CELLS AND TISSUES.

Tissue: A Definition

A group of connected, interdependent cells that cooperate to perform a (common) specific function.

• The building blocks of life are cells.

• Cells form tissues,

• tissues form organs,

• organs form organ-systems,

• and organ-systems form an entire living and breathing organism.

• Cells are structural units that make up plants, animals and single cell organisms.

• The cells of single cell organisms are called prokaryotic cells (prokaryotes).

• A prokaryotic cell does not have a membrane around its nuclear region (for example a bacterium). It has a cell wall, plasma membrane, nucleoid (region of DNA), and cytoplasm with ribosomes.

• Tissues are groups of cells that lie together to accomplish a common function. They are the basic building blocks of organs.

• Tissues are divided into four main groups (epithelial tissue, connective tissue, muscle tissue, and nervous tissue).

• These groups are further subdivided into many subgroups.

• As an example, the epithelial tissue is subdivided into covering and lining epithelia (outer layer of skin, inner surface of heart and blood vessels, inner surface of respiratory cavities, etc.) and glandular epithelia (most of the glands in the body).

Categories of Tissue1. Epithelial Tissue

2. Connective Tissue

3. Muscle Tissue

4. Nervous Tissue

Epithelial Tissues• General FeaturesCellular Layer +

Basement Membrane

Epithelial Tissues• General FeaturesCellular Layer + Basement Membrane

No Blood Supply

Touching Each Other

Rapid Rate of Cell Reproduction

Epithelial Tissues• Structural Classification

Cell Shapes

Columnar

Cuboidal

Squamous

Epithelial Tissues• Structural Classification

Cell Shapes

Cell Layers

Simple

Stratified

Epithelial Tissues• Major Types of epithelial tissue.

Simple Squamous Epithelium

Simple Squamous EpitheliumFunction: Exchange

Sample Locations: Alveoli, capillaries

Stratified Squamous Epithelium

Stratified Squamous Epithelium

Function: Protection

Sample Locations: Skin, Mouth, Repro tracts

Simple Cuboidal Epithelium

Function: Absorption or Secretion

Sample Locations: Kidney, Hormone Glands

Simple Columnar Epithelium

Function: Absorption (or Secretion)

Sample Location: Digestive Tract

Microvilli

STRATIFIED CUBOIDAL AND COLUMNAR EPITHELIUM:

--Are not common. A two-layered cuboidal epithelium is, for example seen in the ducts of the sweat glands. Stratified columnar epithelum are found in the excretory ducts of the mammary glands and the main excretory duct of large salivary glands.

Simple ColumnarNote goblet cells

Pseudostratified Columnar Epithelium

Pseudostratified Columnar Epithelium

Pseudostratified Columnar EpitheliumFunction: Absorption, Secretion, Movement

Sample Locations: Respiratory & Repro Ducts

Transitional EpitheliumFunction: Stretchability

Sample Location: Urinary Bladder

GLANDS: are cells or aggregation of cells whose primary function is secretion.

Exocrine glands:Release the secretory product via a system of ducts that opens upon one of the surfaces of the body which are in contact with the external world (skin, Gastrointestinal tract)

Endocrine gland :Release their secretory product (typically hormones)into the spaces between the secretory cells(extracellular space) from which it enters the blood stream.

Both endocrine and exocrine glands are developmentally derived from epithelia tissue, which form down-growth into the underlying connective tissue. The cells forming this down–growth then develop the characteristics of a mature gland.

Classification of exocrine glands: exocrine glands may be classified: according to cell number and/or the shape -branching pattern of their secretory portions and ducts.

• Both endocrine and exocrine glands are developmentally derived from epithelia tissue, which form a down-growth into the underlying connective tissue.

• The cells forming this down-growth then develop the special characteristics of the mature gland.

• Exocrine glands maintain the connection with the body surface whereas it is lost by endocrine glands.

Unicellular Glands

– consist of a single secretory cell. In mammals the only example of unicellular exocrine glands are goblet cells, which occur in the epithelium of many mucous membranes.

– Goblet cells secrete the glycoprotein mucin, which by the uptake of water is converted into a slimy substance, mucus.

•  

Multicellular glands

• The secretory portion may have a variety of shapes. Secretory cells may form

• tubes in tubular glands,

• acini in acinar glands or

• alveoli in alveolar glands.

Secretory Mechanisms• The secretory cells can

release their secretory products by one of three mechanisms:

• Merocrine secretion

corresponds to the process of exocytosis. Vesicles open onto the surface of the cell, and the secretory product is discharged from the cell without any further loss of cell substance.

When the cells release their secretory products, the membranes of secretory granules fuse to the cells membrane, and the granule contents spill out of the cell in a process called EXOCYTOSIS.

• Apocrine secretion designates a mechanism in which part of the apical cytoplasm of the cells is lost together with the secretory product.

• This mechanism is used by apocrine sweat glands, the mammary glands and the prostate.

• Holocrine secretion designates the breakdown and discharge of the entire secretory cell. It is only seen in the sebaceous glands of the skin.

Apocrine

• Product of secretion is shed with the whole cell,a process that involves destruction of the secretion-filled cells.

• Cytocrine secretion;

• Is the transfer of secretory materal from one cell to another.i.e transfer of melanosomes.

Simple tubular

Coiled tubular

Lateral and basal foldings:

Also transcytosis or transepithelial transport.

Lateral and basal infoldings are found in epithelial cells that are involved in active transport of ions and water

- the folding contain Na + / K+ ATPase to provide energy

e.g. epithelium of kidney tubules, intestine and ducts of salivary glands,