1. “Introduction to the Human Skeletal System – Bone Histology”
Laboratory Exercise
“Introduction to the Human Skeletal System – Bone Histology”

2. Human Skeletal System - Introduction
In this exercise, you will examine the histology of bone, describe and identify bones according to their shapes, describe the gross anatomy of a typical long bone and learn the markings found on bones.
These terms should be familiar to you: spongy or compact bone, trabeculae, osteons, central canal, lamellae, lacunae, [bone types] – (long, short, flat, irregular, sesamoid, sutural), diaphysis, epiphyses, epiphyseal line, marrow, [bone markings] – foramen, fissure, meatus, sinus, fossa, alveolus, sulcus, head, facet, condyle, epicondyle, process, trocanter, tubercle, tuberosity
This laboratory exercise correlates with Chapter 6 of your textbook.

3. What procedures are we doing?
Examine the histology of compact bone
Observe different bone shapes
Study the anatomy of a long bone
Examine different bone markings

4. Resources available…
Anatomy and Physiology Laboratory Study Pages; submenu: Skeletal

5. Laboratory Lecture

6. An Introduction to the Skeletal System
The Skeletal System includes:
Bones and Cartilage
Ligaments and Connective Tissues
Adipose (fat)
Vasculature (blood)
Nervous tissue

7. Functions of the Skeletal System
The Skeletal System has six primary functions
Physical support (framework using long bones)
Storage of minerals (calcium and phosphate)
Storage of fat (yellow marrow)
Blood Cell production (red marrow – hematopoetic)
Protection (organ guardianship using flat bones)
Leverage (force of motion using long bones)

8. Bone (Osseous) Tissue
Bone is a dense, supportive connective tissue that is mostly composed of a (98%) solid extracellular matrix with several types of (2%) specialized cells

9. Components of the Solid Matrix
Calcium Salts are an inorganic component of the solid matrix
Two thirds of the bone matrix is calcium phosphate
Calcium phosphate reacts with calcium hydroxide and calcium carbonate to form crystals of hydroxyapatite, Ca10(PO4)6(OH)2
other minerals are also reactive in this matrix (fluoride, potassium, magnesium), which can pose health-related concerns (radioactive isotopes)
The crystals are very hard and resist compression, but they are inflexible and can be broken with a side-way impact
Collagen fibers are the organic component of the solid matrix
One third of the bone matrix is collagen
They are flexible and tough; they can be twisted and bent
But they offer little resistance to compression
Together, calcium salts and collagen fibers interlock with each other, giving bone its strength, flexibility and shatter-resistant properties…

10. Bone-specific Cell types
Bone Cells make up only 2% of bone mass
Bone contains four types of cells
Osteoprogenitor (osteogenic) cells
Osteoblasts
Osteocytes
Osteoclasts

11. Osteoprogenitor cells are mesenchymal stem cells that divide and differentiate to produce osteoblasts
They are located in endosteum, the inner cellular layer of periosteum (which is the outer cellular layer of bone)
These cells assist in fracture repair

12. Osteoblasts are immature bone cells that secrete compounds which form the osteoid matrix (osteogenesis)
The osteoid matrix is fluid; it is not yet calcified to form bone
Osteoblasts that are surrounded by bone become osteocytes

13. Osteocytes are mature bone cells that maintain bone matrix
Osteocytes reside in lacunae between layers (lamellae) of the matrix; they are connected by cytoplasmic extensions through canaliculi
They do not divide, but help maintain and repair damaged bone

14. Osteoclasts secrete acids and protein-digesting enzymes that dissolve the bone matrix and release stored minerals (osteolysis)
They are giant, multinucleate cells (many nuclei per cell)
Derived from stem cells that produce macrophages

15. the great balance: homeostasis
Bone building (by osteoblasts) and bone recycling (by osteoclasts) must balance  homeostasis!
More breakdown than building, bones become weak
Exercise, particularly weight-bearing exercise, causes osteoblasts to build bone

16. Classification of Bones
Bones are classified by their:
Shape
Bone Markings (surface features)
Internal tissue organization (compact or spongy)

17. 1. There are six types of bone shape
Short bone: small & thick
Carpal and tarsal bones
Flat bone: thin & plate-like
Bones of the cranium, sternum, scapula and ribs
Long bone: long and thin
Bones of arm, leg, hand, feet, fingers and toes
Irregular bone: complex shapes
Bones of the spinal vertebrae and face
Sesamoid bone: small & flat
Bones wrapped inside of a tendon – near joints of the hand, knee (patella) and feet
Sutural bone: small & irregular
Found between cranial bones

18. …a little more about the structure of long bones
Structure of a Long Bone
Diaphysis (pr. di-AF-i-sis)
Is the “shaft” of a long bone
Consists of a heavy wall of compact bone (or dense bone), and
A central cavity that is filled with yellow bone marrow in adults (medullary, or marrow, cavity)
Epiphyses (pr. ē-PIF-i-ses)
Are the two wide parts at each end that articulate with other bones
They are covered with a compact bone cortex, and
A central cavity that is mostly spongy bone (cancellous or trabecular) with a red bone marrow filling
OF NOTE: Articular cartilage covers parts of the epiphyses that articulate with other bones
Metaphyses (pr. me-TAF-i-ses)
Are where diaphysis and epiphyses meet
It has a cartilagenous epiphyseal plate (pre-pubescent) which is the site of expansive bone growth (genesis)

19. …a little more about the structure of flat bones
Structure of a Flat Bone
Resembles a sandwich of spongy bone that is between two layers of compact bone
Rich in red bone marrow
Within the cranium, the layer of spongy bone between the compact bone is called the diploë (pr. dip-low-ë)
An example is the parietal bone of the skull

20. A word about bone marrow: soft connective tissue
Adult Bone
Marrow Distribution
Red bone marrow
Contains hematopoietic (hemopoietic) stem cells for the production of red blood cells, white blood cells and platelets
It is used in bone marrow transplantation (commonly pulled from the hip bone of the donor)
Children contain mostly red bone marrow in the medullary cavity & epiphyses of long bones and spongy bone of flat bones
In adults, red bone marrow is restricted to mostly flat bones – the skull, vertebrae, ribs, sternum and ilium

21. Yellow bone marrow is not hematopoetic
Adult Bone
Marrow Distribution
Yellow bone marrow
Is widespread in adults and consists of fat (adipose) cells that store energy reserves:
It is found in the medullary cavity and epiphyses of long bones and in the spongy bone of flat bones
It is not found in skull flat bones, the vertebrae, ribs, sternum and ilium, the proximal epiphysis of the humerus and the femur
Yellow bone marrow is not hematopoetic

22. 2. Bone Markings Bone markings refer to surface features of bone.
They help in naming parts of bone, and being landmarks for physicians and scientists to locate internal structures
Depressions, grooves or tunnels run along and into bone
These indicate conduits for vascular and nervous tissue
Elevations or projections form out from the bone
Where tendons and ligaments attach (very strong!)
Where adjacent bones articulate (at joints)

24. 3. Internal Tissue Organization (compact or spongy)
Compact Bone composes 80% of bone
It is dense, supporting body weight and movement
The osteon is the basic unit of compact bone:
Osteocytes reside in a lacunae cavity and are arranged in concentric lamellae, which surround a central canal that contains blood vessels and nerves
Perforating canals are perpendicular to the central canal and carry blood vessels into the bone and the marrow
Canaliculi (shown in later slide) are narrow passageways containing cytoplasmic extensions of osteocytes; they connect osteocytes to each other and to the vasculature
Circumferential and Interstitial lamellae are wrapped around the long bone or between the osteons
Circumferential
lamellae
Osteon
Interstitial
lamellae
Concentric
lamellae
Central
canal
Perforating
canals

25. NOTE: the criss-crossed orientation of collagen fibers (strength) and the canaliculi networking of osteocytes from the central canal

26. NOTE: Look at how few lacunae (the residence of osteocytes) there are in the osteon! Remember that mature bone only has a cellular content of about 2% (the rest is mineral and collagen)!

27. Trabeculae of spongy bone Canaliculi opening on surface
Spongy Bone composes 20% of bone; it is lighter than compact bone
Spongy bone does not have osteons, it is composed of an open network of trabeculae that can withstand stress from many directions
Concentric lamellae are present, but are not arranged as osteons
The trabeculae have no blood vessels, they are fed by canaliculi
The space between the trabeculae is filled with red bone marrow, blood vessels and nerves
Forms red blood cells by the process of hematopoesis
Supplies nutrients to the osteocytes in the lamellae
Trabeculae of spongy bone
Canaliculi opening on surface
Concentric
Lamellae

28. Except for joint cavities, compact bone is covered with a membrane of periosteum
The outer layer is fibrous, dense irregular connective tissue
It provides protection and repair to bone
Anchors blood vessels and nerves
Serves as an attachment site for ligaments and tendons
The inner layer is a cellular layer containing cells (osteogenic, osteoblasts and osteoclasts)
Sharpey’s Fibers (perforating fibers) are collagen fibers that anchor the periosteum to bone and with fibers of joint capsules, tendons and ligaments

29. The endosteum in an incomplete cellular layer that lines the medullary (marrow) cavity of compact bone
It covers the trabeculae of spongy bone and lines central canals
It contains osteoblasts, osteogenic cells and osteoclasts
It is active in bone growth and repair

30. Extra material…

31. Types of Bone Fractures
Fractures are cracks or breaks in bones that are caused by physical stress, and are named according to their external appearance, location and the nature of the crack or the break in the bone
The broadest general categories are:
Closed (simple) fractures that are completely internal and can only be visualized by x-rays
Open (compound) fractures that project through the skin, which can complicate the condition with secondary infection
Many fractures fall into more than one category, because terms overlap…

32. Transverse fractures break a bone shaft along its long axis
ulna

33. Displaced fractures produce new and abnormal bone arrangements.
(Nondisplaced fractures retain the normal alignment)

34. Compression fractures occur in vertebrae subjected to extreme stresses
Compression fractures occur in vertebrae subjected to extreme stresses. (like falling on your backside in an abrupt “sitting” manner)

35. Spiral fractures are produced by twisting stresses that spread along the length of the bone.
tibia

36. Epiphyseal fractures tend to occur where the bone matrix is undergoing calcification and the chondrocytes are dying. Clean transverse fractures heal well (for obvious reasons) if treated properly.
femur

37. Comminuted fractures shatter the affected area into many bony fragments.
femur

38. Greenstick fractures are a partial fracture, where one side of the shaft is broken and the other side is bent. This happens frequently in children.
radius

39. Colles fractures are a break in the distal portion of the radius
Colles fractures are a break in the distal portion of the radius. (typically caused by extending your hand to cushion a fall)

40. Pott’s fractures occur at the ankle and affect both bones of the leg.
(this is a common sports injury)