Histology I - Chapter 2 PDF
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This document provides an overview of histology, focusing on the structure and function of cells. It includes definitions, descriptions, and diagrams related to cell membranes, organelles, and mitochondria.
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HISTOLOGY I ------------------------------------------------------------------------------------------------------------------ Definition: The cell is the structural unit of any lining organisms. It is formed of two basic parts cytoplasm and nucleus....
HISTOLOGY I ------------------------------------------------------------------------------------------------------------------ Definition: The cell is the structural unit of any lining organisms. It is formed of two basic parts cytoplasm and nucleus. |Page8 HISTOLOGY I ------------------------------------------------------------------------------------------------------------------ Cytoplasm The cytoplasm is formed of a matrix containing organic and inorganic chemicals. The cytoplasm contains organelles, fibrillar elements known as cytoskeleton and inclusions. Organelles Are metabolically active structures in the cytoplasm carrying out specific functions. I- Cell membrane: Each cell is bounded by a cell membrane. This membrane is crucial important to life because the cell must separate itself from the outside environment. It is not visible with light microscope. By electron microscope it appears as thin dense line 7-10 nm in thickness. At high magnification it appears as a trilaminar structure of two thin electron – dense lines separated by light electron-lucent layer. Each layer is about 2.5 nm. The whole structure is known as the unit membrane. Molecular structure: | P a g e 01 HISTOLOGY I ------------------------------------------------------------------------------------------------------------------ Cell membrane is formed of lipids, proteins and carbohydrates. A- Lipids: Are mainly phospholipids and some cholesterol. The phospholipids molecules are arranged in two layers. Each molecule is formed of a polar hydrophilic head and two long nonpolar hydrophobic tails. The molecules are arranged with their heads directed outwards and the tails are directed inward. B- Proteins: Proteins are of two types: i- Integral proteins which are incorporated within the lipid bilayer. Some of these proteins are partially embedded in the lipid bilayer and some are long and extend across the lipid bilayer to protrude from the outer and the inner surfaces and called transmembrane proteins. ii- Peripheral proteins which are located on the inner aspect of the membrane. C- Carbohydrates: Are polysaccharides molecules attached to the proteins and phospholipids of the outer surface of the membrane forming the cell coat or glycocalyx. | P a g e 00 HISTOLOGY I ------------------------------------------------------------------------------------------------------------------ Functions of cell membrane: 1- It surrounds the cell separating its contents from the surrounding environment. 2- It permits the movement of substances in and out the cell by the following mechanisms: a. Passive diffusion as water and some ions. b. Active transport, it requires energy. c. Facilitated diffusion as glucose and amine acids. d. Selective transport which depends upon the presence of receptors. | P a g e 01 HISTOLOGY I ------------------------------------------------------------------------------------------------------------------ 3- Phagocytosis: It is the ability to engulf particulate matter. 4- Pinocytosis: It is the ability to engulf droplets of fluids. 5- Exocytosis: It is the ability to extrude wastes and secretory vesicles. 6- Cell coat acts as adhesive to hold adjacent cells together. 7- Cell membrane can be modified to form microvilli, cilia and flagella. II- Mitochondria: They are rod-shaped or spherical organelles. They are bounded by two membranes. The outer one is smooth while the inner one projects inside to form shelves called cristae. The inner membrane has also globular structures connected to it by stalks. In some cells the cristae are tubular in shape. The mitochondria have two spaces: one present between the two membranes called intermembrane space and the other is enclosed by the inner membrane called matrix space. This space is filled with matrix containing DNA, RNA, ribosomes and protein. | P a g e 02 HISTOLOGY I ------------------------------------------------------------------------------------------------------------------ Functions: 1- Mitochondria are regarded as the power house of the cell. It is the site of oxidative phosphorylation to supply the cell with energy. 2- Mitochondria contain the enzymes for Krebs cycle and form ATP from its precursors ADP to supply the cell with energy. 3- Mitochondria control the calcium level in the cell. III- Ribosomes: Are small particles formed of proteins and ribosomal RNA (rRNA). Each ribosome is formed of two subunits: large and small. They are formed in the nucleolus and are released through the nuclear pores. In the cytoplasm they are present either free or attached to the membrane of rough endoplasmic reticulum (RER). Ribosomes may be linked together by a long molecule of messenger RNA (mRNA) to form polyribosomes (polysomes). Ribosomes are basophilic because of its content of acids. Functions: Ribosomes are the sites where the process of protein synthesis occurs. The protein needed in the cell is synthesized by free ribosomes while protein secreted by the cell is synthesized by attached ribosomes. IV- Endoplasmic reticulum: It is a system of interconnected tubules, vesicles and flat saccules (cisternae) to form network and are bounded by a limiting membrane. Two types are present: rough and smooth. | P a g e 03 HISTOLOGY I ------------------------------------------------------------------------------------------------------------------ A- Rough endoplasmic reticulum (RER): Present in all cells especially in protein secreting cells. The outer surface of its limiting membrane is studded with ribosomes. Functions: The synthesized proteins are segregated from the cytoplasm inside the lumen of RER and then delivered inside the transfer vesicles that are pinched off from RER to reach Golgi apparatus. B- Smooth endoplasmic Reticulum (SER): It appears as a network of branching tubules. Its limiting membrane has no ribosomes. Functions: 1- Synthesis of fatty acids, other lipids and steroid hormones. 2- Synthesis and breakdown of glycogen. 3- Detoxification of toxins and drugs. 4- It regulates the calcium ions distribution in striated muscles to control muscle contraction. V- Golgi complex (Apparatus): It is not seen in routine histological sections but can be detected as pale unstained area called negative Golgi image in cells with basophilic cytoplasm. It appears as granular or fibrillar network in sections stained with silver. By EM it appears as membrane-bounded curved stack of 4 to 10 parallel | P a g e 04 HISTOLOGY I ------------------------------------------------------------------------------------------------------------------ cisternae. The Golgi stack has convex surface called forming or cis face. The opposite side is concave and called maturing or trans face. Transfer vesicles bud from RER fuse with the cis face. Proteins are transferred and modified through Golgi apparatus to reach the trans face. Small vesicles and secretory granules arise from the trans face to remain in the cytoplasm or discharge their contents outside the cell. | P a g e 05 HISTOLOGY I ------------------------------------------------------------------------------------------------------------------ Functions: 1- Chemical modification of proteins that synthesized in RER. 2- Concentration and packing of secretory products to form secretory granules. 3- Formation of coated vesicles, secretory granules and lysosomes. 4- Control the distribution of membrane protein between cell organelles and cell wall. VI- Secretory granules (vesicles): They arise from the trans face of Golgi complex and bud off from its stacks. With LM they appear as small granules. With EM they are surrounded by a membrane. VII- Lysosomes: They are round electron-dense bodies surrounded by a membrane and contain hydrolytic enzymes. The newly formed lysosome is called primary lysosome. When fuses with another vesicle it is called secondary lysosome. | P a g e 06 HISTOLOGY I ------------------------------------------------------------------------------------------------------------------ Functions: 1- When bacteria or other materials are engulfed by the process of phagocytosis they will be enclosed within phagosomes. Primary lysosomes fuse with these phagosomes to digest their content. 2- The cell is able to engulf fluid by pinocytosis to form pinocytotic vesicle. Primary lysosomes fuse with this vesicle to digest its content. 3- Lysosomes are involved in the elimination of worn-out organelles by fusion with autophagic vacuole containing these organelles. So lysosomes have been described as suicide bags. | P a g e 07