Development of the skeleton of the head

The main causes of the formation processes of the skull are the progressive development of the brain, sensory organs and reconstruction of the gill apparatus surrounding the initial parts of the digestive and respiratory systems.

The brain skull develops around the developing brain. The brain of the lancelet is surrounded by a thin connective tissue membrane (webbed skull). In the cyclostomes (myxins, lampreys) the cerebral skull in the base of the cartilaginous, and the roof of the skull remains connective tissue. The villagers (shark) brain is in a cartilaginous capsule. In the visceral skull of the selachian, 7 pairs of branchial arches: the first two pairs are called visceral, the others - gill. Sturgeon fish have placoid scales developing due to skin epithelium. In bone-bones, bone plates are superimposed on the cartilaginous skull and, as it were, pushing it out, forming overhead or overcoating bones.

With the emergence of animals on land, the replacement of cartilaginous bone tissue throughout the skeleton became necessary, since the functions of the skeleton became more complex. Progress in their development of the sense organs and chewing apparatus, which have a modeling effect on the formation of the skull. In terrestrial animals, gills are reduced, replaced by respiratory organs - lungs. Slots between gill arches - gill pockets are retained only in the embryonic period, and the material of the gill arches is used to form the visceral skull.

Thus, the base of the skull passes through three successive stages of development: connective tissue (membranous), cartilaginous and bone. The visceral skull and individual bones of the cerebral cranium develop on the basis of the membranous, bypassing the cartilaginous stage. In humans, in connection with the upright walking and lifestyle, the skull acquired a number of characteristic features:

• significantly increased brain capacity of the skull;
• decreased the size of the facial (visceral) skull;
• decreased the mass and size of the lower jaw, which is important for increasing the bite force of the front teeth (with shortening of the jaw) and for articulate speech;
• The large (occipital) opening and the adjacent condyles are shifted anteriorly. As a result, the discrepancy between the size (and mass) of the posterior and anterior parts of the head significantly decreased, and there were great possibilities for its equilibrium;
• considerable development was achieved by the mastoid processes, to which the muscles, which turn their heads, are attached;
• Weakly developed ridges, hillocks on the skull, which is explained by a smaller development of occipital and masticatory muscles.

In phylogenesis, the number of bones of the skull is significantly reduced: some disappear completely, others grow together.

The brain's skull in man develops from the mesenchyme surrounding the rapidly growing brain. The mesenchymal cover turns into a connective tissue membrane - the stage of the membranous skull. In the vault region, this shell is later replaced by a bone. The internal relief of the skull with holes is a consequence of the laying of the mesenchyme around the developing brain, sensory organs, nerves and vessels. Cartilaginous tissue appears only at the base of the skull, near the anterior section of the chord, posteriorly from the future leg of the pituitary gland. The areas of cartilage lying next to the chord have been termed the near-chord (parachordal) cartilage, and ahead of the prechordal plates and skull rails. These cartilages later fuse into one common plate with a hole for the pituitary and with cartilaginous auditory capsules formed around the labyrinths of the labyrinths of the organs of hearing and balance. The deepening for the organ of vision is between the nasal and auditory capsules. Subsequently, the cartilage at the base of the skull is replaced by a bone, with the exception of small areas (synchondrosis) that persist in adults until a certain age.

Thus, in man, the cranial roof (cranium) undergoes two stages in its development: membranous (connective tissue) and bone, and the base of the skull - three stages: membranous, cartilaginous and bone.

The facial skull develops from the mesenchyma adjacent to the primary section of the primary gut. In the mesenchyme between the gill pockets are formed cartilaginous gill arches. Of particular importance are the first two of them - visceral arches, on the basis of which the visceral skull develops.

The first visceral arch (maxillary) in man gives rise to auditory ossicles (a malleus and anvil) and the so-called Meckel cartilage, on the basis of which the lower jaw develops from the mesenchyme.

The second visceral arc (sublingual) consists of two parts - the upper and the lower. The upper part develops the auditory ossicle - the stapes and an anterior styloid process of the temporal bone.

The lower part goes to the formation of small horns of the hyoid bone. Large horns and the body of the hyoid bone are formed from the third arc (I gill). Thus, on the basis of the visceral arches from the connective tissue, small bones of the facial skull and lower jaw develop.

Development and age features of individual bones of the brain and facial parts of the skull

The frontal bone begins to form on the 9th week of the uterine life from the connective tissue (endesmally), from the two points of ossification that appear in the places corresponding to the future frontal tubercles. In a newborn this bone consists of two almost symmetrical halves joined by a median suture. The growth of these halves of the frontal bone occurs in the 2-7th year of the child's life. The ovary of the frontal sinus appears in the first year of life.

In the sphenoid bone, the ossification points begin to appear at the 9th week of intrauterine development. Most of the bone develops on the basis of cartilage, in which 5 pairs of ossification points are formed. Connected tissue origin has the most lateral sections of large wings and medial plates of pterygoid processes (with the exception of the pterygoid hook). Wedge-shaped shells also have connective tissue origin, they are formed near the posterior sections of the nasal capsules. The points of ossification merge with each other gradually. At the time of birth, the sphenoid bone consists of 3 parts: central, comprising the body and small wings, large wings with a lateral plate of the pterygoid process and medial plate. These parts are fused into a single sphenoid bone after birth, at the 3rd-8th year of life. In the 3rd year, a sphenoid sinus begins to form in the body of this bone.

The occipital bone - basilar and lateral parts of it, as well as the lower part of the occipital scales develop on the basis of cartilage, in which one ossification point appears (in each part). The upper part of the occipital scales is formed on a connective tissue basis, in it two points of ossification form on the 8th-10th week. Merging them into one bone occurs after birth, in the 3-5th year of life.

In the parietal bone, which develops from the connective tissue, the ossification point is found at the 8th week of intrauterine life at the site of the future parietal hillock.

Latticed bone is formed on the basis of cartilage of the nasal capsule from 3 points of ossification: medial and two lateral. A perpendicular plate develops from the medial plate, and latitudinal labyrinths from the lateral. The growth of these parts in a single latticed bone occurs after birth (at the 6th year of life).

The temporal bone develops from the points of ossification appearing in the cartilaginous auditory capsule at the 5th-6th month of intrauterine life (the future pyramid), as well as from the scaly (in the ninth week) and drum (in the 10th week) . The styloid process develops from the cartilage of the second visceral arch; he receives 2 points of ossification (before birth and in the 2nd year of life of the child). Parts of the temporal bone, as a rule, begin to fuse after birth, their fusion lasts up to 13 years. The styloid process grows in 2-12 years.

The basis for the formation of the upper jaw is the right and left maxillary processes and the middle nasal processes (the frontal process) that join them. At the end of the second month of intrauterine life, several points of ossification appear in the connective tissue of the appendages. One of them is laid in that part of the future alveolar process that contains dental alveoli for incisors. This is the so-called incisive bone. Growth of bone rudiments, except for the area of the "incisors", occurs in the intrauterine period. The maxillary sinus begins to develop on the 5th-6th month of intrauterine life.

Small bones of the facial skull (palatine bone, vomer, nose, tear, zygomatic) develop from one, two or even three points of ossification in each bone. These points appear in the connective tissue at the end of the 2nd - the beginning of the 3rd month of intrauterine life. The basis for the formation of the inferior nasal concha, as well as the latticed bone, is the cartilage of the nasal capsule.

The lower jaw develops from the connective tissue around the Meccale cartilage and initially consists of two halves. In each half of the membranous mandible, on the 2nd month of intrauterine life, several ossification points appear. Gradually, these points grow together, and the cartilage within the emerging bone dissolves. Both halves of the lower jaw coalesce into one bone after birth, at the age of 1-2 years.

In early childhood, when there are no teeth, the angle of the lower jaw is obtuse, its branch is short and as if bent backward. At the age of 20-40 years the angle is close to the straight, the branch of the lower jaw is located vertically. In elderly people, old people whose teeth have dropped out, the angle of the lower jaw becomes blunt, the length of the branch decreases, the alveolar part becomes atrophic.

The hyoid bone is formed on the basis of the cartilage of the second visceral (small horns) and the third (I gill) arc - the body and large horns. Points of ossification in the body and large horns appear before birth (8-10 months), and in small horns - at the 1st and 2nd year of life. The growth of bone parts in one bone occurs in 25-30 years.

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