Cytology PDF - Cell Structures and Functions

# The Cytoplasmic Organelles - They are living and permanent intracellular structures that carry out the vital functions of the cell. - They are classified according to the presence or absence of membrane into membranous and non-membranous organelles. ## Membranous Organelles - Surrounded with me...

# The Cytoplasmic Organelles - They are living and permanent intracellular structures that carry out the vital functions of the cell. - They are classified according to the presence or absence of membrane into membranous and non-membranous organelles. ## Membranous Organelles - Surrounded with membrane - Have metabolic enzymes ### Components 1. Cell Membrane 2. Mitochondria 3. Rough Endoplasmic Reticulum 4. Smooth Endoplasmic Reticulum 5. Golgi Apparatus 6. Lysosomes ## Non Membranous Organelles - Not surrounded by membrane - Free from enzymes ### Components 1. Ribosomes 2. Cytoskeleton - Microtubules - Microfilaments ## The Membranous Organelles ### 1. The Cell Membrane - Also called plasma membrane or plasmalemma. - It is a limiting membrane surrounding the cell cytoplasm externally. - Measures about 7.5-10 nm in thickness. - **LM:** cannot be seen by light microscopy while the cell coat (the external part of the cell membrane) can be demonstrated when stained with PAS (pink) or silver stains (black). - **EM:** It appears as trilaminar membrane as it formed from two electron-dense dark lines separated by an electron lucent clear one. It also called unit membrane. ## 2. Mitochondria - One of the membranous organelles responsible for energy production. - Considered the power house of the cell. - Site: They present in the part of the cytoplasm where the energy is needed as beside rER in secretory cells, between myofibrils in muscle cells, and in the basal infolding in the ion transport cells. - Number: Their number varies according to the activity of the cell. Mitochondria are self-replicating organelles, so they can increase their number as they contain DNA. - **LM:** appear as dark blue dots when stained with Iron Hematoxylin. - **LM:** appear as green dots when stained with genus green. - **EM:** appear as rounded or elongated, double membranous structure of about 1 um in diameter and 10 um in length. - The outer mitochondrial membrane: is smooth and permeable. - The inner mitochondrial membrane: forms shelf-like folds or cristae with variable numbers to increase the surface area for oxidative phosphorylation. - Mitochondrial matrix: contains oxidative enzymes, DNA, RNA, and electron-dense granules rich in calcium. ### Functions: 1. Production of energy from metabolites present in the cytoplasm by Kreb's cycle. 2. Storage of energy as ATP molecules by oxidative phosphorylation. ## 3. Rough Endoplasmic Reticulum (rER) - One of the membranous organelles responsible for protein synthesis. - Present mainly in protein-secreting cells. - Site: Present around the nucleus and associated with the nuclear membrane. - **LM:** Either localized or diffused basophilic dots localized as in pancreatic cells (basal basophilia) or diffused as in plasma cell (diffused basophilia). - **EM:** Appear as parallel flattened membranous sacs or cisternae connected with each other by short canaleculi. These membranous sacs are studded with polyribosomes on a special site which gives this organelle its rough appearance. ### Functions: 1. Synthesis of protein for export outside the cell as polypeptide hormones and digestive enzymes. 2. Synthesis of hydrolytic enzymes of primary lysosomes. 3. Packing of formed proteins in membranous vesicles to be delivered to Golgi apparatus as transfere vesicles. 4. Act as intracellular pathway for some substances. ## 4. Smooth Endoplasmic Reticulum - One of the membranous organelles responsible for steroid synthesis - Site: Well developed in steroid hormone-secreting cells (adrenal cortex, testis, and ovary), fat cells, and liver cells. - **LM:** Cannot be seen. - **EM:** Branching and anastomosing intercommunicating tubules with variable shaped cisternae. They may be continuous with rER but lacking attached ribosomes. ### Functions: 1. Synthesis of steroid hormones as cortisone, estrogen and progesterone. 2. Detoxication of drugs, hormones, and some chemicals in liver cell. 3. Act as a pump releasing calcium ions during muscle contraction. 4. Synthesis of cholesterol and phospholipids. 5. Act as intracellular pathway for some substances. ## 5. Golgi Apparatus - One of the membranous organelles responsible for secretion, so it is well developed in secretory cells. - Site: - In pancreatic cells present supranuclear (in the apical part). - In nerve cells present perinuclear (around the nucleus). - **LM:** It can be demonstrated by silver stains as a network of brownish granules and fibers. In plasma cell, which characterized by diffuse basophilia, Golgi apparatus could be recognized as a pale unstained area near the nucleus called negative Golgi image. - **EM:** Golgi apparatus consists of: - **Golgi stalks or membranous saccules:** several parallel, flat, and curved saccules with dilated ends. These saccules are interconnected with each other. Each saccule has two surfaces: - **Immature convex surface:** receives transfer vesicles which carry crude protein from rER. - **Mature concave surface:** from which the secretory vesicles bud off carrying protein ready for secretion.. - **Transfer or Micrivesicles:** small membrane-bound vesicles carrying crude protein from rER to the immature surface of the Golgi stalks. - **Secretory or Macrovesicles:** large membrane-bound vesicles arise from the mature active surface of Golgi stalks carrying protein ready for secretion. ### Functions: 1. Concentration, storage, and packing of proteins received from rER. 2. Conjugation of protein with carbohydrates, sulfates, and salts. 3. Formation of coated vesicles and primary lysosomes. 4. Renewal and maintenance of the cell membrane. ## 6. Lysosome - One of the membranous organelles contains hydrolytic enzymes and responsible for cellular digestion. - Number: A huge number of primary lysosomes present in the phagocytic cells as monocyte, neutrophils, lymphocytes, and macrophages. - Origin: The hydrolytic enzymes are synthesized in the rER then transferred to Golgi apparatus then released as primary lysosomes. - **LM:** Can not be seen but the acid phosphatase enzyme inside the lysosome could be detected by histochemical reactions. - **EM:** - **Primary lysosomes:** Membrane-bound spherical vesicles contain hydrolytic enzymes. They have homogenous electron density. - **Secondary lysosomes:** When the primary lysosome fuses with any intra cytoplasmic vesicle either exogenous or endogenous it becomes a secondary lysosome. The secondary lysosome appear heterogeneous in electron density due to digested and undigested particles. ### Types of Secondary Lysosomes: 1. **Heterolysosome:** Fusion of the primary lysosome with a phagocytic vesicle as dust, bacteria or virus 2. **Multivesicular body:** Fusion of the primary lysosome with a pinocytotic vesicle 3. **Autophagosome:** Fusion of the primary lysosome with an old organelle as mitochondria or ER leading to its hydrolysis 4. **Residual body:** The undigested materials inside the lysosome as dust retained as a residual body then the cell expelled it by exocytosis (cytostol) or it accumulate inside the cell as lipofuscin pigment in nerve cells. ### Functions: 1. Protection of the cell by digestion of the engulfed bacteria or viruses. 2. Maintain the cell health by continuous removal and elimination of the old organelles. 3. Autolysis after cell death the lysosomal enzymes liberated and hydrolyse the cell constituents. 4. Helps in fertilization of the ova by the acrosomal enzymes. 5. Formation of thyroid hormones by digestion of colloid. ## Non Membranous Organelles ### 1. Ribosomes - Non membranous electron-dense bodies present in all cells but increased greatly in protein-producing cells. - Structure: Formed of rRNA from the nucleolus and cytoplasmic protein. Both structures conjugate together inside the nucleus to form large and small subunits, then they pass to the cytoplasm for protein synthesis. - **LM:** Accumulation of a large number of ribosomes causes cytoplasmic basophilia which may be diffused (embryonic cells), localized (pancreatic cells) or focal (Nissl's granules of nerve cells). - **EM:** A ribosome appears as small electron-dense granule formed of small and large subunits. The large subunit contains a groove in its center housing the polypeptide chain. The two subunits connected together by mRNA. More than one ribosomes are connected together by mRNA forming polyribosomes, the latter may be free or attached. ### Functions: 1. **Free ribosomes:** Scattered singly throughout the cytoplasm. 2. **Attached ribosomes:** Mainly attached to the outer nuclear membrane and the outer surface of rER by the large subunits at glycoprotein receptors. - Synthesize protein to be used inside the cell as glycolytic enzymes. The mRNA that carries information about the polypeptide to be synthesized attached to ribosome between large and small subunits. tRNA picks up the specific amino acids and transports them to the ribosome which translate the code into polypeptide chain that extends through the groove of the large subunit. - Form the proteins that secreted outside the cells as hormones and enzymes. ## 2. Cytoskeleton - The term cytoskeleton refers to a complex network of microtubules and microfilaments together with some linked proteins leading to the formation of what is called microtrabecular lattice. ### A. Microtubules - Fine tubules of variable lengths and regular diameter of about 25nm. - **LM:** Cannot be seen but they could be demonstrated by immunohistochemistry. - **EM:** They appear as fine tubules formed of alpha and beta tubulin protein molecules arrange into 13 protofilaments. ### Functions: 1. Dynamic form with continuous assembly and disassembly helping in cell movement 2. Stable or organized form represented by centerioles, cilia, and flagella. 3. Determination of the cell shape. 4. Formation of the mitotic spindle during cell division. 5. Intracellular transport of macromolecules. 6. Form the centerioles, cilia, and flagella. ### B. Microfilaments - Thread like structures distributed allover the cytoplasm and constitute a part of the cytoskeleton. - **LM:** Some of them appear in ordinary sections (Tonofibrils & Myofibrils), others require a special stain (Neurofibrils are argyrophilic). They appear as very fine lines. - **EM:** They appear as very fine thin threads of varying length and diameter. In C.S they appear as minute dots. Regarding their arrangement, they are either: - **Regularly:** Arranged to form myofibrils in muscle cells and tonofibrils in epithelial cells. - **Irregularly:** Arranged to act as a cytoskeleton. ### Functions: 1. Microfilaments in every cell act as a cytoskeleton. 2. Myofibrils in muscle cells contract muscle fibers. 3. Tonofibrils in epithelial cells connect adjacent cells together. ## Cytoplasmic Inclusions - Temporary, nonliving, and nonessential metabolically inactive materials in the cytoplasm. - They exist in 3 different forms: stored food, pigments, and secretory granules. ### 1. Stored Food - Biochemical compounds produced by the cell itself and stored inside it till the time of need. It includes carbohydrates and fats. #### a. Carbohydrates - Cytoplasmic inclusion stored in the cells of liver and muscle in the form of glycogen. - **LM:** As red granules when stained with Best's carmine or pink granules when stained by PAS stain. - **EM:** They appear as rosette-like aggregations. #### b. Fats: - Cytoplasmic inclusions stored in special cells called fat cells or adipocytes as large globules and occasionally present in liver cells as small droplets. - They can easily demonstrated by **LM:** as black spots when stained with Sudan black or osmic acid stains, and orange colour when stained with Sudan III. These stains applied after freezing technique as the fat dissolved during preparation with paraffin technique. ### 2. Pigments - Coloured intracellular inclusions either produced within the cell itself (endogenous) or taken from outside (exogenous). - **Type: a- Endogenous pigments**: They are coloured inclusions produced by the cell itself as hemoglobin in RBCs, melanin in melanocytes and lipofuscin in cardiac & nerve cells. - **Type: b- Exogenous pigments**: They are coloured inclusions engulfed or phagocytosed by the cell from outside as dust particles in lung, tattoo marks in skin and carotene pigments in other cells. ### 3. Secretory Granules - One of the cytoplasmic inclusions which occur in secretory cells as a result of their activity (endogenous in origin). - **LM:** Some granules appear in ordinary sections (zymogen granules stained pink with PAS). - **EM:** Each granule appears as vesicle surrounded by a unit membrane and filled with the secretory substance. ### Functions: - Secretory granules possess so many functions depending on the nature of their contents. ## Nucleus - The largest and most prominent and important component of the cell. - Present in all true cells - **Shape:** The nucleus may be spherical, oval, flattened, lobulated, segmented, or kidney-shape. - **Position:** The nucleus usually takes a central position but sometimes it may be eccentric, basal, or peripheral. - **Number:** The cell may be mononucleated (one nucleus in each cell), binucleated (two nuclei in each cell as liver cell) or multinucleated (more than two nuclei in each cell as skeletal muscle cell). ### Components of the Nucleus: 1. Nuclear membrane 2. Nuclear chromatin 3. Nuclear sap 4. Nucleolus #### 1. Nuclear Membrane: - A dark basophilic line surrounding the other nuclear components - **By EM:** It appears as a double-walled membrane formed of outer and inner nuclear membranes separated by perinuclear space of about 50nm. The nuclear membrane is interrupted at intervals by the nuclear pores. - **The outer nuclear membrane:** Appears rough due to presence of attached ribosomes on its outer surface. It also continuous with rER. - **The inner nuclear membrane:** Attached to the peripheral chromatin and continuous with the outer membrane at the nuclear pores. - **The nuclear pores:** They are circular openings where the inner and outer membranes fuse. They are formed of special protein called nucleoporins. #### 2. Nuclear Chromatin: - It constitutes the genetic materials of the cell consists of chromosomes that formed mainly of DNA and histones. - **LM:** It appears as basophilic masses distributed allover the nucleus and attached to the inner nuclear membrane. It has two forms: - **Euchromatin:** Lightly stained appears pale basophilic. It contains the active genes that responsible for protein synthesis. The euchromatic nucleus called vesicular nucleus. It is present in active cells. By EM it appears as fine granules. - **Heterochromatin (condensed or inactive):** Appears dark basophilic as it represents the coiled chromosomes containing inactive genes. By EM it appears as coarse electron dense granules. ### Functions: 1. Carries the genetic information of the organism. 2. Controls the process of protein synthesis. 3. Formation of different forms of RNA. #### 3. Nuclear Sap: - A colloidal fluid that filled the spaces between the nuclear chromatin and the nucleolus. - It consists of nucleoproteins, enzymes, sugars, and some ions. - It provides a medium for nucleic acid synthesis and transport of RNA to the cytoplasm. #### 4. The Nucleolus: - A rounded or oval deeply basophilic mass formed of rRNA and surrounded by chromatin material. The nucleus contains one or more nucleoli. - **By EM:** It appears as a non-membranous oval spongy structure formed of dark and light areas. - **The dark area consists of:** - Pars granulosa: Mature rRNA - Pars fibrosa: newly formed rRNA. - Pars amorpha (nuclear organizer): DNA filaments - **The light area formed of nucleolar sap.**