Histology: Tissues and Extracellular Matrix

• Tissues are groups of cells organized to perform specific functions.
• Organs are composed of four basic tissue types.
• Cells
• Also contains the Extracellular matrix which includes fluid-like components

Four Basic Tissue Types

• Epithelial
• Connective
• Muscular
• Nervous

Cells, Extracellular Matrix, and Main Functions of Tissues

• Nervous Tissue: Intertwining elongated processes; No Extracellular Matrix; Transmission of nervous impulses.
• Epithelial Tissue: Aggregated polyhedral cells; Small amount of Extracellular Matrix; Living of surface or body cavities (for covering), glandular secretion.
• Muscle Tissue: Elongated contractile cells; Moderate amount of Extracellular Matrix; Movement/Locomotion.
• Connective Tissue: Several types of fixed and wandering cells such as bone/cartilage and blood cells; Abundant Extracellular Matrix; Support, providing nutrients and protection to epithelium.

Epithelium

• Characterized by close cell apposition and presence at a free surface
• Epithelial cells are always contiguous, arranged in single or multiple layers
• Simple: single laver
• Stratified: two or more layers
• Usually joined by specialized cell-to-cell junctions, creating a barrier

Epithelium continued

• Intercellular space is minimal with few structures other than junctional attachments
• Sub-classifications are based on cell shape and number of layers

Cell Shapes

• Squamous: flattened
• Cuboidal: almost have equal sides
• Columnar: taller than they are wide

Layer Descriptions

• Simple: single layer
• Stratified: multiple layers
• Pseudostratified: appears stratified due to varying cell heights, but is a single layer

Connective Tissue

• Characterized by its extracellular matrix
• Cells are separated by spaces occupied by the the extracellular matrix (ECM)
• Loose connective tissue associates with most epithelia
• Dense connective tissue is densely packed with collagen fibers for strength

Muscle Tissue

• Categorized by the ability of its cells to contract
• Grouping for muscle and nervous tissue is based on functional property
• Grouping for epithelial and connective tissue, it's based in morphology
• Cardiac, smooth, and skeletal muscles are classified based on function
• Muscle cells aggregate into distinct bundles
• Muscle cells are elongated and oriented with long axes in the same direction
• Classification can be by contractile ability such as whether it is voluntary/nonvoluntary

Nerve Tissue

• Nerve tissue made of nerve cells (neurons) and supporting cells
• Neurons are defined by axons and dendrites

Central Nervous System

• The central nervous system (CNS) is made of the brain and spinal cord
• Supporting cells are called Schwann (neurilemmal) cells and satellite cells

Histogenesis of Tissues

• In development, tissue and organs originate from three germ layers
• Ectoderm
• Mesoderm
• Endoderm

Ectodermal Derivatives

• Ectoderm is the outermost of the three germ layers
• Two major classes:
• Surface ectoderm
• Neuroectoderm

Surface Ectoderm Derivatives

• Epidermis and its derivatives (hair, nails, sweat glands)
• Sebaceous glands
• Parenchyma and ducts of the mammary glands
• Cornea and lens epithelia of the eye
• Enamel organ and the enamel of teeth
• Components of the internal ear
• Adenohypophysis (anterior lobe of pituitary gland)
• Mucosa of the oral cavity

Neuroectoderm Derivatives

• The neural tube and its derivatives, including components of the central nervous systems, ependyma (epithelium lining the cavities of the brain and spinal cord), pineal body, posterior lobe of pituitary gland (neurohypophysis), and the sensory epithelium of the eye, ear, and nose
• The neural crest and its derivatives, including components of the peripheral nervous system

Mesodermal Derivatives

• Mesoderm is the middle of the three primary germ layers.
• Connective tissues
• Striated muscles and smooth muscles
• Heart, blood vessels, and lymphatic vessels
• Spleen
• Kidneys and gonads
• Mesothelium
• Adrenal cortex
• Bone and cartilage

Endodermal Derivatives

• Innermost layer and forms the wall of the primitive gut by giving rise to epithelial portions/linings of organs from the gut tube

Endodermal Derivatives include

• Alimentary canal epithelium
• Extramural digestive gland epithelium
• Lining epithelium of the urinary bladder, part of the urethra
• Respiratory system epithelium
• Thyroid, parathyroid, and epithelial components of the thymus gland
• Parenchyma of the tonsils
• Lining epithelium of the tympanic cavity and auditory (Eustachian) tubes

Epithelial Tissue Functions

• Covering, lining, and protection of surfaces (e.g., skin)
• Absorption (e.g., the intestines)
• Secretion (e.g., glands)
• Contractility (e.g., myoepithelial cells)

Epithelial Tissue Functions include

• Protection – epidermis of the skin
• Absorption - epithelium of the small intestine
• Excretion – epithelium of the kidney
• Secretion - glandular epithelium
• Sensory reception - neuro-epithelium
• Lubrication – goblet cells and sebaceous glands
• Reproduction – seminiferous tubules and germina epithelium of the ovary
• Some cells are contractile (myoepithelial cells)

Characteristics of Epithelial Cells

• Lamina propria supports the epithelium by providing nutrition and binding to underlying structures
• Loose connective tissue is usually found beneath the epithelium.
• Papillae are small invaginations or irregularities in the connective tissue surface to increase contact area

Poles of Epithelial Tissue

• Basal Pole: contacts connective tissue
• Apical Pole: faces a space
• Lateral Surfaces: adjoin adjacent cells

Basement Membrane

• A felt-like sheet of extracellular material
• Basal lamina: network of fine fibrils
• • Lamina lucida: connects to the epithelium
• • Lamina densa: is nearer to the underlying connective tissue
• Reticular lamina: a diffuse and fibrous layer

Macromolecular Components of Basal Laminae

• Laminin: glycoprotein molecules that self-assemble into a lace-like sheet near basal poles
• Type IV collagen: monomers containing three polypeptide chains that self-assemble to form a felt-like sheet
• The laminin and type IV collagen network are held together by adhesive glycoprotein entactin/nidogen and by perlecan.
• Laminin is in the lamina lucida, while type IV collagens are in the lamina densa

Diffuse Meshwork of Reticular Laminae

• Type III collagen (reticular fibrils)
• Type VII collagen (anchoring fibrils)

Basement Membrane Functions

• Anchors the epithelium
• Serves as barrier against malignant human cells
• Functions structurally and in filtration
• Influences cell polarity
• Regulates cell proliferation and differentiation
• Influences cell metabolism and survival
• Organizes the proteins in the adjacent plasma membrane
• Serves as pathways for cell migration
• Contains information necessary for cell-to-cell interactions

Basal Lamina and Basement Membrane

• "Basal lamina” denotes the lamina densa and adjacent layers, as seen with TEM
• “Basement membrane” denotes structures seen with a light microscope

Intercellular Adhesion and Other Junctions

• Junctions function to:
• Prevent material flow (occluding junctions)
• Provide adhesion (adhesive or anchoring junctions)
• • They interact with actin and intermediate filaments, and provide mechanical stability
• Create channels for communication (gap junctions)

Intercellular Junction and Communication

• For cell-to-cell communication the function is diffusion of molecules
• Endocytosis is involved

Three Distinct Zones of Connetions

• Zonula occludens (Occluding junction): important in transporting epithelium and maintaining structural integrity
• Zonula adherens (intermediate junction)
• *"Terminal web"
• • Serves for microfilament insertion used for the core of the microvilli
• Macula adherens (desmosomes): appear as dense dots that adhere to the cytoskeleton of the cell surface

LM Entire Structure

• In light microscopy the entire connection structure is called "terminal bar"
• Zonula adherens and macula adherens are adhesive or anchoring junctions

Nexus Functions

• Concerned with cell-to-cell communication (gap junction)
• "Communicating junction"
• Each membrane has polygonal projections (connexions)
• Adhesive function
• Present in osteocytes, smooth, and cardiac muscles and neurons
• Absent in skeletal muscle and blood

Junction Components

• Major transmembrane proteins
• Tight junction: occludens, claudins
• Adherent junction: E-cadherin, catenin proteins
• Desmosome: cadherin family proteins
• Gap junction: connexin proteins

Cytoskeletal Components by Junction Type

• Tight Junction - Actin filaments
• Adherent junction - Actin filaments
• Desmosome - Intermediate filaments (keratin)
• Gap junction - None

Major Functions

• Tight junctions seal adjacent cells together
• Provides cytoskeleton points linking
• Allows the transport of small molecules and ions

Specialization of the Apical Cell Surface

• Microvilli Delicate are vertical striations in a refractile border and is prominent in cells
• Functions:
• Increase the efficiency of absorption
• Act in kidney function for absorption

Stereocilia & Cilia

• Stereocilia are long processes projecting into the lumen from each cell and is found in the lining of the epididymis
• Stereocilia promote absorption by amplifying cell surface
• Cilia are larger than microvilli
• Primary cilia are enriched with receptors
• Motile Cilia is found only in epithelia to propel fluid or mucus

Main Groups of Epithelia include

• Covering epithelia
• Secretory (glandular) epithelia

Epithelia Classifications

• The classifications are according to:
• Number of cell layers
• • Simple
• • Pseudostratified
• • Stratified

Epithelia Contd

• Cell morphology
• • Squamous
• • Cuboidal
• • Columnar
• • Transitional

Covering Epithelia

• I. Simple Squamous Epithelium
• Very thin, flat cells
• Mosaic pattern
• Attenuated cytoplasm with central bulging nucleus
• Examples:
• • Endothelium: lining of blood vessel, lymph vessels, cavities of the heart
• • Mesothelium: lining serious cavities

Simple Squamous Epithelium Distribution

• Mesenchymal: lines the interior chamber of the eye, perilymph spaces of the internal ear, subdural and subarachnoid spaces
• Flattened cells: lining the pulmonary alveoli, bowman's capsule

Simple Squamous Epithelium Functions

• Facilitates the distribution of the viscera
• Active transport by pinocytosis
• Secretion of biologically active molecules

Simple Cuboidal Functions are

• Covering
• Secretion
• II. Simple Cuboidal Epithelium
• Row of square or rectangular profile
• Nuclei tend to aligned at the same level in all of the cells, "box-like”, “cube-like"

Simple Cuboidal Examples are

• Covering the ovary
• Thyroid follicles
• Choroid plexus
• Pigmented epithelium of the retina

III. Simple Columnar Epithelium is made of:

• A membrane that is composed of cylindrical cells possessing an appreciable height beside length and width
• Nuclei are at the same level
• Associated with secretion or absorption

Simple Columnar Epithelium Distribution

• Simple plain tall columnar: mucosa of the stomach, small and large intestine, gallbladder, bigger ducts of glands
• Simple plain low columnar: smaller ducts of glands, some excretory tubules of kidney

Functions of Columnar Epithelium

• Protection
• Lubrication
• Absorption
• Secretion

Stratified Epithelia

• Classified as Stratified Squamous
• • I.a. Stratified Squamous Epithelia (Keratinized)(I.b. Stratified Squamous Epithelia (Non-keratinized)
• Stratified Cuboidal
• Stratified Columnar

Stratified Squamous Function

• Cells become more irregular in shape and flatten as they accumulate keratin
• Cells become hardened, cornified, and removed closer to the skin surface
• Thin packets of keratin-lacking

Stratified Squamous Keratinized Function

• Epidermis
• Protection
• Prevents water loss

Stratified Squamous Non-Keratinized Function

• Lines cavities where loss of water is not a problem
• Flattened cell of the surface contain less keratin and retain nuclei
• Non-keratinized is non-keratinized

Stratified Squamous Non-Keratinized Distribution

• Mouth
• Esophagus
• Larynx
• Vagina
• Anal Canal

Stratified Squamous Non-Keratinized Functions

• Protection
• Secretion
• Prevents water loss

Stratified Cuboidal Epithelia

• Examples of distribution are:
• Excretory ducts of salivary and sweat glands
• Developing ovarian follicles

Stratified Cuboidal Epithelia Function

• Protection
• Secretion

Example of Stratified Columnar Epithelia

• Conjuctiva

Stratified Columnar Epithelia Function

• Protection

Transitional Functions

• IV. Transitional (Urothelium) Epithelium
• Very thin
• Many layers (contracted stage)
• Deepest layer is one or two rows
• Is Protections

Transitional Distrubution

• Blatter
• Ureter
• Rena calyces

PSEUDOSTRATIFIED EPITHELIA Function

• Only one layer
• Tall coulmar
• Lining trachea
• Nasal cavitiy

PSEUDOSTRATIFIED EPITHELIA Location

• Lning trachea
• Bronchi
• Nasal Cavity

PSEUDOSTRATIFIED EPITHELIA Function

• Portection and Secretion

Secretory Epithelia and Glands

• Epithelial cells can be specialized for secretion in structure and the cells can be called "gland"

Secretory Products

• The molecules usually stored in the cells within small membrane by vesicles called secretory granules

Exocrine/Endocrine glands

• Two major categories:
• Exocrine: deliver their product to into external or internal structures
• Endocrine: release the products into blood or lymph for transport

Acrine glands

• Secretion is not discharged from the cells such as granular leukocytes and phagocytes

Secratory Products

• Serous: acinar cells of pancreas
• Mucous : gastric glands

Gland Structures

• Multicellular: arranged in cords, bundles, or islet
• Connective tissue stroma: support
• Arrangement follicle

Endocrine

• Secratory hormone direct

Hormones different types includes

• Polypeptide protein hormoness
• Glycoprotein hormone
• Steroid
• Biochemical

Hormone Structure

• Structure; Nucleous

Classification of Glands

• Unicellular: one cell as a goblet

Exocrine glands

• Simple one tubes multiple tubes

Simple Tubes

• Simple tubular
• Branched Tubular -Coiled Tubular
• Arcina Areolar

Features and Examples based on Shape and Function

• Simple Tubular: elonngated ( Colon)
• Branched Tubular: secretory multiple (gland in the uretus)
• Coiled tubular - very long sweat glands

Acinar Alevolar Features

• Round sace example is small mucous glands

Branched Arcinal

• Multpel ex; sebacous gland

Compound Feature

Several elongated

Tubular

• Several elobrated;
• Subucosal in the duodenum

Acinar Alevolar Features

• Several ducts mammory

Tululoacina Featurs

• Ducts both tubukar anc acinar

Exocrine Gland - Secretery products

• Merocrine
• Holocrine
• Apocrine

Merocrine secretion

• Is also called eccrine; is called typical exocytosis
• mantain the integrity

Holocrine secretion

• Involved in shedding secretery; being disruputed
• Skin being sheded

Apocrine

• partial destruction CellPartial Destruction destrcitoin

Acording to Nature

Serous Galnd

Stain intensilty

Mucous Galnd

• Larger sell in in the Sublnguatl