Histology

Question 1

What are the histological characteristics of cartilage?

Answer 1

Modified CT
Firm, gel-like ground substance
Not penetrated by blood vessels (a vascular)
Covered by perichondrium

Question 2

What are the characteristics of perichondrium?

Answer 2

It is a fibro-cellular membrane which consists of two layers:
-An outer fibrous
-An inner vascular and cellular layer

Functions of perichondrium:
-Growth of cartilage at its periphery (appositional)
-Nourishment of Chondrocytes

Inside of joint cavities, the articular cartilage is devoid of perichondrium and is nourished by diffusion of oxygen and nutrients from the synovial fluid

Question 3

What are the components of cartilage?

Answer 3

1) the extra cellular matrix:
Responsible for firmness and flexibility.
It is produced by cartilage cells composed of:
-Intercellular ground substance
-Fibres: mainly type II collagen fibrils. It may contain also type I collagen or elastic fibres

2) Cartilage cells: Chondroblasts and chondrocytes

Question 4

What are the characteristics of chondroblasts?

Answer 4

Site: inner cellular layer of perichondrium

Shape:
-flattened oval cells
-basophilic cytoplasm (high in RER/protein)
-oval nuclei

Functions:
-they secrete the cartilage matrix (appositional growth)
-they become imprisoned inside the lacunae forming chondrocytes

Question 5

Describe the different transformations of chondrocytes within the lacunae

Answer 5

1) young chondrocytes:
-Found underneath the perichondrium
-flat elliptical
-their long axis parallel to perichondrium

2)mature chondrocytes:
-deeply situated within the matrix
-spherical in shape

3)cell nests:
Form from mature chondrocytes undergoing mitosis

Question 6

What are the characteristics of chondrocytes?

Answer 6

Origin: from chondroblasts

Function:
-they maintain the cartilage matrix
- they undergo mitosis (interstitial growth) thus the cartilage expands from within

Question 7

Name the three types of cartilage

Answer 7

Hyaline cartilage
Elastic cartilage
White fibrocartilage

Question 8

What are the characteristics of hyaline cartilage?

Answer 8

-smooth and firm
-covered by perichondrium
-gains nutrition from perichondrium
-extracellular matrix:
Pale basophilic with glassy, translucent appearance
It is homogenous with no apparent fibres
-chondrocytes:
Widely-scattered
Numerous
Cell nests present
1-8 chondrocytes per lacunae
-sites of hyaline cartilage:
1)cartilaginous skeleton in the embryo
2) epiphyseal plates in growing age
3) costal cartilages
4) nose, larynx trachea and bronchi
5) covers the articular surfaces of nearly all synovial and cartilaginous joints

Question 9

What are the characteristics of elastic cartilage?

Answer 9

-very flexible
-bear mechanical stress without permanent deformation
-covered by perichondrium
-gains nutrition from perichondrium
-matrix:
Fresh state (yellowish)
Large number of interlacing elastic fibres
-demonstrated in histological sections by orcein and VVG stains
-cells:
Numerous and crowded
1-3 chondrocytes per lacunae
-sites of elastic cartilage:
1)the auricle of the ear (ear pinna)
2)external auditory meatus
3)small cartilages in larynx
4)epiglottis

Question 10

What are the characteristics of white fibro cartilage?

Answer 10

-great strength combined with flexibility and rigidity
-not covered by perichondrium
-nutrition: from the surrounding dense CT of the joint capsule and ligaments
-extracellular matrix:
Fresh state (white)
H&E stain (thick acidophilus bundles)
-cells:
Few and rounded
Lie in rows between the collagen bundles
1-2 chondrocytes per lacunae
-sites:
1) the intervertebral disc
2) the symphysis pubis
3) the tempero mandibular joint

Question 11

How is muscular tissue classified

Answer 11

1) skeletal muscle: striated and voluntary
2) cardiac muscle: striated and involuntary
3) smooth muscle: not striated, involuntary and visceral

Question 12

What are the characteristics of cardiac muscle fibres under a light microscope?

Answer 12

-Long cylindrical fibres which branch and anastomose extensively
-same section in different directions
-fibres show fainter striations
-each fibre is composed of several short columnar cells called cadriomyocytes
-each cardiomyocyte has a single oval central nucleus
-cardiomyocytes are joined end to end at the intercalated discs

Question 13

What are the characteristics of skeletal (striated) muscles?

Answer 13

Skeletal: It’s contraction moves the skeleton
Voluntary: it contracts according to will
Striated: under the microscope, the muscle fibres show transverse dark and light bands

Question 14

How do skeletal muscles appear underneath a light microscope?

Answer 14

Long cylindrical discs parallel to each other
Multinucleated; the nuclei are elongated, oval and are peripherally situated lying under the sarcolemma
The sarcophagi is acidophilic: it is filled with numerous longitudinally oriented myofibrils

Question 15

Differentiate between the 2 types of myofibrils (thick myosin and thin actin)

Answer 15

Thick myosin:
-extends from one end of the A band to the other with free both ends
-M-line = T.S filaments keep myosin in place

Thin actin:
-Attached to Z line and extends in ton the margins of H-bands
-Interdigitate with part of myosin in peripheral A -band

Question 16

What are T-tubules of the skeletal muscles?

Answer 16

Tubular invaginations that encircle each myofibril
Found at the A-I band junction

Question 17

Describe the sarcoplasmic reticulum

Answer 17

A branching network of SER surrounding each myofibril
Contains:
-Sarcotubules which is a network around the myofibrils that is dilated at the A-I junction forming:
-Terminal cisternae which are two terminal cisternae located at the level of the A-I junction and separated by a T-tubule (triad)
Each sarcomere in the skeletal muscle is encircled by two triads

Question 18

What is the function of T-tubules?

Answer 18

They regulate Ca ion concentration Worthing the myofibrils
Liberation of Ca =sliding of actin over myosin filaments (contraction occurs)
Storage of Ca =prevents interaction between actin and myosin (no contraction)

Question 19

What are the histological components of a bone?

Answer 19

Bone covering
Bone matrix
Bone cells
Bone lamelllae

Question 20

Describe the histological structure of primary bones (immature bone)

Answer 20

-It is an immature bone first formed during fetal development and bone repair after injury

-it has abundant osteocytes and irregular bundles of collagen, which are later replaced and organised as secondary bones except in certain areas

• the mineral content of primary bone is also much less than that of secondary bone

Question 21

Describe the histological structure of secondary bones

Answer 21

-It is mature bone composed of parallel or concentric bony lamellae of collagen
-osteocytes in their lacunae are dispersed at regular intervals between, or occasionally within lamellae
-osteocytes communication: canaliculi, housing osteocytes processes, connect neighbouring lacunae with one another, forming a network of inter communicating channels that facilitate the flow of nutrients, hormones, ions and waste products to and from osteocytes. In addition, osteocytes processes within these canaliculi make contact with similar processes of neighbouring osteocytes and form gap junctions, permitting theses cells to communicate with each other.
- Because the matrix of secondary bone is more calcified, it is stronger than primary bone.

Question 22

What are the types of secondary (lamellar) bone?

Answer 22

1) compact bone; in diaphysis of long bones, outer tables of flat bones (clavicle)

2) Cancellous bone; in epiphysis of long bones and central part of flat bones

Question 23

What are the types of bone coverings?

Answer 23

The periosteum:
The outer fibrous layer helps distribute vascular and nerve supply to bone, whereas the inner cellular layer possesses osteoprogenitor cells and osteoblasts

The endosteum:
Surrounds the bone marrow cavity. It is formed only of a cellular layer

Question 24

Describe the histological structure of compact bones

Answer 24

Types of lamellae:
-Outer circumferential lamellae: are deep to the periosteum, forming the outermost region of the diaphysis and contain sharpeys fibres anchoring the periosteum to the bone
-Inner circumferential lamellae: are analogous to but not as extensive as outer circumferential lamellae and completely encircle the marrow cavity.
-Osteons (haversion canal systems): forms the bulk of compact bone; each system is composed of 4-20 cylindrical lamellae concentrically arranged around a vascular space known as the haversion canal that contains a neuronas cular bundle. Each haversion canal is lined by a layer of osteoblasts and osteoprogenitor cells.
-Interstitial lamellae: remnants of osteons remaining as irregular arcs of lamellar fragments
-Volkmanns canals: connect haversion canals of adjacent osteons to each other. These vascular spaces are oriented oblique to or perpendicular to haversion canals

Question 25

Describe the structure of bone lamellae

Answer 25

Cylindrical in shape (round outline)
Parallel to the long axis of the bone
Each lamella is formed of osteocytes and collagen bundles

Question 26

Describe the histological features of cancellous (spongy) bones

Answer 26

Sites:
-epiphysis of long bones
-the space between the outer and inner tables of flat bones

Formed of randomly oriented and branching bone trabeculae, surrounding multiple bone marrow cavities

Within bone trabeculae, collagen bundles and osteocytes are regularly arranged in parallel layers. No haversion systems are observed within cancellous bone

Question 27

Describe the histogenesis of bone

Answer 27

1) Intramembranous ossification:
-Most flat bones are formed by intramembranous bone formation. This process occurs in a richly vascularised mesenchymal tissue, whose cells contact each other via long processes. Then, mesenchymal cells differentiate into osteoblasts that begin secretion of bone matrix in the form of bone trabeculae.

• the initially formed bone is the primary immature woven bone with irregularly oriented collagen bundles.

2) Endochondral ossification:
-It takes place in most of long and short bones. It occurs on a template of hyaline cartilage

-It takes place in the diaphysis of long bones (primary Center of ossification) and in the epiphysis (secondary Center of ossification)

Question 28

What are the events that occur in diaphysis?

Answer 28

It involves the transformation of the perichondrium into periosteum. As a result of increased vascularity, chondrogenic cells differentiate into osteoblasts, which starts the deposition of subperiosteal bone collar

Consequently the hyaline cartilage model is deprived of blood supply because it is surrounded by the bone collar, resulting in degeneration of hypertrophied chondrocytes and leave empty lacunae.

Next oseteoclasts make an opening in the bone collar, thus allowing entry if a vascular bud that is formed of boood capillaries and osteoprogenitor cells.

Osteoprogenitor cells differentiate into osteoblasts that start bone formation on the calcified cartilage matrix. Next, calcified cartilage matrix and calcified bone matrix are reserved by osteoclasts with formation of the central bone marrow cavity.

Finally continuous thickening of the subperiosteal bone collar occurs in the diaphysis that eventually replaces the entire cartilage

Question 29

What is the difference between diaphysis and epiphysis

Answer 29

The same events occur but in epiphysis, no bone collar formation occurs

Question 30

What are the steps that occur in bone fracture healing?

Answer 30

Bone fracture involves tears in the periosteum and endosteum, bone matrix loss and possible displacement of broke bone ends

Stages of bone healing after a fracture:
1) hematoma and inflammation:
-hemorrhage occurs at the site of the fracture due to damaged blood vessels resulting in formation of a blood clot or hematoma.

-the blood clot will be invaded by new blood capillaries and fibroblasts from the surrounding connective tissue and start forming granulation tissue within 7-14 days

-inflammation takes place with invasion of macrophages that remove dead cells and damaged tissue

• the area of bone matrix around the zone of the fracture, on both sides, is deprives of its blood supply with consequent death of osteocytes and loss of bone tissue.

-periosteum and endosteum at the site of fracture are highly vascular tissues, thus more resistant to degeneration. Osteoprogenitor cells in both layers start to proliferate.

2) Callus stage:
-First, proliferation of osteoprogenitor cells in the endosteum and mesenchymal stem cells from the bone marrow, differentiate into chondroblasts with subsequent formation of soft callus (internal callus) of white fibrocartilage like tissue at the area of damage (within 2-3 weeks)

-at the same time, the proliferating osteoprogenitor cells in the deeper layer of the periosteum, close to the bone, differentiate into osteoblasts (bone forming cells) because they exist close to well vascularised tissue and start formation of primary (woven) bone. On the other hand, the proliferating osteoprogenitor cells in the central zone are exposed to limited blood supply and oxygen tension, thus differentiate into chindroblasts and form areas of hyaline cartilage. As such, an external callus is formed of both hyaline cartilage and woven bone.

-the soft callus is essential for stabilisation if the fracture bone ends

-Bony callus formation (1-4 months):
The fibrocartilaginous callus is converted into a bony callus of spongy bone. It takes about 2 months for the broken bone ends to be firmly joined together after the fracture. Primary bone formation is initiated by:
A) intramembranous ossification through the osteoprogenitor cells in the periosteum
B) intracartilaginous ossification
Thus, histological examination of a repairing fractured bone reveals areas of cartilage together with areas of intramembranous and endochondral ossification. As repair proceeds a hard bone callous is formed. It is made up of irregular bone trabeculae of primary bone that unites the extremities of the fractured bone.

3) bone remodelling (4-12 months):
The bony callus is then remídeles by osteoclasts and osteoblasts, with excess material on the exterior of the bone and within the medullary cavity being removed by osteoclasts. Compact bone is added to create bone tissue that is similar to the original, unbroke bone