A&P: Structural Organization

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Structural Organization

Understanding the various primary tissue types in the human body is essential for understanding the structure and function of organs.

Four Basic Tissue Types

The four basic tissue types are nervous, muscular, epithelial, and connective, each with distinct functions and characteristics.

Connective Tissue

Provide structure and support, consisting of various cell types and extracellular fibers.

Loose, dense

The two connective tissue types.

Muscle Tissue Types

Classified into three subtypes: skeletal, cardiac, and smooth, each responsible for different types of movement.

Skeletal Muscle

Striated, multinucleated, and voluntarily controlled.

Cardiac Muscle

Striated, has intercalated discs, and no voluntary control.

Smooth Muscle

Nonstriated, unnucleated, and no voluntary control.

Nervous Tissue

Composed of neurons and glial cells, responsible for information processing and communication.

Epithelial Tissue Characteristics

• Cellularity

• Covers body surfaces

• Have an exposed surface

• Cellular organization

• Cellular connections

• Avascular

• Innervated

• Capable of Regeneration

Cellularity

Close-packed cells with limited extracellular material.

Have an exposed surface

The surface that is exposed is called apical (free) surface. I.e, skin exposed to air; stomach lining exposed to food.

Cellular Organization

Epithelia tissue attaches at the basal surface— the surface of cells that are anchored in place. Material is called the basement membrane.

• Made up of two layers: basal lamina and reticular lamina

  • Can be further divided into lamina lucia and lamina densa.

Deep to the basement membrane is the reticular lamina. Between the epithelial cells (where epithelial cells are connected to each other) is the lateral surface.

All epithelial tissue is supported by connective tissue.

Basal Surface

Epithelia tissue attaches at the __.

Basement Membrane

Made up of two layers: basal lamina and reticular lamina

• Can be further divided into lamina lucia and lamina densa.

• Made up of extracellular material secreted by epithelial cells— important role in supporting/guiding cell migration in tissue repair.

Reticular Lamina

Deep to the basement membrane.

Lamina Surface

Between epithelial cells.

Cellular Connections

Between epithelia tissue are different cellular connections— tight junctions, desmosomes, and gap junctions.

Avascular

Contains no blood vessels.

Innervated

Receives nervous innervation (includes nerves).

Regeneration

Epithelia tissue is replaced rapidly by cell division. Cell loss due to friction and contact with hostile environments.

Protection, absorption, filtration, and secretion

Functions of Epithelial Tissue.

Protection

Acts as a barrier and protects underlying structures from abrasion

Prevents substances from moving through it (toxic molecules and microorganisms).

Absorption

Contain carrier proteins which regulate the absorption of materials.

Filtration

Permits passage such as oxygen or carbon dioxide through.

Secretion

Epithelial tissue can secrete sweat or enzymes.

Simple Layers

Single cell layer, areas of absorption and filtration. Each cell extends from the basement membrane to the free surface. Facilitates absorption and filtration.

Stratified Layers

Two or more cell layers, areas of high abrasion.

• Regenerate from below via mitotic division

  • Basal cell divide

  • Move apically to replace older surface cells

• Durable

• Provide protection

Squamous

Flattened, scale-like.

• Squished

Cuboidal

Boxlike, as tall as wide.

• Cube

Columnar

Tall

• Column

Simple Epithelia

This kind of epithelia consists of a single layer of cells, facilitating absorption and filtration.

Stratified Epithelia

This kind of epithelia have two or more layers, providing protection in areas of high abrasion.

Transitional Epithelium

This type of epithelium accommodates fluctuations in fluid volume and changes shape to increase surface area.

Pseudostratified Epithelium

One layer, but looks like more due to different sizes of cells. Has cilia on apical side.

Simple Squamous Epithelia

Function:

• Diffusion, secretion, and filtration

• Needed when fluids or air must readily travel for diffusion

Location:

• Endothelium

  • Lining of the lymphatic system

  • Lining of all organs in cardiovascular system

• Mesothelium

  • Serous membrane linings of ventral body cavity

• Alveoli or capillary beds

Simple Cuboidal Epithelia

Function:

• Secretion and absorption

Location:

• Kidneys (nephron cells)

• Plexus of the brain

• Lining of lung bronchioles

• Surfaces of ovaries

Microvilli may be present to increase surface area.

Simple Columnar Epithelia

Function:

• Absorption (specifically pinocytosis), protection, and secretion

Location:

• Digestive tract, GI tract

Modifications:

• Dense microvilli on apical surface

• Goblet cells that secret protective lubricant

Pseudostratified Columnar Epithelia

• Single layer of cells that vary in height

• Only tallest reach apical surface

  • Always ciliated

    • Cilia propel trapped matter out

  • Goblet cells secrete mucus

    • Mucus traps particulate matter

  • Nuclei are located at different heights

Function:

• Absorption and secretion

Location:

• Respiratory system (nasal cavities)

• Reproduction

Stratified Squamous Epithelia

• Surface cells are squamous

• Deep layers consist most often of cuboidal

• Stratified squamous is categorized as non keratinized (moist) or keratinized. 

  • Non keratinized are living cells in the deepest and superficial layers— a layer of fluid covers superficial layers, making them moist

  • Keratinized are living cells only in the deepest levels, and the superficial layers are composed of dead cells

Function:

• Protects against abrasion; mechanical protection wherever there is friction

Location:

• Areas of abrasion

  • I;e, tongue or epidermis

• Forms external surface of the body

  • Extends into all body openings

  • Outer layer (epidermis) is keratinized

• Surface cells are flattened and atrophied

Stratified Cuboidal Epithelia

Rare

Function:

• Movement and secretion

Location:

• Glands (mammary, sweat)

• Ureter

Stratified Columnar Epithelia

Rare

Function:

• Protective and secretion

Location:

• Location of eyelids

• Urethra

• Pharynx

• Anus

Transitional Epithelia

Function:

• Accommodates fluctuations in the volume of fluid in an organ/tube

  • Need for more surface area

Location:

• Lining of urinary organs

  • Apical cells can change shape to accommodate stretching

Glandular Epithelium

Consists of specialized cells that secrete products, classified into exocrine and endocrine glands.

Classification of Glandular Epithelia

There are three specific distinctions of glands— duct structure, secretory structure, and mode of secretion.

Glands

They consist of one or more cells that make/secrete a product.

• Secretion refers to the aqueous product of glandular cells and the process of making that product.

  • Formation involves active processes.

    • Made in ER, packed in Golgi, and secreted by exocytosis

Exocrine Glands

Secrete products via ducts onto body surfaces or cavities.

Endocrine Glands

Ductless and secrete hormones directly into the bloodstream.

Duct Structure

The duct is the tube in contact with the epithelial tissue free surface, which transports secreted material. 

• The duct can be:

  • Simple— single, unbranched duct

  • Compound— branched duct

Secretory Portion

Found deeper within the epithelium, composed of cells responsible for producing the secreted material (secretory cells).

• The secretory portions of the glands can be:

  • Tubular— straight, narrow tube with the same width as the duct

  • Acinar— a saclike structure whose width is greater than the width of the duct

They both are supported by connective tissue, which supplies blood and nervous fibers.

Unicellular, simple, and compound

Three Categories of Exocrine Glands

Unicellular Glands

Goblet cells that secrete mucus.

Simple Glands

Multicellular glands with a single, unbranched duct.

Compound Glands

Multicellular glands that have several branched ducts. The secretory portions can be tubular or acinar, or a mixture of both.

Simple Tubular

Glands forming a straight tube with no branching.

Single Branched Tubular

Glands with several tubular secretory portions branching from single duct.

Simple Acinar

Glands with a single saclike secretory portion.

Simple Branched Acinar

Several acinar secretory proportions branching from single duct.

Compound Tubular

Glands with multiple ducts, each with a narrow tubular secretory portion.

Compound Acinar

Glands with multiple ducts, each with several saclike secretory portions.

Compound Tubuloalveolar

Glands with multiple ducts, each with several tubular and acinar secretory portions.

Modes of Secretion

Exocrine glands can secrete via merocrine, apocrine, or holocrine modes, each with distinct mechanisms.

Merocrine Glands

Secrete via exocytosis without altering secretory cell. It is the most common gland.

Apocrine Glands

Accumulate products just beneath the free surface.

• Top of the cell is removed, and products are released.

  • The cell is then repaired

Holocrine Glands

Accumulate products until the cell bursts— releases secretory products, and then dies.