Connective Tissue: Bone & Ossification Flashcards
What is the principle connective tissue cell of bone?
The osteocyte. It is the ‘dormant’ version of osteoblasts which build bone. The osteocyte is involved in bone maintenance, and lives in lacunae
What is bone technically and what makes it up?
It is mineralized connective tissue. It is roughly 35-50% ECM, made up of GAG ground substance and Type 1 collagen fibers (very eosinophilic). It is roughly 50-65% hydroxyapatite, the inorganic component of bone.
Can bone grow interstitially?
No, only cartilage can. Although both cartilage and bone exhibit appositional growth
What are the two types of bone tissue?
Compact (dense) - bone with few spaces, forms the outside
Spongy (cancellous, trabecular, medullary) - thin spicules of bone tissue (trabeculae) in a meshwork that forms inside of bone. Contains the marrow and blood vessels
What is a long bone?
Longer in one dimension than another. Has a central marrow cavity and layer of compact bone.
What is a short bone?
Nearly equal in length and diameter, has marrow cavity i.e. carpal bones of hand
What is a flat bone?
Thin and platelike bones with intervening layer of spongy bone, i.e. bones of calvaria (skull). Two layers of relatively thick compact bone
What are irregular bones?
Bones that don’t fit into the other categories, i.e. vertebrae
What are the three major regions of the long bone?
- Diaphysis - middle region
- Epiphysis - End region, expanded
- Metaphysis - between diaphysis and epiphysis
What makes up the diaphysis of a long bone?
The center is generally hollowed out and filled with bone marrow, called the medullary cavity.
There is only a small amount of spongy bone facing the marrow cavity, most of the spony bone is in the epiphysis. Covered with periosteum and endosteum
What makes up the metaphysis and epiphysis?
It has a core of spongy bone. The bone marrow occupies the spaces between the trabeculae of bone in the spongy bone. Outer shell of compact bone is thinner than in diaphysis. The epiphysis is not covered with periosteum wherever the bone articulates in the joint -> it is covered with articular cartilage (hyaline) with no perichondrium
How do you embed the organic components of bone?
Soak in acidic solutions or decalcify with EDTA
Can be stained with H&E once minerals have been removed.
How does organic bone typically stain?
It is eosinophilic because of the large amount of Type I collagen. The osteocytes can be seen trapped inside lacunae
How is an inorganic section of bone prepared?
Bone is dried, chipped, and polished, but not stained. Appears gray or yellow/gold
What is the periosteum and its two layers?
Covers all the external surfaces of the bone except articulating surfaces.
Outer layer is dense irregular connective tissue containing fibroblasts.
Inner layer is in contact with the surface of the bone and contains osteoprogenitor cells which could help start appositional growth
What are osteoprogenitor cells?
The precursors of osteoblasts, which produce bone. When they are in the periosteum, they are also called periosteal cells!
Where do muscles and tendons anchor?
The periosteum
What connects the periosteum to the compact bone?
Type 1 collagen fibers called Sharpey’s fibers
What are endosteal cells?
Osteoprogenitor cells found in the endosteum, can differentiate into osteoblasts. Remember, the endosteum lines all the inner surfaces of bones
What are circumferential lamellae?
Layers of mineralized matrix of compact bone that run around entire bone at inner and outer surfaces
What are osteons?
What limits their size?
They are parallel sheets of mineralized matrix around a central (Haversian) canal. They run parallel to the axis of the long bone. There are typically 5 but up to 20 lamallae (concentric ring layers) around an osteon. Size is limited by diffusion of nutrients and oxygen from the Haversian canal blood vessels
What is the cement line?
Outermost layer of osteon, sometimes visible in ground bone sections as a refractile annulus (or ring)
What are interstitial lamallae?
Fragments of old osteons found between neighboring osteons. Basically, ring shapes without Haversian canals
How to lacunae appear differently in ground bone vs. demineralized sections?
Ground bone: dark against a light background
Demineralized: White spaces with osteocytes sometimes visible within them
What are canaliculi? Where are they best seen?
Canals connecting lacunae together. Each contains an osteocyte process that forms a gap junction with a neighboring osteocyte’s process. Best seen in ground bone sections as dark
How is blood supplied to the compact bone?
Mainly centrifugally (from center outward). However, some blood is supplied by periosteal arteries
How does blood enter the bone?
Through openings called nutrient foramina. The arteries pierce the periosteum and compact bone to enter the marrow cavity
What is a Volkmann’s canal and how can you tell?
They are channels going into compact bone which run perpendicular to Haversian canals. They can be identified because they are not surrounded by concentric lamellae of bones
What is inside a Haversian canal?
Blood vessels, nerves, loose connective tissue, and is lined by endosteum
How does blood exit the bone?
Blood exits the compact bone via Volkmann’s canals through the outer circumferential lamellae and joins the venous system in the periosteum
Can osteons be found in the spongy bone?
Yes, if the spicules of lamellar bone are large enough.
What are the two types of bone marrow and what organizes them?
They are organized by reticular connective tissue.
Red marrow - mainly pressent during development, form red and white blood cells during hemopoiesis
Yellow marrow - contains mainly adipocytes, mostly in adults.
What are three locations of osteoprogenitor cells?
The mesenchyme (not recognizable by H&E) during development, the inner layer of the periosteum, and the endosteum. They function to become osteoblasts
How can osteoblasts be identified?
They are basophilic cuboidal / columnar cells always seen on the surface of bone or bone spicules. It has a basophilic cytoplasm because much rough ER is required for osteoid production
What is osteoid?
Nonmineralized material secreted into extracellular matrix by osteoblasts. It consists of Type 1 collagen, and other proteins including phosphate-containing compounds, osteopontin, osteocalcin, and alkaline phosphatase.
What is the function of osteocalcin and alkaline phosphatase?
Osteocalcin binds Ca+2 in the extracellular matrix, and alkaline phosphatase liberates phosphate ions in the matrix
What is a matrix vesicle and what is its function?
Secreted by osteoblasts by exocytosis, they are small vesicles with osteocalcin and alkaline phosphatase in their membranes. They start precipitating calcium phosphate due to the high surrounding ion concentration, and displace hydroxyapatite (calcium phosphate) crystals across the membrane
How do osteocalcin and osteopontin work together?
They assist in the nucleation and proliferation of hydroxyapitate crystals around type I collagen fibers within the matrix. The matrix vesicles get the precipitation to begin, then these proteins work in the matrix
How does an osteoclast attach to bone?
integrin alpha-v, beta-3 receptor in osteoclast membrane binds to osteopontin in the bone matrix, forming a ring seal which creates a specialized compartment between the surface of the bone and the osteoclast
What is the structure of the osteoclast enclosed by the ring seal?
The ruffled border. It is plasma membrane lying against bone which is deeply infolded
How does an osteoclast break down the mineral component of the bone matrix?
Uses carbonic anhydrase to convert water and CO2 to bicarbonate, and H+. The ruffled border pumps in H+ and Cl- to break down the inorganic material (calcium phosphate) with hydrochloric acid
How does an osteoclast break down the organic material of the bone matrix?
Uses lysosomal hydrolases pumped out of ruffled border, including matrix metalloproteases and cathepsin K
What do you call the depression at the surface of the bone that osteoclasts create?
Howship’s lacunae
What do osteoclasts do with broken down bone?
Transcytose it from ruffled border into surrounding vasculature. Can also use amino acids from degraded collagen
What hormone stimulates osteoclast activity?
Parathyroid hormone, in response to decreased blood calcium
How does parathyroid hormone activate osteoclasts?
PTH binds to osteoblast. Osteoblast increase expression of RANK ligand (RANKL) and secrete macrophage colony stimulating factor (M-CSF). Monocytes are stimulated by M-CSF to become macrophages and fuse to form immature multinucleated osteoclasts. RANK on this cell binds RANKL of osteoblast to complete this differentiation
What inhibits osteoclast activity?
Calcitonin, which is released by C-cells of thyroid in response to high blood calcium, deactivates them
Why is bone well vascularized and innervated?
The solid matrix prevents diffusion of oxygen and nutrients from the blood
What is the initial weak bone structure formed in the process of ossification called?
Woven bone. It is the same for both endochondral and intramembranous ossification
What bones typically use intramembranous ossification?
Flat bones of face and skull, mandible, and clavicle
What are the steps of intramembanous ossification?
Regions of mesenchyme condense and become more vascularized. The mesenchyme differentiates into osteoprogenitor cells, which then differentiates into osteoblasts. Osteoblasts basically proceed as normal, building randomly through appositional growth as the bone spicules grow larger. The woven bone is eventually remodelled into lamellar bone.
What is endochondral ossification?
The model of the desired bone structure is initially formed as hyaline cartilage before being replaced by mineralized bone tissue
What is the first step of endochondral ossification?
Mesenchyme differentiates into chondrocytes to form a “center of chondrification”. The hyaline cartilage is surrounded by perichondrium, and a small cartilage model is completed.
What forms the primary center of ossification and what does it become?
A bony collar is first formed around the midregion of the hyaline cartilage model, as osteoblasts are formed in perichondrium to make a periosteum. This will become the diaphysis.
How do the chondrocytes mineralize the hyaline cartilage in the primary center of ossification?
They hypertrophy. They begin secreting Type X instead of Type II collagen, as well as osteocalcin and alkaline phosphatase, along with VEGF (vascular endothelial growth factor) and metalloproteases (to dissolve cartilage matrix). These cells mineralize cartilage the same way do osteoblasts, and then apoptose.
How is bone formed by the mineralized cartilage matrix?
VEGF secreted by hypertrophic chondrocytes brings in blood vessels carrying cells that will differentiate to osteoblasts. Monocytes are also brought through which form osteoclasts. Two two cell types work together to remodel the bone framework that is already there.
What is the last step of development for the primary center of ossification?
The spongy bone or hollow medullary cavity are completed so hemopoietic cells from the blood can form marrow
Where is the secondary center of ossification and why is it important for joints?
It is in the epiphysis of long bones. It is important for joints because ossification of cartilage stops whenever the spongy bone is fully formed for marrow to come in, and the articulating surfaces remain hyaline (articular) cartilage
What occurs at the epiphyseal plate?
It is the growth plate, where the diaphyseal and epiphyseal ossification centers meet. This is where bones lengthen
What are the five zones of the epiphyseal plate?
- Zone of reserve cartilage - living hyaline cartilage - basophilic. Secrete indian hedgehog (Ihh) to keep next zone dividing, via diffusion
- Zone of proliferation - stacks of dividing chondrocytes
- Zone of hypertrophy - chondrocytes hypertrophy when not stimulated by Ihh. Begin staining paler / white
- Zone of calcification - typically matrix becomes more basophilic when calcified. Lacunae empty
- Zone of resorption - closest to diaphysis, you’ll sometimes see the intersection between acellular basophilic cartilage and eosinophilic bone containing osteoblasts / osteocytes
What stimulates the slowing of growth of long bones?
Increased levels of gonadal steroids (estrogen, testosterone) in adolescence. Epiphyseal plate becomes epiphyseal line
What is a diarthrotic joint?
A synovial joint. Most common in the body
Where are synovial cells and what do they secrete?
There are 1 or 2 layers of synovial cells in the synovial membrane which secrete synovial fluid. This is high in glycoproteins and hyaluronic acid which is very viscous for lubrication. They overly vascular connective tissue.
How is a fracture repaired?
Blood collects at site of fracture and clots, stimulating inflammatory response. Macrophages, WBCs and fibroblasts rapidly invade to form granulation tissue. A fibrocartilaginous callus is first formed, then a bony callus is formed from the degradation of the first soft callus. It is eventually remodelled into lamellar bone
How is a soft callus converted into a bony callus?
A soft callus is made of mostly fibrocartilage. It is ossified by osteoblasts, and trabeculae of bone and endosteum grow on all sides until this hard or bony callus is finally completed (had to be broken down by osteoclasts). It is only temporary though because it is woven bone. Needs to be remodelled
