Histology Lectures - First Lecture - PDF

1st semester Histology Dr. Ramzi 1st Lecture Histology is the microscopic study of the structure of biological tissues using special staining techniques combined with light and electron microscopy. Cytology is the study of the structural components of the cell....

1st semester Histology Dr. Ramzi 1st Lecture Histology is the microscopic study of the structure of biological tissues using special staining techniques combined with light and electron microscopy. Cytology is the study of the structural components of the cell. Histology is the study of the integration of cells to form tissues and organs. Cell The cell is the basic structural unit of the tissues and organs of the body. Various cell shapes include spherical, stellate, spindle, polyhedral, squamous, cuboidal or columnar. Cell sizes range from a single micrometre to several centimetres in diameter.  Comparison between Eukaryotic cell and Prokaryotic cell Characters Eukaryotic Cell Prokaryotic Cell Nucleus Present Absent Number of chromosomes More than one One--but not true chromosome: Plasmids Cell Type Usually multicellular Usually unicellular True Membrane bound Nucleus Present Absent Example Animals and Plants Bacteria Genetic Recombination Meiosis and fusion of gametes Partial, undirectional transfers DNA Microtubules Present Absent or rare Endoplasmic reticulum Present Absent Mitochondria Present Absent Ribosomes larger smaller Vesicles Present Present Golgi apparatus Present Absent Vacuoles Present Present Cell size 10-100µm 1-10µm Cell Membrane Each cell in the body is bounded by a cell membrane (plasmalemma) which provides a barrier and controls movement of substances into and out of the cell. 1. The cell membrane is a lipid bilayer with embedded proteins. 2. Integral proteins are tightly bound within the membrane and often extend across it as transmembrane proteins. 3. These transmembrane proteins frequently form ion channels or carrier proteins that transport molecules across the cell membrane. 4. Peripheral proteins, located on the cytoplasmic surface of the cell membrane, are more loosely bound to other membrane proteins or lipids. 5. A glycocalyx, comprised of carbohydrates on the outer surface of the cell membrane, functions in cell recognition, adhesion, absorption and antigenicity. Nucleus 1. More than one nucleus can be present in a cell. 2. Within this spherical structure, deoxyribonucleic acid (DNA) is transcribed and ribonucleic acid (RNA) is synthesized. 3. The surrounding nuclear envelope is formed by two adjacent bilaminar lipid membranes with embedded proteins. 4. Scattered nuclear pores, which perforate the envelope, regulate passage of substances between the cytoplasm and the nucleus. Chromatin, primarily comprised of DNA, is located within the nucleus. 5. The inert form of chromatin, heterochromatin, stains intensely while euchromatin, which is actively involved in protein production, stains lightly. 6. Nuclear chromatin condenses to form chromosomes during cell division. 7. Within the nucleus is the nucleolus that is the site of rRNA synthesis. 8. The number and size of cell nucleoli are related to the amount of protein synthesis occurring within the cell. DNA and RNA  They are made up of chains of nucleotides. Nucleotides are composed of three subunits: a nitrogenous base, a five-carbon sugar, and a phosphate group. In DNA the sugar is deoxyribose and in RNA the sugar is ribose.  DNA and RNA nucleotides are linked in such a way as to form a “backbone” of alternating sugar and phosphate groups. The nitrogenous bases project out of this backbone and in DNA they are weakly bonded to nitrogenous bases on an opposing strand.  In this way, DNA forms a double-stranded molecule, the basic structure of which is analogous to a twisted ladder in which the vertical poles are composed of alternating sugar and phosphate groups and the horizontal rungs are paired nitrogenous bases.  DNA’s molecular structure is therefore called the double helix. RNA, however, is a single-stranded molecule that has no opposing strand. The single strand of RNA is similar in structure to each of the strands found in DNA  Four kinds of nitrogen base are found in DNA and RNA nucleotides. The three that are found in both RNA and DNA are adenine (A), cytosine (C), and guanine (G). However, only DNA contains thymine (T), and only RNA contains uracil (U).  In addition, the structure of each nitrogenous base permits the bonding of only certain pairs of nucleotides. For example, thymine can only bond to adenine, and cytosine can only bond to guanine. DNA and RNA  Uracil, the RNA base, can only bond to the DNA base adenine. These nitrogenous bases and their corresponding bonding parameters are the foundation for the storage of genetic information.  Chromatin appears as light or dark fibers in the nucleoplasm of the nucleus and is made up of DNA and globular proteins called histones. A single strand of DNA winds around eight histone molecules, forming a granule called a nucleosome. The nucleosomes are held together by short strands of DNA called linker DNA. Not only do the histone proteins help keep the DNA strand organized and untangled, they also expose small sections of the DNA to the outside nucleoplasm. These sections of DNA are called genes.  By changing shape, the histones can expose different genes at different times. The exposed genes determine what proteins will be made by the cell. In this way, histones play an important role in the control of gene expression. We call this process gene regulation.  The DNA contains all of the important instructions required for the synthesis of thousands of different proteins, but not all of them are made: only a small percentage are  The histones help to determine which segments of the DNA will be expressed and therefore which proteins will be made Cytoplasm The cytoplasm, which surrounds the nucleus and the organelles of the cell, varies in composition of water, protein, carbohydrates and salts. A cytoskeleton of 1. Microfilaments 2. Intermediate filaments 3. microtubules These above provides structure for cell shape and movement. The cell organelles are: Golgi bodies, endoplasmic reticulum, lysosomes, peroxisomes, microtubules, filaments, chloroplast. The cell inclusions are: pigment granules, fat droplets, secretory products, glycogen, lipids and crystalline inclusions. Cell organelles Function Nucleus It stores genetic material (DNA or RNA) of the cell. Mitochondrion It involves in energy production. Golgi apparatus It involves in protein modification and export. Endoplasmic Reticulum (ER) It involves in lipid production, protein production, and detoxification. Lysosomes It contains various hydrolytic enzymes (recycling and security). Chloroplast It involves in photosynthesis (glucose production). Cytoskeleton It provides cell stability and helps in movement. Microtubules It helps in cell movement. Intermediate Filaments It provides structural stability to the nuclear envelope. Microfilaments It helps in cell movement. *Cell organelles are living structure of the cell while cell inclusions are non living material of the cell. Cell organelles form metabolic machinery of the cell while cell inclusions are products of metabolism. Cell surface modifications: 1. Microvilli are finger like projections of the cell membrane that increase the surface area of absorptive cells located in the lining of digestive tract. 2. Microfilaments constitute the core structure of the microvilli. 3. Stereocilia represent a long variant of microvilli, limited to the epididymis and ear *Both microvilli and stereocilia are nonmotile. 4. Ciliated cells have numerous, 2- to 10-µm long motile cilia, which are on the cell surface. In the respiratory tract, cilia transport mucus and particles in an oral direction. 4. Flagellated cells, like spermatozoa, normally have a single flagellum. The structure of cilia and flagella is very similar. Both cilia and flagella are delimited by the cell membrane and contain a central region (axoneme). 5. Another cilium-like structure, the kinocilium, is found in the ear.