Ossification

Phylogenetically dual bone development:

Chondrogenic (deeper) and desmogenous (superficial, covering) were often combined to form larger combined units.

Both methods of bone formation were preserved in human embryonic development:

ossification desmogenous = endesmal (in ligament)
chondrogenic ossification - division into two groups according to the place of origin in the cartilage:
- perichondrial − surface ossification from the perichondrium
- enchondral − ossification inside the cartilage

Ossification process[edit | edit source]

osteoblasts − cells from the mesenchyme, produce non-calcified precursors of the ground substance → turn into osteoid by polymerization
- osteoblasts become stuck in this mass
- bone beams − structures that are created by osteoblasts, further increase by apposition
osteocytes − typical bone cells, it arises from immobile osteoblasts (which are stuck in osteoid)
Osteoblasts
osteoclasts - break down bone

Bone growth
- broken bone is replaced by new bone → bone remodeling
- Bone breakdown occurs in two ways. On the basis of the genetic program, the bone is given the desired shape and on the basis of its loading, the internal architecture is formed in the spongiosa.

it replaces the original cartilaginous bone model, which is destroyed by this bone formation

Enchondral ossification

perichondral ossification (in the middle of the length of the bone)
- in the perichondrium, osteoblasts differentiate, which produce the bone mantle (surrounds the body of the future bone)
enchondral ossification − inside the cartilage (changes will take place in the cartilage before that)
- mesenchyme buds with blood vessels penetrate into the cartilage from the periosteum → differentiation of primitive bone marrow cells and osteoblasts (enchondrally form bone tissue and the remains of cartilage are broken down)
- his is how an ossification nucleus is formed inside the cartilage (in the middle of the length of the bone) → middle part of a long bone = diaphysis, diaphysis
  - from the diaphysis, ossification spreads to both ends of the bone

the ends of long bones ossify from separate ossifying nuclei − they are formed enchondrally only
- in cartilage, the same changes as during ossification of the diaphysis, formation of an ossification core → epiphysis, epiphysis − at one or two ends of a long bone
growth (epiphyseal) plates − between the epiphyses and the diaphysis, they are maintained throughout the bone growth (disappears around the age of 18)
- only in them the bone grows in length - the cartilage creates new material
- one of them (proximal or distal) is more involved in bone growth than the other (e.g. proximal is more active in the humerus and lower leg bones, distal is more active in the forearm and femur)
- at the same time, the bone beams break down in the diaphysis → cavitas medullaris, a medullary cavity is created.
growth in thickness − by apposition from the periosteum and partly also from the endosteum
apposition must be supplemented by bone resorption (remodeling)

enchondrally - in the middle of the cartilaginous base
spread of ossification to the bone surface (occurs throughout the growth period)
formation of perichondral mantle lamella (at the end of the growth period) → formation of superficial compact bone

the mucous membrane of the nasal cavity is inserted into some skull bones after birth
the mucous membrane of the middle ear cavity is inserted into part of the temporal bone
the spongiosa of the bone recedes in front of the mucous membrane, the cavity enlarges
with the original cavity (nasal, middle ear) from which the mucous membrane has been removed, the cavity in the pneumatic bone remains connected only by a narrow passage