Histology Chapter 8: Bone

Functions of bone (3)

Provides solid support for the body, protects vital organs in the cranial and thoracic cavities, encloses medullary cavities containing bone marrow where blood cells form

Why is bone called a mineral reservoir?

It stores and releases calcium, phosphate, and other ions in a controlled fashion to maintain constant concentrations in body fluids.

Bones form _____that multiply forces generated by _____ _____ _____ and transform them into movement.

Levers, skeletal muscle contraction

Composition of bone

A specialized connective tissue composed of calcified ECM and three major cell types; osteoclasts, osteoblasts, and osteocytes

Osteoblast function

Form and create new bone tissue, derived from mesenchymal stem cells

Osteocyte function

Act as the skeleton’s security and communication system, maintain the calcified matrix

Osteoclast function

Dissolve and break down old or damaged bone tissue, multinucleated cells derived from monocyte fusion

Products of osteoblasts

Type 1 collagen, proteoglycans and matricellular glycoproteins such as osteonectin, inorganic components of the matrix

Location of active osteoblasts

Bone matrix surfaces, bound by integrins, typically as a single cuboidal layer joined by adherent and gap junctions

Osteoid

Unique collagen-rich material that osteoblasts secrete between the cell layer and preexisting bone surface before calcification

What are the 3 fates of osteoblasts after secretory activity ends?

Differentiate as entrapped osteocytes, flatten as bone lining cells, or most commonly undergo apoptosis

Osteocalcin

A vitamin K dependent polypeptide secreted by osteoblasts that binds calcium ions and concentrates this mineral locally

Matrix vesicles

Membrane=enclosed vesicles rich in alkaline phosphatase, released by osteoblasts, that raise local PO4 concentration (calcium dependent)

Hydroxyapatite crystals

First visible step in calcification that results from the high calcium, high PO4 microenvironment from osteocalcin and matrix vesicles (where hard bone is formed)

Hydroxyapatite crystals frow by _____ and eventually merge into a _____ mass of calcified material embedding the _____ _____.

Accretion, confluent, collagen fibers

Primary bone tumors

Bone cancer that originates directly from bone cells, uncommon (~0.5% of all cancer deaths)

Example of primary bone tumor and what can it arise from?

Osteosarcoma, can arise in osteoprogenitor cells

Metastatic bone tumors

The skeleton is a frequent site of secondary tumors, arising when cancer cells travel via blood or lymph from breast, lung, prostate, kidney, or thyroid malignancies

Formation of osteoblasts

Osteoblasts become surrounded by secreted matrix, differentiate as osteocytes, enclosed in lacunae, extend dendritic processes into canaliculi

Because metabolites cannot diffuse through calcified matrix, exchange between osteocytes and capillaries depends on _____.

Canaliculi

What structure allows osteocytes, osteoblasts, and bone lining cells to physically connect and communicate across the lacunar-canaliculi network?

Gap junctions

What allows osteocytes to detect mechanical load and microdamage, allowing them to trigger remedial activity in osteoblasts and osteoclasts?

Mechanosensing

_____ are normally the most abundant bone cells, with less RER, smaller Golgi, and more condensed chromatin than osteoblasts.

Osteocytes