Lecture 01: The tissue level of organization
The basics:
Anatomy ​– Study of Structures
Physiology ​– Study of function
Tissue ​– group of cells with a common embryonic origin that carry out
specialized activities
◦ They can be hard (bone), soft (cartilage, connective tissue), or liquid (blood)
◦ Histology ​– the study of tissues
◦ Pathologist ​– physician who studies tissues to help diagnose disease
Cell Junctions
Cell junction​ - the border between two cells
◦ there are 5 common structures found where cells meet
▪ 1. Hemidesmosomes ​use transmembrane proteins to bind cells to a
basement membrane
Found in​ epithelial membranes
▪ 2. Occluding Junctions ​– weblike strands of transmembrane proteins that
bind two cells together tightly
Found in​ stomach, intestinal, and bladder epithelium
▪ 3. Adhesion belt ​– Plaques on the inside of a cell’s membrane provide a
place for transmembrane proteins to bind
The plaques contain microfilaments in long ribbons which look like a
belt encircling the cell
Found in​ places where cells contract, like in epithelium of sm. intestines
▪ 4. Gap Junctions ​– tunnels that allow cells to communicate with each other
directly
Found in​ nerves and muscles where impulses need to spread rapidly
from cell to cell
▪ 5. Desmosomes ​– similar to hemidesmosomes but bind two cells together
instead of binding cells to a basement membrane
Found in​ places where cells contract hard! Like heart muscles cells and
epidermis
4 types of tissues:
Epithelial tissue
Connective tissue
Muscle tissue
Nervous tissue
Epithelial tissue ​:
A flat covering that covers the body in sheets, lines organs and cavities
Cells closely paced, held tightly together
Always have one free (apical) surface and one bound (basal) surface
4 Major functions
◦ 1​. provide physical protection
◦ 2. ​control permeability (e.g. act like a filter)
◦ 3. ​provide sensation (e.g. the feeling of touch, heat, cold etc)
◦ 4. ​provide specialized secretions (e.g. sweat, sebum, etc)
surfaces
◦ 1. Apical surface​: the top
▪ the top, it is often facing a lumen (internal space) or facing the exterior of
the body
▪ often covered in cilia or microvilli (tiny hairlike projections)
◦ 2. basolateral surfaces​: the bottom and sides
▪ the basal surface is where the cell attaches to the ​basal lamina
basal lamina​:​ ​a complex structure made by the epithelial cells and the
connective tissue below them
the basal lamina has two layers
◦ 1. clear layer
◦ 2. dense layer
Epithelial tissue has nerves, but is avascular (it has no blood
vessels)
◦ Blood is supplied by the underlying connective tissue
Epithelial tissue divides rapidly so that it is always being
renewed and repaired
Epithelium can be classified in two major ways –
◦ 1. ​By the ​number​ of layers
▪ Simple – a single layer
Function in diffusion, osmosis, filtration, or
absorption
▪ Pseudostratified – a single layer that looks like it has
several layers (but it doesn’t)
Nuclei are at different levels so it has a stratified appearance
▪ Stratified – many layers to provide
protection to underlying tissues
◦ 2. ​By the ​shape​ of the cells it is
composed of
▪ Squamous – Flat cells
Allows for rapid passage or
substances
▪ Cuboidal – cube shaped cells
May have microvilli
Function in secretion or
absorption
▪ Columnar – column shaped
May have cilia or microvilli
Function in secretion or absorption
▪ Transitional – cells can change shape!
Exist in the bladder where things stretch considerably
Examples of Squamous epithelium
◦ Simple squamous epithelium
▪ Single layer, arranged like floor tiles
▪ Nucleus is centrally located
▪ Found​ at sites of filtration
(kidneys), and diffusion (air sacs in
lungs)
◦ Stratified squamous epithelium
▪ Several layers, that are flat in the
apical layer
▪ New cells are made and pushed up
where they dehydrate, darken, and
die
▪ Can be keratinized (skin) or
non-keritinized (mouth &
esophagus)
Examples of Cuboidal epithelium...
◦ Simple cuboidal epithelium
▪ Single layer, cube shaped cells
▪ Nucleus centrally located
▪ Found in​ thyroid gland (secretion)
and kidneys (absorption)
◦ Stratified cuboidal epithelium
▪ Apical layers are cuboidal
▪ Rare, ​found in​ sweat glands and esophageal glands
Example of Transitional epithelium...
◦ Transitional epithelium
▪ when relaxed cells are cuboidal,
when stretched cells are squamous
▪ Found in​ bladder, and in urine
collecting chambers in kidneys
Examples of columnar epithelium...
◦ Simple columnar epithelium
▪ Single layer, column shaped
non-ciliated – no cilia, have
microvilli instead
◦ secrete mucus via goblet cells
◦ line digestive, reproductive,
& urinary tracts
ciliated – have cilia
◦ also secrete mucus via goblet cells
◦ line respiratory tract (ciliary escalator)
◦ Pseudostratified columnar epithelium
▪ Appears to have several layers, but all cells attached to basement membrane
▪ Ciliated – secrete mucus (with goblet cells)
▪ Non-ciliated – lack goblet cells
◦ Stratified columnar epithelium
▪ Apical layers are columnar
▪ Also rare, found in esophagus, urethra, conjunctiva of eye
Epithelium can be divided into two major types:
◦ 1. ​Covering and lining epithelium
▪ e.g. like the skin or the lining of internal organs
▪ can be classified in two major ways, layer arrangements, and cell
◦ 2. ​Glandular epithelium
▪ The parts of glands that secrete things (like hormones, sweat, oil, etc.)
▪ Two types of glands
Endocrine ​– secrete hormones ​directly into the bloodstream
◦ Example: thyroid gland
Exocrine ​– secrete products (mucus, sweat, oil, earwax, saliva, etc) into
ducts
◦ Example: sweat glands
Connective tissue
Acts like “insulating glue”
Protects, supports, binds tissues together, and stores energy
Supplied with nerves just like epithelium, but unlike epithelium, C.T. is ​highly
vascular​ (exception: cartilage and tendons)
Most abundant & widely distributed tissues in the body
Consists of cells and extracellular matrix (the stuff located between cells)
◦ Most of the time, there is a ​lot ​of matrix (exception: adipose tissue)
◦ Often, the matrix is the basis for classifying the tissue
◦ 3 major types of fibers in the extracellular matrix
▪ collagen fibers ​– very strong & thick, but not very stretchy
found in​ most types of C.T. (especially in bone, cartilage, tendons, and
ligaments)
▪ reticular fibers ​– thin, strong, and form branching networks
found in​ framework of many soft organs (like spleen & lymph nodes)
▪ elastic fibers ​– thin, unbranching, & very stretchy
found in​ skin, blood vessel walls, lung tissue
◦ the extracellular matrix that isn’t fibers is called ​ground substance
▪ can be fluid, semi-fluid, gelatinous, or calcified
Classifying connective tissues
◦ 5 Major kinds
▪ 1. Loose connective tissue – comes in three varieties
Areolar C.T.
◦ Most widely distributed tissue
in the body
◦ Contains all three fiber types
◦ Located in subcutaneous layer
of skin, many organs
Adipose Tissue
◦ Contains fat cells (adipocytes)
◦ Insulates body and stores
energy
Reticular Connective Tissue
◦ Fine interlaced reticular fibers
and cells
◦ Forms stroma of liver, spleen,
& lymph nodes
▪ 2. Dense connective tissue – heals
poorly due to minimal blood supply
Very fibrous, collagen fibers are packed closely together
Fewer cells than loose connective tissue
Three types:
◦ Dense regular connective tissue
– bundles of fibers are arranged
in parallel
▪ Appears wavy, make up
tendons & most ligaments
◦ Dense irregular connective
tissue – ​fibers are irregularly
arranged
▪ Found in​ dermis of skin and
heart, where pulling forces
exerted in many different
directions
◦ Elastic connective tissue ​–
contain branching elastic fibers
▪ Found in​ lung tissue and arteries
▪ 3. Fluid connective tissue
Blood ​– the stuff in your veins & arteries
◦ Composed of red blood cells, white blood cells, platelets, and plasma
(the liquid portion & extracellular matrix
lymph (more on this stuff in A&P2)
▪ 4. ​Cartilage ​– dense network of collagen fibers and elastic fibers in ground
substance
no blood vessels or nerves – means that cartilage repairs slowly
Chondrocytes ​– cells found in spaces called lacunae
Cartilage is surrounded by a membrane called ​pericondrium ​which
contains all of the blood vessels needed to nourish the cartilage
Three types
◦ Hyaline cartilage ​– most abundant cartilage in the body
▪ Provides flexibility
& support, reduces
friction
▪ Located at the ends
of long bones, nose,
larynx, trachea
◦ Elastic Cartilage ​–
contains elastic fibers
▪ Most flexible
cartilage type
▪ Found in​ pinna of
ear, epiglottis
◦ Fibrocartilage ​–
strongest type of
cartilage
▪ Found in
intervertebral discs
in spine, pubic
symphysis
▪ 5. Bone tissue – the tough stuff that muscles pull on
Provide support for your entire body (more on this stuff later this
semester)
Muscular tissue
Consists of elongated cells called
muscle fibers
Generates physical forces, produce
heat
◦ Three types
▪ 1. Skeletal
Attached to bones, have
striations, used for voluntary
movement
▪ 2. Cardiac
Make up the heart, have
striations & intercalated discs
▪ 3. Smooth
Spindle shaped, non-striated,
not under voluntary control
Found in​ iris of eyes, small
intestine, uterus, blood vessels
Nervous tissue
Send information very fast because of electrical excitability
◦ Information travels in waves called ​action potentials​ (APs)
▪ APs propagate along nerves to other nerves or muscles
◦ Two types
▪ 1. ​Neurons (send the signals)
▪ 2. ​Neuroglia (tend to neurons, neuron nannies)
◦ Basically the raddest stuff ever
Tissue Repair:
Tissue heals faster in young adults
Surgery of a fetus normally leaves no scars
◦ Young tissues have a better nutritional state, blood supply, and higher
metabolic rate
▪ Blood flow can be increased by exercise
◦ Breakdown & loosening of collagen and elastic fibers contribute to aging
process