Unit one: tissue level of organization

1. Tissues Level of Organization
BY : GAGANDEEP KAUR

2. Body tissues
 Epithelial tissue covers body surfaces and lines hollow organs, body cavities, and
ducts. It also forms glands.
 Connective tissue protects and supports the body and itsorgans. Various types of
connective tissue bind organs together, store energy reserves as fat, and help
provide immunity to disease-causing organisms.
 Muscular tissue generates the physical force needed to make body structures move
and generates body heat.
 Nervous tissue detects changes in a variety of conditions inside and outside the
body and responds by generating action potentials (nerve impulses) that activate
muscular contractions and glandular secretions.

3.  Tissues of the body develop from three primary germ layers, the first tissues formed
in the human embryo called the ectoderm, endoderm, and mesoderm. Epithelial
tissues develop from all three primary germ layers. All connective tissue and most
muscle tissues derive from mesoderm. Nervous tissue develops from ectoderm.

4. Epithelial tissue
An epithelial tissue or epithelium consists of cells arranged in continuous sheets, in
either single or multiple layers. Because the cells are closely packed and are held
tightly together by many cell junctions, there is little intercellular space between
adjacent plasma membranes. Epithelial tissue forms coverings and linings throughout
the body. It is never covered by another tissue, so it always has a free surface.
Epithelial tissues have three major functions:
(1) selective barriers that limit or aid the transfer of substances into and out of the body;
(2) secretory surfaces that release products produced by the cells onto their free
surfaces;
(3) protective surfaces that resist the abrasive influences of the environment.

5. ❖Apical (free) surface of an epithelial cell faces
the body surface, a body cavity, the lumen
(interior space) of an internal organ, or a tubular
duct that receives cell secretions. Apical
surfaces may contain cilia or microvilli.
❖Lateral surfaces of an epithelial cell face the
adjacent cells on either side, may contain tight
junctions, adherens junctions, desmosomes,
and/or gap junctions.
❖Basal surface of an epithelial cell is opposite
the apical surface, and the basal surfaces of the
deepest layer of cells adhere to extracellular
materials.

6. Basement Membrane
basement membrane is a thin extracellular layer that commonly consists of two layers,
the basal lamina and reticular lamina.
 The basal lamina (lamina thin layer) is closer to— and secreted by—the epithelial
cells. It contains proteins such as laminin and collagen, as well as glycoproteins and
proteoglycans. The laminin molecules in the basal lamina adhere to integrins in
hemidesmosomes and thus attach epithelial cells to the basement membrane.
 The reticular lamina is closer to the underlying connective tissue and contains
proteins such as collagen produced by connective tissue cells called fibroblasts. The
basement membrane functions as a point of attachment and support for the
overlying epithelial tissue.

7. Types Of Epithelial Tissues
Epithelial tissue may be divided into two types.
(1) Covering and lining epithelium forms the outer covering of the skin and some
internal organs. It also forms the inner lining of blood vessels, ducts, and body
cavities, and the interior of the respiratory, digestive, urinary, and reproductive
systems.
(2) Glandular epithelium makes up the secreting portion of glands such as the thyroid
gland, adrenal glands, and sweat glands.

8. Covering and lining epithelial tissue
1. Arrangement of cells in layers.
a. Simple epithelium is a single layer of cells that
functions in diffusion, osmosis, filtration, secretion, or
absorption. Secretion is the production and release of
substances such as mucus, sweat, or enzymes.
Absorption is the intake of fluids or other substances
such as digested food from the intestinal tract.

9. CONTD……..
b. Pseudostratified epithelium appears to have multiple layers of
cells because the cell nuclei lie at different levels and not all cells
reach the apical surface. Cells that do extend to the apical surface
may contain cilia; others (goblet cells) secrete mucus.
Pseudostratified epithelium is actually a simple epithelium because
all its cells rest on the basement membrane.
c. Stratified epithelium (stratum layer) consists of two or more
layers of cells that protect underlying tissues in locations where
there is considerable wear and tear.

10. Cell shapes
a. Squamous cells are arranged like floor tiles and are thin, which allows for the rapid
passage of substances.
b. Cuboidal cells are as tall as they are wide and are shaped like cubes or hexagons.
They may have microvilli at their apical surface and function in either secretion or
absorption.
c. Columnar cells are much taller than they are wide, like columns, and protect
underlying tissues. Their apical surfaces may have cilia or microvilli, and they often are
specialized for secretion and absorption.
d. Transitional cells change shape, from flat to cuboidal and back, as organs such as the
urinary bladder stretch (distend) to a larger size and then collapse to a smaller size.

11. Combining the two characteristics (arrangements
of layers and cell shapes)
Simple epithelium
A. Simple squamous epithelium
B. Simple cuboidal epithelium
C.Simple columnar epithelium
(nonciliated and ciliated)
D.Pseudostratified columnar
epithelium (nonciliated
and ciliated)
Stratified epithelium
A. Stratified squamous epithelium
(keratinized and nonkeratinized)*
B. Stratified cuboidal epithelium*
C. Stratified columnar epithelium*
D. Transitional epithelium

12. GLANDULAR EPITHELIUM
 The function of glandular epithelium is secretion, which is accomplished by glandular
cells that often lie in clusters deep to the covering and lining epithelium. A gland may
consist of a single cell or a group of cells that secrete substances into ducts (tubes),
onto a surface, or into the blood. All glands of the body are classified as either
endocrine or exocrine.
 The secretions of endocrine glands enter the interstitial fluid and then diffuse directly
into the bloodstream without flowing through a duct. These secretions, called
hormones, regulate many metabolic and physiological activities to maintain
homeostasis. The pituitary, thyroid, and adrenal glands are examples of endocrine
glands.

13. CONTD……..
 Exocrine glands secrete their products into ducts that empty onto the surface of a
covering and lining epithelium such as the skin surface or the lumen of a hollow
organ. The secretions of exocrine glands include mucus, sweat, oil, earwax, saliva,
and digestive enzymes.
 Examples of exocrine glands include sweat glands, which produce sweat to help lower
body temperature, and salivary glands, which secrete saliva. Saliva contains mucus
and digestive enzymes among other substances.

14. Structural Classification of Exocrine Glands
 Exocrine glands are classified as unicellular or multicellular. As the name implies,
unicellular glands are single-celled. Goblet cells are important unicellular exocrine
glands that secrete mucus directly onto the apical surface of a lining epithelium.
 Most glands are multicellular glands, composed of many cells that form a distinctive
microscopic structure or macroscopic organ. Examples include sudoriferous,
sebaceous (oil), and salivary glands.
Multicellular glands are categorized according to two criteria:
(1) whether their ducts are branched or unbranched
(2) the shape of the secretory portions of the gland.

15.  If the duct of the gland does not branch, it is a simple gland. If the duct branches, it
is a compound gland. Glands with tubular secretory parts are tubular glands; those
with rounded secretory portions are acinar glands, also called alveolar glands.
Tubuloacinar glands have both tubular and rounded secretory parts.

16. Simple glands
A. Simple tubular. Tubular secretory part is straight and attaches to a single unbranched duct.
Example: glands in the large intestine.
B. Simple branched tubular. Tubular secretory part is branched and attaches to a single
unbranched duct. Example: gastric glands.
C. Simple coiled tubular. Tubular secretory part is coiled and attaches to a single unbranched
duct. Example: sweat glands.
D. Simple acinar. Secretory portion is rounded and attaches to a single unbranched duct.
Example: glands of the penile urethra.
E. Simple branched acinar. Rounded secretory part is branched and attaches to a single
unbranched duct. Example: sebaceous glands.

17. Compound glands
A. Compound tubular. Secretory portion is tubular and attaches to a branched duct.
Example: bulbourethral (Cowper’s) glands.
B. Compound acinar. Secretory portion is rounded and attaches to a branched duct.
Example: mammary glands.
C. Compound tubuloacinar. Secretory portion is both tubular and rounded and attaches
to a branched duct. Example: acinar glands of the pancreas.

18. Functional Classification of Exocrine Glands
 Secretions of merocrine glands are 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. Most exocrine
glands of the body are merocrine glands. Examples
include the salivary glands and pancreas.

19. CONTD……
 Apocrine glands accumulate their
secretory product at the apical surface of
the secreting cell. Then, that portion of
the cell pinches off from the rest of the
cell to release the secretion. The
remaining part of the cell repairs itself
and repeats the process. Electron
microscopy has recently confirmed that
this is the mechanism of secretion in the
mammary glands.

20. CONTD…….
 The cells of holocrine glands accumulate a
secretory product in their cytosol. As the
secretory cell matures, it ruptures and becomes
the secretory product The sloughed off cell is
replaced by a new cell. One example of a
holocrine gland is a sebaceous gland of the skin.