Connective Tissue Lecture: Histology MBG5008

• Connective Tissue is taught by Asst. Prof. Ayca İLTER in the Molecular Biology and Genetics Department.
• Email contact: ayca.ilter@istinye.edu.tr
• Lecture: Reproductive Biology (Fundamentals of Histology) MBG5008
• Lectures are on Fridays from 12.10-14.00.
• Office hours are on Fridays from 11.00-12.00.
• Room number 835 (B section, 8th floor)

Introduction to Connective Tissue

• Functions of Connective Tissue detailed include:
• Development
• Content
  • Cell Types
  • Fibers
  • Ground Substance
• Identified types of Connective Tissue

Functions of Connective Tissue

• Maintains the form of organs throughout the body.
• Provides a matrix that supports and physically connects other tissues and cells together in organs.
• The Ground substance allows nutrients and waste materials to spread to cells.
• The extracellular matrix (ECM) is the main component of Connective Tissue.
• The ECM comprises Protein Fibers and Ground substance.

Development of Connective Tissue

• Connective tissues originate from embryonic mesenchyme.
• Tissue develops from the middle layer of the embryo which is the mesoderm.
• Mesodermal cells migrate from their site of origin in the embryo, surrounding and penetrating developing organs.
• Undifferentiated mesenchymal stem cells give rise to all types of connective tissue, as well as specialized connective tissues, bone, and cartilage.
• Mesenchymal stem cells are multipotent, used in regenerative treatments for damaged tissue regeneration in certain patients.

Content of Connective Tissue

• Cells present are Permanent Resident and Transient types.
• Extracellular matrix includes Fibers and Ground Substance.

Cells of Connective Tissue: Fibroblasts

• Are named as the most common type in Connective Tissue
• There are two levels of activity: active fibroblasts and quiescent fibroblasts/fibrocytes.
• Active fibroblasts have large euchromatic nuclei, are spindle-shaped, and contain rough endoplasmic reticulum (RER) and a well-developed Golgi apparatus.
• Quiescent Fibroblasts (Fibrocyte) are smaller than active fibroblasts, contain much less RER, and have a darker, more heterochromatic nucleus.

Cells of Connective Tissue: Adipocytes

• Designated as fat cells- Mesenchymal-derived
• Typically found in the connective tissue of many organs.
• Adipocytes specialize in concentrating triglycerides as lipid droplet(s).
• Triglycerides are a storage form of energy.
• Fat cells predominate and are known as adipose tissue.
• White and Brown are the two types of Adipose Tissue (BAT).
• BAT has numerous mitochondria containing iron-rich enzymes, which gives mitochondria a reddish-brown color. Capillary networks enhance the brown appearance.

Cells of Connective Tissue: Macrophages & The Mononuclear Phagocyte System

• Exhibit a Well-developed phagocytic ability
• Capable of removing dead cells, tissue debris, or other particulate material
• Is a Histocyte (Pathology)
• Contains a well-developed Golgi apparatus and many lysosomes.
• Derive from monocytes.
• Along with other monocyte-derived cells, they comprise a family of cells called the mononuclear phagocyte system.

Cells of Connective Tissue: Locations and Functions

• Monocytes are located in the blood and are precursors to macrophages.
• Macrophages are located in connective tissue and lymphoid organs and produce cytokines and chemotactic factors. They participate in inflammation, antigen processing, and presentation.
• Kupffer cells are in the liver and have the same function as macrophages.
• Microglial cells are located in the central nervous system and have the same function as macrophages.
• Langerhans cells are located in the epidermis of the skin and are involved in antigen processing and presentation.
• Dendritic cells are in the lymph nodes and spleen and are involved in antigen processing and presentation
• Osteoclasts located in bone perform localized digestion of bone matrix.

Cells of Connective Tissue: Mast Cells

• Oval or irregularly shaped connective tissue cells.
• Abundant Secretory granules
• Capable of Metachromasia, containing basic dyes, and staining blue to purple or red.
• Mast cells function in the localized release of many bioactive substances with roles in the local inflammatory response and innate immunity, therefore playing a part in tissue repair.
• Secretory Granules contained within Mast Cells include Histamine, Serine Proteases and Cytokines.
• Heparin acts locally as an anticoagulant.
• Histamine promotes increased vascular permeability and smooth muscle contraction
• Serine Proteases activate various mediators of inflammation
• Eosinophil and neutrophil chemotactic factors attract those leukocytes
• Cytokines are polypeptides directing activities of leukocytes and other cells of the immune system
• Phospholipids are precursors for conversion to prostaglandins and leukotrienes, and Lipid mediators of the inflammatory response

Cells of Connective Tissue: Mast Cell Secretion

• The initial binding event is the antigen/allergen match with mast cells.
• The initial response involves an IgE antibodies.
• Second exposure: IgE-IgE Receptor is released.
• Activates adenylate cyclase, which leads to the phosphorylation of specific proteins and entry of Ca+2.
• This leads to the release of Heparin and histamine by Exocytosis (subcellular process to transport materials out).
• Phospholipases -production and release of leukotrienes

Cells of Connective Tissue: Plasma Cells

• Are B-lymphocyte–derived, antibody-producing cells.
• Large, ovoid cells containing basophilic cytoplasm attributed to richness in RER.
• Lighter areas Euchromatin give the appearance of a clock face.
• Plazma Cells are more abundant in infected tissues
• Produce antibodies that mediate immunity

Cells of Connective Tissue: Leukocytes-White Blood Cells

• A transient Cell Population, lacking hemoglobin
• Are key players in the defense against invading microorganisms
• Leave the blood migrating between the endothelial cells lining venules and migrate to connective tissue via diapedesis.
• Normal level in 1mm³ human blood is approx. 8000.
• Classified as Granulocytes- polymorphic nuclei
• Agranulocytes- spherical but not lobulated

Content of Connective Tissue: Fibers - Collagen

• Collagen fibers are extremely strong and resistant to normal shearing and tearing.
• Collagen is the most abundant protein in the human body.
• Mainly produced by fibroblasts.
• Acidophilic collagen fibers stain pink.

Content of Connective Tissue: Collagen Types

• Type I collagen fibers are located in skin, tendon, and bone and provide resistance to tension.
• Type II collagen fibers are located in cartilage and provide resistance to pressure.
• Type III collagen fibers are located in skin, muscle, and blood vessels and provide structural maintenance in expansible organs. Frequently found with type I.
• Type IV collagen fibers form sheet-forming collagens located in all basal and external laminae to support epithelial cells and provide filtration.
• Type VII collagen fibers have linking/anchoring collagens in epithelial basement membranes and anchors in underlying reticular lamina.

Content of Connective Tissue: Collagen Structure

• Has a subunit that is 8.6 nm
• Triple helix is the defining feature
• Contains two α1 and one α2 chains
• Mainly produced by fibroblasts
• Synthesis begins in RER then synthesized collager
• Is secreted by exocytosis to ECM
• Vitamin C is required for its synthesis

Connective Tissue Disorders: Collagen Deficiency

• Ehler Danlos Syndrome can occur
• Related to genetic disorder that is autosomal dominant
• Mutation in COL5A1, COL5A2, COL3A1 gene
• The result of connective tissue defect include
  • Excessive elasticity of the skin
  • Vulnerability
  • Extreme flexibility of joints

Content of Connective Tissue: Fibers - Elastic

• Have rubber-like properties allowing them to be much bending and stretching
• Form sparse networks interspersed with collagen bundles
• Elastin also occurs as fenestrated sheets called elastic lamellae in the wall of large blood vessels or arteries.

Content of Connective Tissue: Elastic Fibers Structure

• Tropoelastin- Precursor of elastin
• Tropoelastin aggregates into organized spheres on the cell surface before being cross-linked into elastic fibers

Connective Tissue Disorders: Elastic Fibers

• Lack of resistance can lead to Marfan Syndrome
• Is a genetic disorder that is autosomal dominant
• Mutation in the fibrillin-1 gene, which encodes the structure of elastic fibers
• Aortic enlargement (dilation of the main blood vessel that transports blood from the heart to the rest of the body) - can be life-threatening
• Mostly seen in the heart, blood vessels, bones, joints and eyes

Content of Connective Tissue: Fibers - Reticular

• Form an extensive network (reticulum)
• Type III collagen makes up the fibers which are heavily glycosylated
• They stain black argyrophilic (argyros;silver)
• Produced by fibroblasts
• Occur in the reticular lamina of basement membrane
• Serve as the supportive stroma
• Abundant in (bone marrow) and some lymphoid organs (eg, spleen and lymph nodes)
• Reticular cells (Fibroblast cells secreting reticular fibers)

Content of Connective Tissue: Ground Substance - General

• Components of extracellular matrix are ground susbstances include Glycosaminoglycanns, proteoglycans and multiadhesive glycoproteins

Ground Substance of ECM: Glycosaminoglycans (GAG)

• Made of of repeating disaccharide units are
• A hexosamine (e.g., glucosamine or galactosamine).
• A uronic acid (e.g., glucuronic acid or iduronic acid) or galactose.
• Classified into sulfated (e.g., chondroitin sulfate, heparan sulfate) and non-sulfated types (e.g., hyaluronic acid)
• Most ubiquitous GAG is hyaluronic acid (hyaluronan)

Ground Substance of ECM: Proteoglycans

• core protein + sulfated GAGs
• Synthesized on RER
• Mature in the Golgi for GAG side chains
• Secreted from cells by exocytosis.
• Major proteoglycan is aggrecan
• Main cartilage component

Ground Substance of ECM: Multiadhesive Glycoproteins

• Have Multiple binding sites for cell surface receptors (integrins) and for other matrix macromolecules
• Integrin+Laminin- Assembly and maintenance of basal lamina
• Integrin+Fibronectin- Cell adhesion and migration

ECM: Intergins functions and makeup

• Integrins are transmembrane links by which cells adhere to components of the ECM by binding a matrix protein and the actin cytoskeleton via talin inside.
• A integrin molecule heterodimer, with α and β chains.
• The integrin head portion extends from the cell surface into the ECM where it interacts with sites on fibronectin, laminin, collagens, or other matrix components.

Types of Connective Tissue

• Connective Tissue (C. T.) Proper
  • Loose C.T.
  • Dense Irregular C.T.
  • Dense Regular C.T.
• Specialized C. T.
  • Reticular C. T.
  • Adipose Tissue
  • Cartilage
  • Bone
  • Blood
• Embryonic Connective Tissues
  • Mesenchyme
  • Mucoid (mucous) C.T.

Types of Connective Tissue Proper

• Loose and Dense terms refer to the amounts of collagen present
• Loose Connective Tissue
  • Has Less collagen randomly distributed
  • Supports microvasculature, nerves, and immune defense cells creating Lamina propria, which is beneath the epithelial lining of digestive tract.
• Dense irregular connective tissue
  • Contains Much collagen randomly distributed, thus supports and protects organs against tearing in the capsule or the submucosa layer of digestive tract
• Dense regular connective tissue
  • Is Almost completely filled with parallel bundles of collagen; and provides strong connections within the musculoskeletal system, and resistance to force e.g. Tendons, Ligaments

Connective Tissue: Specialized

• A Delicate network of reticulin (reticular fibers) / Collagen Type III with attached fibroblasts (reticular cells)
  • Supports blood-forming cells, many secretory cells, and lymphocytes in most lymphoid organs
  • e.g. Bone marrow, liver, pancreas, adrenal glands, all lymphoid organs except the thymus

Connective Tissue: Embryonic

• Undifferentiated cells evenly distributed in a matrix containing stem/progenitor cells for all adult connective tissue cells
• Forms much of the mesodermal layer of early embryo
• Random fibroblasts with sparse collagen fibers in viscous matrix supports large blood vessels in the matrix in the umbilical cord

Notes

• Mesoderm is one of the three primary germ layers (ectoderm, mesoderm, and endoderm).
• Mesenchyme is a type of tissue derived from the mesoderm (or ectoderm in some cases) that contributes to connective tissues
• It is composed of loosely associated cells that can differentiate into various types of cells, contributing to the formation of various organs and tissues in the body
• Mucoid Tissue (Wharton's Jelly)
  • Definition: A specialized form of embryonic mucous connective tissue that replaces mesenchyme in the umbilical cord as development progresses.