DEV2022 – LECTURE 5
Two developmental pathways:
It means that there are certain parts of the body that develops the skeletal
system by process 1 and process 2.
The bones of the calvarian (the bones of the skull) plus some aspects of the face
and jaws and the clavicle are developed directly from mesenchymal tissue.
Everything else is developed by an intermeditary tissue which we know as
cartilage. The end result which is mature bone is the same – identical tissue
developed by two completely different tissues.
Bone consists of:
Matrix in which the fibres and crystals and minerals and water can sit
Androgens + Estrogens regulate bone health:
We have a growth spurt at the time of puberty. During puberty, these sex
hormones are produced which causes bone growth. So bone growth and
lengthening is depended upon the secretion of estrogen + testosterone at
puberty. Also responsible for bone development at old age.
Estrogen is withdrawn at menopause.
Although Mesoderm is predominantly responsible for development of skeleton,
the neural crest (ectoderm derived special fourth germ layer) also contributes to
aspects of the cranio-facial skeleton.
Mesoderm at the midline contributes to some parts of the cranio-facial skeleton,
but mostly the appendicular skeleton. The notochord is mesodermally derived, it
is central defining axis at the early embryo. It ends up being trapped into the
frontal, paraxial, temporal, zygoma, maxilla, part of the mandible (developed by
IMO within the mesenchymal process).
Growing fetal skull:
red = growing bone
yellow = connective tissue (mesenchymal tissue)
Those plates – frontal, paratial, part of the exipital, zygoma – transition from
embryo to fetus are pieces of mesenchymal connective tissue. Within them, some
of these Mesenchymal connective tissue. CTs are programmed by gene activity to
turn into bone forming cells – osteoblasts. The gene of the osteoblasts are to
make fine matrix (yellow materials being deposited between them). Immature
bone matrix is weak and poorly mineralized, so they’re called osteoid. Osteiod is
first laid down.
With the passage of time, the osteoid will become mineralized and the primitive bone forming will eventually remodel itself osteons (osteons). The end product
of this ossification is to form strong compact bone on the outside of skull, and
honey-comb on the inside, where bone marrow lives, where all the bone tissue is
Growth of the osteoid means that the fingerlike projections of growing weak
bones will interconnect and form bridges and strut which is what we’ll see in
Another part of the clavicle and mandible relies on cartilage. The development of
occipital bone relies on intermeditary cartilage bone and membrane bone.
As you grow older, the contents inside the skull is resorbed – taken away, and on
the outside of the bone, new layers are added, so it increases in size.
Long bone development:
In this type of development, mesoderm is a starting point in all long bone
We next get the involvement of cartilage. The reason why cartilage is involved is
- Cartilage cells can multiply and proliferate, whereas bone cannot. When you
fracture a bone, the way the bone is replaced is by the intermeditary of cartilage.
- Cartilage secretes a matrix. The matrix of cartilage in long bone growth is the
substrate, scaffold, platform upon which osteoblasts will lay down bone to
manufacture the elongation of bone growth.
The panels of limbs are made of connective tissue – mesenchymal kind. Those
panels have developed a cartilage matrix and eventually the cartilage matrix now
appears to be forming for bone – mesenchyme, cartilage, bone. This explains why
radiographs of children, there are pieces of skeleton missing in various joints –
they have yet to be ossified. Many of those developing bones are cartilage
because they have no yet been developed fully yet.
The developing skeleton has been stained blue. Blue is specific for cartilage.
E10.5 = mesenchymal tissue
E12.5 = it is programmed to form condesnation of this mesenchymal tissue
which resembles the digits. Those primitive digits made from connective tissue
are then stained blue.
So cartilage is the key element in EO.
ossification centre = where bone appears first. When its long bone, it appears in
the middle. Miniature cartilage tissue model:
The hyaline cartilage will be invaded by blood vessels in its wasteful central
region. Those vascular elements will form the first sign of bone formation, so
they will be responsible for the appearance and differentiation of osteoblasts -
the cells that form cell matrix. They begin to appear in the middle of the shaft of
the primitive cartilage. We call this the primary ossification centre where bone is
initially being laid down. Cartilage is acting like a scaffold where the osteoblasts
are secreting their matrix.
Embryonic fetal bone development:
Bone = cartilage
Ossification centre = middle, where in the 2 and 3 picture, it has