Histology Chapter 1 Review of the cell PDF

Department : Medical Laboratory Technology Histology Chapter 1 : Review of the cell Dr. Sandra Yassine Academic year : 2024-2025  PHE211 Histology Course description : Thi...

Department : Medical Laboratory Technology Histology Chapter 1 : Review of the cell Dr. Sandra Yassine Academic year : 2024-2025  PHE211 Histology Course description : This course has been designed to provide students with the study of the fine structure of the cell and the extracellular matrix as well as the study of the basic tissue types (I.e., epithelial, connective, cartilage, bone, muscle and nervous tissue). Light microscopic analysis of tissue sections on slides will also be emphasized to teach students how the cellular organization of organs enables them to perform their specific functions. Objectives : 1. Acquire knowledge of normal histology. 2. Describe the structural and functional characteristics of the major classes of tissues and their associations in various ways. 3. Understand how the tissues are integrated or modified to perform specific coordinated functions. 2 Chapter 1 : Review of the cell 3  Chapter 1 : Review of the cell Histology : from its derivation, (the Greek histos – tissue, logia – study or science of), means the science of tissues and organs. It provides a basis for later study of anatomy and pathology. All diseases result from disorders in cellular function. When diseases such as cancer or inflammation affect a tissue, there are often specific changes in the microscopic structure of the tissue. The microscopic study of these changes is known as histopathology or anatomical pathology. Obviously a sound knowledge of normal structure is essential for an understanding of pathology. Objectives of this chapter : - Overview of the structure and the function of the cell and the extracellular matrix. - Overview of the composition of the cell and the function of each component : ▪ Nucleus. ▪ Cytoplasm : Organelles (membranous and non-membranous), Inclusions. 4  Introduction to the cell The cell : - Is the basic structural and functional unit of all living tissues, having the capacity to perform all the essential life functions. - Based on the presence or absence of the nucleus, cells are subdivided into two types: ▪ Eukaryotic cells : refer to organisms whose cells consist of cytoplasm and a defined nucleus bounded by a nuclear membrane; this includes plants, fungi and animals, both unicellular and multicellular. The cytoplasm contains variable numbers of several different recognizable structures called organelles, each with a defined function. ▪ Prokaryotic cells : refer to bacteria and archaea, whose cells do not have a membrane-bound nucleus and the nuclear material is spread within the cytoplasm; they also have other major structural differences. 5  Introduction to the cell Prokaryotic vs Eucaryotic cells : Prokaryotic cell (unicellular) Eukaryotic cell (unicellular or multicellular) 6  Introduction to the cell Levels of cellular organization : - Cells with a similar structure and function come together to form tissue. - Tissues with a similar function come together to form an organ. - Organs with a similar function come together to form a system. 7  Introduction to the cell Types of cells : The cells are of different types with a unique function of its own. Estimates are that more than 200 different types of cells are present in an adult human. Some of them are as follows : 1. Stem cells : These are the primary cells from which all the other cells arise. 2. Muscle cells : skeletal, smooth and cardiac cells. 3. Bone cells : cells that make up the bone. 4. Cartilage cells : cells that make up the cartilage. 5. Blood cells : Platelets, red blood cells, white blood cells. 6. Sex cells : They are two types, sperms and ovum. 7. Nervous cells. 8. Fat cells (adipocytes). Within the different tissues and organs of the body, the constituent cells exhibit a wide range of functional specialization. Despite this range of morphological forms, cells conform to a basic structural model. 8  Cell structure Each Eukaryotic cell : - Is divided into two components : ▪ The nucleus Protoplasm ▪ The cytoplasm - All cells are bounded by an external limiting membrane called the plasma membrane which serves as a dynamic interphase between the internal environment of the cell and various external environments. The nucleus is separated from the cytoplasm by a nuclear membrane. The cytoplasm has a fluid base (matrix) which is referred to as the cytosol or hyaloplasm. 9  Cell structure The Cell Membrane : - Also known as the plasma membrane or plasmalemma. - Approx. 8 nm thick. - Boundary that separates the cell and its content from the surrounding, and gives it a definite shape. In case it breaks, the content of the cell will spill out. - Controls traffic into and out of the cell it surrounds. - Shows a selective permeability, allows some substances to cross it easily than other substances which enables the cell to maintain an internal environment different from the external environment. Structure of the plasma membrane : 10  Cell structure The Cell Membrane: Composition of the plasma membrane : - Consists of a phospholipid bilayer combined with a variety of proteins like integral membrane protein, peripheral membrane protein, and glycoproteins, in a fluid mosaic arrangement. - It also contains cholesterol and glycolipids. N.B : The different types of proteins help in the active transport of chemicals, food, and wastes. 11  Cell structure The Cell Membrane: Composition of the plasma membrane : Phospholipid molecules consist of : ▪ a polar, hydrophilic (water-loving) head. ▪ a non-polar, hydrophobic (water-hating) tail. - The polar heads are mainly derived from glycerol conjugated to a nitrogenous compound such as choline, ethanolamine or serine via a phosphate bridge. - The phosphate group is negatively charged whereas the nitrogenous group is positively charged. - The non-polar tail of the phospholipid molecule consists of two long-chain fatty acids each linked to the glycerol component of the polar head. 12  Cell structure The Cell Membrane: Composition of the plasma membrane : - On the external surface of the plasma membrane of Eukaryotic cells, many of the membrane proteins and some of the membrane lipids are conjugated with short chains of polysaccharide. - These glycoproteins and glycolipids respectively project from the surface of the bilayer forming an outer coating known as glycocalyx that provides mechanical and chemical protection for the plasma membrane. 13  Cell structure The cytoplasm : - Consists of the cytosol (hyaloplasm) and the organelles. - Contains : ▪ a number of organelles which have distinctive structure and functions (Golgi apparatus, mitochondria, ribosomes, lysosomes, endoplasmic reticulum, etc.). ▪ Inclusions: small structures within the cell cytoplasm. A few examples being, melanin, glycogen, lipids etc. ▪ Cytoskeleton : a network of fibers that extend through the cytoplasm in the cell. There are 3 basic structures of the cytoskeleton : Microtubules, Microfilaments and Intermediate filaments. - The cytosol present in the cytoplasm is watery, but the cytoplasm is gel-like because of the presence of organelles. Cytoskeleton : Provides structural support for the cell and its organelles as well as providing a mechanism for cellular and intracellular movement. 14  Cell structure The Nucleus “control center of the cell”: - Is the largest organelle, accounts for about 10 % of the cell’s volume. - Usually occupies the central portion of the cell. - Contains DNA arranged in chromosomes. - Programmed to control all the functions of the cell. Hence it is referred to as the “brain” of the cell. - Most cells have a single nucleus, though some have none (ie. red blood cells), and some have several (i.e. skeletal muscle). Function of the nucleus : 1. Essential for the vitality and division of the cell. 2. Site of storage of genetic information. 3. Site of formation of the three types of RNA (rRNA , mRNA , tRNA). 15  Cell structure The Nucleus : Structure of the nucleus : - Is separated from the rest of the cytoplasm by a nuclear membrane (envelope), a phospholipid bilayer surrounding the genetic material. The nuclear envelope (nuclear membrane) : - Consists of two membranes : The inner nuclear membrane (INM) the outer nuclear membrane (ONM) separated by a perinuclear space (20-40 nm thick). ▪ The outer nuclear membrane is continues with the ER membrane. ▪ The inner nuclear membrane have proteins which bind the underlying nuclear lamina. The nuclear membrane shows innumerable perforations called nuclear pores, specialized to have selective permeability, required for transport of molecules between the nucleus and the cytoplasm. 16  Cell structure The Nucleus : Structure of the nucleus : Chromatin : - Formed of DNA and protein. - Based on the level of packaging → two forms of chromatin : ▪ Euchromatin (pale) : extended genetically active chromatin, involved in transcribing RNA to produce proteins used in cell function and growth. ▪ Heterochromatin (dark) : condensed inactive chromatin, typically not transcribed. Functions of chromatin : 1. Carries genetic information. 2. Directs protein synthesis. 17  Cell structure The Nucleus : Structure of the nucleus : The nuclear lamina : - Fibrous meshwork of proteins, underlying the inner nuclear membrane. - Physically connected to the INM by lamina-associated integral membrane proteins. - Provide structural support to the nucleus and serve as a site for chromatin attachment. Nucleoplasm : - Is a viscous fluid found inside the nucleus, made up mostly of water, a mixture of various proteins (DNA and RNA polymerase), and dissolved ions. 18  Cell structure The Nucleus : Structure of the nucleus : Nucleolus /nucleoli : - One or more small spherical, non-membranous, dark-staining bodies within the nucleus. - Involved in Ribosomal RNA (rRNA) synthesis and ribosome subunit assembly. - Is the site in the nucleus where ribosomal RNA is transcribed. It is then linked to the subunits of the ribosome, and transported out of the nucleus through nuclear pores. - Associated with nucleolar organizer regions (contains DNA coding for rRNA). Nucleolus : is the primary site of ribosome subunit biogenesis in eukaryotic cells. 19  Cell structure The Nucleus : Structure of the nucleus : 20  Cell structure : Organelles vs Inclusions It is IMPORTANT to know the difference between Organelles and Inclusions Organelles : are specialized structures, ESSENTIAL for vital processes of the cell. Two types of Cytoplasmic Organelles : Membranous and Non-Membranous organelles. ▪ Membranous (membrane-limited): Mitochondria, Endoplasmic Reticulum, Golgi Apparatus, Lysosomes, Secretory Vesicles. ▪ Non-Membranous : Ribosomes, Centrosome and centrioles, Cytoskeleton. Inclusions : structures that are not essential for vitality of cells, may be present or absent. Examples are lipids, glycogen and pigments like melanin & lipofuscin. 21  Cell structure The Mitochondria : Mitochondria : - Scattered free in the cytoplasm. - Bounded by two membranes : a smooth outer one, and an inner one which is thrown into folds called cristae projecting into the inner cavity. The cavity is filled with mitochondrial matrix, which contains enzymes. Also contains its own DNA. - The inner mitochondrial membrane is closely applied to the outer membrane leaving a narrow intermembranous space, which extends into each crista. Function : Mitochondria structural features Generation of ATP : Mitochondria provide all the energy needed by a cell to move, divide, contract, produce secretory products, and all other functions of a cell; this occurs by breaking down the food that helps in making the ATP, which is the principal fuel for all cell activities that require energy. N.B : The number of mitochondria in cells is highly variable: the liver cells contain as many as 2000 mitochondria whereas inactive cells contain very few. 22  Cell structure The Endoplasmic reticulum : The endoplasmic reticulum : - Consists of an interconnecting network of membranous tubules and flattened sacs (cisternae) which ramifies throughout the cytoplasm, enclosing a single internal space (lumen). - Occurs in two morphologically distinct forms : ▪ Smooth (or agranular) endoplasmic reticulum (SER). ▪ Rough (or granular) endoplasmic reticulum (RER). - The lumen of the RER is continuous with that of the SER. 23  Cell structure The Endoplasmic reticulum : The Rough-endoplasmic reticulum – rER : - Much of its surface is studded with ribosomes giving a rough or granular appearance leading to the name rough or granular endoplasmic reticulum (rER or gER). Function : - Plays a major role in protein synthesis by ribosomes on its outer surface. It synthesizes secretory proteins, Integral membrane proteins and lysosomal enzymes → Cells active in secretion will have lots of rER. - Quality checkpoint in the process of protein production. N.B : Proteins which are utilized within the cytosol are synthesized on free ribosomes. 24  Cell structure The Endoplasmic reticulum : The Rough-endoplasmic reticulum – rER : Proteins synthesis : - Proteins are synthesized by the ribosomes on the external surface of the rER and are then passed into reticular lumen. - The ER folds in on itself to form small membrane-bound vesicles containing protein, known as transport vesicles. These transport vesicle bud off from the endoplasmic reticulum and move to the cytoplasm and then coalesce with the Golgi apparatus, where further processing for the proteins occurs. Proteins synthesis : 25  Cell structure The Endoplasmic reticulum : The Smooth-endoplasmic reticulum – sER : - Consists of an irregular network of membranous tubules and vesicles devoid of ribosomes. - It forms part of the intracellular membrane system being continuing with the rough endoplasmic reticulum. Function : - Plays a major role in lipid biosynthesis and intracellular transport. - In liver cells, sER also plays a major role in the metabolism of glycogen and detoxification of various noxious metabolic products and alcohol. - In highly contractile cells (muscle cells), sER is involved in storage and release of calcium ions which activates the contractile mechanism. 26  Cell structure The Endoplasmic reticulum : The Smooth-endoplasmic reticulum – sER : Lipid biosynthesis : - Lipids are synthesized by all cells in order to repair and replace damaged or worn membranes. Cells may also synthesize lipids : ▪ as a means of strong excess energy (as cytoplasmic droplets). ▪ For lipid transport (e.g., chylomicron production by cells of the small intestine). ▪ For steroid hormones synthesis. 27  Cell structure The Golgi apparatus : The Golgi apparatus : - Also called Golgi body or Golgi complex. - Is typically located close to the nucleus. - Consists of 5-8 membranous sacs called the Cisternae. The lumens of these cisternae are separate from each other and from that of the ER. - Has two faces : ▪ Cis : Convex (receiving or forming or entry) face, receives transfer vesicles. ▪ Trans : Concave (maturing or exit) face, forms secretory vesicles. Secretory vesicles : are vesicles leaving the trans face of the Golgi. These secretory vesicles are of much greater dimensions than the transfer seen at the Golgi forming face. 28  Cell structure The Golgi apparatus : The Golgi apparatus : - The Golgi receives synthetic products from the ER, modifies them, and exports them to a variety of destinations. For example, secreted proteins are packaged into secretory vesicles and liberated at the cell surface by exocytosis. The sequence of events from protein synthesis on the rough ER to final protein distribution 29  Cell structure The Golgi apparatus : The Golgi apparatus : Function : 1. Organelle primarily responsible for the processing and delivery of proteins and other molecules like lipids to different parts of the cell → Also called post office of the cell or shipping department. 2. Involved in modification of certain proteins and contains the enzymes required for the synthesis of glycoproteins. These include plasma membrane glycoproteins forming the glycocalyx. 3. Responsible for the elaboration of membrane glycolipids. 4. Responsible for lysosomes formation. 30  Cell structure The ribosome : The ribosomes : - Are minute cytoplasmic organelles, each composed of two subunits of unequal size. Each subunit consists of a strand of RNA (ribosomal RNA) with associated ribosomal proteins; the ribosomal RNA strand and associated proteins are folded to form a condensed, globular structure. - Main site of proteins synthesis → said to be the protein factory of a cell. Ribosome assembly : - Takes place within the cytoplasm. - Different ribosomal proteins imported from the cytoplasm are conjugated with ribosomal RNA to form the ribosomal subunits, which then pass into the cytoplasm; through the nuclear pores, before being assembled into fully active ribosomes. Protein synthesis (Translation) : - Occurs on free ribosome or on ribosomes attached to the rER. 31  Cell structure Lysosomes : Lysosomes : - Spherical membranous vesicles (0.2 to 0.4 µm diameter), with an acidic internal PH (ph5.0), for degradation. - Originate from mature surface of the Golgi apparatus. - Contain around 40 different types of hydrolytic enzymes, including proteases, glycosidases, nucleases and lipases. - Also known as the digestive apparatus of the cell, responsible for the degradation of : ▪ Extracellular material that has entered the cell by phagocytosis and require digestion. ▪ Organelles that require degrading because they have reached the end of their active life, a process called “autophagy” in which cell old organelles are marked for destruction. 32  Cell structure Endosomes : - Endocytic vesicles formed after endocytosis. 33  Cell structure Peroxisomes : - Are small vesicles, single membrane-bound organelles found in the eukaryotic cells. - Derived from rough ER. - Contain digestive enzymes for breaking down toxic materials (such as hydrogen peroxide H2O2) damaging to the cell and oxidative enzymes for metabolic activity. 34  Cell structure Centrioles & Centrosomes : - 2 cylinders, perpendicular to each other made of 9 triplets of microtubules arranged in a cylindrical manner. - Essential structures that play in role in the cell division and replication as well as formation of Cilia & Flagella. 35  Cell structure Inclusions : Storage products and pigments (non membranous structure that exist in the cytoplasm).. ▪ Carbohydrates, stored in cytosol as deposits of the polysaccharide glycogen. ▪ Fats, stored, mostly by the fat cells (adipocytes), as free droplets in the cytosol. ▪ Cellular Pigments : - Red blood cells (erythrocytes) have the red iron-containing pigment hemoglobin. - Skin cells have the brown-black pigment melanin. 36  Cell structure The cytoskeleton : - Supporting framework contained within a cell’s cytoplasm and is made up of proteins. - Provides cellular scaffolding upon which cellular organization is arranged. - Essential for intracellular transport of organelles. - Is composed of 3 types of fibers : ▪ Microtubules (MTs) : the thickest of the 3 components of the cytoskeleton. ▪ Microfilaments (MFs) : also called actin filaments, are the thinnest components. ▪ Intermediate filaments : are fibers with diameters in a middle range. Function : - Maintains the cell’s shape and structural stability (protects against deformation). - Provides various types of cell movement. - Involved in chromosome separation in mitosis and meiosis. 37  Cell structure The cytoskeleton : Microfilaments : - Extremely fine strands (5 nm in diameter) of a protein known as actin. - Each actin filament consists of two strings of actin subunits twisted together. - Are important in cell shape and cell motility. Microtubules : - (25 nm in diameter) are much larger than microfilaments, but like them, are made up of globular protein subunits, alpha and beta tubulin, which polymerize to form a hollow tubule. - Form a rigid internal skeleton. - Act as a framework along which motor proteins can move and organize structures in the cell. 38  Cell structure The cytoskeleton : Intermediate filaments : - Intermediate in size between actin filaments and microtubules. They are about 8 to 10 nm in diameter. - Are made up of fibrous proteins that are organized into tough assemblage (coiled coils). The type of intermediate filament found in a cell type, is typical of that type of cell. For example : ▪ Epithelial cells contain keratin. ▪ Neurons contain neurofilaments. ▪ Nuclear lamina contain Lamins. - Have 2 major structural functions : ▪ Stabilize cell structure and maintain cell-cell connectivity (desmosomes). ▪ Resist tension. 39  Extracellular matrix Eukaryotic cells in tissues are embedded in an extracellular matrix, a large network of proteins and other molecules that surround, support, and give structure to cells and tissues in the body. The extracellular matrix also called ECM: 1. Helps cells attach to, and communicate with nearby cells. 2. Plays an important role in cell growth, cell movement. 3. Is involved in repairing damaged tissue. Abnormal changes in the extracellular matrix may lead to the development of certain diseases, such as cancer. The extracellular matrix of cancer cells can affect how they grow and spread. 40  Extracellular matrix The extracellular matrix is composed of : ▪ Amorphous intercellular substances (ground substances). ▪ Formed intercellular substances : - Collagen fibers. - Elastic fibers. - Reticular fibers. - Minerals. 41  Extracellular matrix The extracellular matrix : ▪ Amorphous intercellular substances (ground substances) : - Is an amorphous gel-like material, composed of glycosaminoglycans (GAGs), proteoglycans and glycoproteins. - Glycosaminoglycans : Large linear negatively charged polysaccharides, constructed of repeating disaccharide units, that have the capability of binding large quantities of water. - Proteoglycans: Proteins heavily glycosylated by glycosaminoglycans. - Glycoproteins: Proteins to which carbohydrates are bound through glycosidic bonds (may or may not be negatively charged). ▪ Formed intercellular substances : - Collagen fibers. - Elastic fibers. - Reticular fibers. - Minerals. 42  Extracellular matrix - Components of the ECM are produced intracellularly by resident cells and secreted into the ECM via exocytosis. - Once secreted, they then aggregate with the existing matrix. The ECM is composed of an interlocking mesh of fibrous proteins and glycosaminoglycans (GAGs). 43