Although the foundations of laryngeal morphology have been established in the period of organogenesis, recent studies have shown that significant aspects of maturation also occur during critical fetal periods (8 weeks at birth) and postnatal development.
Of particular importance in this period are maturational events that include changes in laryngeal position in relation to contiguous structures of the aerodigestive tract.
In most mammals, the larynx at all levels of postnatal development, measured from the upper end of the epiglottis to the lower end of the cricoid cartilage, corresponds to the level from the basioccipital or upper margin first vertebra (C1) to the third or fourth cervical vertebra (C3-C4). This laryngeal position corresponds to the high position of the hyoid bone, tongue and pharyngeal constrictors.
For example, the tongue at rest lies almost entirely in the oral cavity, with no portion taking part in any anterior portion of the pharyngeal wall. Thus, the supralaryngeal region of the pharynx is remarkably small, with little or no oral component. Another peculiarity is that the epiglottis borders or directly overlaps the soft palate. The epiglottis can then pass behind the soft palate to open directly into the nasopharynx, creating what is often called the intranasal larynx. This configuration provides an airway directly from the outside through the nostrils and through the nasal cavity to the nasopharynx, larynx, trachea, and lungs. Fluids, and even some semisolid material, can pass to either side of the larynx and nasopharynx via the isthmus of the jaws. Although many studies have pointed to the high position of the infant larynx, only recently have insights begun to identify the late fetal period as the crucial time for defining the position of the larynx sufficient to allow the epiglottic-palatal area overlap. In fact, recent studies have shown that fetal life is a critical period for the development of future upper respiratory and digestive tract structures and functions. The second trimester (13-26 weeks of gestation), in particular, has been shown to be a period of intense developmental activity. Studies have shown that by week 15 of development, earlier than previously reported, the epiglottis is already present, indicating that an epiglottic sketch may be visualized earlier than classically estimated. During this period, the larynx lies high in the neck, in the region generally corresponding to the area (from the epiglottic tip to the lower edge of the cricoid) from the level of the basiocciput to the third cervical vertebra. At 21 weeks, the epiglottis is found to be almost in apposition to the uvula of the soft palate. Between 23 and 25 weeks, the epiglottis and soft palate are found to overlap for the first time, thus providing the previously described 'interlocking'.
The establishment of this anatomical relationship allows the creation of essentially separate respiratory and digestive pathways that will function as such in the newborn. Recent ultrasound investigations have shown patterns of upper respiratory activity strongly indicative of an initial '2-pipe' operating system functioning even before birth, i.e., the larynx remains highly positioned and intranasal during fetal swallowing movements. Thus, this period between 23-25 weeks may reflect a critical time in the development of the entire upper respiratory tract. This may be very significant, as much as how the shape of the basicranium has been shown experimentally to be related to a direct relationship to the position of the larynx in the neck. Important changes also occur in the lower respiratory tract during this same period of fetal life, for example, the maturation of the pulmonary glandular epithelium occurs during this period.
This epithelium is responsible for the production of surfactant in the fetal lung, a substance that has been shown to be essential for independent respiratory function. The simultaneous occurrence of these phenomena suggests that the timing of upper and lower respiratory tract maturation is closely linked. Understanding these stages with increasing precision may be of great importance for the intervention and treatment of premature infants.
Infants and toddlers until about 1 1/2 to 2 years of age continue to maintain a high larynx. As already explained, its location runs from the basioccipital - C1 and extends to the upper margin C3-C4 in infants, descending slightly to the level between C2-C5 at about 2 years of age. The tongue at rest lies entirely within the oral cavity.
The maintenance of the high laryngeal position in infants and young children allows for the existence of widely separated air and digestive pathways similar to those described in other mammals. These pathways may allow the infant to breathe and swallow some fluids almost simultaneously. Because of this position of the larynx, infants breathe through the nose. From a linguistic point of view, the high position of the larynx strongly influences the ability to modify sounds, severely limiting the range of sounds that can be produced. Although the larynx remains high until about the second year, the first episodes of oral breathing, occur within the first half year of life. The period between 4 and 6 months in particular may represent a crucial phase of aerodigestive tract activity. At this time, the neuromuscular control mechanisms of the larynx and pharynx are beginning to change and the first real structural 'descent' of the larynx has also occurred. This transition period may indicate a time of potential respiratory instability due to the transition from one respiratory rhythm to another. It should be remembered that this is also a time of remarkable maturation in the central nervous system itself. The combination of nervous system maturation and development in respiratory patterns can predispose the child to a number of developmentally related problems. The onset of sudden infant death syndrome (SIDS or crib death), for example, may be related to these early changes in the upper airway and subtle changes in larynx position and central and peripheral neuromotor control of the larynx.
The postnatal descent of the larynx into the neck is one of the most characteristic aspects of human ontogeny. Its permanent descent, which appears to begin in the second year, results in a larynx with a position corresponding to the level between the upper border of C3 and the lower border of C5 in a 7-year-old child and to the lower border of C3 or upper part of C4 to the upper border of C7 with adulthood. This change in larynx position dramatically alters our breathing and swallowing patterns. The permanently lowered larynx also produces an enlarged supralaryngeal area of the pharynx. This resulting pharyngeal expansion, in turn, allows greater modification of sounds produced to the vocal cords. The descent of the larynx thus provided the ability to produce fully anatomical articulate speech.
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