Microscopic observation of the bone structure allows us to recognize:
- a fibrous or non-lamellar bone tissue
- and a lamellar type bone tissue.
The fibrous bone tissue, or woven fibers, is an immature bone and is normally found in the embryo, in newborns, in metaphyseal areas (see below) and during the healing of fractures. Once laid, the fibrous tissue is readily reabsorbed and replaced with lamellar-type bone tissue.
Under the microscope, the fibrous bone tissue appears as a series of fibers intertwined in the three dimensions of space in an almost random manner. The meshes of this "three-dimensional web" consist of large collagen fibers of considerable thickness (5-10 μm in diameter).
The non-lamellar bone is, on the whole, more elastic and less consistent than the lamellar one, due to the lower quantity of minerals and the lack of a preferential orientation of the collagen fibers.
The lamellar bone tissue forms the mature bone that derives from the remodeling of the fibrous or pre-existing bone tissue. Compared to the previous one, it is a more organized tissue, with an ordered and parallel orientation of the collagen fibers, which are arranged in overlapping layers, called bony lamellae.
Between a lamella and the other, small communicating spaces between them: the gaps, host the cells that, by means of a system of canaliculi, come into contact with the areas of the bone from which they can receive nutritive materials.
Between the two types of tissue, lamellar is the most widespread and constitutes almost all of the compact bone and a good part of the spongy bone.
The two types of bone tissue (lamellar and non-lamellar) are distinguished by the arrangement of collagen fibers, ordered in the first type and not ordered in the second.
In adults, all bone is lamellar; we find the non-lamellar type during ossification or during fracture repair.
Lamellar bone is in turn divided into spongy bone and compact bone. The basic composition is the same but their three-dimensional arrangement is different. This diversification makes it possible to optimize the weight and bulk of the bones according to the various stresses to which they are subjected.
SPONGEOUS OR TRABECULAR BONE
Presence of trabeculae
The spongy bone is found mainly in the inner part of the bones, at the level of the short bones, of the flat bones and of the epiphyses of the long bones.
The trabeculae, variously oriented and intersected between them, delimit cavities, called medullary cavities, which contain red marrow (hematopoietic) and yellow (fat).
The spongy fabric gives the bone lightness, thanks to its alveolar structure, e
This type of bone is more abundant in the spine, ribs, jaw and wrist. It constitutes only 20% of the skeletal mass, but represents the most active metabolic component.
COMPACT BONE OR CORTIC BONE
Presence of osteons
The compact bone forms the outer (more superficial) portion of the short bones, flat bones and long bones; it also constitutes the diaphyses of the latter. It is a hard, solid, compact bone, precisely, because it has no macrospically evident cavities; small channels are reserved for blood vessels, cells and their processes, necessary to keep it alive.
It constitutes 80% of the skeletal mass. It presents itself with the lamellar structure organized in osteons .
OSTEONS are the structural units of compact bone.
Inside them, the bone cells (osteocytes) are distributed in cavities shaped like a biconvex lens called bone gaps. The most evident characteristic of the osteone is given by the presence of columns of lamellas (from 4 to 20) concentric that delimit a central channel. Inside this channel, known as Havers, nerves and vessels, both blood and lymphatic, run.
Taken together, lamellas and canal form the Haversian system (synonymous with osteone). The various systems communicate with each other (anastomosis), with the medullary cavity and with the free surface of the bone through the channels arranged transversely and obliquely, called Volkmann channels.
In the periosteum we recognize two types of channels:
- Longitudinal (of Havers) in which the blood capillary flows.
- Transversal (by Volkmann): they come from the periosteum and from the endostium and they end up in the longitudinal ones.
The compact bone gives rigidity, hardness and resistance to mechanical stress.
Most compact bone is located at the level of the long bones of the lower and upper limbs.
