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By the end of this section, you will be able to:
  • Discuss the two types of embryonic bone development within the skull
  • Describe the development of the vertebral column and thoracic cage

The axial skeleton begins to form during early embryonic development. However, growth, remodeling, and ossification (bone formation) continue for several decades after birth before the adult skeleton is fully formed. Knowledge of the developmental processes that give rise to the skeleton is important for understanding the abnormalities that may arise in skeletal structures.

Development of the skull

During the third week of embryonic development, a rod-like structure called the notochord    develops dorsally along the length of the embryo. The tissue overlying the notochord enlarges and forms the neural tube, which will give rise to the brain and spinal cord. By the fourth week, mesoderm tissue located on either side of the notochord thickens and separates into a repeating series of block-like tissue structures, each of which is called a somite    . As the somites enlarge, each one will split into several parts. The most medial of these parts is called a sclerotome    . The sclerotomes consist of an embryonic tissue called mesenchyme, which will give rise to the fibrous connective tissues, cartilages, and bones of the body.

The bones of the skull arise from mesenchyme during embryonic development in two different ways. The first mechanism produces the bones that form the top and sides of the brain case. This involves the local accumulation of mesenchymal cells at the site of the future bone. These cells then differentiate directly into bone producing cells, which form the skull bones through the process of intramembranous ossification. As the brain case bones grow in the fetal skull, they remain separated from each other by large areas of dense connective tissue, each of which is called a fontanelle    ( [link] ). The fontanelles are the soft spots on an infant’s head. They are important during birth because these areas allow the skull to change shape as it squeezes through the birth canal. After birth, the fontanelles allow for continued growth and expansion of the skull as the brain enlarges. The largest fontanelle is located on the anterior head, at the junction of the frontal and parietal bones. The fontanelles decrease in size and disappear by age 2. However, the skull bones remained separated from each other at the sutures, which contain dense fibrous connective tissue that unites the adjacent bones. The connective tissue of the sutures allows for continued growth of the skull bones as the brain enlarges during childhood growth.

The second mechanism for bone development in the skull produces the facial bones and floor of the brain case. This also begins with the localized accumulation of mesenchymal cells. However, these cells differentiate into cartilage cells, which produce a hyaline cartilage model of the future bone. As this cartilage model grows, it is gradually converted into bone through the process of endochondral ossification. This is a slow process and the cartilage is not completely converted to bone until the skull achieves its full adult size.

Questions & Answers

yellow marrow has been identified as
Raychelle Reply
Which of the following accurately describe external resipration
Gwendolyn Reply
from the heart to the lungs
I think it's not outside organ of respiratory all respiratory organ are inside of human body
diffusion of CO2 and oxygen at a pulmonary capillary surrounding an alveolar sac.
In other words, from the external environment to the lungs (alveoli) then to the pulmonary capillary then to the heart. So this is a stage of inhalation. inhale = external respiration.
What kind of discussion
horyaal Reply
what is the function of the placenta
Nchimunya Reply
The placenta acts to provide oxygen and nutrientsto the fetus, whilst removing carbon dioxide and other waste products.
Also it's the barrier through which the mother is connected to the fetus.
I want to discuss... atherosclerosis.., everything about it, about to treatment n prevention at age 50 +
Please participate in discussion
ok let's discuss now
first define the word artherosclerosis
a disease of the arteries characterized by the deposition of fatty material on their inner walls.
hardening of the arteries, due to fats..
Q = which type of fat utilized for this.., I.e LDL, HDL, TG, VLDL...?
Atherosclerosis is a condition of deposition of plaque inside the artries
Plaque include such as fat, chalestrol, calcium etc
Thank you kumar...., is there any way that we can protect these plaques without any medicines., I. e exercises n food stuffs
go to the gym
what is chylomicrons?
how can plaque buildup in The angina or vessels ?
Atherosclerosis is a condition of deposition of plaque inside the artries
atherosclerosis can also be caused by tortuousness of arteries with old age as contributing factor.
You tube Prof Fink...he is an amazing lecturer and does a brilliant job on arteriosclerosis
what are cell
Chigozie Reply
The basic structural and functional unit of any living thing. Each cell is a small container of chemicals and water wrapped in a membrane. 
cell is the structural and functional basic unit of life
So every living thing was Created From a Cell
from a fusion of two cells , the sperm and the egg
What is the only bone that doesn't have any articulation?
that magical fusion of cells
why body immune system attack and destroy the body own cell during type 1 diabetes?
Sanamacha Reply
It's an autoimmune disease... targeting the pancreas
what are the three genetic defects of pregnancy?
Belinda Reply
structure of fallopian tubes
Akash Reply
what z the role played by the transport system
zinitha Reply
how fats are digested in the human body
Nabukwasi Reply
Fat digestion begins in the stomach but some argue in that it starts in the mouth. Reason is because the sublingual gland secretes an enzyme called lingual lipase. However, this enzyme is not activated until it comes into contact with gastric fluids (HCl). In the stomach, HCl breaks down the lipid..
due to body heat
into smaller molecules. Going from a triglyceride and a fatty acid to a monoglyceride and a a fatty acid no longer bound to one another. This is known as lipolysis.
After lipolysis in the stomach from gastric and lingual lipase, an acidic chyme is produced after stomach churning the bolus. The chyme exits the stomach at the pyloric sphincter and enters the first section of the small intestine known as the duodenum.
in the duodenum. An alkaline mucus from goblet cells neutralizes the acidic chyme to prevent acid burns. After that, the pancreas and gallbladder secrete a number of enzymes to continue lipolysis. Bile from the gallbladder enters the duodenum via common bile duct. The acinar cells in the pancreas...
secretes pancreatic lipase after enteroendocrine cells in the duodenum secrete a stimulator hormone called CCK. Cck stimulates bile synthesis and secretion as well as pancreatic lipase.
Bile emuslifies the lipid, allowing the lipases to continue lipolysis
this breakdown continues until it reaches the jejunum of the small intestines. At this point, the lipid has been broken down small enough to absorbed into the blood stream. So villi in the jejunum, absorb the contents.
ileum, the last small intestine region, absorbs anything that wasn't absorbed previously. Like minerals, vitamins, bile salts, water soluble material. Villi here complete that task. Fatty acid and glycerol however, are absorbed by lacteals. small lymph vessels. And are transported to the liver.
That concludes lipid digestion. Anything else that remains is deficated after it travels through the large intestines.
parents with blood group AB & 0,,,what will b the blood group of their offspring
what are the different branches of anatomy
hopefully that helped.
which ion is low of blood level?
Ezra Reply
what is coagulation?
feng Reply
liquid turning to solid... blood clots.
coagulation : liquid blood into blood clots caused with a coagulant.
when the blood turn from liquid form to solid
it said to coagulate by the action of active plasma protein called *fibrin*
I.e liquid inform of blood when to solid
it is the process by which blood becomes more viscous or becomes thick
cloting of blood cells
clot of blood
the process of forming semi solid lumps in a liquid
conversion of blood to solid state
semi solid., rather than solid form
what is pivot functioned
to less thefriction
how to calculate the micrograph
Ampong Reply
it can be used to knw the disease condition
Akbar Reply
which gland secret tears
lacrimal glands
explain the blood supply to the brain
Brenda Reply
There are two paired arteries which are responsible for the blood supply to the brain; the vertebral arteries, and the internal carotid arteries. These arteries arise in the neck, and ascend to the cranium.
two arteries main vertebral arteries & internal carotid artery

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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