Sound Development

Features of the larynx at birt

At birth, a baby's larynx is 2 cm long and 2 cm wide, which is about a third of the size of an adult. It is much softer and more flexible than that of an adult. The layers in the vocal cords of adults have not yet formed. There is no difference in the anatomy of the larynx between the sexes at birth

Development of the larynx in terms of voice formation

The development of the larynx in terms of voice formation is divided into three stages: Topographic (descent of the larynx), Morphological (increase in volume and change in shape) and Histological (changes in the lamina propria layers of the vocal cords).

Topographic Development of the Larynx

At birth, the newborn larynx is positioned higher in relation to the palate and mandible than at any other time in life. In infants, the larynx is so high that the epiglottis touches the soft palate, allowing for both breathing and feeding simultaneously. The descent of the larynx begins immediately after birth. Radiographs taken serially on individuals aged between one month and fifteen years have shown that the dimensions of the airway change most significantly during the first year and puberty. The ratio between pharyngeal height and oral cavity length decreases from 1.5 to 1 from birth to around 6-8 years, remaining constant thereafter. During the postnatal development phase, the relative positions of the vocal folds, hyoid body, mandible, and hard palate do not significantly change in relation to each other. The lower boundary of the cricoid cartilage is located at the cervical third to fourth vertebra levels at birth (C3 C4). By the age of five, the larynx descends to the level of C7. Measurements taken between ages 15-20 show that the larynx remains at the C7 level. The descent of the larynx elongates the vocal tract, reduces resonance frequency, and creates the perception of larger body size. Similar findings have been demonstrated in studies conducted on chimpanzees and deer. Research indicates that men's lower-pitched voices are perceived by women as having larger body dimensions. There is a developmental relationship between the descent of the larynx (as the pharyngeal cavity elongates, lower fundamental frequencies gain more resonance) and the average pitch drop. The descent of the larynx is evolutionarily significant for both swallowing and voice production.

Morphological Development of the Larynx

Morphological changes that begin at birth continue throughout life. At birth, the thyroid cartilage and hyoid bone touch each other and move away along the vertical axis. In infants, the epiglottis is elevated and swollen. The aryepiglottic folds are thick. The epiglottis is omega-shaped and rests against the root of the tongue. With growth, the epiglottis expands, hardens, and the omega shape flattens. The angle formed at the front by the thyroid cartilage's plates becomes approximately 90 in males during puberty, while remaining about 120° in females. The increase in the length of the thyroid cartilage is three times greater in boys than in girls. During the first year of life, the entrance to the larynx widens, transitioning from a T-shape to a round-oval shape.

The laryngeal airway is protected by the hard and rounded structure of the cricoid cartilage. If the negative pressure in the vocal tract increases sufficiently, the newborn larynx may collapse due to its soft cartilages and loose ligaments. The laxity of the subepithelial connective tissues and their proportionally high vascularisation increase the tendency for fluid to accumulate. Therefore, the incidence of subglottic or supraglottic obstruction associated with inflammatory oedema is high in infants. The hyoid begins to ossify by the age of two years. The other cartilages begin to harden with puberty, starting with the thyroid cartilage, which is previously hyaline, followed by ossification of the cricoid cartilage. The arytenoid cartilage, which is hyaline and elastic, then ossifies. In males, in the seventh and eighth decades, all laryngeal cartilages ossify, except for the elastic epiglottis, quiniform and corniculate cartilages. In women, complete ossification of the larynx never occurs.

Information about the length of the vocal cords, glottic width and subglottic sagittal and transverse diameters in infancy, adolescence and adulthood can be seen in Table 1. In infants, half of the vocal cords are cartilaginous and half are membrane-like and form a wide glottis, whereas in adults the membrane ratio increases to two thirds, increasing the flexibility of the vocal cords. Glottic width and subglottic diameters increase with age.

Histological Development of Larynx

The most important histological changes occur in the sensitive structures of the vocal cords, which include the thyroarytenoid muscle and mucosa. Until one to four years of age, the mucosa of children is thick and has a smooth structure, lacking the stratification seen in adults. Adult-like differentiation of the lamina propria becomes apparent during adolescence, but histological changes continue throughout life. In the elderly, the lamina propria becomes thickened and oedematous, while the density of elastin and collagen fibres decreases.

Histological changes in the vocal muscle are also important. In newborns, the muscle fibres are thin. They thicken in adulthood, but gradually atrophy after the age of 40. In newborns there are far fewer type 1 fibres (short, fast contraction) than type 2 fibres (prolonged contraction). With growth, type 1 fibres become more predominant and help to better control the voice and produce long-lasting sound.