A&P II Chapter 4: Histology

Tissue

group of similar cells & extracellular matrix working together to perform a function

Epithelial tissues

lot of cells tightly packed together w/ very little extracellular matrix between the cells (space), These cells lack blood vessels (no vascular system) and get their nutrients by diffusion, and these cells divide very frequently because they are prone to damage, Found on the outside of our body where tissue is exposed to the external world, anything bringing in things from the external world (digestive, respiratory, body cavities, tube of small intestines, etc.) 

Functions of epithelial tissues

• Physical protection  

• Selective permeability – gatekeeper of the body controlling what comes in and out 

• Secretion

Basement membrane

two layers of ECM that anchor an epithelial tissue to the connective tissue 

Apical side

part of cell exposed to the external environment

basal side

bottom of epithelial side in direct contact with basement membrane (made of molecules that create a layer between epithelial layer and whatever is underneath)

Simple epithelia

single layer of cells, found where we do a lot of absorption and secretion

Simple squamous epithelium

single layer of flat cells

Simple cuboidal epithelium

single layer of cube-shaped cells

simple columnar epithelium

tall and rectangular in a section, may have microvilli and cilia extensions; known for having goblet cells

pseudostratified columnar epithelium

some nuclei are higher than others (looks stratified when it is not because all cells are connected to basement membrane) 

Stratified epithelia

many layers of cells stacked on top of each other, found where we need more protection; cells at the top are different than cells at the bottom, so give them a name based on the TOP of the surface

Keratinized

nucleus starts to disappear, as cells get pushed to the surface they start to die because they fill with keratin

Gland

structure that makes and secretes a product

Exocrine glands

secretes a product through a duct to the external surface of the body or tract 

Endocrine glands

makes hormones which are communication molecules, they are secreted into the bloodstream and travel through the bloodstream

Neuroepithelium

found in the head that give special senses (in the nose for smell, in the eye for light, etc.)

Goblet cell

found in epithelium lining and digestive/respiratory tracts, creates mucus 

Merocrine secretion

product is secreted by exocytosis

Endocrine glands

secretes a hormone directly into the bloodstream to influence functions of distant target cells 

Connective tissues

cellular Velcro connecting all body tissues to each other allowing transport of substances through the body; Cells are not attached to one another, spread out and between the cells there is a lot of extracellular matrix

Functions of connective tissue

Connective tissue is things like tendons & ligaments, cartilage, bone, blood; Function to connect body parts, protect & bind organs and used for transport & immune protection 

Connective tissue proper

generally functions to connect and support other tissues 

Extracellular matrix

variety of substances, including protein fibers and other molecules that dissolve in a water solutions

Protein Fibers

collagen, reticular and elastic; large molecules, made by the cells in the connective tissue, help support the tissue

Collagen fibers

the body makes the most of this protein, long and unbranched, strong proteins

Reticular Fibers

similar to collagen but not as big and they branch

Elastic Fibers

very small, look thin and wavy, gives connective tissue elasticity 

Fibroblast

Cell within connective tissue proper that produces components of extra cellular matrix

Adipocytes

cell type in adipose tissue, each cell contains large, lipid droplets in the cytoplasm 

Mast cells

tissue bound immune cell found in many types of connective tissue that secretes inflammatory mediators

Phagocytes

cells of the immune system that can ingest foreign substances and dead cells

Muscle tissues

cells contract & generate force

Nervous tissues

cells that generate, send and receive messages

Extracellular matrix

substance in tissues outside of the cells that consists of extracellular fluid, ground substance and protein fibers 

Ground substance

water solution and all things dissolved in the water solution, may be a liquid, semisolid or solid depending on the composition and amount of water present 

Glycosaminoglycans

long, straight polysaccharide chains 

Proteoglycans

Glycosaminoglycans bound to a protein core

Cell adhesion molecules

glycoproteins in the extracellular matrix that holds cells in a tissue together 

Collagen fibers

protein fibers in the ECM of a tissue that are made of the protein collagen and give a tissue tensile strength 

Elastic fibers

protein fibers in the ECM that give a tissue distensibility and elasticity 

Reticular fibers

thin protein fibers in the ECM that form nets within organs like the spleen & lymph nodes, and form supportive networks around blood vessels and nerves

Cell junctions

where cells in a tissue are linked to one another

Desmosomes

hold cells together in a tissue to increase resistance of tissue to mechanical stress

Gap junctions

Gap junctions

Embryonic - tissue

all adult connective tissue comes from specific type of tissue in embryo called mesenchyme, which has stem cells that divide and give rise to connective tissue in the baby 

• Mucus connective tissue - found in umbilical cord 

Loose connective tissue

does not have as much collagen or fibers, more space not filled with proteins

Areolar Loose Connective Tissue

loosely arranged collagen (pink), fibroblasts are the primary cell, found around organs and between muscles

Adipose Loose Connective Tissue

closely packed adipocytes (fat cells), large and don’t pick up stain well; found under skin and around internal organs

Reticular Loose Connective Tissue

matrix of reticular fibers (black) that are thin and branch to create a network, have many fibroblasts and white blood cells; found in organs like liver & kidney

Dense regular connective tissue

not much space as there is so much protein present, collagen & elastic fibers, collagen is stacked in parallel bundles and fibroblasts are in lines, has nuclei in between collagen fibers; found in tendons & ligaments

Dense Elastic Connective tissue

found where you need more stretch like in the walls of arteries; collagen is straight and elastic fibers are wavy

Supporting connective tissue

cartilage & bone, in cartilage, the ECM is more gel-like and flexible than bone

Cartilage

tough & flexible tissue, absorbs shock and resists tension, found where there are many pores, difficult to repair because it does not get a good blood supply

Chondroblast

immature cartilage cell, then mature to become chondrocytes

Perichondrium

oxygen & nutrients diffuse from blood vessels in the perichondrium through the ECM to supply those cells

Types of cartilage 

Hyaline, fibrocartilage and elastic

Hyaline cartilage

the most abundant cartilage, thin collagen fibers, chondrocytes in lacunae, perichondrium is present (only in hyaline & elastic)

Fibrocartilage

weight-bearing cartilage, important for discs between bones, strongest & least flexible, bundles of collagen, chondrocytes in lacunae, wavy, microscope cannot focus well on it

Elastic cartilage

flexible with elastic fibers, perichondrium is present (only in hyaline & elastic), elastic fibers are darkly stained

Fluid connective tissue

blood & lymph

Cells of connective tissue 

• Resident cells – fixed in extracellular matrix and don't move 

• Wandering cells – as the liquids move, the cells do too 

Muscle Tissue

(myocyte) composed of muscle cells that can contract when stimulated by nervous system

Muscle cells

large contractile proteins inside that slide back and forth to cause a muscle contraction, can convert chemical energy to mechanical energy via ATP, allow us to walk, move, breathe, have a heartbeat, etc.

Skeletal Muscle Tissue

muscles attached to skeleton that allow us to move joints, stimulated by nervous system, signaling is under voluntary or conscious control (if you can think about it and then move it, that is skeletal) 

• Skeletal muscle cells are long and extend the whole length of the muscle 

• Nuclei are pushed to the edges and fibers are unbranched 

• Striated (striped), multinucleate cells 

Cardiac Muscle Tissue

found in the middle layer of the heart and the contraction results in heartbeat, contracts involuntarily

• Cells are shorter than skeletal and branched, they have intercalated discs which are gap junctions that connect two different cardiac muscle cells which serves as a method of communication to make sure cells are contracting at the same time to create a regular heartbeat 

• The dark lines on pictures of cardiac muscles are the intercalated discs 

• It is striated (striped) and it can branch 

Smooth Muscle Tissue

found in hollow organs in the body (blood vessels, eyes, etc.), can control blood pressure and protect airways, cannot consciously control this muscle

• cells are fat in the center and pointy at the ends, found where involuntary contractions occur

Nervous tissue

found in brain, spinal cord and in peripheral nerves 

• Plenty of space between cells and ECM has very few proteins  

• Composed of neurons or glial cells 

Neuron

cell type that makes nervous tissue unique as they can send electric signals

• Have ability to receive and respond to signals, neuron sends an action potential allowing for signaling to other cells, they can signal other neurons, muscle cells to tell them to contract and gland cells to release the substance it makes  

• They don't have the ability to divide and replace themselves 

• Large cells with cellular extensions  

• Have lots of ribosomes as they need to make lots of proteins 

Dendrites

extension of neuron that receives signal (many of these on one side)

Axons

extension of neuron that sends signal (only one on one side of neuron, taper slowly and are longer)

Neuroglial cells

do not send electrical signals but help support the neurons 

• Can divide themselves, anchor neurons in place, monitor composition of extracellular fluid, circular brain and spinal fluid 

Organ

two or more tissues that combine structurally and functionally. The tissues have their own function but work together to achieve the overall organ function (EX: skeletal muscle is an organ, it is composed of skeletal muscle (allowing for contraction) and dense irregular collagenous connective tissue (binding cells together and supporting them so the whole muscle organ can contract) 

True Membranes

within the body meaning thin sheets of cells of one or more tissue type that line body surfaces or cavities

• Consist of epithelial layer on top of connective tissue layer 

• Anchor various organs in place, serve as barriers for protection, immunity 

Serous membranes 

• folded on top if itself  

• Consists of mesothelium (thin layer of simple squamous epithelium), basement membrane and layer of connective tissue 

Synovial membranes 

Lining our freely moveable joints, between bones there is a fluid that reduces friction (synovial fluid) which is produced by cells in the synovial membrane 

Membrane-like structures

mucous and cutaneous membranes, they aren't true membranes

Mucous membrane

structure lining passageways that open to the external world such as respiratory and digestive tract, First layer is the epithelial later, then basement membrane, then connective tissue underneath 

Cutaneous membrane

refers to the skin

Tissue repair

wound healing occurs when dead and damaged cells are removed and replaced with new ones 

• If they cannot divide, connective tissue will divide and produces collagen instead which builds scar tissue 

• Scar tissue changes the function of the tissue so you lose functionality of the muscle in the area due to formation of scars 

• Connective tissue can generally heal by regeneration, they have immature cells that can divide 

• Cardiac muscle and skeletal muscle cannot regenerate so we get scar 

• Nervous tissue also does not regenerate (neurons are damaged in surgeries and it cannot be repaired) 

• Nutrition and blood supply also impacts the regeneration of tissue, you need to make large quantities of proteins for regeneration, tissues without good blood supply will have a harder time healing (people with artery diseases have this problem) 

Transitional stratified epithelium

cuboidal/columnar basal cells and dome-shaped surface cells; cells will start to stretch and look squamous based on the situation (EX: bladder)

Pseudostratified columnar epithelium

technically simple since it is one layer of cells, some nuclei are higher than others but all cells are connected to the basement membrane