What characterizes connective tissues?
They have scattered non-junctional cells in a water-rich extracellular matrix.
What forms the macromolecular framework of the extracellular matrix?
Connective fibers and a hydrated ground substance form it.
Which fibers are found in the connective-tissue matrix?
Collagen, reticular, and elastic fibers are found.
What other components are present in the connective-tissue matrix besides fibers?
Blood capillaries, nerves, and lymphatic vessels are present.
Which cells are considered resident in connective tissue?
Fibroblasts and sometimes adipocytes are resident cells.
Which mobile cells of hematopoietic origin are found in connective tissue?
Macrophages, mast cells, and leukocytes are mobile cells.
What is a fibroblast in connective tissue?
An active connective-tissue cell that divides and produces and degrades extracellular-matrix components.
What shape and features characterize fibroblasts?
They are fusiform or stellate cells with cytoplasmic extensions and an oval nucleolated nucleus.
Which organelles are abundant in fibroblasts?
Rough endoplasmic reticulum and Golgi apparatus are abundant in fibroblasts.
How do fibroblasts regulate extracellular-matrix composition?
By sensing tension and compression and balancing matrix synthesis with degradation.
How does a fibrocyte differ in size and shape from a fibroblast?
A fibrocyte is smaller and more elongated than a fibroblast.
What nuclear features distinguish a fibrocyte?
It has a dense elongated nucleus lacking a nucleolus.
Can a fibrocyte become a fibroblast again?
Yes, a fibrocyte can be reactivated into a fibroblast.
What is the most abundant type of collagen fiber?
Type I collagen fibers.
What is the diameter range of Type I collagen fibers?
1 to 40 μm.
How do Type I collagen fibers appear under H.E. staining?
Yellow-orange.
What is the thickness of Type III collagen fibers?
About 0.2 μm thick.
What type of network do Type III collagen fibers form?
Anastomosing networks.
Where are reticular fibers found in the body?
In basal laminae, around vessels, muscle and adipose cells, liver, and lymphoid and hematopoietic organs.
What are the two main phases of collagen synthesis?
Intracellular synthesis and extracellular assembly.
Where do hydroxylated alpha chains assemble into pro-collagen?
In the rough endoplasmic reticulum.
What prevents intracellular assembly of pro-collagen molecules?
Pro-peptides on the triple-helical pro-collagen molecule.
What modification does the Golgi apparatus perform on pro-collagen?
Glycosylation of pro-collagen.
How is pro-collagen secreted into the extracellular environment?
By exocytosis.
What converts pro-collagen into tropocollagen extracellularly?
Proteolytic cleavage.
Into what do tropocollagen molecules assemble?
Fibrils and then fibers.
What are elastic fibers?
Thin, sinuous, anastomosing fibers that allow connective tissues to recoil after stretching.
How thick are elastic fibers?
About 0.2 μm thick.
Which stain colors elastic fibers dark blue or violet-black?
Orcein stain.
What is the central zone of elastic fibers composed of?
Elastin containing desmosine and isodesmosine.
What composes the peripheral microfibrils of elastic fibers?
Glycoproteins mainly fibrillins 1 and 2.
What enzyme induces tropo-elastin polymerization on microfibrils?
Lysyl oxidase.
What is ground substance?
A strongly hydrated gel of sulfated glycosaminoglycans, proteoglycans, and soluble molecules.
What does ground substance permit to diffuse?
Mineral salts, glucose, lipids, amino acids, hormones, proteins, and catabolic products like urea.
Where do blood monocytes originate before becoming histiocytes?
They originate in the bone marrow.
What size range do activated macrophages reach?
They enlarge to 20 to 50 μm.
How do macrophages contribute to inflammation?
They induce and maintain inflammation through pro-inflammatory cytokines.
Where do mast-cell precursors mature?
They mature in connective tissue in contact with fibroblasts.
What triggers mast cells to release mediators in IgE-dependent allergy?
Recognition of an allergen by IgE bound to mast cells.
What effect does mast cell mediator release have in allergy?
It produces edema, bronchoconstriction, and vasodilation.
How do leukocytes enter connective tissue from capillaries?
By diapedesis crossing capillary walls.
Which leukocyte type commonly indicates an allergic reaction?
Eosinophils commonly indicate an allergic reaction.
What functions do connective tissues provide?
Support, exchange and nutrition, defense, tissue repair, metabolic functions, and control of other tissues.
How are connective tissues classified by their components?
By the relative proportions of cells, fibers, and ground substance.
Which features determine connective tissue classification besides component proportions?
Fiber nature and fiber orientation.
What dominates fibrous connective tissue's fiber composition?
Interwoven, non-oriented collagen fibers dominate fibrous connective tissue.
How are collagen fibers oriented in dense uni-tendoned connective tissue?
All collagen fibers are oriented in one direction in dense uni-tendoned connective tissue.
Where does dense bi- or multi-tendoned connective tissue occur?
It occurs in aponeuroses, fascias, the corneal stroma, and the sclera.
What limits the healing capacity of dense oriented connective tissues?
Their poor vascularization limits their healing capacity.
Where is elastic dense connective tissue mainly found?
It is found in the ligamentum flavum, false vocal cords, and media of large arteries near the heart.
What is the main function of elastic dense connective tissue?
Its main function is mechanical support.
What fibers are associated with collagen in fibrous connective tissue?
Elastic fibers are associated with collagen in fibrous connective tissue.
How does dense bi- or multi-tendoned connective tissue differ from uni-tendoned?
It has superimposed oblique layers of collagen fibers, unlike uni-tendoned tissue.
What is the most widespread connective tissue type?
Loose areolar connective tissue.
What type of adipose tissue is multilocular and produces heat in infants?
Brown adipose tissue.
How do brown adipocytes compare in size to white adipocytes?
Brown adipocytes are smaller than white adipocytes.
What is the defining feature of white adipose tissue's adipocytes?
They contain one large triglyceride vacuole displacing the nucleus.
What hormone does white adipose tissue secrete to inhibit appetite?
Leptin.
What percentage of body weight does white adipose tissue represent in women?
20 to 25%.
What type of collagen fibers form the network in reticular connective tissue?
Type III collagen fibers.
Where is mucoid connective tissue found?
In embryonic mesenchyme, umbilical cord, dental pulp, and vitreous body.
Test your knowledge with 27 questions on Connective Tissue Histology.
1. Which feature best characterizes connective tissue compared with epithelial tissue?
2. What are the two principal components of the connective-tissue extracellular matrix?
Review the complete course in the study sheet for Connective Tissue Histology.
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