BHSC 1310 Module 3

Histology and its purpose

• the process of fixing a specimen, slicing it very thin and viewing it under a microscope

• many time, the specimen is stained with specific dyes or markers depending on what is being looked at

• histology allows us to look microscopically at different tissues and cells

How cells differentiate

• All cells in your body have the same DNA, but they may carry out different functions

• during development as an embryo, out cells respond to certain signals that tell them to go into specific developmental pathway (by turning some genes off and others on)

• These cells develop specific characteristics and are grown-up - cell differentiation

• differentiated cells look different from one another and have different characteristics and functions in the body

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How cells form tissues

• disassociated cells are reassemble into organized tissues through selective cell-cell adhesion

  • when embryonic organs are mixed together, the mixed aggregates can gradually sort out according to their tissue of origin

  • there are cell-cell recognition systems that make similar cells preferentially adhere to one another

Epithelial Tissue

• grouping of cells that covers a body surface or lines a body cavity

• this can also come into the form of glands → secrete substances

• main functions of epithelial tissue:

  • protection

  • absorption

  • filtration

  • excretion

  • secretion

  • reception (sensory receptors)

Describe the features typical of cells in epithelial tissue

• cells in epithelial tissue have special characteristics that help them carry out their functions:

  • polarity

  • specialized contacts

  • supported by connective tissue

  • avascular (no blood vessel)-but has nervous system innervation

  • able to regenerate

How epithelial cells are classified

Classified based on:

• cell shape

  • Squamous (scale-like)

  • Cuboidal

  • Columnar

• Number of cell layers

  • simple (one layer)

  • stratified (multiple layers)

• Surface specialization

  • Ciliated

  • Keratinized

Simple squamous epithelium

• single layer of flat cells → this thin layer of cells creates a small barrier that allows some substances to pass

• functions:

  • allows materials to pass via diffusion or filtration

  • secretes substances that help with lubrication

• locations:

  • glomeruli of kidney (filtration), air sacs of lungs (gas exchange), lining of heart, blood vessels, and lymphatic vessels

Simple Cuboidal epithelium

• single layer of cuboid-shaped cells

• functions → secretion and absorption

• locations → walls of ducts (secretion), kidney tubules, ovaries

Simple columnar epithelium

• single strand of column shaped cells

• functions:

  • secretion and absorption (especially mucus, enzymes and other substances)

  • usually contain specializations like cilia and microvilli

• locations:

  • digestive tract

  • gall bladder

  • bronchi (respiratory system)

  • uterine tubes

Pseudostratified Columnar Epithelium

• cells are of different heights and appear to be stratified but it is a single layer of cells

• functions

  • secretion (mucus)

  • many cells contain cilia - move substances away

• location

  • upper respiratory tract

  • ducts of large glands

  • tubules in testes

Stratified squamous epithelium

• contains two or more layers of flattened cells (basal cells usually are cuboidal/columnar)

• Functions

  • protection, especially from abrasions

  • specialized type contains keratin which makes the cells a bit stronger

• locations

  • lining of esophagus, mouth and vagina

  • epidermis of skin

Stratified cuboidal and columnar epithelium

• contains two and more layers of cuboidal/columnar cells (both have limited distribution in the body)

• Functions

  • helps epithelium transfer from one type to the other

  • limited secretion

• locations

  • mammary glands

  • pharynx

  • male urethra

Transitional Epithelium

• appears like stratified squamous and stratified cuboidal - surface cells usually appear dome-like

• functions

  • varying shapes allow for stretching

• locations

  • lines ureters, bladder and parts of the urethra

Glandular epithelium

• once or more cells that produce and secrete a fluid

• classified by:

  • where the secretion is released

    • endocrine: these glands have no ducts and release their products into the blood (usually hormones)

    • exocrine: these glands have ducts and release their products onto body surfaces or into body cavities

  • number of cells that form the gland

    • unicellular

    • multicellular

endocrine gland

• these glands do not have cuts

• they release their products into the surrounding interstitial fluid and this is picked up by surrounding blood vessels

• these hormones travel to a target tissue as chemical messengers where they will cause this organ to respond in a specific way depending on the hormone released

• Ex) Thyroid gland, Pituitary gland

exocrine gland

• these release their products into ducts that open into body surfaces or cavities

• more abundant than endocrine

• examples:

  • mucous, sweat, salivary

• can be unicellular or multicellular

Unicellular exocrine glands

• mucous cells and goblet cells

  • both produce mucin which can dissolve in water to form mucous

• mucous is a very important substance in the body and helps provide a protective and lubricating coating

Multicellular exocrine glands

• are composed of a duct and a unit of cells creating the secretion

• these are surrounding by connective tissue that supports the gland and allows for proximity to blood vessels and nerves

• classified by structure and their mode of secretion

• structure

  • simple have ducts that do not branch; compound have branching ducts

  • secretory units can form

    • tubes - tubular

    • sacs - alveolar

    • both - tubuloalveolar

Multicellular exocrine glands-mode of secretion

• merocrine → secretes via exocytosis as secretions are produced

  • most sweat glands, pancreas, salivary glands

• apocrine → accumulate product within, apex rupture (cell repairs damage and repeats)

  • there is debate on extent of this in the human body

• holocrine → accumulate product until the cell ruptures (cell must be regenerated by dividing underlying cells)

  • sebaceous (oil) glands

connective tissue

• most abundant and widely distributed tissue in the body

• helps with binding, support, protection, insulation, and transport

• different from other tissues in that it has an extracellular matrix that exists outside of the cells and is composed of non-living components

• four types of connective tissue in the body

  • connective tissue paper

  • cartilage

  • bone

  • blood and lymph

The extracellular matrix (ECM)

• connective tissues are not made solely of cells

• a substantial part of tissue volume is extracellular space, largely filled by the extracellular matrix

• the ECM is closely associated with the cell surface

• functions:

  • binds cells together and supports tissue

  • influences cell growth and development

  • influences cell polarity, which is important for directional transportation of materials

  • influences cell behavior

Components of ECM

• ECM is composed of ground substance and fibers

• ground substance

  • gel-like material that fills the space between cells-helps solutes diffuse between blood vessels and cells

• contain

  • interstitial fluid

  • cell adhesion proteins - attachment

  • proteoglycans attached to glycosaminoglycans (these help with the hydration of the ground substance and attract water) - resists compression and attracts water for diffusion

• Fibrous proteins - embedded in the ground substance

  • collagen-the major proteins of the ECM

    • provides tensile strength to the ECM

  • Elastic fibers - elastin provides resilience to the matrix

    • can recoil after transient stretch

  • reticular fibers - short collagenous fibers

    • branching creates networks that allow some give to tissues

immature cells of connective tissue

• an immature cell is known as a “-blast” and these actively secrete ground substance and matrix fibers

Mature cells of connective tissue

• a mature cell is a “-cyte” and helps maintain matrix (can revert to blast to repair/regenerate)

Connective tissue proper cells - Fibroblasts (become fibrocytes)

Produce fibers and ground substance

Connective tissue proper cells - macrophages

• engulf foreign materials and activate immune system

Connective tissue proper cells - Lymphocytes

• create antibodies

Connective tissue proper cells - mast cells

• secrete histamine which mediates inflammation

Connective tissue proper cells - adipocytes

• fat cells that store triglycerides

Classification of connective tissue proper

• classified by:

  • loose connective tissue

    • Areolar

    • Adipose

    • Reticular

  • Dense connective tissue

    • dense regular

    • dense irregular

    • elastic

Areolar (loose)

• widely distributed and helps support and bind other tissues and is used as packing

• contains fibroblasts which secrete a loose arrangement of collagen fibers

• loose arrangement means abundant space for the ground substance - helps hold more interstitial fluid

• locations

  • under epithelia (lamina propria)

  • surrounds organs (packaging)

  • surrounds capillaries

Adipose (loose)