Connective tissue is a heterogeneous tissue with many cell shapes and tissue architecture. Structurally, all connective tissues contain cells that are embedded in an extracellular matrix stabilized by proteins. The chemical nature and physical layout of the extracellular matrix and proteins vary enormously among tissues, reflecting the variety of functions that connective tissue fulfills in the body. Connective tissues separate and cushion organs, protecting them from shifting or traumatic injury. Connect tissues provide support and assist movement, store and transport energy molecules, protect against infections, and contribute to temperature homeostasis.
Many different cells contribute to the formation of connective tissues. They originate in the mesodermal germ layer and differentiate from mesenchyme and hematopoietic tissue in the bone marrow. Fibroblasts are the most abundant and secrete many protein fibers, adipocytes specialize in fat storage, hematopoietic cells from the bone marrow give rise to all the blood cells, chondrocytes form cartilage, and osteocytes form bone. The extracellular matrix contains fluid, proteins, polysaccharide derivatives, and, in the case of bone, mineral crystals. Protein fibers fall into three major groups: collagen fibers that are thick, strong, flexible, and resist stretch; reticular fibers that are thin and form a supportive mesh; and elastin fibers that are thin and elastic.
The major types of connective tissue are connective tissue proper, supportive tissue, and fluid tissue. Loose connective tissue proper includes adipose tissue, areolar tissue, and reticular tissue. These serve to hold organs and other tissues in place and, in the case of adipose tissue, isolate and store energy reserves. The matrix is the most abundant feature for loose tissue although adipose tissue does not have much extracellular matrix. Dense connective tissue proper is richer in fibers and may be regular, with fibers oriented in parallel as in ligaments and tendons, or irregular, with fibers oriented in several directions. Organ capsules (collagenous type) and walls of arteries (elastic type) contain dense irregular connective tissue. Cartilage and bone are supportive tissue. Cartilage contains chondrocytes and is somewhat flexible. Hyaline cartilage is smooth and clear, covers joints, and is found in the growing portion of bones. Fibrocartilage is tough because of extra collagen fibers and forms, among other things, the intervertebral discs. Elastic cartilage can stretch and recoil to its original shape because of its high content of elastic fibers. The matrix contains very few blood vessels. Bones are made of a rigid, mineralized matrix containing calcium salts, crystals, and osteocytes lodged in lacunae. Bone tissue is highly vascularized. Cancellous bone is spongy and less solid than compact bone. Fluid tissue, for example blood and lymph, is characterized by a liquid matrix and no supporting fibers.
Connective tissue is made of:
cells:
fibroblasts - the most abundant, secrete many protein fibers
adipocytes specialize in fat storage
hematopoietic cells from the bone marrow give rise to all the blood cells
chondrocytes form cartilage
osteocytes form bone.
extracellular matrix contains fluid, proteins, polysaccharide derivatives, and mineral crystals (bone).
protein fibers can be embedded in this matrix:
tough collagen
flexible elastin
meshwork reticulin
The three subgroups of connective tissue are loose connective tissue, dense connective tissue, and specialized connective tissue.
Loose connective tissue fills space between and penetrates into organs. 3 types:
Areolar is the most widely distributed type of loose connective tissue. It contains three types of cells: fibroblasts, which make fibrils for repair; histiocytes or macrophages, which do phagocytosis; and mast cells, which produce the anticoagulant heparin and histamine, an inflammatory substance.
Adipose tissue is loose connective tissue with fat stored in its cells. It protects and insulates.
Reticular tissue forms the framework of the liver, spleen, lymph nodes, and bone marrow.
Fiber arrangements may be:
Regular (parallel): tendons, ligaments, and aponeuroses
Irregular (mixed directions): muscle sheaths & joint capsules (collagenous), artery walls (elastic)
Cartilage. Cells called chondrocytes. Three types of cartilage tissue:
Hyaline cartilage - smooth & clear; in costal cartilages, nasal septum, tracheal rings, covers joints, in growing bones.
Fibrocartilage - very strong; extra collagen fibers; in intervertebral disks.
Elastic cartilage - stretchable and flexible; in ears, epiglottis and auditory tubes.
Bone. Cells called osteocytes, embedded in lacunae matrix of calcium and phosphorous. Two types:
compact or dense
cancellous or spongy.
Teeth are made of dentin; the crown of the tooth is covered with enamel.
Blood is composed of a liquid portion called plasma and the blood cells. Blood cells are formed in red bone marrow, a hematopoietic tissue. They include red and white blood cells and platelets.
Specialised immune cells circulate in the blood and in lymphoid tissues such lymph nodes, thymus, spleen, tonsils, and adenoids, collectively called reticuloendothelial (RE) system.
a type of white blood cell - plasma cells or B lymphocytes that produce antibodies
Kupffer’s cells line the liver and other RE cells also line the spleen and bone marrow.
Macrophage is a term for any phagocytic cell of the RE system.
Microglia cells do phagocytosis in the nervous system; other neuroglia cells do support.
supports other tissues.
provides nourishment: blood carries nutrients.
transports: blood transports enzymes and hormones.
connects various tissues to one another.
provides movement via bones.
protects vital organs (bones of skull and thorax) and provides immunity (lymphoid tissue and white blood cells).
insulates and maintains temperature (adipose tissue).
provides storage areas: bone stores calcium and phosphorous, adipose tissue stores fat.
attaches and separates other tissues of the body.
adipocytes
lipid storage cells
adipose tissue
specialized areolar tissue rich in stored fat
areolar tissue
(also, loose connective tissue) a type of connective tissue proper that shows little specialization with cells dispersed in the matrix
chondrocytes
cells of the cartilage
collagen fiber
flexible fibrous proteins that give connective tissue tensile strength
connective tissue proper
connective tissue containing a viscous matrix, fibers, and cells.
dense connective tissue
connective tissue proper that contains many fibers that provide both elasticity and protection
elastic cartilage
type of cartilage, with elastin as the major protein, characterized by rigid support as well as elasticity
elastic fiber
fibrous protein within connective tissue that contains a high percentage of the protein elastin that allows the fibers to stretch and return to original size
fibroblast
most abundant cell type in connective tissue, secretes protein fibers and matrix into the extracellular space
fibrocartilage
tough form of cartilage, made of thick bundles of collagen fibers embedded in chondroitin sulfate ground substance
fibrocyte
less active form of fibroblast
fluid connective tissue
specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved proteins
ground substance
fluid or semi-fluid portion of the matrix
hyaline cartilage
most common type of cartilage, smooth and made of short collagen fibers embedded in a chondroitin sulfate ground substance
lacunae
(singular = lacuna) small spaces in bone or cartilage tissue that cells occupy
loose connective tissue
(also, areolar tissue) type of connective tissue proper that shows little specialization with cells dispersed in the matrix
matrix
extracellular material which is produced by the cells embedded in it, containing ground substance and fibers
mesenchymal cell
adult stem cell from which most connective tissue cells are derived
mesenchyme
embryonic tissue from which connective tissue cells derive
mucous connective tissue
specialized loose connective tissue present in the umbilical cord
parenchyma
functional cells of a gland or organ, in contrast with the supportive or connective tissue of a gland or organ
reticular fiber
fine fibrous protein, made of collagen subunits, which cross-link to form supporting “nets” within connective tissue
reticular tissue
type of loose connective tissue that provides a supportive framework to soft organs, such as lymphatic tissue, spleen, and the liver
supportive connective tissue
type of connective tissue that provides strength to the body and protects soft tissue
Visit this link to test your connective tissue knowledge with this 10-question quiz. Can you name the 10 tissue types shown in the histology slides?
Click at the bottom of the quiz for the answers.
1. Connective tissue is made of which three essential components?
A) cells, ground substance, and carbohydrate fibers
B) cells, ground substance, and protein fibers
C) collagen, ground substance, and protein fibers
D) matrix, ground substance, and fluid
B
2. Under the microscope, a tissue specimen shows cells located in spaces scattered in a transparent background. This is probably ________.
A) loose connective tissue
B) a tendon
C) bone
D) hyaline cartilage
D
3. Which connective tissue specializes in storage of fat?
A) tendon
B) adipose tissue
C) reticular tissue
D) dense connective tissue
B
4. Ligaments connect bones together and withstand a lot of stress. What type of connective tissue should you expect ligaments to contain?
A) areolar tissue
B) adipose tissue
C) dense regular connective tissue
D) dense irregular connective tissue
C
5. In adults, new connective tissue cells originate from the ________.
A) mesoderm
B) mesenchyme
C) ectoderm
D) endoderm
B
6. In bone, the main cells are ________.
A) fibroblasts
B) chondrocytes
C) lymphocytes
D) osteocytes
D
1. One of the main functions of connective tissue is to integrate organs and organ systems in the body. Discuss how blood fulfills this role.
Blood is a fluid connective tissue, a variety of specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved proteins in a liquid extracellular matrix. Blood contains formed elements derived from bone marrow. Erythrocytes, or red blood cells, transport the gases oxygen and carbon dioxide. Leukocytes, or white blood cells, are responsible for the defense of the organism against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some cells have the ability to cross the endothelial layer that lines vessels and enter adjacent tissues. Nutrients, salts, and waste are dissolved in the liquid matrix and transported through the body.
2. Why does an injury to cartilage, especially hyaline cartilage, heal much more slowly than a bone fracture?
A layer of dense irregular connective tissue covers cartilage. No blood vessels supply cartilage tissue. Injuries to cartilage heal very slowly because cells and nutrients needed for repair diffuse slowly to the injury site.