Tissues are a group of similar cells of the same origin that carry out a specific function together. Humans have four different types of basic tissues. Connective tissues such as bone tissue are made up of fibrous cells and give shape and structure to organs. Muscle tissue is made up of cells that can contract together and allow animals to move. Epithelial tissues make up the outer layers of organs, such as the skin or the outer layer of the stomach. Nervous tissue is made of specialized cells that transmit information through electrochemical impulses, such as the tissue of nerves, the spinal cord, and the brain.

1. Tissue Level of Organization

2. Tissue
 A tissue is a group of cells of common embryonic origin that function together to carry
out specialized activities.
 Hard (bone), semisolid (fat), or liquid (blood)
 Histology is the science that deals with the study of tissues.

3. Types of Tissue
 Epithelial Tissue: Covers body surfaces and lines hollow organs, body cavities, ducts,
and forms glands
 Connective Tissue: Protects, supports, and binds organs. Stores energy as fat
provides immunity
 Muscular Tissue: Generates the physical force needed to make body structures move and
generate body heat
 Nervous Tissue: Detect changes in the body and responds by generating nerve impulses

4. Development of Tissue
 Tissues of the body develop from three primary germ layers
 Ectoderm, Endoderm, and Mesoderm
 All connective tissue and most muscle tissues derive from mesoderm
 Nervous tissue develops from ectoderm

5. Cell Junction
 Contact points between the plasma membranes of tissue cells
 5 most common types:
1. Tight junctions
2. Adherens junctions
3. Desmosomes
4. Hemidesmosomes
5. Gap junctions

6. Tight Junction
 Web-like strands of transmembrane proteins
 Fuse cells together Seal off passageways between adjacent cells
 Common in epithelial tissues of the stomach, intestines, and
urinary bladder
 Help to retard the passage of substances between cells and
leaking into the blood or surrounding tissues

7. Adherens Junction
 A dense layer of proteins called plaque
 Resist separation of cells during contractile activities
 Located inside of the plasma membrane attached to
both membrane proteins and microfilaments of the
cytoskeleton
 Transmembrane glycoproteins called cadherins insert
into the plaque and join cells
 In epithelial cells, adhesion belts encircle the cell.
 https://youtu.be/vpYsjMDwsRk?si=xcwMvSmIM-GP_rQt

8. Desmosome
 Contain plaque and cadherins that extend into the
intercellular space to attach adjacent cells together
 Desmosome plaque attaches to intermediate filaments
that contain the protein keratin
 Prevent epidermal cells from separating under
tension and cardiac muscle cells from pulling apart
during contraction

9. Hemidesmosome
 Resemble half of a desmosome
 Do not link adjacent cells but anchor cells to the
basement membrane
 Contains transmembrane glycoprotein integrin
 Integrins attach to intermediate filaments and the
protein laminin present in the
basement membrane

10. Gap Junction
 Connect neighboring cells via tiny fluid-filled
tunnels called connexons
 Contain membrane proteins called connexins
 Plasma membranes of gap junctions are
separated by a very narrow intercellular gap
(space)
 Communication of cells within a tissue
 lons, nutrients, waste, chemical, and electrical
signals travel through the connexons from
one cell to another

11. Epithelial Tissue
 Epithelial tissue consists of cells arranged in continuous sheets, in either single or
multiple layers
 Closely packed and held tightly together
 Covering and lining of the body
 3 major functions:
1. Selective barrier that regulates the movement of materials in and out of the body
2. Secretory surfaces that release products onto the free surface
3. Protective surfaces against the environment

12. General features of epithelial cells
 Surfaces of epithelial cells differ in structure and
have specialized functions
 Apical (free) surface: Faces the body surface,
body cavity, lumen, or duct
 Lateral surfaces: Faces adjacent cells
 Basal surface: Opposite of apical layer and
adheres to extracellular materials

13.  Basement membrane: Thin double extracellular layer that serves as the point of attachment
and support for overlying epithelial tissue
 Basal lamina:
o Closer to and secreted by the epithelial cells
o Contains laminin, collagen, glycoproteins, and proteoglycans
 Reticular lamina:
o Closer to the underlying connective tissue
o Contains collagen secreted by the connective tissue cells

14. Epithelial Tissue
 Own nerve supply
 Avascular or lacks its own blood supply
 Blood vessels in the connective tissue bring in nutrients and eliminate waste
 High rate of cell division for renewal and repair
 Numerous roles in the body (i.e. protection and filtration)
 Covering and lining epithelium: Outer covering of skin and some internal organs
 Glandular epithelium: Secreting portion of glands (thyroid, adrenal, and sweat glands)

15. Covering and Lining of epithelium
 Normally classified according to:
1. Arrangement of cells into layers
2. Shapes of cells

16. Classification according to shape
 Cells vary in shape depending on their function
 Squamous epithelium: Thin cells, arranged like floor tiles. Allows for rapid passage of
substances
 Cuboidal epithelium: As tall as they are wide, shaped like cubes or hexagons. May have
microvilli. Function in secretion or absorption
 Columnar epithelium: Much taller than they are wide, like columns. May have cilia or
microvilli. Specialized function for secretion and absorption
 Transitional epithelium: Cells change shape, and transition for flat to cuboidal. Organs such
as the urinary bladder stretch to a larger size and collapse to a smaller size

17. Arrangement of cells in layers
 Consist of one or more layers depending on the function
 Simple epithelium: Single layer of cells that function in diffusion, osmosis, filtration, secretion, or
absorption
 Pseudostratified epithelium: Appears to have multiple layers because cell nuclei are at different
levels. All cells do not reach the apical surface
 Stratified epithelium: Two or more layers of cells that protect underlying tissues in areas
of wear and tear

18. Simple epithelium
 Simple squamous epithelium
 Simple cuboidal epithelium
 Simple columnar epithelium (nonciliated and ciliated)
 Pseudostratified columnar epithelium (nonciliated and ciliated)

19. Simple squamous epithelium
 Single layer of cells that resembles a tiled floor on the surface
 Nucleus is centrally located and appears flattened oval or sphere
 Found at sites for filtration or diffusion

20. Simple cuboidal epithelium
 Cuboidal-shaped cells
 Cell nuclei are round and centrally located
 Found in the thyroid gland and kidneys
 Functions in secretion and absorption

21. Simple columnar epithelium
 Column-shaped cells
 Oval nuclei at near base
 Nonciliated and ciliated

22. Nonciliated simple columnar epithelium
 Contains columnar cells with microvilli at their apical surface and goblet cells
 Secreted mucus serves as a lubricant for the lining of the digestive, respiratory, reproductive, and
urinary tracts
 Also prevents the destruction of the stomach lining by acidic gastric juices

23. Ciliated simple columnar epithelium
 Columnar epithelial cells with cilia at the apical surface
 In the respiratory tract, goblet cells are interspersed among ciliated columnar epithelia
 Secreted mucus on the surface traps inhaled foreign particles. Beating cilia moves particles to the
throat for removal by coughing, swallowing, or sneezing
 Cilia also moves oocytes to the uterine tubes

24. Pseudostratified columnar epithelium
 Appears to have several layers because the nuclei of the cells are at various depths
 All the cells are attached to the basement membrane in a single layer, some cells do not
extend to the apical surface.
 Ciliated cells bear cilia and secrete mucus
 Nonciliated cells lack cilia and goblet cells

25. Stratified Epithelium
 Stratified epithelium has two or more layers of cells.
 The name of the specific kind of stratified epithelium depends on the shape of the cells in the apical
layer.
 Stratified squamous epithelium (Keratinized and Nonkeratinized)
 Stratified cuboidal epithelium
 Stratified columnar epithelium (nonciliated and ciliated)
 Transitional epithelium

26. Stratified squamous epithelium
 Several layers of cells that are flat in the
apical layer
 New cells are pushed up toward the apical
layer
 As cells move further from the blood supply
they dehydrate, harden, and die
 Keratinized form contains the fibrous protein
keratin: Found in superficial layers of the skin
 Nonkeratinized form does not contain keratin:
Found in mouth and esophagus

27.  Fairly rare type of epithelium
 Apical layers are cuboidal: Functions in protection
Stratified cuboidal epithelium

28.  Also very uncommon
 Columnar cells in the apical layer only
 Basal layers has shortened, irregular shaped cells
 Functions in protection and secretion
Stratified columnar epithelium

29. Transitional epithelium
 Found only in the urinary system
 Variable appearance. In a relaxed state, cells appear cuboidal
 Upon stretching, cells become flattened and appear squamous
 Ideal for hollow structures subjected to expansion

30. Glandular Epithelium: Endocrine gland
 Secretions, called hormones, diffuse directly into the bloodstream
 Function in maintaining homeostasis

31.  Secrete products into ducts that empty onto the surfaces of epithelium
 Skin surface or lumen of a hollow organ
 Secretions of the exocrine gland include mucus, sweat, oil, earwax, saliva, and digestive enzymes
 Examples of glands include sudoriferous (sweat) glands
Glandular Epithelium: Exocrine gland

32. Structural classification of exocrine gland
 Multicellular glands are categorized according to two criteria:
 Ducts are branched or unbranched
 Shape of the secretory portion of the gland
1. Simple gland duct does not branch
2. Compound gland duct branches
3. Tubular glands have tubular secretory parts
4. Acinar glands have rounded secretory parts
5. Tubuloacinar glands have both tubular and rounded secretory parts

34. Functional Classification of Exocrine Glands
 The functional classification of exocrine glands is based on how their secretions are
released.
 Merocrine glands: synthesized on ribosomes attached to rough ER; processed, sorted, and
packaged by the Golgi complex; and released from the cell in secretory vesicles via exocytosis.
 Apocrine glands: accumulate their secretory product at the apical surface of the secreting cell.
 Holocrine glands: accumulate a secretory product in their cytosol.

35. Connective Tissue
 Most abundant and widely distributed tissues in the body
 Numerous functions
 Binds tissues together
 Supports and strengthens tissue
 Protects and insulates internal organs
 Compartmentalize and transport
 Energy reserves and immune responses

36.  Three characteristics of Connective Tissue
1. Extra cellular matrix
2. Fibers
3. Cells of various types
Characteristics of Connective Tissue

37.  Extracellular matrix is the material located between the cells: Consists of protein fibers
and ground substance
 Connective tissue is highly vascular
 Supplied with nerves: Exception is cartilage and tendon. Both have little or no blood
supply and no nerves
Extra cellular matrix

39. Connective Tissue cells
 Fibroblasts
o Secrete fibers and components of ground
substance
 Adipocytes (fat cells)
o Store triglycerides (fat)
 Mast cells
o Produce histamine
 White blood cells
o immune response
o Neutrophil and Eosinophils
 Macrophages
o Engulf bacteria and cellular debris by
phagocytosis
 Plasma cells
o secrete antibodies

40.  Ground substance
o between cells and fibers
o Fluid, semifluid, gelatinous, or calcified
o Functions to support and bind cells, store water, and allow exchange between blood and
cells
o Complex combination of proteins and polysaccharides
 Fibers
1. Collagen fibers
2. Elastic fibers
3. Reticular fibers

41. Classification of connective tissue
 Embryonic connective tissue
 Mesenchyme and mucous connective tissue
 Mature connective tissue
 Loose connective tissue
 Areolar, adipose, and reticular
 Dense connective tissue
 Dense regular, dense irregular, and elastic
 Cartilage
 Hyaline, fibrocartilage, and elastic cartilage
 Bone tissue
 Liquid connective tissue
 Blood and lymph