Dr.Waqas Nawaz PMAS arid agriculture university rawalpindi

1. Chapter 4
Tissues and Histology
• Tissues - collections of similar cells and the substances
surrounding them
• Tissue classification based on structure of cells, composition
of noncellular extracellular matrix, and cell function
• Major types of adult tissues
– Epithelial
– Connective
– Muscle
– Nervous
• Histology: Microscopic Study of Tissues
– Biopsy: removal of tissues for diagnostic purposes
– Autopsy: examination of organs of a dead body to
determine cause of death

2. Embryonic Tissue
• 3 major germ layers that form the embryonic disc (source of stem cells)
– Endoderm
• Inner layer
• Forms lining of digestive tract and derivatives
– Mesoderm
• Middle layer
• Forms tissues as such muscle, bone, blood vessels
– Ectoderm
• Outer layer
• Forms skin and neuroectoderm

3. I. Epithelial Tissue
• Cellularity - Consists almost
entirely of cells
• Covers body surfaces, lines
hollow organs, and forms glands
– Outside surface of the body
– Lining of digestive, respiratory
and urogenital systems
– Heart and blood vessels
– Linings of many body cavities
• Polarity - Has apical, basal, and
lateral surfaces
• Rests on a basement membrane
• Specialized cell contacts bind
adjacent cells together
• Avascular - no blood vessels
• Regenerative -Replaces lost cells
by cell division

4. Functions of Epithelia
• Protecting underlying structures; e.g.,
epithelium lining the mouth
• Acting as barriers; e.g., skin
• Permitting the passage of substances; e.g.,
cells lining air sacs in lungs and nephrons in
kidney
• Secreting substances; e.g., pancreatic cells
• Absorbing substances; e.g., lining of stomach
and small intestine

5. Special Characteristics of
Epithelia
Figure 4.1

6. Classification of Epithelium
• Number of layers of cells
– Simple- one layer of cells. Each extends from
basement membrane to the free surface
– Stratified- more than one layer.
– Pseudostratified- tissue appears to be stratified,
but all cells contact basement membrane so it is in
fact simple
• Shape of cells
– Squamous- flat, scale-like
– Cuboidal- about equal in height and width
– Columnar- taller than wide

7. Classifications of Epithelia

8. Simple Squamous Epithelium
Figure 4.3a

9. Simple Cuboidal Epithelium
Figure 4.3b

10. Simple Columnar Epithelium
Figure 4.3c

11. Pseudostratified Ciliated
Columnar Epithelium
Figure 4.3d

12. Stratified Epithelia
• Contain two or more layers of cells
• Regenerate from below
• Major role is protection
• Are named according to the shape of
cells at apical layer

13. Stratified Squamous
Epithelium
• Description
– Many layers of cells – squamous in shape
– Deeper layers of cells appear cuboidal or
columnar
– Thickest epithelial tissue – adapted for
protection

14. Stratified Squamous
Epithelium
• Specific types
– Keratinized – contain the protective protein
keratin
• Surface cells are dead and full of keratin
– Non-keratinized – forms moist lining of
body openings

15. Stratified Squamous
Epithelium
• Function – Protects underlying tissues
in areas subject to abrasion
• Location
– Keratinized – forms epidermis
– Non-keratinized – forms lining of
esophagus, mouth, and vagina

16. Stratified Squamous
Epithelium
Figure 4.3e

17. Transitional Epithelium
Figure 4.3h

18. Epithelium: Glandular
• A gland is one or more cells that makes and secretes
an aqueous fluid
• Two types of glands formed by infolding of epithelium:
– Endocrine: no contact with exterior of body; ductless; produce
hormones (pituitary, thyroid, adrenals, pancreas)
– Exocrine: open to exterior of body via ducts (sweat, oil)
• Exocrine glands classified either by structure or by the
method of secretion
• Classified by structure
– Unicellular: goblet cells
– Multicellular: sweat, oil, pituitary, adrenal

19. Multicellular Exocrine Glands
• Classified on the basis of types of
ducts or mode of secretion
• Types of ducts
– Simple: ducts with few
branches
– Compound: ducts with many
branches
• If ducts end in tubules or
sac-like structures: acini.
Pancreas
• If ducts end in simple
sacs: alveoli. Lungs

20. Lateral Surface Features
• Tight junctions
• Desmosomes
• Gap junctions

21. Membrane Junctions: Tight Junction
• Integral proteins of adjacent
cells fuse together
• Completely encircle the cell
and form an adhesion belt.
• Form an impermeable
junction.
• Common near apical region

22. Lateral Surface Features –
Cell Junctions
• Desmosomes – two disc-like plaques
connected across intercellular space
– Plaques of adjoining cells are joined by
proteins called cadherins
– Proteins interdigitate into extracellular
space
– Intermediate filaments insert into plaques
from cytoplasmic side

23. Membrane Junctions: Desmosome
Linker proteins extend
from plaque like teeth of
a zipper.
Intermediate filaments
extend across width of
cell.
• Common in superficial layers of skin; skin
peels after a sunburn
• Reduces chance of tearing, twisting, stretching
Figure 3.5b

24. Membrane Junctions: Gap Junction
• Connexon proteins are trans-
membrane proteins.
• Present in electrically excitable
tissues (heart, smooth muscle)

25. Basal Feature: The Basal
Lamina
• Noncellular supporting sheet between the
epithelium and the connective tissue deep to it
• Consists of proteins secreted by the epithelial
cells
• Functions:
• Acts as a selective filter, determining which
molecules from capillaries enter the epithelium
• Acts as scaffolding along which regenerating
epithelial cells can migrate
• Basal lamina and reticular layers of the
underlying connective tissue deep to it form the
basement membrane

26. Epithelial Surface Features
• Apical surface features
– Microvilli – finger-like extensions of plasma
membrane
• Abundant in epithelia of small intestine and kidney
• Maximize surface area across which small
molecules enter or leave
– Cilia – whip-like, highly motile extensions of
apical surface membranes
• Movement of cilia – in coordinated waves

27. Connective Tissue
• Most diverse and abundant tissue
• Main classes
– Connective tissue proper
– Cartilage
– Bone tissue
– Blood
• Characteristics
– Mesenchyme as their common tissue of origin
(mesenchyme derived from mesoderm)
– Varying degrees of vascularity
– Nonliving extracellular matrix, consisting of ground
substance and fibers
– Cells are not as abundant nor as tightly packed together
as in epithelium

28. Connective Tissue: Embryonic Origin
Figure 4.5

29. Functions of Connective Tissue
• Enclose organs as a capsule and separate
organs into layers. Areolar
• Connect tissues to one another. Tendons and
ligaments.
• Support and movement. Bones.
• Storage. Fat.
• Insulation. Fat.
• Transport. Blood.
• Protection. Bone, cells of the immune
system.

30. Structural Elements of Connective Tissue
• Ground substance – unstructured
material that fills the space between cells
• Fibers – collagen, elastic, or reticular
• Cells – fibroblasts, chondroblasts,
osteoblasts, hematopoietic stem cells,
and others

31. Connective Tissue Cells
• Fibroblasts - secrete the proteins needed for fiber
synthesis and components of the extracellular matrix
• Adipose or fat cells (adipocytes). Common in some
tissues (dermis of skin); rare in some (cartilage)
• Mast cells. Common beneath membranes; along small
blood vessels. Can release heparin, histamine, and
proteolytic enzymes in response to injury.
• Leukocytes (WBC’s). Respond to injury or infection
• Macrophages. Derived from monocytes (a WBC).
Phagocytic; provide protection
• Chondroblasts - form cartilage
• Osteoblasts - form bone
• Hematopoietic stem cells - form blood cells
• Undifferentiated mesenchyme (stem cells). Have
potential to differentiate into adult cell types.

32. Extracellular Matrix - ECM
• ECM has 3 major components
1. Protein fibers 2. Ground substance 3. Fluid
• Protein fibers
– Collagen fibers. Composed of the protein collagen.
Strong, flexible, inelastic; great tensile strength (i.e.
resist stretch). Perfect for tendons, ligaments
– Elastic fibers. Contain molecules of protein elastin that
resemble coiled springs. Returns to its original shape
after stretching or compression. Perfect for lungs,
large blood vessels
– Reticular fibers. Formed from fine collagenous fibers;
form branching networks (stroma). Fill spaces
between tissues and organs.

33. Ground Substance
• Interstitial (tissue) fluid within which are one or more of
the molecules listed below:
– Hyaluronic acid: a polysaccharide. Very slippery;
serves as a good lubricant for joints. Common in
most connective tissues.
– Proteoglycans: protein and polysaccharide complex.
Polysaccharides called glyocosaminoglycans
(chondroitin sulfate, keratin sulfate). Protein part
attaches to hyaluronic acid. Able to trap large
amounts of water.
– Adhesive molecules: hold proteoglycan aggregates
together. Chondronectin in cartilage, osteonectin in
bone, fibronectin in fibrous connective tissue.
• Functions as a molecular sieve through which nutrients
diffuse between blood capillaries and cells

34. Embryonic Connective Tissue
• Mesenchyme: source of all
adult connective tissue.
– Derived from mesoderm
– Delicate collagen fibers
embedded in semifluid matrix
• Mucus: found only in the
umbilical cord. Wharton’s
jelly.

35. Areolar Connective Tissue
Figure 4.12b

36. Adipose Tissue
Figure 4.12c

37. Reticular Connective Tissue
Figure 4.12d

38. Dense Irregular Connective
Tissue
Figure 4.12e

39. Dense Regular Connective
Tissue
Figure 4.12f

40. Elastic Connective Tissue
• Bundles and sheets of collagenous and elastic fibers
oriented in multiple directions
• In walls of elastic arteries (aorta), lungs, vocal
ligaments
• Strong, yet elastic; allows for recoil of tissue after
being stretched

41. Connective Tissue: Cartilage
• Composed of chondrocytes (cells) located in matrix-
surrounded spaces called lacunae.
• Type of cartilage determined by components of the matrix.
• Firm consistency.
• Ground substance: Proteoglycans and hyaluronic acid
complexed together trap large amounts of water
(microscopic sponges). Allows tissue to spring back after
being compressed.
• Avascular and no nerve supply. Heals slowly.
• Perichondrium. Dense irregular connective tissue that
surrounds cartilage. Fibroblasts of perichondrium can
differentiate into chondroblasts (cartilage-forming cells)
• Types of cartilage
– Hyaline
– Fibrocartilage
– Elastic

42. Hyaline Cartilage
Figure 4.12g

43. Elastic Cartilage
Figure 4.12h

44. Fibrocartilage
Figure 4.12i

45. Bone Tissue
Figure 4.12j

46. Blood Tissue
Figure 4.12k

47. Muscle Tissue
• Characteristics
– Cells are referred to as fibers
– Contracts or shortens with force when
stimulated
– Moves entire body and pumps blood
• Types
– Skeletal:attached to bones
– Cardiac: muscle of the heart.
– Smooth: muscle associated with tubular
structures and with the skin. Nonstriated and
involuntary.

48. Skeletal Muscle Tissue
Figure 4.14a

49. Cardiac Muscle Tissue
Figure 4.14b

50. Smooth Muscle Tissue
Figure 4.14c

51. Nervous Tissue
Figure 4.15

52. Tissues and Aging
• Cells divide more slowly
• Collagen fibers become more irregular in structure, though
they may increase in number
– Tendons and ligaments become less flexible and more fragile
• Elastic fibers fragment, bind to calcium ions, and become
less elastic
– Arterial walls and elastic ligaments become less elastic
• Changes in collagen and elastin result in
– Atherosclerosis and reduced blood supply to tissues
– Wrinkling of the skin
– Increased tendency for bones to break
• Rate of blood cell synthesis declines in the elderly
• Injuries don’t heal as readily