salivary glands

Histology

The glands are enclosed in a capsule of connective tissue and internally divided into lobules. Blood vessels and nerves enter the glands at the hilum and gradually branch out into the lobules.

Ducts

In the duct system, the lumens formed by intercalated ducts, which in turn join to form striated ducts. These drain into ducts situated between the lobes of the gland (called interlobar ducts or excretory ducts).

All of the human salivary glands terminate in the mouth, where the saliva proceeds to aid in digestion. The saliva that salivary glands release is quickly inactivated in the stomach by the acid that is present there.

Anatomy

The salivary glands are situated at the entrance to the gastrointestinal system to help begin the process of digestion.

Parotid Glands

Main article: Parotid gland

The parotid glands are a pair of glands located in the subcutaneous tissues of the face overlying the mandibular ramus and anterior and inferior to the external ear. The secretion produced by the parotid glands is serous in nature, and enters the oral cavity through the Stensen's duct after passing through the intercalated ducts which are prominent in the gland. Despite being the largest pair of glands, only approximately 25% of saliva is produced by the glands.Saliva contains a mixture of enzymes like salivary amylase(ptyalin), matase(trace amounts), lysozyme etc., salts and water. Saliva helps converting starch into maltose which is then converted patially to glucose by the maltase.

Submandibular Glands

Main article: Submandibular gland

The submandibular glands are a pair of glands located beneath the floor of the mouth, superior to the digastric muscles. The secretion produced is a mixture of both serous and mucous and enters the oral cavity via Wharton's ducts. Approximately 70% of saliva in the oral cavity is produced by the submandibular glands, even though they are much smaller than the parotid glands.

Sublingual Gland

Main article: Sublingual gland

The sublingual glands are a pair of glands located beneath the floor of the mouth anterior to the submandibular glands. The secretion produced is mainly mucous in nature, however it is categorized as a mixed gland. Unlike the other two major glands, the ductal system of the sublingual glands do not have striated ducts, and exit from 8-20 excretory ducts. Approximately 5% of saliva entering the oral cavity come from these glands.

Eukaryotic Cell Definitions: = Typically Found Only In Plant Cells = Typically Found In Animal Cells

• Golgi Apparatus: A series (stack) of flattened, membrane-bound sacs (saccules) involved in the storage, modification and secretion of proteins (glycoproteins) and lipids destined to leave the cell (extracellular) and for use within the cell (intracellular). The Golgi apparatus is abundant in secretory cells, such as cells of the pancreas.

• Golgi Vesicle: A membrane-bound body that forms by "budding" from the Golgi apparatus. It contains proteins (glycoproteins), such as digestive enzymes, and migrates to the cell (plasma) membrane. Golgi vesicles fuse with the cell membrane and discharge their contents into the exterior of the cell through a process called exocytosis. Some Golgi vesicles become lysosomes which are involved in intracellular digestion.

• Pinocytotic Vesicle: A membrane-bound vacuole formed by a specific type of endocytosis called pinocytosis. The plasma membrane invaginates (pinches inwardly) to form a vesicle that detaches and moves into the cytoplasm. Macromolecular droplets and particles up to 2 micrometers in diameter enter the cell within these pinocytotic vesicles. Larger particles (including bacteria) enter special white blood cells (phagocytes) through a form of endocytosis called phagocytosis. The Amoeba is a unicellular protist that ingests food (including algal cells) by phagocytosis.

• Lysosome: A membrane-bound organelle containing hydrolytic (digestive) enzymes. Lysosomes originate as membrane-bound vesicles (called Golgi vesicles) that bud from the Golgi apparatus. They are primarily involved with intracellular digestion. Lysosomes fuse with vesicles (small vacuoles) formed by endocytosis. The contents of these vesicles are digested by lysosomal enzymes. Autodigestion by lysosomes also occurs during embryonic development. The fingers of a human embryo are webbed initially, but are separated from each other by lysosomal enzymes. Cells in the tail of a tadpole are digested by lysosomal enzymes during the gradual transition into a frog.
• Peroxisome: A membrane-bound organelle that contains specific enzymes imported from the cytoplasm (cytosol). For example, certain peroxisomes contain the enzyme catalase which rapidly breaks down toxic hydrogen peroxide into water and oxygen. This reaction can be easily demonstrated by pouring some hydrogen peroxide on raw meat or an open wound.
• Glycolysis: An anaerobic oxidation pathway outside of the mitochondria in which glucose is oxidized to pyruvate with a net gain of 2 ATP molecules. Pyruvate is converted into a 2-carbon acetyl group which enters the Krebs cycle within the mitochondria.

• Mitochondrion: Membrane-bound organelle and the site of aerobic respiration and ATP production. Energy from the step-by-step oxidation of glucose (called the Krebs or citric acid cycle) is used to produce molecules of adenosine triphosphate (ATP). The Krebs cycle starts when a 2-carbon acetyl group combines with a 4-carbon group to form a 6-carbon citrate. Including glycolysis (which occurs outside the mitochondria), a total of 38 ATP molecules are generated from one molecule of glucose.

In eukaryotic cells, including the cells of your body, ATP is produced within special membrane-bound organelles called mitochondria. During this process, electrons are shuttled through an iron-containing cytochrome enzyme system along membranes of the cristae which result in the phosphorylation of ADP (adenosine diphosphate) to form ATP (adenosine triphosphate). ATP is the vital energy molecule of all living systems which is absolutely necessary for key biochemical reactions within the cells. The actual synthesis of ATP from the coupling of ADP (adenosine diphosphate) with phosphate (PO₄) is very complicated and involves a mechanism called chemiosmosis. The electron flow generates a higher concentration (charge) of positively-charged hydrogen (H+) ions (or protons) on one side of the membrane. When one side of the membrane is sufficiently "charged," these protons recross the membrane through special channels (pores) containing the enzyme ATP synthetase, as molecules of ATP are produced. In the membranes of prokaryotic bacterial cells, ATP is produced by a similar process. In fact, some biologists believe that mitochondria and chloroplasts within eukaryotic animal and plant cells may have originated from ancient symbiotic bacteria that were once captured by other cells in the distant geologic past. This fascinating idea is called the "Endosymbiont Theory" (or "Endosymbiont Hypothesis" for those who are more skeptical). Chloroplasts and mitochondria have outer phospholipid bilayer membranes and circular DNA molecules like those of prokaryotic bacterial cells. In addition, the layers of thylakoid membranes in the grana of chloroplasts are remarkably similar to photosynthetic cells of cyanobacteria. Acquiring cells and genomes from other organisms is known as symbiogenesis. According to L. Margulis and D. Sagan (Acquiring Genomes: A Theory of the Origins of Species 2002), symbiogenesis is a major factor in the evolution of life of earth. In fact, the author's state that long-term genomic mergers result in much greater evolutionary change than DNA mutations and natural selection.

The Parts of a Typical Microscope

The Parts of a Typical Microscope

'Microscopes"

"Microscopes" can largely be separated into three classes: optical theory microscopes (Light microscope), electron microscopes (e.g.,TEM), and scanning probe microscopes (SPM).

Optical theory microscopes are microscopes which function through the optical theory of lenses in order to magnify the image generated by the passage of a wave through the sample. The waves used are either electromagnetic (in optical microscopes) or electron beams (in electron microscopes). The types are the Compound Light, Stereo, and the electron microscope.

Optical microscopes

Main article: Optical microscope

Optical microscopes, through their use of visible wavelengths of light, are the simplest and hence most widely used type of microscope.

Optical microscopes typically use refractive lenses of glass and occasionally of plastic or quartz, to focus light into the eye or another light detector. Mirror-based optical microscopes operate in the same manner. Typical magnification of a light microscope, assuming visible range light, is up to 1500x with a theoretical resolution limit of around 0.2 micrometres or 200 nanometers. Specialized techniques (e.g., scanning confocal microscopy) may exceed this magnification but the resolution is diffraction limited. Using shorter wavelengths of light, such as the ultraviolet, is one way to improve the spatial resolution of the microscope as are techniques such as Near-field scanning optical microscope.

A stereo microscope is often used for lower-power magnification on large subjects.

Various wavelengths of light, including those beyond the visible range, are sometimes used for special purposes. Ultraviolet light is used to enable the resolution of smaller features as well as to image samples that are transparent to the eye. Near infrared light is used to image circuitry embedded in bonded silicon devices as silicon is transparent in this region. Many wavelengths of light, ranging from the ultraviolet to the visible are used to excite fluorescence emission from objects for viewing by eye or with sensitive cameras.

• phase contrast microscope:Phase contrast microscopy is an optical microscopy illumination technique in which small phase shifts in the light passing through a transparent specimen are converted into amplitude or contrast changes in the image.

A phase contrast microscope does not require staining to view the slide. This microscope made it possible to study the cell cycle.

Electron Microscope

Three major variants of electron microscopes exist:

• Scanning electron microscope (SEM): looks at the surface of bulk objects by scanning the surface with a fine electron beam and measuring reflection. May also be used for spectroscopy.
• Transmission electron microscope (TEM): passes electrons completely through the sample, analogous to basic optical microscopy. This requires careful sample preparation, since electrons are scattered so strongly by most materials.This is a scientific device that allows people to see objects that could normally not be seen by the naked or unaided eye.
• Scanning Tunneling Microscope (STM): is a powerful technique for viewing surfaces at the atomic level.

The SEM, TEM, STM are include in the scanning probe microsocpy.

Flagella

Salmonella enterica. TEM about 10,000X. Salmonella is an enteric bacterium related to E. coli. The enterics are motile by means of peritrichous flagella.

Flagella

Flagella are filamentous protein structures attached to the cell surface that provide the swimming movement for most motile procaryotes. Procaryotic flagella are much thinner than eucaryotic flagella, and they lack the typical "9 + 2" arrangement of microtubules. The diameter of a procaryotic flagellum is about 20 nanometers, well-below the resolving power of the light microscope. The flagellar filament is rotated by a motor apparatus in the plasma membrane allowing the cell to swim in fluid environments. Bacterial flagella are powered by proton motive force (chemiosmotic potential) established on the bacterial membrane, rather than ATP hydrolysis which powers eucaryotic flagella. About half of the bacilli and all of the spiral and curved bacteria are motile by means of flagella. Very few cocci are motile, which reflects their adaptation to dry environments and their lack of hydrodynamic design.

The ultrastructure of the flagellum of E. coli is illustrated in Figure 3 below (after Dr. Julius Adler of the University of Wisconsin). About 50 genes are required for flagellar synthesis and function. The flagellar apparatus consists of several distinct proteins: a system of rings embedded in the cell envelope (the basal body), a hook-like structure near the cell surface, and the flagellar filament. The innermost rings, the M and S rings, located in the plasma membrane, comprise the motor apparatus. The outermost rings, the P and L rings, located in the periplasm and the outer membrane respectively, function as bushings to support the rod where it is joined to the hook of the filament on the cell surface. As the M ring turns, powered by an influx of protons, the rotary motion is transferred to the filament which turns to propel the bacterium.

Figure 3. The ultrastructure of a bacterial flagellum (after J. Adler). Measurements are in nanometers. The flagellum of E. coli consists of three parts, filament, hook and basal body, all composed of different proteins. The basal body and hook anchor the whip-like filament to the cell surface. The basal body consists of four ring-shaped proteins stacked like donuts around a central rod in the cell envelope. The inner rings, associated with the plasma membrane, are the flagellar powerhouse for activating the filament. The outer rings in the peptidoglycan and outer membrane are support rings or "bushings" for the rod. The filament rotates and contracts which propels and steers the cell during movement.

Flagella may be variously distributed over the surface of bacterial cells in distinguishing patterns, but basically flagella are either polar (one or more flagella arising from one or both poles of the cell) or peritrichous (lateral flagella distributed over the entire cell surface). Flagellar distribution is a genetically-distinct trait that is occasionally used to characterize or distinguish bacteria. For example, among Gram-negative rods, Pseudomonas has polar flagella to distinguish them from enteric bacteria, which have peritrichous flagella.

Figure 4. Different arrangements of bacterial flagella. Swimming motility, powered by flagella, occurs in half the bacilli and most of the spirilla. Flagellar arrangements, which can be determined by staining and microscopic observation, may be a clue to the identity of a bacterium. See Figure 6 below.

Flagella were proven to be organelles of bacterial motility by shearing them off (by mixing cells in a blender) and observing that the cells could no longer swim although they remained viable. As the flagella were re-grown and reached a critical length, swimming movement was restored to the cells. The flagellar filament grows at its tip (by the deposition of new protein subunits) not at its base (like a hair).

Procaryotes are known to exhibit a variety of types of tactic behavior, i.e., the ability to move (swim) in response to environmental stimuli. For example, during chemotaxis a bacterium can sense the quality and quantity of certain chemicals in its environment and swim towards them (if they are useful nutrients) or away from them (if they are harmful substances). Other types of tactic response in procaryotes include phototaxis, aerotaxis and magnetotaxis. The occurrence of tactic behavior provides evidence for the ecological (survival) advantage of flagella in bacteria and other procaryotes.

Detecting Bacterial Motility

Since motility is a primary criterion for the diagnosis and identification of bacteria, several techniques have been developed to demonstrate bacterial motility, directly or indirectly.

1. flagellar stains outline flagella and show their pattern of distribution. If a bacterium possesses flagella, it is presumed to be motile.

Figure 5. Flagellar stains of three bacteria a. Bacillus cereus b. Vibrio cholerae c. Bacillus brevis. (CDC). Since the bacterial flagellum is below the resolving power of the light microscope, although bacteria can be seen swimming in a microscope field, the organelles of movement cannot be detected. Staining techniques such as Leifson's method utilize dyes and other components that precipitate along the protein filament and hence increase its effective diameter. Flagellar distribution is occasionally used to differentiate between morphologically related bacteria. For example, among the Gram-negative motile rod-shaped bacteria, the enterics have peritrichous flagella while the pseudomonads have polar flagella.

2. motility test medium demonstrates if cells can swim in a semisolid medium. A semisolid medium is inoculated with the bacteria in a straight-line stab with a needle. After incubation, if turbidity (cloudiness) due to bacterial growth can be observed away from the line of the stab, it is evidence that the bacteria were able to swim through the medium.

Julius Adler exploited this observation during his studies of chemotaxis in E. coli. He prepared a gradient of glucose by allowing the sugar to diffuse into a semisolid medium from a central point in the medium. This established a concentration gradient of glucose along the radius of diffusion. When E. coli cells were seeded in the medium at the lowest concentration of glucose (along the edge of the circle), they swam up the gradient towards a higher concentration (the center of the circle), exhibiting their chemotactic response to swim towards a useful nutrient. Later, Adler developed a tracking microscope that could record and film the track that E. coli takes as it swims towards a chemotactic attractant or away from a chemotactic repellent. This led to an understanding of the mechanisms of bacterial chemotaxis, first at a structural level, then at a biomolecular level.

Figure 6. Bacterial cultures grown in motility test medium. The tube on left is a non motile organism; the tube on right is a motile organism. Motility test medium is a semi-soft medium that is inoculated with a straight needle. If the bacteria are motile, they will swim away from the line of inoculation in order to find nutrients, causing turbidity or cloudiness throughout the medium. If they are non motile, they will only grow along the line of inoculation. www.jlindquist.net/ generalmicro/dfmotility.html.

3. direct microscopic observation of living bacteria in a wet mount. One must look for transient movement of swimming bacteria. Most unicellular bacteria, because of their small size, will shake back and forth in a wet mount observed at 400X or 1000X. This is Brownian movement, due to random collisions between water molecules and bacterial cells. True motility is confirmed by observing the bacterium swim from one side of the microscope field to the other side.

Wet mount of the bacterium Rhodospirillum rubrum, about 1500X mag. Click here or on the image for a short video from the Department of Microbiology and Immunology, University of Leicester, that illustrates swimming motility of this photosynthetic purple bacterium.

Figure 7. A Desulfovibrio species. TEM. About 15,000X. The bacterium is motile by means of a single polar flagellum. Of course, one can determine the presence of flagella by means of electron microscopy. Perhaps this is an alternative way to determine bacterial motility, if you happen to have an electron microscope.

Primary Structure of Biological Macromolecules Determines Function

Drawing of a typical bacterial cell, by Vaike Haas, University of Wisconsin-Madison Primary Structure of Biological Macromolecules Determines Function Procaryotic structural components consist of macromolecules such as DNA, RNA, proteins, polysaccharides, phospholipids, or some combination thereof. The macromolecules are made up of primary subunits such as nucleotides, amino acids and sugars (Table 1). It is the sequence in which the subunits are put together in the macromolecule, called the primary structure, that determines many of the properties that the macromolecule will have. Thus, the genetic code is determined by specific nuleotide base sequences in chromosomal DNA; the amino acid sequence in a protein determines the properties and function of the protein; and sequence of sugars in bacterial lipopolysaccharides determines unique cell wall properties for pathogens. The primary structure of a macromolecule will drive its function, and differences within the primary structure of biological macromolecules accounts for the immense diversity of life. Table 1. Macromolecules that make up cell material Macromolecule Primary Subunits Where found in cell Proteins amino acids Flagella, pili, cell walls, cytoplasmic membranes, ribosomes, cytoplasm Polysaccharides sugars (carbohydrates) capsules, inclusions (storage), cell walls Phospholipids fatty acids membranes Nucleic Acids (DNA/RNA) nucleotides DNA: nucleoid (chromosome), plasmids rRNA: ribosomes; mRNA, tRNA: cytoplasm Procaryotic Cell Architecture At one time it was thought that bacteria and other procaryotes were essentially "bags of enzymes" with no inherent cellular architecture. The development of the electron microscope in the 1950s revealed the distinct anatomical features of bacteria and confirmed the suspicion that they lacked a nuclear membrane. Procaryotes are cells of relatively simple construction, especially if compared to eucaryotes. Whereas eucaryotic cells have a preponderance of organelles with separate cellular functions, procaryotes carry out all cellular functions as individual units. A procaryotic cell has five essential structural components: a nucleoid (DNA), ribosomes, cell membrane, cell wall, and some sort of surface layer, which may or may not be an inherent part of the wall. Structurally, there are three architectural regions: appendages (attachments to the cell surface) in the form of flagella and pili (or fimbriae); a cell envelope consisting of a capsule, cell wall and plasma membrane; and a cytoplasmic region that contains the cell chromosome (DNA) and ribosomes and various sorts of inclusions (Figure 1). Figure 1. Cutaway drawing of a typical bacterial cell illustrating structural components. See Table 2 below for chemical composition and function of the labeled components. Table 2. Summary of characteristics of typical bacterial cell structures Structure Flagella Function(s) Swimming movement Predominant chemical composition Protein Pili Sex pilus Stabilizes mating bacteria during DNA transfer by conjugation Protein Common pili or fimbriae Attachment to surfaces; protection against phagotrophic engulfment Protein Capsules (includes "slime layers" and glycocalyx) Attachment to surfaces; protection against phagocytic engulfment, occasionally killing or digestion; reserve of nutrients or protection against desiccation Usually polysaccharide; occasionally polypeptide Cell wall Gram-positive bacteria Prevents osmotic lysis of cell protoplast and confers rigidity and shape on cells Peptidoglycan (murein) complexed with teichoic acids Gram-negative bacteria Peptidoglycan prevents osmotic lysis and confers rigidity and shape; outer membrane is permeability barrier; associated LPS and proteins have various functions Peptidoglycan (murein) surrounded by phospholipid protein-lipopolysaccharide "outer membrane" Plasma membrane Permeability barrier; transport of solutes; energy generation; location of numerous enzyme systems Phospholipid and protein Ribosomes Sites of translation (protein synthesis) RNA and protein Inclusions Often reserves of nutrients; additional specialized functions Highly variable; carbohydrate, lipid, protein or inorganic Chromosome Genetic material of cell DNA Plasmid Extrachromosomal genetic material DNA Figure 2 . Electron micrograph of an ultra-thin section of a dividing pair of group A streptococci (20,000X). The cell surface fimbriae (fibrils) are evident. The bacterial cell wall is seen as the light staining region between the fibrils and the dark staining cell interior. Cell division in progress is indicated by the new septum formed between the two cells and by the indentation of the cell wall near the cell equator. The streptococcal cell diameter is equal to approximately one micron. Electron micrograph of Streptococcus pyogenes by Maria Fazio and Vincent A. Fischetti, Ph.D. with permission. The Laboratory of Bacterial Pathogenesis and Immunology, Rockefeller University.

Cell As A Basic Unit Of Life

The one-celled organism amoeba proteus

A single-celled bacteria of the type: E. coli

A human red blood cell

A plant cell from the leaf of a poplar tree

The cell is one of the most basic units of life. There are millions of different types of cells. There are cells that are organisms onto themselves, such as microscopic amoeba and bacteria cells. And there are cells that only function when part of a larger organism, such as the cells that make up your body. The cell is the smallest unit of life in our bodies. In the body, there are brain cells, skin cells, liver cells, stomach cells, and the list goes on. All of these cells have unique functions and features. And all have some recognizable similarities. All cells have a 'skin', called the plasma membrane, protecting it from the outside environment. The cell membrane regulates the movement of water, nutrients and wastes into and out of the cell. Inside of the cell membrane are the working parts of the cell. At the center of the cell is the cell nucleus. The cell nucleus contains the cell's DNA, the genetic code that coordinates protein synthesis. In addition to the nucleus, there are many organelles inside of the cell - small structures that help carry out the day-to-day operations of the cell. One important cellular organelle is the ribosome. Ribosomes participate in protein synthesis. The transcription phase of protein synthesis takes places in the cell nucleus. After this step is complete, the mRNA leaves the nucleus and travels to the cell's ribosomes, where translation occurs. Another important cellular organelle is the mitochondrion. Mitochondria (many mitochondrion) are often referred to as the power plants of the cell because many of the reactions that produce energy take place in mitochondria. Also important in the life of a cell are the lysosomes. Lysosomes are organelles that contain enzymes that aid in the digestion of nutrient molecules and other materials. Below is a labelled diagram of a cell to help you identify some of these structures.

There are many different types of cells. One major difference in cells occurs between plant cells and animal cells. While both plant and animal cells contain the structures discussed above, plant cells have some additional specialized structures. Many animals have skeletons to give their body structure and support. Plants do not have a skeleton for support and yet plants don't just flop over in a big spongy mess. This is because of a unique cellular structure called the cell wall. The cell wall is a rigid structure outside of the cell membrane composed mainly of the polysaccharide cellulose. As pictured at left, the cell wall gives the plant cell a defined shape which helps support individual parts of plants. In addition to the cell wall, plant cells contain an organelle called the chloroplast. The chloroplast allow plants to harvest energy from sunlight. Specialized pigments in the chloroplast (including the common green pigment chlorophyll) absorb sunlight and use this energy to complete the chemical reaction:

6 CO₂ + 6 H₂O + energy (from sunlight) C₆H₁₂O₆ + 6 O₂

In this way, plant cells manufacture glucose and other carbohydrates that they can store for later use.

Organisms contain many different types of cells that perform many different functions. In the next lesson, we will examine how individual cells come together to form larger structures in the human body.