BINUCLEATE Relaxed (contracted) Distended (stretched)
Glandular Epithelium- Exocrine Glands Epithelial cells that produce secretory products are called glands. Glands may be unicellular, like the goblet cell of the small intestine, but most are multicellular.Exocrine glands are multicellular glands that deliver their secretory product into a duct. Exocrine glands are classified according to the type of secretory element (acinar, tubular, or tubulo-acinar), and the degree of branching (simple vs. compound). Types of exocrine glands:
Glandular Epithelium- Endocrine Gland Endocrine glands are multicellular glands that have no duct. They release their secretory product directly into the surrounding extracellular space, from which it diffuses into capillaries. We will study endocrine glands in detail in later lab exercises. Back to Exocrine glands…
Goblet Cell- merocrine (eccrine) Look for goblet-shaped, mucus-secreting cells, located within the intestinal mucosal epithelium (4X). Each of these cells is itself a unicellular gland. Release occurs by exocytosis of mucigen granules, i.e. a merocrine (eccrine) mechanism. Notice that the nuclei of these goblet cells (arrows, 40X; schematic; EM) have been displaced basally by the large amount of intracellular mucus. Note that the mucigen granules distend the apical portions of the cells. You may see some mucus(*) that has been released into the gut lumen. Red stain is many goblet cells (4x) *=mucus already released Arrows=nuclei (unstained)
Salivary Gland Serous alveolar cells The small, dark-blue granules in the serous acini are granules of protein that will be released into the saliva by exocytosis (i.e. merocrine secretion). Mucous alveolar cells Duct cells
Sebaceous Glands- holocrine Note that these glands (5X; 40X) are found in association with hair follicles. The arrow in the 40X image points to a hair shaft. Study the cells (*) within the acini of these glands; note their cytoplasmic appearance. The frothy material, an oily substance called sebum, is released by a holocrine process. Sebum is extracted from tissues during routine histological processing; therefore, sebum-secreting cells stain poorly with H&E. ACINUS= cluster of cells that resembles a many-lobed “berry”
Sweat glands- merocrine (eccrine) Identify the secretory (arrows) and duct (double arrows) portions of these glands (2X; 50X) deep within the dermis. Look for finger-like projections extending across the sweat gland alveoli. These myoepithelial cells (schematic; actin-stained LM; 100X) ("basket cells") are epithelial cells that have contractile properties; here, they are found in association with sweat glands. The nuclei (double arrow) of these basket cells lie between the basement membrane of the epithelium and the secretory cells. Arrow=myoepithelial cells acting on secretory region (contraction squeezes product out to duct)
Mammary Gland- merocrine & apocrine Merocrine and apocrine secretion both occur during lactation; lipid globules are released by an apocrine process, while milk protein is secreted by a merocrine process (schematic). Examine both active (4X) and inactive (4X) mammary gland sections, and compare secretory lobules in these two conditions (schematic). Note the abundant C.T. (connective tissue) in resting mammary gland.
Cilia Too small to be seen with light microscopy! The epithelium is coated with a layer of mucus that is constantly moved upwards by the active beating of cilia. Note the dense distribution of cilia (40X; 100X; schematic; EM) on the apical surfaces of many of these epithelial cells.
Microvilli Normal microvilli are too small to be seen with light microscope! BUT: High concentrations of microvilli, such as those of the inner lining of the small intestine, form structures that can be seen by LM; these structures are called "striate border"or "brush border". Unusually large microvilli, called "stereocilia", are large enough to be visible by LM.
Microvilli- Striate Border of Small Intestine Notice the pronounced ribbon-like layer at the apex of the intestinal epithelial cells (schematic; 40X, white arrows; 100X; EM). The density and uniform length of the microvilli give it this appearance. You may be able to see the striations that give this structure its name; these are bundles of microvilli. Note that individual microvilli are too small to see by LM Notice how goblet cells (* in the 100X micrograph) interrupt the striate border.
Microvilli- Brush Border of Kidney Lumen Organized layers of microvilli may be called striate (striped) borders, or brush borders. Within the cortex of the kidney, examine the lumenal surface (i.e. the inner surface of the epithelium that lines the lumen) of proximal convoluted tubules (PCTs) to find the prominent brush border that characterizes this region of the renal nephron. (schematic; 20X; 100X; EM) In many cases, the lumen (double arrows in the 100X micrograph) appears to be nearly occluded by the brush border. Notice that the brush border in the kidney is somewhat less regular than the striate border of the small intestine, although both structures are made up of dense arrays of microvilli.
Stereocilia Note that these microvilli are considerably longer than those of striate and brush borders, and that they tend to clump (1X; 10X; 40X). Compare these stereocilia with cilia found in the efferent ductules (5X; 40X). It should be apparent that cilia and stereocilia cannot easily be distinguished in the light microscope Stereocilia are large microvilli, NOT cilia! Looks very similar to stereocilia…hard to distinguish
Terminal Bar (aka junctional complex in light microscopy) "Terminal bar" (100X; 100X) is a light-microscopic term for junctional complex (schematic; EM), an apical grouping of tight junctions, adherent junctions and desmosomes. In cross sections, the terminal bars will appear as a row of small dots at the apical border of the epithelium; you may also see it form an open mesh in a fortuitous oblique section. You should scan the field to find a good example.
Desmosomes During histological preparation, epithelial cells of the stratum spinosum layer tend to shrink, increasing the intercellular spaces. Desmosomes (macula adherens, 100X) remain intact, creating "bridges" (100X; EM) between the cells, and making the cells look spiny (hence the name, "spinosum").
Basement membrane (aka Basal Lamina) In the elastic-stained specimen, look for the thick yellowish band below the epithelial cells that line the inside of the trachea (100X). This is the thickest general basement membrane in the body. Note that the basement membrane is extracellular (EM). Bear in mind that all epithelia rest on a basement membrane Basal lamina of kidney