Download Histology Connective Tissues General Concepts Composition Cells

Histology
Connective Tissues
 General Concepts
 Composition
1. Cells. Each type of connective tissue has it own characteristic complement of
one or more of a wide variety of cells.
2. Extracellular matrix. Synthesized and secreted by resident “blast” cells
specific for each connective tissue type (e.g., fibroblasts and chondroblasts);
the matrix is composed of:
a. Fibers. Collagen, elastic and reticular
b. Ground substance. An amorphous substance that can exist as a liquid, gel,
or flexible or rigid solid, conferring unique structural properties to each
connective tissue.
 Functions

1. Provides substance and form to the body and organs
2. Defends against infection
3. Aids in injury repair
4. Stores lipids
5. Provides a medium for diffusion of nutrients and wastes
6. Attaches muscle to bone and bone to bone
Types of connective tissue. Classified by the relative abundance, variety, and
content of their components
1. Connective tissue proper
2. Cartilage
3. Bone
4. Special. Includes adipose, elastic, reticular, and mucoid connective tissues as
well as blood and hematopoietic tissue.
 Connective Tissue Proper
 General Concepts
1. Connective tissue proper comprises a very diverse group of tissues, both
functionally and structurally.
a. Structural functions of connective tissue proper
1) Forms a portion of the wall of hollow organs and vessels and the
stroma of solid organs.
2) Forms the stroma of organs and subdivides organs into functional
compartments.
3) Provides padding between and around organs and other tissues.
4) Provides anchorage and attachment (e.g., muscle insertions)
b. Provides a medium for nutrient and waste exchange
c. Lipid storage in adipocytes
d. Defense and immune surveillance function via lymphoid and phagocytic
cells.
2. All connective tissues are composed of two basic components, which vary
widely among different types of connective tissue. The components of
connective tissues are:
a. Cells (e.g., fibroblasts and macrophages)
b. Extracellular matrix
1) Fibers (e.g., collagen and elastic fibers)
2) Ground substance.
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Histology
Connective Tissues
 Cells of Connective Tissue:
1. Fibroblasts
a. Synthesize and maintain fibers and ground substance
b. Major resident cell in connective tissue proper
c. Active and inactive fibroblasts
2) Active fibroblast
a) Large, euchromatic, oval nucleus
b) Cytoplasm not usually visible but contains abundant rough
endoplasmic reticulum and Golgi
c) Elongated, spindle-shaped cells
d) High synthetic activity
2) Inactive fibroblast
a) Small, heterochromatic, flattened nucleus
b) Reduced cytoplasm and organelles
c) Low synthetic activity
2. Macrophages
a. Derived from blood monocytes; monocytes enter connective tissue from
the bloodstream and rapidly transform into macrophages that function in
phagocytosis, antigen processing, and cytokine secretion.
b. Comprise the mononuclear phagocyte system of the body; include Kupffer
cells in the liver, alveolar macrophages in the lung, microglia the central
nervous system, Langerhan’s cells in the skin, and osteoclasts in bone
marrow
c. Structure
1) Heterochromatic, oval nucleus with an indentation in the nuclear
envelope and marginated chromatin.
2) Cytoplasm usually not visible unless it contains phagocytosed material.
3. Mast cells
a. Mediate immediate hypersensitivity reaction and anaphylaxis by releasing
immune modulators from cytoplasmic granules, in response to antigen
binding with cell surface antibodies.
b. Structure
1) Round to oval-shaped cells
2) Round, usually centrally located nucleus
3) Well-defined cytoplasm filled with secretory granules containing
immune-modulatory compounds (e.g., histamine and heparin).
4. Plasma cells
a. Secrete antibodies to provide humoral immunity
b. Derived from B-lymphocytes
c. Structure
1) Oval-shaped cells
2) Round, eccentrically located nucleus with heterochromatin clumps
frequently arranged like the numerals on a clock-face
3) Basophilic cytoplasm due to large amounts of rough endoplasmic
reticulum
4) Well-developed Golgi complex appears as a distinct, unstained region
in the cytoplasm near the nucleus and, for that reason, is often referred
to as a “negative Golgi.”
5. Adipose cells (adipocytes, fat cells)
a. Store lipids
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Histology
Connective Tissues
b. Types
1) Yellow fat (unilocular)
a) Each cell contains a single droplet of neutral fat (triglycerides) for
energy storage and insulation.
b) Minimal cytoplasm, present as a rim around the lipid droplet.
c) Flattened, heterochromatic, crescent-shaped nucleus that conforms
to the contour of the lipid droplet Can occur singly, in small
clusters or forming a large mass, which is then referred to as
adipose connective tissue.
2) Brown fat (multilocular)
a) Cells contain numerous, small lipid droplets.
b) Large numbers of mitochondria
c) Present mostly during early postnatal life in humans, abundant in
hibernating animals for heat production
6. White blood cells (WBCs, leukocytes):
These cells enter and leave the blood stream to migrate through, and function
in, connective tissues. The most common WBCs encountered in connective
tissue proper are lymphocytes, neutrophils, and eosinophils. For a complete
discussion of blood cells
a. Lymphocytes (T and B lymphocytes)
1) Small spherical cells with sparse cytoplasm and a round
heterochromatic nucleus, often with a small indentation.
2) B cells enter connective tissue where they transform into plasma cells
and secrete antibodies. T cells are primarily located in lymphatic
tissues and organs; however, T cells can be present in connective tissue
proper under certain circumstances (e.g., organ transplantation).
b. Neutrophils (polymorphonuclear leukocytes, PMNs)
1) Spherical cells with a heterochromatic nucleus with three to five lobes.
2) Pale-staining cytoplasmic granules.
3) Highly phagocytic cells that are attracted to sites of infection
c. Eosinophils
1) Spherical cells with a bilobed nucleus
2) Cytoplasmic granules stain intensely with eosin.
3) Modulate the inflammatory process
 Extracellular Matrix
1. Fibers
Fiber type
Composition
Properties
Collagen
Collagen I, II
Inelastic, eosinophilic
Reticular
Elastic
Collagen III
Elastin
Inelastic,branched, argyrophilic
Elastic, eosinophilic
2. Ground substance
a. Functions
1) Forms a gel-like matrix of variable consistency in which cells and fibers
are embedded
2) Provides a medium for passage of molecules and cells migrating through
the tissue
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Histology
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3) Contains adhesive proteins that regulate cell movements
b. Components
1) Tissue fluid. Contains salts, ions and soluble protein
2) Glycosaminoglycans (GAGs)
a) Long, unbranched polysaccharides composed of repeating disaccharide
units, which are usually sulfated.
b) Large negative charge of the sugars attracts cations, resulting in a high
degree of hydration. The matrix formed ranges from a liquid
passageway to a viscous shock absorber.
c) GAGs are generally attached to proteins to form proteoglycans.
d) Proteoglycan aggregate. Many proteoglycans are attached to
hyaluronic acid, which is itself a glycosaminoglycan.
3) Adhesive glycoproteins. For example, fibronectin and laminin.
 Classification of Connective Tissue
A. Connective Tissue Proper
1. Loose (areolar)
a. Highly cellular, numerous cell types present.
b. Fewer and smaller caliber collagen fibers compared with dense
c. Abundant ground substance, allows for diffusion of nutrients and wastes
d. Highly vascularized
e. Provides padding between and around organs and tissues
2. Dense
a. Fewer cells, mostly fibroblasts
b. Highly fibrous with larger caliber collagen fibers, provides strength.
c. Minimal ground substance
d. Poorly vascularized
e. Types
1) Dense, irregular connective tissue. Fiber bundles arranged in an
interlacing pattern; forms the capsule of organs and the dermis of the
skin.
2) Dense regular connective tissue. Parallel arrangement of fiber bundles;
restricted to tendons and ligaments.
3. Adipose connective tissue. Consists of accumulations of adipocytes that are
partitioned into lobules by septa of connective tissue proper. Provides energy
storage and insulation.
4. Blood and hematopoietic (blood-forming) tissues (LAB10)
5. Elastic connective tissue. Regularly arranged elastic fibers or sheets (e.g., the
vocal ligament)
6. Reticular connective tissue. A loosely arranged connective tissue whose fibers
are reticular fibers. Forms the stroma of hematopoietic tissue (e.g., bone
marrow) and lymphoid organs (e.g., lymph node and spleen).
7. Mucoid connective tissue. Embryonic connective tissue with abundant ground
substance and delicate collagen fibers; present in the umbilical cord.
B. Supportive Connective Tissues
1. Cartilage (LAB 6)
2. Bone (LAB 6)
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