Human Tissues

What is histology?

The study of tissues and how they are arranged into organs.

What is a tissue?

A group of similar cells and cell products that work together to perform a specific role.

What are the four primary tissues?

Epithelial, connective, nervous, and muscular tissue.

How do the four primary tissues differ?

In the types and functions of their cells, the characteristics of the matrix around the cells, and the relative amount of space cells and matrix occupy.

What is the matrix composed of?

Water, fibrous proteins, glycoproteins, and proteoglycans.

What is epithelial tissue?

A flat sheet of closely adhering cells, one or more cells thick, with the upper surface usually exposed to the environment or to an internal space.

What are the functions of epithelium?

Covering the body surface, lining body cavities, forming the external and internal linings of many organs, and constituting most gland tissue.

What is the role of loose connective tissue in epithelium?

Providing a blood supply for nutrients and waste removal.

What is the basement membrane?

A structure that anchors an epithelium to the underlying connective tissue.

How are epithelia classified?

Into two broad categories: simple and stratified.

What is a simple epithelium?

An epithelium where every cell touches the basement membrane.

What is a stratified epithelium?

An epithelium where some cells rest on top of other cells and do not contact the basement membrane.

What are the three types of simple epithelia?

Simple squamous (thin, scaly), simple cuboidal (squarish, round), and simple columnar (tall, narrow).

What is pseudostratified columnar epithelium?

An epithelium where shorter cells do not reach the free surface, but all cells reach the basement membrane.

What do simple columnar and pseudostratified columnar epithelia often produce?

Mucus, which is secreted by goblet cells.

What is stratified epithelium?

Epithelium with 2-20 layers, deepest layer on basement membrane.

What are the three types of stratified epithelia based on surface cell shapes?

Stratified squamous, stratified cuboidal, stratified columnar (rare).

What was the fourth type of stratified epithelium thought to represent?

Transitional stage between stratified squamous and stratified columnar.

Which type of stratified epithelium is the most widespread in the body?

Stratified squamous epithelium.

Describe the deepest layers of cells in stratified squamous epithelium.

Cuboidal to columnar, undergo continual mitosis.

What happens to daughter cells in stratified squamous epithelium?

They push towards the surface and become flatter.

What is the process called when cells migrate to the surface and flake off?

Exfoliation or desquamation.

What is keratinized epithelium?

Epithelium covered with compact, dead squamous cells packed with keratin and coated with a water repellent.

What is nonkeratinized epithelium?

Epithelium without a surface layer of dead cells.

Where is nonkeratinized epithelium found?

Tongue, esophagus, vagina, and some internal membranes.

Where is transitional epithelium found?

Limited to the urinary tract.

What are the surface cells of transitional epithelium called?

Umbrella cells.

What is the outer layer of transitional epithelium composed of?

Phospholipids with dense patches of lipid rafts.

What proteins are embedded in the lipid rafts of transitional epithelium?

Uroplakins, impermeable to urine.

What is connective tissue mostly composed of?

Fibers and ground substance.

What is the function of connective tissue?

Connect, support, bind, and protect organs.

Give an example of connective tissue binding organs.

Tendons bind muscle to bone; ligaments bind bones together.

What does bone provide in terms of support?

Support for the body.

What does cartilage support?

Other structures.

What is the function of connective tissue in terms of physical protection?

Protect delicate organs.

What is the function of connective tissue in terms of immune protection?

Attacking foreign invaders.

What is the function of bones in the body?

Bones provide a lever system for movement.

How do cartilages on bone surfaces contribute to joint movements?

Cartilages on bone surfaces ease joint movements.

What is the function of fat in the body?

Fat forms the body's major energy reserve.

What is the function of brown fat?

Brown fat generates heat in infants and children.

What is the function of blood in the body?

Blood transports gases, nutrients, wastes, and other materials.

What is fibrous connective tissue?

Fibrous connective tissue consists of cellular and fibrous components in a ground substance.

What are the cellular components of fibrous connective tissue?

Cellular components include fibroblasts, macrophages, leukocytes, plasma cells, mast cells, and adipocytes.

What is the function of fibroblasts?

Fibroblasts produce the fibers and ground substance of the tissue.

What is the function of macrophages?

Macrophages engulf and destroy bacteria, foreign particles, and dead or dying cells; they also respond to antigens.

What are leukocytes?

Leukocytes, or white blood cells (WBCs), enter connective tissues from the bloodstream and search for bacteria.

What is the function of plasma cells?

Plasma cells synthesize antibodies.

What is the function of mast cells?

Mast cells near blood vessels secrete heparin and histamine.

What is the main cell type in adipose tissue?

Adipocytes, or fat cells, are the main cell type in adipose tissue.

What are the fibrous components of fibrous connective tissue?

Fibrous components include collagenous fibers, reticular fibers, and elastic fibers.

What are collagenous fibers made of?

Collagenous fibers are made of collagen and are tough, flexible, and resist stretching.

What are reticular fibers?

Reticular fibers are thin collagen fibers coated with glycoprotein, forming a sponge-like framework in some organs.

What are elastic fibers made of?

Elastic fibers are made of elastin and can recoil from stretching like a rubber band.

What is the ground substance in fibrous connective tissue?

Ground substance occupies the space between cells and fibers and usually has a gelatinous consistency.

What is Glycosaminoglycan (GAG)?

A long polysaccharide made of amino sugars and uronic acid that can absorb and hold water.

Give examples of Glycosaminoglycans (GAGs).

Chondroitin sulfate, heparin, and hyaluronic acid.

What are Proteoglycans?

Gigantic molecules with a central core of protein and bristle-like outgrowths of GAGs.

What is the function of Proteoglycans?

They form thick colloids and create strong structural bonds between cells and extracellular macromolecules.

What are adhesive glycoproteins?

Protein-carbohydrate complexes that bind plasma membrane proteins to collagen and proteoglycans outside the cell, holding tissues together.

What are the two broad categories of fibrous connective tissue?

Loose connective tissue and dense connective tissue.

What is the main characteristic of loose connective tissue?

Much of the space is occupied by ground substance.

Give examples of loose connective tissue.

Areolar and reticular tissues.

What is the main characteristic of dense connective tissue?

Fiber occupies more space than cells and ground substance.

Give examples of dense connective tissue.

Dense regular connective tissue and dense irregular connective tissue.

Describe areolar tissue.

Loosely organized fibers, abundant blood vessels, and a lot of seemingly empty space.

What types of fibers are found in areolar tissue?

Randomly organized collagenous fibers, with elastic and reticular fibers also present.

Where is areolar tissue found?

Underlying nearly all epithelia, including the skin and membranes of the body, and surrounds blood vessels and nerves.

What is reticular tissue?

A mesh of reticular fibers and fibroblasts that forms the structural framework (stroma) of many organs.

What are the two properties of dense regular connective tissue?

Closely packed collagen fibers with little open space, and a parallel arrangement of fibers.

What is the function of the parallel arrangement of fibers in dense regular connective tissue?

An adaptation to directional pull, such as on tendons and ligaments.

What types of cells are found in dense regular connective tissue?

Fibroblasts.

What is elastic tissue?

A dense regular connective tissue found in the vocal cords and some ligaments of the vertebral column.

What is the function of elastic tissue in arteries?

Allows expansion and recoil of arteries.

What is the structure of dense irregular connective tissue?

Closely packed collagen fibers running in random directions.

What is the advantage of random orientation in dense irregular connective tissue?

Resistance to unpredictable stresses.

Where is dense irregular connective tissue found in the body?

Mostly in the dermis, protective capsules around organs, sheath around bones, nerves, and cartilage.

What is adipose tissue?

Tissue dominated by adipocytes (fat cells).

What is the turnover of stored triglycerides in adipose tissue?

Constant turnover between synthesis and hydrolysis, energy storage and use.

What is the most abundant type of fat tissue in adults?

White fat.

What is the size range of adipocytes?

70 to 120 µm, but can be larger in obese individuals.

What are the functions of adipose tissue?

Thermal insulation, anchoring, and cushioning.

Do women or men have more fat relative to body weight?

Women.

What is brown fat?

Heat-generating tissue found in fetuses, infants, and children.

What gives brown fat its color?

Abundance of blood vessels and certain enzymes in its mitochondria.

What happens when brown fat is oxidized?

All of the energy is released as heat, not ATP synthesis.

Why do hibernating animals accumulate brown fat?

In preparation for winter.

What is the structure of cartilage?

Flexible rubbery matrix with chondroblasts and chondrocytes.

What are chondroblasts?

Cells that secrete matrix and become trapped in lacunae as chondrocytes.

What is the characteristic of cartilage that affects its healing process?

Cartilage is usually free of blood vessels, so diffusion of nutrients and wastes is slow and chondrocytes have low rates of metabolism and cell division; cartilage therefore heals slowly.

How is cartilage classified?

Cartilage is classified based on the predominant type of fiber.

What are the characteristics of hyaline cartilage?

Hyaline cartilage is clear and glassy in microscopic appearance; the collagen fibers are very fine and invisible.

What are the characteristics of elastic cartilage?

Elastic cartilage has conspicuous elastic fibers.

What are the characteristics of fibrocartilage?

Fibrocartilage has coarse, readily visible bundles of cartilage.

What surrounds elastic cartilage and most hyaline cartilage?

Elastic cartilage and most hyaline cartilage are surrounded by a sheath of dense irregular connective tissue called the perichondrium.

What does the term 'bone' refer to?

The term 'bone' refers either to an organ such as the femur and mandible, composed of multiple tissue types, or to bone tissue (osseous tissue), which makes up most of the mass of the bones.

Where is spongy bone found?

Spongy bone fills the heads of long bones and forms the middle layer of flat bones such as the sternum.

What is the characteristic of compact (dense) bone?

Compact (dense) bone is a calcified tissue with no spaces visible to the naked eye; spongy bone, when present, is always covered by compact bone.

How is compact bone arranged?

Compact bone is arranged in cylinders of tissue that surround central (haversian or osteonic) canals that run longitudinally through long bones; blood vessels and nerves travel through the canals.

How is bone matrix deposited?

Bone matrix is deposited in concentric lamellae around each central canal.

What is a central canal and its surrounding lamellae called?

A central canal and its surrounding lamellae are called an osteon.