Welcome to lab classes in basic histology Introducing lab class (in spanish) .

The aim of these lab classes is to provide students with educational resources necessary to acquire basic practical skills of each subject, ie recognize, locate and describe the types of cells, tissues and structures of each organ.

Each of these practical sessions are structured around three main elements:
- Definition of learning objectives.
- Description of structures, tissues and cell types of the system.
- Exercises location of structures / cell types in virtual slide (which is in other link).

LEARNING OBJECTIVES
- Identify the compact bone and spongy bone.
- Identify the growth plate and articular cartilage.
- Identify the bone matrix.
- Identify the osteons and Haversian canals.
- Differentiate between osteoblasts, osteocytes and osteoclasts.

HANDBOOK
Bone is a type of connective tissue where the extracellular matrix is impregnated with calcium and phosphate salts. Main functions of bone are support and protect the body and its organs, and be the major reservoir of calcium and phosphate in the body.

MACROSCOPIC STRUCTURE OF BONE Humerus. General structure of the bone
We identified two different types of bone, compact bone and spongy or trabecular bone. Compact bone Compact bone appears as a solid massa, while cancellous bone Cancellous (trabecular) bone tissue spicules forming a network of trabecular bone or radial spaces delimiting the spinal marrow.
The long bones such as femur, are composed of two distinct parts. On one side we find the shaft, which consists of a long cylinder of compact bone with a hollow inside, called medullary cavity, which is filled by the bone marrow, while we also find two epiphyses, one at each end of the bone, consisting mainly of spongy bone covered by a thin layer of compact bone Compact bone Cancellous (trabecular) bone tissue .
In a growing person, the epiphyses are separated of the diaphysis by the cartilaginous growth plate Endochondral overview formation . Both the growth plate and adjacent cancellous bone are responsible of longitudinal bone growth.
While on the outside the bones are covered by a connective tissue called periosteum Compact bone , except for the top of the epiphysis, where we find the articular cartilage (hyaline cartilage) and the attachment points of tendons; the cavity medular of the shaft and the spaces within the spongy bone are lined by a connective tissue called endosteum Endochondral overview formation , characterized by its osteogenic potential.

MICROSCOPIC STRUCTURE OF MATURE BONE
We identified two types of bone based on the arrangement of mineralized collagen fibers, depending on the degree of bone maturation. In mature bone, which is very resistant to mechanical stress, we find the lamellar bone with a regular arrangement of collagen fibers Osteone . In developing bones are the woven bone, characterized by irregular alignment of collagen fibers Primary formation of osteone . This bone is weak in relation to mechanical stress as it is formed rapidly, but then is replaced by lamellar bone. The woven bone is also formed during repair of a broken bone.
The lamellar bone, the most common in an adult, mineralized bone matrix is deposited in layers or sheets Structure of the osteone . Between the lamellae we found small holes or gaps where they leave the tubules in radial arrangement and which penetrate through the gaps of adjacent plates. In these gaps we find one of the cell types characteristic of bone, the osteocytes.
The laminar bone has four different patterns.
1.- The osteones or Haversian systems, which consists of concentric lamellae around a longitudinal vascular channel Structure of the osteone .
2.- The observed interstitial sheets between the osteones.
3.- The external concentric lamellae are observed in the external surface of compact bone beneath the periosteum.
4.- The internal concentric lamellae are observed on the inner surface of compact bone below the endosteum.
Vascular channels of compact bone present two approaches in relation to laminar structures. We find the longitudinal channels that run through the center of the osteons, called Haversian canal, where capillaries and postcapillary veins run, and transverse or oblique channels called Volkmann's canal that connect the Haversian canals and contain blood vessels that provides from bone marrow and periosteum.

BONE MATRIX
Bone matrix consists of a 35% organic material (osteoid) and 65% of inorganic compounds. The protein is predominantly type I collagen, which in the compact lamellar bone presents a very orderly arrangement in successive layers concentric to the axis of Haversian canal. Found in bone matrix we found others noncollagenous proteins including osteocalcin, osteopontin and osteonectin, all synthesized by osteoblasts.
Inorganic components of bone matrix preferably represented by deposits of calcium phosphate crystals (hydroxyapatite) distributed along the length of the collagen fibers through an assembly process assisted by non-collagenous proteins.

CELLULAR COMPONENT OF BONE Osteoclasts and osteocytes
The actively growing bone has two different cell lines, the line of osteoprogenitor cells, including osteoblasts and osteocytes, and osteoclasts line.
Osteoprogenitor cells with mesenchymal origin, have stem cell properties, as they have proliferative potential and differentiation capacity. These cells persist throughout postnatal life and is reactivated in the adult during the repair of bone fractures and other injuries.
Progenitor cells give rise to osteoblasts by a mechanism regulated by growth factors and others, and are present in the inner layer of the periosteum i endosteum.
Osteoblasts differentiate into osteocytes when trapped inside a gap due to mineralization of the matrix that they produce.

OSTEOBLASTS AND OSTEOCYTES Osteoblasts and osteocytes
Osteoblasts are cuboidal or prismatic epithelial-like cells, which form a monolayer that covers all active sites of bone formation. These cells are highly polarized and that deposit osteoid along the osteoblalsts interface with bone matrix, and initiate and control subsequent mineralization of osteoid.
After completing the bone formation, osteoblasos flatten and become osteocytes, highly branched cells whose cell body occupies bone gaps, and connected together by GAP junctions by cytoplasmic processes that run inside of the tubules. Nutrients diffuse to the osteocytes through the tubules from the blood vessels in the Haversian canals. The dense network of osteocytes is supported by the GAP junctions present in the cell connect and also thanks to the molecules that circulate in the extracellular spaces conetcan gaps between them.
The life of the osteocytes depends on the proper diffusion of nutrients and life of the bone matrix depends on the osteoctros. The osteocytes may remain for years as the vascularization is continuous.

OSTEOCLASTS Osteoclasts
Osteoclasts are large and highly polarized cells that occupy a shallow socket called Howship lacuna. The domain cell in contact of Howship lacuna presents profund folds in the cell membrane, called the ruffled border. While the osteoclast cell is not active the ruffler border disappears and remains in a resting phase.
Osteoclasts derived from monocytes that reach the bone through the bloodstream and fuse to form multinucleated osteoclasts cells up to 30 nuclei, a process regulated by osteoblasts and stromal cells from bone marrow.
Osteoclasts play a role in bone remodeling. This process involves removal of bone matrix in different places, followed by their replacement by new bone matrix by osteoblasts warehouses. Once the osteoclast attaches to bone matrix in response to metabolic demand of calcium of the body, generate an acidic environment necessary for bone resorption, a process that involves first the dissolution of the inorganic components (demineralization) that occurs in an acidic environment, followed by enzymatic degradation of the organic matrix.

EXERCICES
- Localize osteocytes in this virtual microscope slide Pulverized compact bone .
- Localize the Haversian systems and the Haversian canal in this virtual microscope slide Pulverized compact bone .
- Localize osteocytes and osteoblasts in these virtual microscope slides Compact Bone in longitudinal section Compact Bone in transversal section Spongy bone (trabecular) .
- Localize an osteoclasts in this virtual preparation slide Compact Bone in longitudinal section .
- Observe the different structure of bone tissue between Compact Bone in longitudinal section Compact Bone in transversal section and Spongy bone (trabecular) .
- Localize the haversian canals in longitudinal section Compact Bone in transversal section .
- Localize the growth cartilage tissue in this virtual microscope slide Endochondral ossification .
- Localize the chondrocytes in the growth cartilage tissue Endochondral ossification .
- Observe as chondrocytes change their morphologies along the grwoth connective tissue Endochondral ossification .
- Localize osteoclasts in this virtual microscope slide Endochondral ossification .