Skeletal development of the head

The main reasons for the morphogenetic processes of the skull are the progressive development of the brain, sensory organs and the restructuring of the gill apparatus surrounding the initial sections of the digestive and respiratory systems.

The cranium develops around the developing brain. The brain of the lancelet is surrounded by a thin connective tissue membrane (membranous skull). In cyclostomes (hagfish, lampreys), the cranium is cartilaginous in the area of the base, and the roof of the skull remains connective tissue. In selachians (sharks), the brain is in a cartilaginous capsule. In the visceral skull of selachians, there are 7 pairs of gill arches: the first two pairs are called visceral, the rest are called gill. Sturgeons have placoid scales that develop due to the epithelium of the skin. In bony fish, bone plates are superimposed on the cartilaginous skull and seem to displace it, forming superimposed, or integumentary, bones.

With the emergence of animals on land, the replacement of cartilaginous tissue with bone throughout the skeleton became necessary, since the functions of the skeleton became more complex. The sensory organs and the masticatory apparatus progress in their development, which have a modeling effect on the formation of the skull. In land animals, the gills are reduced, being replaced by respiratory organs - the lungs. The gaps between the gill arches - the gill pockets are preserved only in the embryonic period, and the material of the gill arches goes to the formation of the visceral skull.

Thus, the base of the skull goes through three successive stages of development: connective tissue (membranous), cartilaginous and bone. The visceral skull and individual bones of the cranial skull develop on the basis of the membranous, bypassing the cartilaginous stage. In humans, due to upright posture and lifestyle, the skull has acquired a number of characteristic features:

• the capacity of the cranial cavity has increased significantly;
• the size of the facial (visceral) skull decreased;
• the mass and size of the lower jaw have decreased, which is important for increasing the bite force of the front teeth (when the jaw is shortened) and for articulate speech;
• the large (occipital) opening and the adjacent condyles are shifted forward. As a result, the discrepancy in size (and mass) of the posterior and anterior sections of the head is significantly reduced and greater opportunities for its balance are created;
• the mammillary processes, to which the muscles that turn the head are attached, have reached significant development;
• The ridges and tubercles on the skull are less developed, which is explained by the lesser development of the occipital and chewing muscles.

During phylogenesis, the number of skull bones decreases significantly: some disappear completely, others grow together.

The human cranium develops from the mesenchyme surrounding the rapidly growing brain. The mesenchyme is transformed into a connective tissue membrane - the stage of the membranous skull. In the area of the vault, this membrane is subsequently replaced by bone. The internal relief of the skull with openings is a consequence of the formation of mesenchyme around the developing brain, sensory organs, nerves and vessels. Cartilaginous tissue appears only at the base of the skull, near the anterior part of the notochord, behind the future stalk of the pituitary gland. The areas of cartilage lying next to the notochord are called parachordal cartilages, and in front - prechordal plates and cranial crossbars. These cartilages subsequently grow together into one common plate with an opening for the pituitary gland and with cartilaginous auditory capsules formed around the rudiments of the labyrinths of the organs of hearing and balance. The depression for the organ of vision is located between the nasal and auditory capsules. Subsequently, the cartilage at the base of the skull is replaced by bone, with the exception of small areas (synchondrosis), which persist in adults until a certain age.

Thus, in humans, the vault (roof) of the skull goes through two stages in its development: membranous (connective tissue) and bone, and the base of the skull goes through three stages: membranous, cartilaginous and bone.

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

The first visceral arch (jaw) in humans gives rise to the auditory ossicles (malleus and incus) and the so-called Meckel's cartilage, on the basis of which the lower jaw develops from the mesenchyme.

The second visceral arch (hyoid) consists of two parts - upper and lower. From the upper part the auditory ossicle - the stapes and the styloid process of the temporal bone develop.

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

Development and age-related characteristics of individual bones of the cranial and facial parts of the skull

The frontal bone begins to form in the 9th week of gestation from connective tissue (endesmally), from two ossification points that appear in places corresponding to the future frontal tubercles. In a newborn, this bone consists of two almost symmetrical halves connected by a median suture. Fusion of these halves of the frontal bone occurs in the 2nd to 7th year of life. The rudiment of the frontal sinus appears in the first year of life.

In the sphenoid bone, ossification centers begin to appear in the 9th week of intrauterine development. Most of the bone develops on the basis of cartilage, in which 5 pairs of ossification centers are formed. The most lateral parts of the greater wings and the medial plates of the pterygoid processes (except for the pterygoid hook) are of connective tissue origin. The sphenoid conchae are also of connective tissue origin; they are formed near the posterior parts of the nasal capsules. The ossification centers gradually merge with each other. By the time of birth, the sphenoid bone consists of 3 parts: the central part, including the body and lesser wings, the greater wings with the lateral plate of the pterygoid process, and the medial plate. These parts fuse into a single sphenoid bone after birth, in the 3rd to 8th year of life. In the 3rd year, the sphenoid sinus begins to form in the body of this bone.

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

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

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

The temporal bone develops from ossification points that appear in the cartilaginous auditory capsule in the 5th-6th month of intrauterine life (future pyramid), as well as from the squamous (in the 9th week) and tympanic (in the 10th week) parts that develop through connective tissue. The styloid process develops from the cartilage of the second visceral arch; it receives 2 ossification points (before birth and in the 2nd year of the child's life). The parts of the temporal bone, as a rule, begin to grow together after birth, their fusion continues until the age of 13. The styloid process grows in the 2nd-12th year.

The basis for the formation of the upper jaw is the right and left maxillary processes and the middle nasal processes (frontal process) that fuse with them. At the end of the 2nd month of intrauterine life, several ossification points appear in the connective tissue of the processes. One of them is laid in the part of the future alveolar process that contains the dental alveoli for the incisors. This is the so-called incisive bone. Fusion of bone rudiments, except for the "incisive bone" area, occurs in the intrauterine period. The maxillary sinus begins to develop in the 5th-6th month of intrauterine life.

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

The lower jaw develops from connective tissue around the Meckel cartilage and initially consists of two halves. In each half of the membranous lower jaw, several ossification points appear in the 2nd month of intrauterine life. Gradually, these points grow together, and the cartilage inside the forming bone is resorbed. Both halves of the lower jaw grow together into one bone after birth, in the 1st or 2nd year of life.

In early childhood, when there are no teeth yet, the angle of the lower jaw is obtuse, its branch is short and seems to be bent backwards. At the age of 20-40 years, the angle is close to right, the branch of the lower jaw is located vertically. In elderly people, old people who have lost their teeth, the angle of the lower jaw becomes obtuse, the length of the branch decreases, the alveolar part atrophies.

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

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