Connective Tissue Overview

• Connective tissue shares cellular and extracellular (matrix) components with epithelium and other basic tissues.
• Connective tissue has abundant extracellular material but fewer cells, unlike epithelial, muscle, and nerve tissues.
• In connective tissue, cells aren't directly attached, and they are scattered within the extracellular substance.
• Blood vessels and nerve fibers are abundant in the extracellular substance.
• Connective tissues come from the mesoderm, with some head exceptions that come from the ectoderm.

Types of Connective Tissue

• Connective tissue proper
• Extracellular Substance (Connective Tissue Matrix)
• Special types of connective tissue like cartilage, bone, blood, and hemopoietic tissue

Connective Tissue Proper

• Found throughout the body, acting to connect body parts while facilitating movement
• Forms the stroma which is the support system of tissues, while parenchyma refers to the functional elements.
• It envelopes muscles, acts as channels for vessels and nerves, promotes substance exchange, and defends against organisms

Extracellular Substance

• Connective tissue matrix includes ground substance plus fibers.
• Ground substance is amorphous, homogenous, transparent, and hydrated.

Ground Substance Components

• Water is stabilized by proteoglycans, hyaluronic acid, mineral salts, and glycoproteins.
• Proteoglycans impart a gelatinous nature.
• Hyaluronic acid results in acidic characteristics in connective tissue.
• Mineral salts sustain ground tissue function.
• Glycoproteins helps with diffusion of materials and waste.
• Proteoglycans are composed of a core protein with glycosaminoglycans (GAGs).

Common GAGs

• Keratan sulfate
• Chondroitin-4-sulfate
• Chondroitin-6-sulfate
• Dermatan sulfate
• Heparin sulfate
• Multiple GAGs create large molecules bonded with hyaluronic acid via link-proteins.
• Glycoproteins include fibrillin, fibronectin, laminin, and thrombospondin that mediates cell migration.

Extracellular Fibers

• They supports and provide strength to connective tissue.

Types of Fibers

• Collagen
• Elastic
• Reticular

Collagen Fibers

• They are a key feature of all connective tissue types.
• They're the primary fiber in collagenous connective tissue, often called routine connective tissue.
• Collagen appears colorless in living tissues but looks white, specifically in tendons.
• H&E stains show pink, while Masson's trichrome dye turns it blue.
• Collagen constitutes about 25% of the total dry weight in the body.
• Types I, II, and III collagen are the most common forms in connective tissue out of ~28 types.
• Type I results in the collagen fibers that exhibit high tensile strength with flexibility and inelasticity.

Collagen Fiber Formation

• Fibroblasts and mesenchymal cells produce procollagen.
• Extracellular enzymes process procollagen into tropocollagen by eliminating extra amino acids.
• Tropocollagen molecules form collagen fibrils that aggregate to form collagen fibers.

Elastic Fibers

• They're flexible fibers that connect and branch.
• H&E stains give it a yellow or purple, while orcein stains it black
• Elastic fibers combine elastin in the center with fibrillin microfibrils.
• Elastic fibers can double in length.
• They are located in elastic cartilage, walls of arteries (elastic lamellae), vertebra ligaments.

Reticular Fibers

• Collagen Type III creates the reticular fibers, an element separate from collagen fibers.
• The fibers aren't visible with H&E, but silver staining can highlight them.
• Connective tissue covers muscles
• They are critical for body defense, housing cells to protect against organisms
• Enveloping muscles acts as an avenue for blood vessels and nerves to pass into and out of body parts and provides cells to defend the body against organisms.

Resident Cells

• They permanently reside within connective tissue.

Mesenchymal Cells

• Pluripotent stem cells able to produce fibroblasts, adipose cells, reticular cells, cartilage, bone, and muscle cells.
• Prevalent in embryos and umbilical cords, but scarce in adults.
• While similar to fibroblasts, these feature coarser chromatin and less organelles.

Fibroblasts & Fibrocytes

• Fibroblasts, in their active state, create extracellular matrix components, like fiber, non-fiber, elastic, and reticular fibers.
• Fibrocytes are inactive fibroblasts.
• Functionally fibroblasts secrete proteins, glycoproteins, glycosaminoglycans, collagen, and elastic fibers.
• Active secretory cells include basophilic cytoplasm with well-developed rough endoplasmic reticulum [rER]
• Possess ovid, pale nucleus with fine chromatin and a nucleolus
• Able to divide during mitosis, but divide infrequently
• Reactivate in wound recovery

Reticular Cells

• These cells make type III collagen (reticular fibers).
• Larger than fibroblasts
• Possess long cytoplasmic processes.

Adipose Cells/Adipocytes (Fat Cells)

• Store fat, providing insulation and cushioning.

Mast Cells

• Have granules containing histamine and heparin involved with immune responses
• Histamine produces dilated capillaries and increases permeability
• The cells start as CFU-Mast cells in bone marrow, then move to connective tissue.
• They are found in the lamina propria and under skin, and near veins.

Macrophages

• They're derived from monocytes and participate in the mononuclear phagocyte system (MPS).
• Histiocytes refers to when macrophages are in connective tissue.
• Morphology
• They contain large, basophilic cells with an indented nucleus.
• Antigen-presenting cells (APCs) can trigger immune response.

Types of Macrophages

• Fixed Macrophages are attached to collagen fibers.
• Free Macrophages move extracellularly.
• Resident Macrophages are responsible for the tissue's regular population.
• Inflammatory Macrophages are produce as a stimulus reaction from monocytes.

Plasma Cells

• They're slightly larger than RBCs, produce immunoglobulins (antibodies) and play a part in the body's immune response.
• Plasma cells are common in tissues exposed to bacteria, such as the digestive tract lamina propria.
• Plasma cells (Plasmocytes) contain basophilic cytoplasm and an eccentric nucleus with "clock-face" chromatin.

Leukocytes/White Blood Cells

• Leukocytes are nucleated blood cells with all leukocytes produced in bone marrow, lymphocytes are produced bone marrow and lymphoid tissues.
• Once in circulation, from capillary squeezing they operate in connective tissues by amassing in inflamed areas for immune response.

Types of Leukocytes

• Granulocytes (Neutrophils, eosinophils, basophils)
• Agranulocytes (Monocytes, lymphocytes.)

Adipose Tissue

• During lipid droplet formation, lipids combine to form larger droplets.

Types of Adipose Tissue

• White Adipose Tissue is made of a single large fat droplet, creating a signet ring look.
• Brown Adipose Tissue has multiple small fat droplets.
• Brown adipose is formed by mesenchymal cells or intermediate cells before complete differentiation
• Develop from mesenchymal cells or preadipocytes, not mature fat cells.

Mast Cells

• They are big, oval cells with a centralized nucleus and multiple cytoplasmic granules.
• The metachromatic granules will stain purple in a blue toluidine bath.
• Histamine is a vessel dilator and increases space.
• Develops from Colony-Forming Unit-Mast Cell (CFU-Mast) in bone marrow, and connective tissue.
• Common in the GI and respiratory tract lamina and under the skin, near the blood vessels.

Macrophages

• They are large cells with basophilic cytoplasm and indented nuceli.
• Support with phagocytosis/digestion action

Types of Connective Tissue

• Collagenous

Ordinary Connective Tissue

• The connective tissue is most abundant in the body.
• Its main fiber is collagen (Type I) and the main cell is fibroblast.
• There are also cells of fibroblasts, macrophages, masts cells, and fat cells

Loose Connective Tissue

• Has a rich ground substance, few collagen fibers, bundles of collagen elastin mix with fluid matrixes.
• It is found in the hypodermis, tunica adventitia, and lamina properia

Dense Connective Tissue

• Tissues can be collagenous or elastic

Specialized Connective Tissue

• Cartilage
  • hyaline
  • fibrocartilage
  • elastic
• Bone
  • endochondral ossification
  • intramembranous ossification

Areolar Connective Tissue

• Under the skin and beneath muscles as well as found in blood vessels and covers internal organs
  • fibroblasts: creates reticulin, elastic, collagen
  • collagen is thickest
  • elastic is thinnest

Hyaline Cartilage

• Consists of cells of chondroblasts, nests of cells called chondrites, matrixes, and perichondrium
• Can be found in the joint cavities, ribs, nose, and skeleton

Fibrocartilage

• Unlike others, has little lacunae
• Fibers seem to go one way
• Resembles striations of a steak
• It can be found in the Intervertebral disc and the pubic symphysis

Adipose Tissue

• Its main cell is adipocytes (fat cell) and used for storage of energy, insulation, and supports parts

Reticular Tissue

• Composed with lymphocytes, fibers, and reticular cells
• It forms a network of support.
• Can be found in the liver, lymphoid organs, and pancreas

Elastic Tissue

• It contains parallel bundles of elastic fibers.
• Has a few collagen fibers and fibroblasts
• Can be found in vertebral columns, suspensory ligaments, vocal chords
• Made of elastin and a little bit of collagen

Mucous Tissue

• It has jelly-like ground substance
• Its fibers have very few cells
• Can be found in embryos

Muscle Cells & Tissues

• Skeletal muscle contributes to the functions of the skeleton. Can be found on skeletons, heat and postures are striated multinucleated and are parallel
• Cardiac muscles are for the hearts to pump that are connected and involuntary.
• Smooth muscles are in the GI tract to for peristalsis, involuntary and do not have striations. They are long and spindle shaped

Skeletal Muscle Tissues

• Form skeletal muscles (organs) with attachment from bones, voluntary except in pharynx and esophagus, functions on quick, forceful movements by shortening the sarcomeres due to actin and filaments.

Origin of Skeletal Muscle Fibers

• Origin is formed by combining myoblasts which results in myosatellite cells.
• Nuclei are abundant and found peripherally
• Sarcoplasm is what contains cell organelles and lipids
• Striation alternates within the bands

Muscle Filaments/Myofilaments Details

• Thick or thin filaments and aligned with Z to M lines.
• They are made with actin, tropomyosin, troponin, and myosin.

Key Structures in Myrofibrils

The following key structure found in myrofibrils include:

• Z band helps hold structures and creates dark lines
• The lighter zone is called H (Helle) Band,
• M-lines are middle sections

Muscle Filaments Compositions

• At the sarcomere's middle and spanning the A-band are thick filaments containing the protein (myosin)
• Thin filaments containing F-actin, tropomyosin, and troponin attach to the Z-line in peripheral.

Thin/Actin Fillament Details

• Main is F-actin protein, which has two strands twisted together to bind to myosin.
• Protein complex on actin controlling muscle contraction.

The Thick Fillament

• Connects to action and comprises over half of the muscle protein. Includes large polypeptide chains.

Sliding Muscle Functions

Shortening of sarcomeres is possible, resulting from thin actin and thick filaments passing one another.

• Thick remains still, while thin is propelled to the sarcomere central

Main Features

• The zones narrow since sections contract.
• The muscle section shortens

Sarcoplasmic Reticulum and T-Tubules

• Tubular and the "electrical drain" for calcium transportation
• Calcium ions stored until muscle response happens

Neuron Impulse Functions

Nerve activation is key since nerve force determines t-tubule response, then calcium ions, leading to contraction

• Later a response reverses by ceasing delivery/delivery of signals for reversal

Red vs White Fibers

• Types are red for endurance and white as fatigable.

Internal Muscle Organs

• The Intrafusal fibers run through the exterior fibers.
• The Extrafusal surrounds internal ones and regulates.
• Function helps detect level / rate of muscle flexing.

Function of Tension Receptors

Golgi Tendon organ helps monitor muscle state, measure pull/stress.

• By nerve action they measure tension throughout joints and muscles.

Long Muscle Features

• Cardiac muscle is limited to heart and it receives power for contractions.
• Electrical coupling permits quick signals pass.

Structure of Cardiac Tissue

• Has a transverse portion and a lateral

Major Intercalated Disk Function for Contractions

• Intercalated discs sustain cells together and help impulses spread
• Signals quickly flow to and from them, leading too proper actions.

Self-Excitation

Since cardiac muscle is self regulating/ self contained, contraction begins from SA-Node in heart.

• Stimulation from nerve activity and acetylcholine reduces rates

Smooth Muscle Definition

• Smooth muscle is in blood vessels and functions in the organs, more than striated muscles. Can exist in several layers.

Smooth Muscle Structures

• Main characteristic is filament and cells made most by protein which creates actions with filaments. No striations.
• Smooth muscle controls the body and contracts over time and with filaments working and gliding.

Key Features

• Allow rapid signal transport and action across units
• They mechanically bond during contracts.
• Signal relays help contractions in the tissues
• These tissues operate well since all parts are synced. This gives well and regulated operations.

The Heart

• Special traits give powerful and powerful movements.

Nervous Tissue Traits

• Tissue has densely configured cells/structure.
• Support actions by tissue and CNS / PNS functions