CONNECTIVE TISSUES
Most of the body is made of connective tissues. While epithelia cover and protect,
connective tissues “connect” and store. Connective tissues fill the spaces between
other tissues and form organs.
In histological slides, we usually see connective tissues between layers of other
tissues — for example, between the epithelium and a layer of muscle in the wall
of a hollow organ, like the stomach or intestines.
Connective tissues differ significantly from epithelial tissues; they have relatively
few cells that are not necessarily connected, and these cells are immersed in a
considerable quantity of extracellular matrix.
This matrix is a composite of “filler material,” also called ground substance, and
varying quantities of fibers.
Extracellular matrices can be very diverse, from loosely arranged fibers and a large
amount of ground substance in loose connective tissue, to very organized fibers
with minimal ground substance in tendons, to fibers covered in crystallized
calcium salts in the bone.
FUNCTIONS OF CONNECTIVE TISSUES
In general, connective tissues:
•
form the bulk of organs.
•
fill the spaces between other tissues and bind organs together.
•
support organs, by forming fascia and sheaths, ligaments, and tendons.
•
form support structures in the joints, such as joint capsules, synovial
membranes, and tendon sheaths.
•
store calcium, fat, immune cells, water, and many bodily chemicals, such
as growth factors and hormones.
•
insulate (mostly due to stored fat, but also by forming the dermis of the
skin).
•
transport substances through the body via blood.
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TYPES OF CONNECTIVE TISSUES
Despite their diversity, based on the properties of the extracellular matrix,
connective tissues can be grouped into four major types and then further divided
into smaller classes.
“ordinary” connective tissues
“specialized” connective tissues
1. connective tissue proper
o loose connective tissue
▪ areolar
▪ reticular
▪ adipose
o dense connective tissue
▪ regular
▪ irregular
▪ elastic
2. cartilage
o
o
o
hyaline
fibrocartilage
elastic
3. bone
•
compact
o spongy
4. blood and blood-forming tissues
o
GROUND SUBSTANCE
Ground substance is a gel-like material that fills the spaces between connective
tissue cells. It is a mixture of glycoproteins, glycoaminoglycans (such as
hyaluronic acid), and proteoglycans.
Ground substance can be very fluid, as in the vitreous humor in the posterior
cavity of the eye, or calcified, as in a bone.
The characteristics of ground substance determine the permeability of a
connective tissue and its ability to store water.
FIBERS IN CONNECTIVE TISSUES
There are three types of fibers in connective tissues:
•
Collagen Fibers
•
Elastic Fibers
•
Reticular Fibers
Collagen fibers, found in tendons, provide tensile strength and ressitance to
longitudinal stretching (i.e. they are strong but only in one direction). In
contrast, elastic fibers are stretchy in all directions, and help tissues return to their
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previous shapes after extension (a property called elasticity). Elastic fibers are
made of two proteins, elastin and fibrillin.
Reticular fibers are very fine collagen fibers that form a net-like structure, the
reticulum, which wraps around organs and protects their integrity.
PRODUCTION AND MAINTENANCE OF CONNECTIVE TISSUES
Connective tissues are formed by undifferentiated cells (whose names end in “blast”), that create new ground substance and connective tissue fibers.
When these “-blast” cells become surrounded by the ground substance they are
producing, they are considered mature and are referred to by names that end in “cyte.” Thus, a chondroblast is the cell that builds new cartilage within developing
fetuses and in the epiphyseal plates of growing bones.
Once that chondroblast is surrounded by the cartilage it is itself producing, it is
considered mature, and is referred to as a chondrocyte. It is this chondrocyte that
maintains the ground substance in post-pubescent adults.
Bone matrix is generated by osteoblasts that are found only on the surface of the
bone. Once osteoblasts become trapped within the matrix, they lose the ability to
divide and become osteocytes.
Fibroblasts are the most common cells within “ordinary” connective
tissue. Fibroblasts produce and maintain ground substance and connective tissue
fibers and can migrate through the extracellular matrix to the places which need
to be renewed or repaired.
The mature fibroblasts located in tissues that don’t actively repair and grow are,
in fact, fibrocytes (but we keep the same name). Fibroblasts are spindle shaped,
with a single, well-visible nucleus.
LOOSE CONNECTIVE TISSUE
Areolar tissue is the most common type of connective tissue. It contains all three
types of fibers (collagen, elastic, and reticular), which are distributed randomly
and crisscross in all directions.
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Areolar tissue wraps around muscles, fills the spaces between muscle fascicles,
and surrounds blood vessels and internal organs — especially those in the
abdominal cavity.
It is usually found just underneath the epithelial layer. Areolar tissue contains
fibroblasts and a large number of macrophages, mast cells, and plasma cells.
Figure : Areolar tissue
Reticular tissue forms a mesh-like, supportive framework for soft organs such
as the liver, the spleen, and lymphatic tissues. It has a high number of reticular
fibers that other cells can use as a scaffold for organ formation.
Figure : Reticular tissue
Adipose tissue consists primarily of adipocytes, or fat cells, which store
triglycerides in the form of a fat droplet. There are two types of adipocytes: white
and brown.
White adipocytes store lipids as a single, large droplet in the middle of the cell,
with the nucleus pushed to the periphery. This forms a characteristic shape that is
sometimes called a “signet ring”. Subcutaneous fat is composed of white
adipocytes.
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Brown adipocytes are very rare in humans and practically present only in fetuses
and very young babies. They are involved in heat production and help babies
adjust to life outside the womb.
Brown adipocytes look more like “normal” cells with multiple small fat droplets
dispersed in the cytoplasm.
Figure : Adipose tissue
DENSE CONNECTIVE TISSUES
Dense connective tissue contains more collagen fibers than loose connective
tissue.
Dense regular connective tissue has fibers arranged in neatly organized, parallel
bundles. It has both collagen and elastic fibers, which provide great tensile
strength and elasticity in one direction.
This tissue has very few fibroblasts and almost no other cells. Dense regular
connective tissue is found in tendons and ligaments.
Figure : Dense regular tissue
Dense irregular connective tissue is composed of haphazardly arranged, densely
packed collagen bundles with many purple stained fibroblasts present.
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Collagen and elastic fibers spread in many directions, providing increased
elasticity. Dense irregular connective tissue is found in the dermis layer of the
skin.
Figure : Dense irregular tissue
CARTILAGE
Cartilage is a very dense and resilient tissue that is mainly composed of collagen
fibers within a ground substance made mostly of chondroitin sulfate.
It is produced by chondroblasts that become chondrocytes once they are embedded
in the matrix. Mature chondrocytes are localized in groups within cartilage.
Cartilage has very few cells and is avascular (lacking blood vessels) and a neural
(lacking nerves).
Nutrition is provided to chondrocytes via diffusion, causing it to heal very slowly.
There are several types of cartilage, based upon the relative amounts of collagen
fibers and proteoglycan matrix.
Hyaline cartilage is the most common — and the weakest — and is found in the
ribs, nose, larynx, and trachea.
It appears glassy in histological slides. In the embryo, bone begins as hyaline
cartilage before ossifying into bone.
Figure : Hyaline cartilage
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Fibrocartilage has many collagen fibers, making it the strongest type of cartilage.
It is found in intervertebral discs, joint capsules, ligaments, and the pubic
symphysis.
Under the microscope, the cartilage fibers appear as wispy lines arranged in an
orderly fashion with chondrocytes spaced throughout.
Figure 7: Fibrocartilage
Elastic cartilage is very springy and yellow, and is found in the epiglottis, the
external ear, and the larynx.
It consists of fiber bundles that appear dark under the microscope and has many
disc-shaped cells interspersed throughout that are white in color.
Figure : Elastic cartilage
BONES
Bone is a dense connective tissue that has calcified extracellular matrix, making
bone a rigid and strong tissue with the ability to disperse tensile forces along the
longitudinal axis of a bone, but not from the side. Side impacts quite often cause
fractures.
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There are four types of cells found within bone tissue. Osteoblasts, that deposit
the bone, osteocytes that maintain it, and osteoclasts that resorb the bone to make
space for new bone formation.
Osteoprogenitor cells (also called osteogenic cells) are undifferentiated stem
cells that develop into osteoblasts, when activated.
Osteoblasts themselves become osteocytes, once surrounded by the bony matrix
they secrete.
Osteoclasts develop from monocytes and macrophages that arrive at the bone with
the blood supply.
Because they develop from a different stem cell line, osteoclasts differ in
appearance from other bone cells.
As with all other connective tissues, bone tissue has cells, fibers, and ground
substance. The cells account for about 2% of the bone mass.
The rest of the bone consists of concentric layers of collagen fibers and the ground
substance – mostly hydroxyapatite crystals, a form of calcium phosphate – which
adheres to the collagen fibers.
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Both collagen and hydroxyapatite contribute to the resilience of bone: collagen
gives bone flexibility and integrity, and hydroxyapatite provides strength.
Low levels of collagen make for brittle bones; low levels of hydroxyapatite result
in bones that are weak and easily bendable.
During bone deposition, osteoblasts produce collagen fibers, the organic portion
of the bone matrix.
This newly laid, not-yet-calcified bone tissue is called osteoid.
Collagen fibers provide a surface onto which the hydroxyapatite crystals
adhere. Hydroxyapatite crystals form when calcium phosphate and calcium
carbonate combine with small quantities of other inorganic salts, including
magnesium hydroxide, fluoride, and sulfate.
During the process of deposition, osteoblasts get trapped within the matrix they
are secreting.
Trapped osteoblasts mature into osteocytes, slowing their metabolic processes and
collagen production.
Osteocytes do not add new bone, instead maintaining the mineral concentration of
the surrounding matrix. Bone can only be added to the surface of a bone, or at the
ends of the bone shaft, at the epiphyseal plates.
At these sites, new osteoblasts are replenished via the differentiation of
osteoprogenitor cells on the bone surface.
The functions and locations of the four types of bone cells are summarized in the table below.
CELL TYPE
FUNCTION
LOCATION
OSTEOPROGENITOR
CELLS
develop into osteoblasts
deep layers of the periosteum and the
marrow
OSTEOBLASTS
bone formation
growing portions of bone, by the
periosteum and endosteum
OSTEOCYTES
maintain mineral concentration
of matrix
entrapped in matrix
OSTEOCLASTS
bone resorption
bone surfaces and at sites of old,
injured, or unneeded bone
table from BC OpenStax Anatomy and Physiology book, under the CC BY license
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COMPACT AND SPONGY BONE
There are two types of bone tissue: compact bone, comprising the shafts of long
bones, and spongy bone, which fills the ends of long bones.
Most bones contain both compact and spongy bone tissue, but their concentration
varies across different bones.
Compact bone is dense and can withstand direct compression, while spongy
bone has open spaces and can support weight, just as studs in the wall of a building
can support sheetrock via the distribution of weight.
Osteons are the microscopic structural units of compact bone. An osteon is a
long, cylindrical structure with a central channel called a Haversian canal. Bone
is a highly vascularized tissue and Haversian canals contain blood vessels,
lymphatics, and nerves.
Deposition of bone occurs in concentric circles around the Haversian canal and is
limited by the availability of nutrients.
New layers of bone are added by the osteoblasts located on the external surface of
the cylinder.
Once osteoblasts are trapped by their own secretions, they mature into osteocytes,
and the next generation of osteoblasts starts depositing the next layer.
Osteocytes are located in lacunae and fully surrounded by bone matrix.
Osteocytes have a characteristic shape, with many processes branching from their
cell body and reaching toward neighboring cells.
During bone formation, the matrix calcifies around these extensions and forms
tiny canals between lacunae called canaliculi.
In canaliculi, the extensions of the osteocytes form cell junctions with their
neighbors and, through gap junctions, are able to pass hormonal signals arriving
in the blood to deeper layers of the bone.
A transverse cross section of a long bone shows a typical arrangement of lacunae
in concentric circles around a central Haversian canal.
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transverse cross section of compact bone tissue; the red arrow indicates a Haversian canal; blue arrows indicate
lacunae
Spongy bone, also known as cancellous bone or trabecular bone, looks like a
sponge under the microscope. Spongy bone also contains osteocytes housed in
lacunae, but they are not arranged in concentric circles.
Instead, the lacunae are found in the loosely arranged bone spikes
called trabeculae.
The trabeculae form along lines where load is exerted on the bone. Spongy bone,
which is lighter than compact bone, helps balance the strength of the bone against
the weight of the skeleton, thus reducing the energy needed to move it.
GROSS ANATOMY OF THE BONE
A long bone has three parts; the diaphysis and two epiphyses, one at each end.
The diaphysis is the cylinder in the middle of the bone.
The hollow inside of the diaphysis is called the medullary cavity, and is filled
with fat or yellow bone marrow.
The walls of the diaphysis are composed of compact bone.
The wider section at each end of the bone is called the epiphysis, which is made
from a thin layer of compact bone and filled with spongy bone.
Red marrow fills the spaces in spongy bone. Each epiphysis meets the diaphysis
at the metaphysis, the narrow area that contains the epiphyseal plate (growth
plate), a layer of hyaline (transparent) cartilage in a growing bone.
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When the bone stops growing in early adulthood, the cartilage is replaced by bone
tissue and the epiphyseal plate becomes an epiphyseal line.
The outer surface of the bone is covered with a fibrous membrane called
the periosteum.
The periosteum contains blood vessels, nerves and lymphatic vessels that nourish
compact bone. On the side of medullary cavity, the bone is covered by a delicate
membrane called the endosteum where bone growth, repair and remodeling
occur.
The periosteum covers the entire outer surface of the bone except where the
epiphyses touch other bones to form joints. In this region, the epiphyses are
covered with articular cartilage, a thin layer of hyaline cartilage that reduces
friction and acts as a shock absorber.
articular cartilage covering the epiphysis of the bone
EPIPHYSEAL PLATE AND LONGITUDINAL BONE GROWTH
Bones grow in length only at the zone between the epiphysis (head of the bone)
and diaphysis (bone shaft). This zone, called the epiphyseal plate, is made of
proliferating cartilage.
It is a layer of hyaline cartilage where ossification occurs in immature bones. On
the epiphyseal side of the epiphyseal plate, cartilage is formed.
On the diaphyseal side, cartilage is ossified, and the diaphysis grows in length.
The epiphyseal plate is composed of the following four zones of cells and activity:
•
The Reserve Zone
•
The Proliferative Zone
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•
Zone Of Maturation And Hypertrophy
•
Zone Of Calcified Matrix
The reserve zone is the region closest to the epiphyseal end of the plate and
contains small chondrocytes within the matrix.
These chondrocytes do not participate in bone growth but secure the epiphyseal
plate to the bone tissue of the epiphysis.
The proliferative zone is the next layer toward the diaphysis and contains stacks
of slightly larger chondrocytes. It makes new chondrocytes to replace those that
die at the diaphyseal end of the plate.
Chondrocytes in the next layer, the zone of maturation and hypertrophy, are
older and larger than those in the proliferative zone. The more mature cells are
situated closer to the diaphyseal end of the plate.
The longitudinal growth of bone is a result of cellular division in the proliferative
zone and the maturation of cells in the zone of maturation and hypertrophy.
Most of the chondrocytes in the zone of calcified matrix, the zone closest to the
diaphysis, are dead because the matrix around them has calcified.
Capillaries and osteoblasts from the diaphysis penetrate this zone, and the
osteoblasts secrete bone tissue on the remaining calcified cartilage. Thus, the zone
of calcified matrix connects the epiphyseal plate to the diaphysis. A bone grows
in length when bone tissue is added to the diaphysis.
Bones continue to grow in length until early adulthood. The rate of growth is
controlled by hormones.
When the chondrocytes in the epiphyseal plate cease their proliferation and bone
replaces the cartilage, longitudinal growth stops. All that remains of the epiphyseal
plate is the epiphyseal line.
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zones of the epiphyseal plate, as seen under a microscope
illustration of zones of the epiphyseal plate
Figure: Spongy bone
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