Histology Notes PDF

## Tissue: Introduction **Definition:** A group of similar cells that work together to perform a specific function. **Components:** * Cells * Extracellular Matrix **Cells:** The living components, where specific cell types are adapted for the unique function of the tissue. **Extracellular Matri...

## Tissue: Introduction **Definition:** A group of similar cells that work together to perform a specific function. **Components:** * Cells * Extracellular Matrix **Cells:** The living components, where specific cell types are adapted for the unique function of the tissue. **Extracellular Matrix:** The non-living part, that surrounds and supports the cells, providing structure and biochemical signals. | Component | Description | |---|---| | Proteins | * Collagen - strength * Elastin - strength | | Glycoproteins | * Proteoglycans - tissue hydration and structure | | Minerals | * Bone tissue | ## Types of Tissue 1. **Epithelial:** Protection, secretion, absorption 2. **Connective:** Support soft body parts 3. **Muscle:** Movement 4. **Nervous:** Conducts impulses ## Embryonic origin of tissues | Germ Layer | Location | Function | |---|---|---| | **Ectoderm** (outermost layer) | * Gives rise to tissue associated with the skin and nervous system. * Epidermis of the skin * Nervous system * Sensory organs * Connective tissues of the face and head | | | **Mesoderm** (the middle layer) | * Muscles * Bone and cartilage * Blood vessels * Kidneys * Dermis of the skin | | | **Endoderm** (innermost layer) | * Internal organs and structures related to the digestive and respiratory systems. | | ### Gastrulation: Results in the formation of the three embryonic layers. ## Epithelial Tissue **Definition:** Is a type of tissue that forms the outer layer of body surfaces, lines hollow organs, and body cavities, and makes up glandular structures. ### Characteristics: | Aspect | Description | |---|---| | Free Surface | Epithelial cells have an exposed surface, open to the outside or to an internal space. | | Basement Membrane | Anchors epithelium to the underlying connective tissue, providing structural support. | | Avascular (No Blood Vessels) | Lacks blood vessels; nutrients and waste exchange occur via diffusion from nearby connective tissue. | | High Regenerative Capacity | Cells readily divide, supporting rapid healing and replacement (e.g., skin healing). | | Tightly Packed Cells | Cells are closely packed with little extracellular space, forming a protective barrier. | ### Basal Membrane Structure | Layer | Description | |---|---| | Lamina Lucida (Rare or Lucid Lamina) | Outermost layer (about 10 nm thick) with sparse fine filaments; lies just below epithelial cells. | | Lamina Densa (Dense Lamina) | Middle layer (about 30 nm thick), made of type IV collagen in a gel-like matrix, providing strength. | | Lamina Fibroreticularis | Deepest layer, connects to connective tissue; composed of type VII collagen fibers. | | Key Biochemical Component | Type IV collagen is essential, especially in the dense lamina, forming a supportive mesh-like network. | ## The Basal Membrane/ Basement Membrane A thin, fibrous, and supportive layer of tissue that lies at the base of epithelial cells and separates them from the underlying connective tissue. ### Background * Thin, dense layer of extracellular matrix * Made of collagen type IV, laminin, and other molecules * Composition dynamic, differs with tissue ### Function * Structural support * Prevention of tumor invasion * Regulation of cell behaviour * Angiogenesis ## Basal Membrane Structure A specialized extracellular matrix that supports and separates epithelial cells from underlying connective tissue. ### Structure * **Lamina Lucida (Rare Lamina):** Immediately beneath the epithelium. Contains sparse, fine filaments, giving it a translucent or ‘lucid’ appearance under electron microscopy, and helps anchor epithelial cells to the basement membrane. * **Lamina Densa (Dense Lamina):** The central layer. Contains a dense network of type IV collagen (reticular network, anchoring to connective tissue). Provides structural integrity and filtration properties. Appears as an opaque mass under electron microscopy. * **Lamina Fibroreticularis:** Positioned adjacent to the connective tissue. Made of type III collagen fibers forming a reticular network that anchors the basement membrane to the underlying connective tissue. ## Cell Adhesion and Functions **What is cell adhesion?** It refers to how cells stick together or attach to one another to form tissues and maintain structure. **How does it work?** Proteins on cell membranes interact with each other and with the environment to form specialized structures called *cell junctions* which connect cells or attach them to the surrounding tissue. | Type | What Do they Do? | How are they built? | Key Idea | |---|---|---|---| | **Occlusive Junctions (Tight Junctions)** | Create a seal to prevent leakage of fluids, ions, and molecules between cells. Example: Intestines, where food and water pass through cells, not between them. | Proteins (claudins, occludins) form a belt-like structure around the top part of cells near the apical (top) part of epithelial cells. | They make tissues "waterproof" or create barriers. | | **Anchoring Junctions** | Keep cells firmly attached to each other or the basement membrane. Help tissues resist mechanical forces like stretching or pulling. | Built with specialized proteins that form connections depending on the type: * Desmosomes: Connect intermediate filaments. * Hemidesmosomes: Attach cells to basement membrane. * Adherens Junctions: Connect actin filaments. | Provide mechanical strength and maintain structural cohesion. | | **Communication Junctions (Gap Junctions)** | Enable communication between cells by passing ions, molecules, and signals directly. Example: In the heart, gap junctions allow electrical signals to pass so the heart contracts as one unit. | Made of connexons, channels formed by proteins called connexins. These channels create small pores for molecule passage. | They make cells work together like a team, ensuring coordination and communication. | ### Summary of Cell Junctions: [Epithelial Cell] * **Tight Junction (Occlusive Junction)** * Location: Top (apical region) of cells * Function: Waterproof barrier, prevents leakage * Proteins: Claudins, Occludins * **Adherens Junction (Anchoring Junction)** * Location: Below tight junctions * Function: Connects actin filaments for shape and cohesion * Proteins: Cadherins * **Desmosomes (Anchoring Junction)** * Location: Throughout lateral sides of cells * Function: Strong connections for resistance to stress * Proteins: Cadherins (desmoglein, desmocollin) * **Hemidesmosomes (Anchoring Junction)** * Location: Base of cell, attaches to basement membrane * Function: Anchors cells to underlying connective tissue * Proteins: Integrins * **Gap Junction (Communication Junction)** * Location: Anywhere between cells * Function: Allows passage of ions, nutrients, and signals * Proteins: Connexins (form connexons) [Neighboring Cell or Basement Membrane] ## Epithelial Classification Epithelial tissue are classified based on two criteria: * **Shape of Cells** * **Number of Cell Layers** ### Shape of Cells * **Squamous:** Thin, flat cells. Allow diffusion and filtration. Found in alveoli in the lungs, and lining of blood vessels. * **Cuboidal:** Cube-shaped cells. Secretion and absorption. Found in glands, kidney tubules. * **Columnar:** Tall, elongated cells. Absorption, secretion, and protection. Found lining the stomach and intestine. ### Number of Cell Layers * **Simple Epithelium:** Single layer of cells involved in processes like absorption, secretion, and filtration. *Simple squamous* in blood vessels, *simple cuboidal* in kidney tubules. * **Stratified Epithelium:** Two or more layers of cells. Protection against wear and tear. *Stratified squamous* in the skin, *stratified cuboidal* in sweat glands. | Classification | Description | Examples | |---|---|---| | Simple Squamous Epithelium | Single layer of flat cells | Alveoli, blood vessel lining | | Stratified Squamous Epithelium | Multiple layers of flat cells | Skin, esophagus lining | | Simple Cuboidal Epithelium | Single layer of cube-shaped cells | Kidney tubules, glandular ducts | | Stratified Cuboidal Epithelium | Multiple layers of cube-shaped cells | Sweat glands, salivary glands | | Simple Columnar Epithelium | Single layer of tall, elongated cells | Lining of stomach, intestines | | Stratified Columnar Epithelium | Multiple layers of tall, elongated cells | Male urethra, ducts of some glands | ## Epithelial Classification and Function | Epithelial Type | Function | Examples | Special Types | |---|---|---|---| | Coating/Lining Epithelium | Protect surfaces, absorb, and secrete. | Skin, intestines, respiratory tract | Pseudostratified, Transitional | | Glandular Epithelium | Secretion (enzymes, hormones, mucus). | Sweat glands, thyroid gland | Exocrine and Endocrine Glands | | Sensory Epithelium | Sensory perception (stimuli detection). | Retina, olfactory epithelium | - | ### Coating/Lining Epithelia Divided into three main groups based on how many layers of cells they have: 1. **Simple (Unistratified):** Only one single layer of cells. * **Why important:** Thin layers allow things to easily pass through (gases, nutrients). * **Where Found:** *Lungs (simple squamous epithelium). Function: gas exchange. *Kidneys (simple cuboidal epithelium) Function: filters blood, absorbs molecules. *Stomach and intestines (simple columnar epithelium) Function: absorbs nutrients, secretes mucus.* 2. **Layered (Multilayered):** 2 or more layers of cells. * **Why important:** Protects against damage, like friction or chemicals * **Where Found:** *Skin (Stratified squamous epithelium) Function: protects against injury. *Esophagus (stratified squamous epithelium) Function: protects from scraping. *Sweat glands (stratified cuboidal epithelium) Function: secretion and durability.* 3. **Special Types** * **Pseudostratified Epithelium:** Looks like many layers but is really just one layer (nuclei are different heights). Found in the respiratory tract (trachea). Moves mucus out with cilia. * **Transitional Epithelium:** Can stretch and return to normal. Found in the urinary bladder fills. ## Simple Squamous Epithelium **Description:** Single layer of flattened cells with disc-shaped central nuclei and sparse cytoplasm. The simplest of the epithelia. **Function:** Allows passage of materials by diffusion and filtration in sites where protection is not important. Secretes lubricating substances in serosae. **Location:** Kidney glomeruli, air sacs of lungs, lining of heart, blood vessels, and lymphatic vessels, lining of ventral body cavity (serosae). **Photomicrograph:** Simple squamous epithelium forming part of the alveolar (air sac) walls (125x). ### Summary of Simple Squamous Epithelium | Category | Details | |---|---| | Description | Single layer of thin, flat cells with disc-shaped nuclei and sparse cytoplasm. | | Function | Allows for diffusion and filtration; secretes lubricating substances in serous membranes. | | Locations | * Kidney glomeruli (filtration) * Air sacs of lungs (gas exchange) * Lining of heart, blood vessels, and lymphatic vessels (smooth blood flow) * Lining of ventral body cavities (reduces friction in body cavities) | | Photomicrograph | Shows thin, flat cells with visible nuclei, often from lung alveoli, emphasizing its structure and arrangement. | | Cell Structure | Thin, flat cells with sparse cytoplasm, centrally located nucleus, resting on a basement membrane over connective tissue. | ## Simple Cuboidal Epithelium **Description:** Single layer of cube-shaped (cuboidal) cells with large, spherical, centrally located nuclei. **Function:** Primarily involved in **secretion and absorption**. The cuboidal shape allows for more cytoplasmic volume, which supports metabolic activities. **Location:** Found in **kidney tubules** (helps in filtering blood and reabsorbing essential nutrients), **ducts of small glands** (for secretion), and **surface of ovaries** (protection and secretion). **Photomicrograph:** Shows simple cuboidal cells in cross-section of kidney tubules. **Features:** * Labels include simple cuboidal epithelial cells, basement membrane (supports the epithelium), and connective tissue (provides structural support). * Photomicrograph shown at 430x magnification, providing a detailed view of individual cells and their arrangement around the lumen of kidney tubules. **It’s Involved in secretion and absorption.** **It has more cytoplasm than squamous cells, allowing for higher metabolic activity.** ## Simple Columnar Epithelium **Description:** Single layer of tall cells with round to oval nuclei. Some cells bear cilia. The layer may contain mucus-secreting unicellular glands (goblet cells). **Function:** Absorption, secretion of mucus, enzymes, and other substances. Ciliated type propels mucus (or reproductive cells) by ciliary action. **Location:** Nonciliated type lines most of the digestive tract (stomach to anal canal), gallbladder, and excretory ducts of some glands, ciliated variety lines small bronchi, uterine tubes, and some regions of the uterus. **Summary of Simple Columnar Epithelium** | Aspect | Details | |---|---| | Structure | Single layer of tall, column-shaped cells. Nuclei are oval and located near the base of cells. May contain goblet cells or cilia. | | Function | * Absorption: Nutrients and substances (e.g., digestive tract). * Secretion: Mucus, enzymes, and other substances. * Propulsion: Cilia move mucus or reproductive cells. | | Non-Ciliated Location | Lines most of the digestive tract (stomach to anal canal). Found in gallbladder and excretory ducts of glands. | | Ciliated Location | Lines small bronchi, uterine (fallopian) tubes, and some regions of the uterus. | | Special Features | * Goblet Cells: Produce mucus for lubrication and protection. * Microvilli: Increase surface area for absorption (forming brush borders). * Cilia: Aid in movement of mucus or ova. | | Appearance in Microscopy | Cells are tall with visible nuclei. Goblet cells appear as clear or pale structures. Microvilli look like a dense "brush border" on the apical surface. Cilia are longer and less dense compared to microvilli. | | Example Organs | * Non-Ciliated: Stomach, intestines, gallbladder. * Ciliated: Respiratory tract, fallopian tubes. | ## Stratified Squamous Epithelium **Description:** Thick membrane composed of several cell layers. Basal cells are cuboidal or columnar and metabolically active. Surface cells are flattened (squamous); in the keratinized type, the surface cells are full of keratin and dead, basal cells are active in mitosis and produce the cells of the more superficial layers. **Function:** Protects underlying tissues in areas subjected to abrasion. **Location:** Nonkeratinized type forms the moist linings of the esophagus, mouth, and vagina. Keratinized variety forms the epidermis of the skin, a dry membrane. **Summary of Stratified Squamous Epithelium:** | Aspect | Details | |---|---| | Structure | Composed of several layers of cells. Basal cells: Cuboidal or columnar, metabolically active, and undergo mitosis. Surface cells: Flattened (squamous) and keratinized (in some types). | | Function | Protects underlying tissues in areas subjected to abrasion and friction. | | Types | * Keratinized: Surface cells are dead and filled with keratin for added protection. * Non-keratinized: Surface cells remain moist and alive. | | Location | * Non-keratinized: Found in moist linings of the esophagus, mouth, and vagina. * Keratinized: Forms the epidermis of the skin (a dry membrane). | | Special Features | * Basement membrane: Anchors the basal cells to the underlying connective tissue. * Keratinized layer: Prevents water loss and protects against physical damage (in keratinized types). | | Appearance in Microscopy | Multiple layers are visible. Basal cells appear rounded or cuboidal, while superficial cells are flattened. Keratinized types show a dense, pinkish layer without nuclei on the surface (keratinized cells). | | Example Organs | * Non-Keratinized: Esophagus, oral cavity, vagina. * Keratinized: Epidermis of the skin. | ## Pseudostratified Columnar Epithelium/ Transitional Epithelium ### Pseudostratified Columnar Epithelium **Description:** Single layer of cells of differing heights, some not reaching the free surface; nuclei seen at different levels; may contain mucus-secreting cells and bear cilia. **Function:** Secretion, particularly of mucus; propulsion of mucus by ciliary action. **Location:** Nonciliated type in male’s sperm-carrying ducts and ducts of large glands; ciliated variety lines the trachea, most of the upper respiratory tract. ### Transitional Epithelium **Description:** Resembles both stratified squamous and stratified cuboidal. Basal cells cuboidal or columnar; surface cells dome shaped or squamouslike, depending on degree of organ stretch. **Function:** Stretches readily and permits distension of urinary organ by contained urine. **Location:** Lines the ureters, urinary bladder, and part of the urethra. **Summary of Pseudostratified Columnar and Transitional Epithelium:** | Aspect | Pseudostratified Columnar Epithelium | Transitional Epithelium | |---|---|---| | Structure | - Appears layered (stratified) but is actually a single layer of cells with nuclei at varying levels. - May have cilia and mucus-secreting goblet cells | - Resembles both stratified squamous and stratified cuboidal/columnar. - Basal cells are cuboidal/columnar. - Surface cells are dome-shaped or flattened, depending on stretch. | | Function | - Secretion (particularly mucus). - Propulsion of mucus by ciliary action. | - Stretching: Allows the urinary organs to distend. - Prevents rupture and maintains barrier integrity when stretched. | | Location | - Ciliated type: Lines the trachea and most of the upper respiratory tract. - Non-ciliated type: Found in male sperm-carrying ducts and ducts of large glands. | - Lines the ureters, urinary bladder, and part of the urethra. | | Special Features | - Cilia: Helps move mucus and trapped particles. - Goblet cells: Produce mucus to trap debris. | - Surface cells change shape: Dome-shaped when relaxed. Flattened when stretched (e.g., bladder full). | | Appearance in Microscopy | - Nuclei are visible at different heights. - Ciliated surfaces are visible in ciliated types. - Goblet cells appear as pale structures. | - Rounded surface cells are visible in relaxed states. - Cells appear flattened and elongated in stretched states. - Basal layer rests on a distinct basement membrane. | | Example Organs | - Ciliated: Trachea, respiratory tract. - Non-ciliated: Male reproductive ducts. | - Urinary bladder, ureters, part of the urethra. | ## Connective Tissue **Connective tissue is a fundamental type of tissue in the body that supports, connects, or separates different types of tissues and organs.** **Roles:** * Structural Support * Protection * Insulation * Transport * Storage **It holds things together. It can be found in bones, fat, blood....** ## Types of Connective Tissue **Loose Connective Tissue:** * Areolar * Reticular Tissue * Adipose tissue **Dense Connective Tissue:** * Dense Regular Tissue * Dense Irregular Tissue * Elastic Tissue **Specialized Connective Tissue:** * Cartilage * Bone * Blood * Lymph ## Key Features of Connective Tissue 1. **Cells are not in direct contact with each other:** Unlike epithelial cells, where cells are closely packed, connective tissue cells are spread out. 2. **Surrounded by Matrix:** The matrix is the material outside the cells. * **Fibers:** Collagen, elastin * **Ground Substance:** Fluid, gel, solid This matrix gives connective tissue its properties, such as flexibility or strength. Example: *Bone* - matrix is hard and calcified. *Blood* - (plasma) is fluid. 3. **More Matrix Than Cells:** This makes connective tissue excellent for filling spaces, supporting organs, and protecting tissues. ## Variety of Matrix Type The composition of the matrix determines the type and function of the connective tissue. * **Fluid Matrix** * **Semifluid Matrix** * **Calcified Matrix** ## Connective Tissue Components 1. **Cells:** Produce and maintain the fibers and ground substance, defend the tissue, and store energy. 2. **Fibers:** Provide the tissue’s structure, strength, and flexibility. 3. **Ground Substance:** Acts as a cushion, fills gaps, and ensure hydration and nutrient exchange. ## Components of Connective Tissue | Component | Description | Function | Examples | |---|---|---|---| | **Cells** | | | | | * **Fibroblasts** | Most abundant cells in connective tissue. Spindle-shaped with large oval nuclei. | Synthesize and maintain the ECM (collagen, elastin, proteoglycans, GAGs). Play a role in tissue repair and wound healing. | Tendons, ligaments, dermis of the skin. | | * **Adipocytes** | Specialized cells for fat storage. Large lipid droplet with a small nucleus at the periphery. | Store energy in the form of fat. Provide insulation and cushioning for organs. Secrete hormones like leptin and adiponectin. | Adipose tissue (fat) | | * **Macrophages** | Large, irregularly shaped cells with lysosomes | Engulf and digest foreign particles, dead cells, and pathogens. Secrete cytokines and assist in immune responses and tissue repair. | Lungs, liver, lymphatic tissues. | | * **Mast Cells** | Contain granules filled with histamine and heparin. | Play a key role in allergic reactions and inflammation by releasing histamine and other chemicals that attract immune cells. | Skin, mucous membranes, lungs. | | * **Chondrocytes** | Rounded cells found in lacunae of cartilage | Synthesize and maintain the cartilage ECM (collagen, elastin, proteoglycans). | Cartilage (e.g., articular, elastic, and hyaline cartilage). | | * **Osteocytes** | Mature bone cells embedded in bone matrix. | Maintain bone matrix and communicate with other bone cells to regulate bone mineral content and homeostasis. | Bone tissue (osteons) | | * **Leukocytes** | White blood cells, including neutrophils, lymphocytes, monocytes, eosinophils | Play key roles in immune defense, inflammation, and tissue repair.| Blood, lymphatic tissue. | | **Extracellular Matrix (ECM)** | | | | | * **Collagen Fibers** | Thick fibers made of collagen protein, often arranged in bundles. | Provide tensile strength to tissues, enabling them to resist stretching. | Tendons, skin, bone, cartilage. | | * **Elastin Fibers** | Thin, branching fibers made of elastin protein. | Provide elasticity, allowing tissues to stretch and return to their original shape. | Skin, lungs, blood vessels, elastic cartilage. | | * **Reticular fibers** | Thin, branching fibers composed of Type III collagen | Form fine, supportive networks in soft tissues and organs | Lymphoid organs (spleen, lymph nodes), bone marrow, liver. | | **Ground Substance** | Gel-like material that fills spaces between fibers and cells | Provides hydration, lubrication, and medium for diffusion of nutrients, waste, and gases. Maintain hydration and resistance to compressive forces. | Connective tissue proper, cartilage, bone. | | * **Glycosaminoglycans (GAGs)** | Long, unbranched polysaccharides that attract water to form a gel-like substance | Trap water and help resist compressive forces, contribute to the structural integrity of tissues. | Hyaluronic acid, chondroitin sulfate, dermatan sulfate | | * **Proteoglycans** | Proteins with attached GAGs (e.g., aggrecan) | | Cartilage, skin, bone. | | * **Glycoproteins** | Proteins with carbohydrate chains attached | Act as adhesive molecules, promoting cell attachment to ECM. Facilitate communication between cells and ECM. | Fibronectin, laminin. | ## Blast: Immature Cells These are *active*, immature cells in connective tissue. **Function** * They divide (undergo mitosis) * They actively secrete the matrix (fibers and ground substance) to build the connective tissue. **Examples** * **Fibroblast:** Produce fibers: collagen, elastin * **Chondroblast:** Secrete the matrix in cartilage. ## Cytes: Mature, Differentiated Cells * Less active **Function:** * **Maintain the Matrix** * **Have reduced capacity for cell division** **Examples:** * **Fibrocytes:** Maintain the connective tissue matrix. * **Histiocytes:** A type of white blood cell that are part of the body's immune system. They originate from monocytes. **Plasma Cells:** a type of white blood cell that plays a crucial role in the immune system. Derived from B cells: a type of lymphocyte. ## Embryonic Connective Tissue A type of connective tissue found in the early stages of development. It plays a critical role in the formation of various tissues and organs during embryogenesis. | Type | Definition | Features | Function | Location | |---|---|---|---|---| | **Mesenchyme** | Loosely organized, undifferentiated tissue with star-shaped mesenchymal cells. | * Multipotent cells. * Gel-like ground substance. * Sparse reticular fibers. | * Forms connective tissues (bone, cartilage, blood, etc.). * Crucial for organogenesis. | Found throughout the embryo. | | **Mucous Connective Tissue (Wharton’s Jelly)** | Specialized embryonic connective tissue in the umbilical cord. | * Jelly-like extracellular matrix rich in hyaluronic acid. * Contains fine collagen fibers. | * Provides structural support to the umbilical cord. * Protects umbilical vessels from compression. | Exclusively in the umbilical cord. | ## Embryonic Connective Tissues * There are two embryonic connective tissues: * **Mesenchyme** gives rise to all other connective tissues. * **Mucous C.T. (Wharton’s Jelly)** is a gelatinous substance within the umbilical cord and is a rich source of stem cells. ## Loose Connective Tissue ### Areolar Tissue **Definition:** A type of connective tissue characterized by a loosely arranged extracellular matrix, providing flexibility and support. It is one of the most widely distributed connective tissues in the body. **It's job is:** * **Hold things together** (like glue, but flexible) * **Cushion organs** (like a soft pillow around them) * **Allow movement** (because it’s not stiff) **Description:** A loose arrangement of fibers and cells in a gel-like ground substance. It serves as a flexible and supportive framework that wraps and cushions organs, allowing them to move without damage. **Summary of Areolar Tissue:** | Aspect | Description | |---|---| | Definition | A type of loose connective tissue with a loosely arranged structure of fibers and cells. | | Structure | - Loosely organized collagenous and elastic fibers running in random directions. - Contains reticular fibers in smaller amounts. - Gel-like ground substance. | | Cells Present | * Fibroblasts: Produce fibers and ground substance. * Macrophages: Engulf pathogens and debris. * Mast cells: Release histamine to mediate inflammation. * White blood cells: Protect against infection. * Adipocytes (occasionally): Store fat. | | Fibers Present | * Collagen fibers: Provide tensile strength. * Elastic fibers: Allow stretch and recoil. * Reticular fibers: Support surrounding tissues. | | Ground Substance | Amorphous, gel-like material composed of glycosaminoglycans and proteoglycans. | | Blood Supply | Highly vascular, ensuring efficient nutrient and waste exchange. | | Locations | * Beneath all epithelial tissues (e.g., under the skin). * Surrounds blood vessels, nerves, and capillaries. * Found in the fascia between muscles. * Lamina propria of mucous membranes. | | Functions | * Provides support and elasticity to tissues. * Cushions and protects organs. * Facilitates diffusion of nutrients and waste. * Acts as a pathway for blood vessels, nerves, and immune cells. * Participates in immune defense and inflammation. | | Special Features | - Loosely packed fibers and cells make it flexible yet strong. - Serves as the body's "packing material" filling gaps and holding tissues in place. | ### Reticular Tissue **Consists of a network of fine, branching fibers called reticular fibers.** It plays an essential role in the structural support and organization of various organs, particularly those involved in immune response and filtration. **Summary of Reticular Tissue:** | Aspect | Details | |---|---| | Definition | Reticular tissue is a type of connective tissue that forms a delicate network of fine fibers, providing structural support to various organs, especially those involved in immune functions and filtration. | | Composition | * Cells: Reticulocytes (modified fibroblasts). * Fibers: Reticular fibers (Type III collagen) * Ground substance: Gel-like matrix | | Structure | Network of fine, branched collagen fibers forming a mesh-like structure. | | Location | * Lymphoid organs: Lymph nodes, spleen, tonsils * Bone marrow: Supports blood-forming cells * Liver: Supports liver parenchyma * Kidneys: Supports renal medulla | | Function | * Support: Provides structural support to cells. * Filtration: Traps pathogens, dead cells, and foreign particles (in lymph nodes, spleen). * Cell attachment: Anchors cells in place (important in hematopoietic organs). * Immune response: Supports immune cells (lymphocytes, macrophages) for immune surveillance. | | Types | * **Lymphoid reticular tissue:** Found in lymph nodes, spleen, tonsils (immune function) * **Hematopoietic reticular tissue:** Found in bone marrow (blood cell production) * **Vascular reticular tissue:** Found in liver, kidneys (supports blood vessels) | | Clinical Relevance | * **Reticular fibrosis:** Increased fiber production in conditions like cirrhosis or fibrosis. * **Reticular cell neoplasms:** Cancerous growths, such as reticulum cell sarcoma (a type of lymphoma). | | Histology | Fine, branching fibers appear dark when stained (e.g., silver staining). | ### Adipose Tissue **Adipocytes (white adipose cells): ** Large, specialized cells that store fat. The fat fills the cell, causing the nucleus to be pushed to the periphery. White adipose tissue consists of large, unilocular (one large fat droplet) cells, while brown adipose tissue contains multilocular (many small fat droplets) cells. **It’s a specialized connective tissue that stores energy as fat, insulates the body, and cushions organs.** **There are two types of adipose tissue:** * **White Adipose Tissue (WAT):** The most common type, responsible for energy storage, insulation, and cushioning. * **Brown Adipose Tissue (BAT):** Rich in mitochondria, primarily involved in thermogenesis (heat production) and found in smaller amounts, especially in infants and hibernating mammals. **Summary of Adipose Tissue** | Aspect | Details | |---|---| | Definition | Adipose tissue is a specialized connective tissue that stores energy as fat, insulates the body, and cushions organs. It exists in two forms: white adipose tissue (WAT) and brown adipose tissue (BAT). | | Composition | * Cells: Primarily adipocytes, which store fat as triglycerides. * Fibers; Collagen fibers (provide structure) * Ground substance; Minimal in adipose tissue. | | Structure | Adipocytes are large, specialized cells that store fat. The fat fills the cell, causing the nucleus to be pushed to the periphery. White adipose tissue consists of large, unilocular (one large fat droplet) cells, while brown adipose tissue contains multilocular (many small fat droplets) cells. | | Location | * Subcutaneous fat: Beneath the skin, providing insulation and protection. * Visceral fat: Surrounding internal organs such as the heart, liver, and kidneys. * Bone marrow: Small amounts in the bone marrow (also involved in energy storage). * Other sites: Fat deposits also exist around joints and muscles. | | Function | * Energy storage: Adipose tissue stores energy in the form of triglycerides. * Insulation: Helps maintain body temperature by acting as an insulator. * Protection: Cushions and protects organs like the kidneys, heart, and eyes from mechanical damage. * Endocrine function: Adipose tissue produces hormones like leptin (regulates hunger and metabolism), adiponectin (involved in glucose regulation), and resistin (affects insulin resistance). | | Types | * **White adipose tissue (WAT):** The most common type, responsible for energy storage, insulation, and cushioning. * **Brown adipose tissue (BAT):** Rich in mitochondria, primarily involved in thermogenesis (heat production) and found in smaller amounts, especially in infants and hibernating mammals. | | Histology | * Under a microscope, white adipose tissue appears as large, rounded cells with a clear fat droplet and a peripherally located nucleus. * Brown adipose tissue has smaller cells with multiple fat droplets and abundant mitochondria, which give it a darker color. | ## Dense Connective Tissue ### Dense Regular Connective Tissue **Description:** Characterized by tightly packed collagen fibers that are arranged in a parallel or organized manner. **Summary of Dense Regular Connective Tissue:** | Aspect | Details | |---|---| | Structure | * Collagen fibers are densely packed and aligned in parallel bundles. * Fibroblasts are flattened and sandwiched between collagen fibers, appearing elongated. * Sparse ground substance fills spaces between fibers, providing slight flexibility and support. | | Main Components | * **Collagen fibers:** Primarily type I collagen, providing tensile strength. * **Fibroblasts:** Cells that produce and maintain collagen and extracellular matrix. * **Elastic fibers:** Minimal or absent, leading to reduced elasticity. * **Ground substance:** Composed of proteoglycans and glycosaminoglycans, though in low amounts. | | Arrangement | Fibers are aligned parallel to the direction of force, ensuring strength against unidirectional tension. | | Vascularization | Poor vascularization leads to slow nutrient supply and healing processes. | | Innervation | Contains nerve fibers that sense tension and strain but less innervated compared to other tissues. | | Function | * Withstands tensile forces in a single direction without tearing. * Provides strong attachment and structural support. * Ensures stability in structures requiring consistent tension, such as tendons and ligaments. | | Locations | * **Tendons:** Attach muscles to bones, transferring force during movement. * **Ligaments:** Connect bones to bones, stabilizing joints and limiting unwanted movement. * **Aponeuroses:** Broad, flattened sheets connecting muscles to other muscles or bones. | | Mechanical Properties | * High tensile strength: Resists stretching along the axis of fibers. * Low elasticity: Minimal ability to stretch or recoil compared to elastic connective tissue. | | Healing Ability | Limited regenerative capacity due to low blood supply. | | Color and Appearance | Appears white and glistening (e.g., tendons and ligaments) due to dense collagen content. | ### Dense Irregular Connective Tissue **Description:** Characterized by its densely packed collagen fibers arranged in a random, irregular pattern. It allows the tissue to resist tension and stretching in multiple directions. ### Elastic Tissue **Description:** Characterized by high density of collagen fibers, which provide strength and support. * **It provides flexibility and resilience.** * **It helps form part of the body** like the nose, ears, joints, and trachea/ some parts of the skeleton. ### Cartilage **It’s a specialized form of connective tissue that provides structural support, resilience, and flexibility.** **Key Features of Cartilage** * **Firm matrix:** Composed of collagen and proteoglycans, giving it strength and elasticity. * **Resists compression:** Absorbs shocks and pressure without permanent deformation. * **Avascular:** No