Human Tissue Types

Overview of Tissues

• Tissues are groups of cells performing a common function
• There are four human tissue types

Epithelial Tissue

• Covers body surfaces, lines cavities and organs, and forms glands
• An example is the epidermis
• It has a free surface and a basement membrane
• The basement membrane is a noncellular layer binding epithelial cells to underlying connective tissue
• The basement membrane helps resist stretching

Connective Tissue

• Binds, supports, transports, and stores energy for other tissues.
• Connective tissues serve as a fat storage site
• They play a key role in immunity
• Connective tissues protect and support the body and its organs
• Examples include bone and blood
• Connective tissues have cells embedded in an extracellular matrix
• The extracellular matrix contains cells, protein fibers like collagen, elastic, and reticular fibers, and ground substances
• Reticular fibers are produced by fibroblasts
• Ground substances can be solid (like bone), gelatinous (like cartilage), or fluid (as in blood)
• Cells secrete the extracellular matrix
• Connective tissue is mostly made up of its matrix, as opposed to other cells
• The distribution of cells in connective tissue extracellular matrix is like pieces of fruit in gelatin dessert

Proper Connective Tissue Types

• Includes Loose Connective Tissue: contains many cells in which the matrix fibers are fewer and more loosely woven than in dense connective tissue
• Areolar connective tissue is widely distributed under the skin, around organs and muscles
• Areolar connective tissue supports and cushions
• Areolar connective tissue functions as a universal packing material
• Areolar connective tissue is found between muscles and under the skin
• Adipose connective tissue cells are specialized for fat storage (energy storage), insulation, and cushioning of organs
• Adipose connective tissue is found under the skin and around organs such as the heart and kidneys
• Dense connective tissue forms strong bands with large amounts of tightly woven fibers
• Dense connective tissue is found in ligaments and tendons

Specialized Connective Tissue

• Includes cartilage, bone, and blood

Cartilage

• Cartilage is a tough but flexible connective tissue
• Cartilage lines bones and absorbs shock
• Cartilage is important because bone against bone contact is problematic
• Tough cartilage is found in discs between vertebrae
• Flexible cartilage is found in the ears and nose

Bone

• Bone is a strong connective tissue with specialized cells in a hard matrix
• Bones are alive
• Bones provide structure, protection, and movement
• Red bone marrow stores red blood cells
• Yellow bone marrow stores lipids

Blood

• Blood consists of a liquid matrix (plasma)
• Blood is composed of cells and cell fragments (platelets)
• Blood transports oxygen, CO2, and nutrients dissolved in plasma
• Red blood cells transport oxygen to cells and carry some CO2 away
• Soluble protein fibers in blood are visible only when blood clots

Muscle Tissue

• Muscle tissue cells contract for body movement and movement of substances throughout the body
• There are three types of muscle tissue: skeletal, smooth, and cardiac
• Skeletal muscle tissue is usually attached to bones and is for voluntary contraction/movement
• Skeletal muscle tissue typically consists of cylindrical, striated cells with many nuclei
• Cardiac muscle tissue is only found in the walls of the heart
• Cardiac muscle tissue contractions pump blood and are involuntary
• Cardiac muscle tissue typically has one nucleus and branching cylinders with striations
• Smooth muscle tissue is found in the walls of blood vessels, intestines, and airways
• Smooth muscle tissue contractions are involuntary, reducing blood or air flow
• Smooth muscle tissue is typically non-striated with a single nucleus

Nervous Tissue

• Nervous tissue conducts nerve impulses from one part of the body to another
• They are located in the brain, spinal cord, and nerves
• Two types of nervous tissue include: neurons and neuroglia
• Neurons generate and conduct nerve impulses
• Neuroglia provide nutrients, insulate, and protect neurons; they are far more numerous than neurons

Organs and Organ Systems

• Organs: structures of two or more tissue types working to achieve a specific function
• Organ System: a group of organs with a common function
• The trachea, bronchi, and lungs constitute the respiratory system

Skin (Integumentary System)

• The integument is the outer covering, also known as skin
• The skin is the largest organ
• The skin and its derivatives function together to provide many services

Skin Functions

• Protection: a physical barrier against foreign bacteria, UV rays, and physical/chemical harm
• Limits water loss: Keratin is water-resistant and prevents excessive water loss from underlying tissues
• Temperature regulation is assisted by keratin
• Excretion: Sweat glands remove excess water, salts, etc
• Sensory: Detects external stimuli such as temperature, touch, pressure, and pain
• Sensory: Keeps us informed about our external environment

Skin Layers

• Epidermis: thin, outer layer that forms a protective barrier against environmental hazards
• The epidermis has layers of epithelial cells
• The epidermis is constantly shed and replaced
• The epidermis is keratinized and the outermost layers are dead
• Keratin is what gives the epidermis its protective qualities
• Droplets of tattoo ink are injected into the dermis and are permanent because the dermis is not shed
• Dermis: the inner layer of skin just below the epidermis; composed of dense connective tissue.
• Blood vessels, hair follicles, glands, and nerves are in the dermis
• Tattoos hurt because the dermis is laced with nerves and sensory receptors
• Hypodermis: a layer of loose connective tissue below the epidermis and dermis
• The hypodermis is usually not considered part of the skin

Skin Color

• There are two factors that produce skin color: the quality and distribution of pigment in blood flow

Pigment

• Melanin: pigment produced by melanocytes
• Melanocytes: spider-like cells located at the base of the epidermis
• Melanin can come in yellowish-to-red form OR black-to-brown form
• Skin color is determined through genetic makeup (yellow-to-red OR black-to-brown)

Blood Flow

• Blood flow affects the amount of oxygen level
• Intense embarrassment can increase blood flow, causing a rosy red color
• High O2 = ruby red; Low O2 = deep red-bluish

Accessory Structures

• Accessory structures include: hair, nails, and glands

Hair

• Hair is modified skin cells
• Scalp hair protects from UV radiation
• Nostril hair prevents bugs from entering, etc

Nails

• Nails are modified skin cells hardened by protein keratin
• Nails act as "antennas"

Glands

• The three types of glands are: oil, sweat, and wax
• Oil glands secrete sebum, which lubricates hair and skin and inhibits growth of certain bacteria
• Sweat glands produce sweat, eliminating waste
• Sweat glands regulate body temperature
• Wax glands are modified sweat glands in the ear canal
• Wax glands produce wax that protects the ear by trapping small particles

Homeostasis

• Homeostasis: maintenance of favorable internal conditions despite external conditions
• This is not a static state; it is dynamic

Negative Feedback Loop/Mechanisms

• The homeostatic mechanism in which the outcome of a process feeds back on the system, shutting it down
• If the body is cold, it shivers to produce heat
• There are three components of homeostatic mechanisms: receptor, control center, and effector
• Receptor: detects changes and sends information to the control center
• Receptors detect cold stimulus and send signals to the control center
• Control center: integrates information from the receptors and directs appropriate responses
• The control center is located in the brain/hypothalamus
• The control center, brain/hypothalamus, receives input from the receptors and initiates mechanisms to increase heat production and conserve heat
• Effector: carries out the selected response, usually a muscle or gland
• The effector triggers a series of responses to raise body temperature, such as shivering
• The body’s set point temperature is 98.6 degrees Fahrenheit or 37 degrees Celsius and is determined by the hypothalamus