chapter 4: a&p (tissues)

what is a tissue?
- Group of cells found together within a body that share an embryonic origin
- Histology: microscopic study of tissue appearance, organization, and function

- epithelial tissue
- connective tissue
- muscle tissue
- nervous tissue

how do we study tissues?
1. Samples are fixed: preserved in formalin or frozen
2. Samples are cut into sections
3. Samples are stained: dyed to enhance specific structures or contrasts
4. Samples are viewed
- Light microscopy can view many different tissues and dyes
- Transmission electron microscopy (TEM) views high magnification in grayscale
- Scanning electron microscopy (SEM) allows 3D views of tissue


Tissue Types
- Epithelial: sheets that cover surfaces or line internal cavities
- Connective: bind cells and organs together, function in protection, support, and integration
- Muscle: excitable; response to stimuli and contract
- Nervous: excitable; propagate electrochemical signals to communicate across the body


Epithelial Tissues
- Sheets of cells exposed to the environments
- Coverings and linings: skin, organ system cavities, organ walls
- Glandular epithelium

Characteristics:
- Polarity (Apex & Basal)
- Basal lamina: supporting glycoprotein sheet
- Tight Junctions & Desmosomes
- Highly regenerative
- Avascular (no blood vessels) & Innervated (nerves)


Epithelial Tissue Identification:
1. # of rows → stratified or simple?
2. shapes → squamous, cuboidal, or columnar?

Epithelial Types
- Simple Squamous: endothelium of vessels, mesothelium of serous membranes (ex: lungs, kidneys, capillary)
- Simple Cuboidal: secretion & absorption (ex: kidney, glands)
- Simple Columnar: absorption & secretion (ex: airway, gi tract)

- Stratified Squamous: most common (ex: skin, mouth)
- Stratified Cuboidal & Columnar: in some glands, but are both very rare


Other Epithelia
- Pseudostratified: single layer of cells attached to basal lamina, but apical surfaces reach different heights, nuclei at different heights (ex: testes, airway)


- Transitional: stratified in range of cuboidal and squamous shapes to enable stretch (ex: bladder)


Glandular Epithelium
- Gland: one or more cells that produce and secrete a specific product
- Endocrine glands: ductless glands, secrete hormones directly into tissues
- (ex: thyroid hormone, insulin, epinephrine/adrenaline, testosterone, estrogen)
- Exocrine glands: ducts lead to the epithelial surface
- (ex: mucus, sweat, saliva, breast milk)

Glandular Structure
- Unicellular Glands: goblet cells
- Multicellular glands: (pockets (alveolar/acinar) or tubes


Secretion Methods
- Merocrine: vesicles emptied into extracellular space
- Apocrine: release portion of cell
- Holocrine: cell ruptures and is destroyed


Connective Tissues
- loosely dispersed cells in a matrix with extensive extracellular material (ex: blood, cartilage, bone, etc)
- Ground Substance: material filling space between cells
1. interstitial fluid
2. adhesion proteins
3. proteoglycans
(glycoprotein)
- Protein Fibers throughout

- support & connect other tissues
- protection & immune defense
- transport
- storage

Tissue → Organs



Connective Tissue: Proper
- Fixed Cells:
1. Fibrocytes (Fibroblasts)
- secrete protein, abundants

2. Adipocytes
- store adipose/fat
- can either be brown or white

3. Mesenchymal Cell
- adult STEM cells
- WBC & RBC develop

4. Macrophages
- immune cell & eat pathogens & signal immune system

5. Mast Cells
- immune cells & release histamine


Extracellular Matrix
- Collagen:
resist, stretch, tendons & ligaments
- Elastic Fiber (Elastin):
stretch & compress → og shape; dermis
- Reticular Fibers:
organ walls


Loose Connective Tissue
- Adipose: metabolism, fast capillary
1. white: insulation, protection from injury
2. brown: “baby fat” ; thermogenic metabolism

- Reticular: support soft organs

- Areolar: cushion organs, mediates inflammation
- between muscles & vessels, under epithelia

Dense Connective Tissue
- Dense Regular: attaches muscles and bones to each other
- parallel collagen, little elastin

- Dense Irregular: structural strength in multiple directions
ex: dermis, digestive tract

- Elastic: regular connective tissue with high levels of elastin

• allows recoil after stretching
  ex: large arteries, bronchial tubes
  
  
  Supportive Connective Tissue

• Cartilage: chondrocytes in lacunae, ground substance full of proteoglycans
  - Avascular & not innervated
  1. Hyaline — strong, dense (ex: nose, ribcage, embryonic skeleton)
  2. Fiber — thick bundle (ex: intervertebral discs)
  3. Elastic — bendy (ex: ear)
  
  

• Bone: collagen fibers embedded in hydroxyapatite (calcium phosphate); highly vascular
  
  
  Fluid Connective Tissue

• Blood: erythrocytes transport oxygen, leukocytes fight pathogens, platelets promote blood clotting
  

• Lymph: white blood cells; highly permeable vessels to regulate fluid balance
  
  
  Muscle Tissues

• Skeletal Muscle: attached to bones; motion, thermal homeostasis
  - myocyte bundles with striations of actin and myosin
  - multinucleated
  - AKA “voluntary” muscle & activated by nervous system
  

• Cardiac Muscle: heart walls; contract without stimulation
  - cardiomyocytes with single nuclei connected by intercalated discs, striated
  

• Smooth Muscle: involuntary movements of internal organs
  - spindle-shaped cells, single nuclei, no striations
  
  
  
  Nervous Tissue

• Neuron: propagates information via electrochemical signals (action potential)
  - sheathed in myelin (myelin sheath)
  - Synapse: gap between neurons
  

• Neuroglia: support neurons
  - Astrocytes: homeostasis of CNS
  - Oligodendrocytes: produce myelin for CNS
  - Schwann Cells: produce myelin for PNS
  
  
  Membranes

• Cutaneous Membrane: skin, keratinized epidermis attached to this connective tissue (dermis)
  - exposed to air, is very dry
  

• Mucous Membrane: AKA mucosae, line all body cavities that open to outside (e.g., digestive & respiratory tracts)
  - bathed in secretions → “wet”
  - epithelial sheets or lamina propria
  - adapted for absorption & secretion
  

• Serous Membranes: AKA serosae, closed ventral body cavities
  - visceral & parietal layers separated by serous fluid
  - simple squamous epithelium over thin areolar connective tissue
  
  Tissue Repair

• Regeneration: replacement of destroyed tissue with correct tissue type
  

• Fibrosis: replacement of destroyed tissue with scar tissue
  
  1. Inflammation: chemicals cause white blood cells, fluid, clotting proteins to leak into space; clot seals off injury
  2. Organization: restored blood supply by new capillaries, new collagen production, macrophages clear debris
  3. Permanent Repair: new tissue matures