Fetal development and variations in the cartilages surrounding the human external acoustic meatus

Anatomy and Development of the Mammalian External Auditory Canal: Implications for Understanding Canal Disease and Deformity

The mammalian ear is made up of three parts (the outer, middle, and inner ear), which work together to transmit sound waves into neuronal signals perceived by our auditory cortex as sound. This review focuses on the often-neglected outer ear, specifically the external auditory meatus (EAM), or ear canal. Within our complex hearing pathway, the ear canal is responsible for funneling sound waves toward the tympanic membrane (ear drum) and into the middle ear, and as such is a physical link between the tympanic membrane and the outside world. Unique anatomical adaptations, such as its migrating epithelium and cerumen glands, equip the ear canal for its function as both a conduit and a cul-de-sac. Defects in development, or later blockages in the canal, lead to congenital or acquired conductive hearing loss. Recent studies have built on decades-old knowledge of ear canal development and suggest a novel multi-stage, complex and integrated system of development, helping to explain the mechanisms underlying congenital canal atresia and stenosis. Here we review our current understanding of ear canal development; how this biological lumen is made; what determines its location; and how its structure is maintained throughout life. Together this knowledge allows clinical questions to be approached from a developmental biology perspective.

Development and growth of auricular cartilage and muscles: A study using human fetuses

OBJECTIVES

The auricle is a key target in pediatric plastic surgery and is considered to develop from a ring- or funnel-like arrangement of six hillocks in the embryo. However, there has been no report showing the morphologies of the auricular muscle and cartilage after midterm in humans.

METHODS

We examined histological sections of 20 near-term human fetuses (29-40 weeks) and those from 7 midterm fetuses (15-16 weeks).

RESULTS

At midterm, the auricular cartilage was a single wavy plate with the helicis major muscle (HMM). The superior and posterior auricular muscles (SAM, PAM) were inserted into the middle parts, and the anterior auricular muscle (AAM) was inserted into the lowest part of the cartilage plate, while the tragus and antitragus were not clearly identified. In near-term fetuses, the cartilage plate varied in size and shape between specimens. The scapha and antihelix were separated from the cartilage plate with major or minor involvement of the HMM from the initial mass along the helix. The SAM inserted to the crus helix or the developing scapha, while the insertion sites of the AAM and PAM into the helix were stable. The tragus-antitragus cartilages were well-developed and they sandwiched a deep notch of skin below the helix tail. The antitragicus muscle was more evident than the tragicus muscle. An unnamed muscle was evident along the external acoustic meatus. The other intrinsic muscles, including the transverse and oblique muscles, might develop from the HMM after birth.

CONCLUSIONS

Development of the auricle was advanced after midterm. However, a single wavy plate-like cartilage was maintained until late-stage. Near term, the antihelix and scapha developed from the plate-like core of the auricle and the tragus and antitragus were added in the antero-inferior side of the cartilage plate. Establishment of muscle arrangements was markedly delayed compared to cartilage development. Altogether, the classical concept of an initial funnel-like arrangement of cartilage anlagen might have been biased by studies of adult morphology.

Low-set ears and associated anomalies in human foetuses

OBJECTIVES

To determine the prevalence of low-set ears (LSE) in a group of human foetuses, to analyse the associated anomalies, and to review the development mechanisms possibly involved.

METHODS

A total of 1759 human foetuses from spontaneous abortion were evaluated. Foetuses were obtained from the Foetuses and Embryos Collection of the Embryology Department of the Faculty of Medicine of the Autonomous University of Nuevo León. The Ethics Committee gave its approval for this study (EH-230-16). The position of the auricles was determined according to the standards recommended by the Elements of Morphology. Two study groups were created: foetuses with LSE and foetuses with normal ears. In both groups, a detailed examination of the external morphology was performed, followed by thoraco-abdominal micro dissection. Statistical analysis was performed.

RESULTS

Two hundred two of the foetuses presented LSE (1148 per 10,000). In this group, 68.8% did not present associated anomalies, while 31.2% had an associated anomaly. The most frequently affected organ was the heart (53.6%), followed by the digestive tract (23.9%), urinary system (16.9%), head and neck (4.2%), and limbs (1.4%). In the group of foetuses with normal ears, only 7.4% of the specimens had associated anomalies, which was a significant difference compared with the LSE group.

CONCLUSIONS

Based on the obtained results, we consider that LSE can be used as a sensitive indicator of major anomalies. It is recommended to include a systematic assessment of the position of the auricles in the initial clinical evaluation of any newborn.

Expression of carbonic anhydrase IX in human fetal joints, ligaments and tendons: a potential marker of mechanical stress in fetal development?

Carbonic anhydrase type IX (CA9) is known to express in the fetal joint cartilage to maintain pH against hypoxia. Using paraffin-embedded histology of 10 human fetuses at 10-16 weeks of gestation with an aid of immunohistochemistry of the intermediate filaments, matrix components (collagen types I and II, aggrecan, versican, fibronectin, tenascin, and hyaluronan) and CA9, we observed all joints and most of the entheses in the body. At any stages examined, CA9-poisitive cells were seen in the intervertebral disk and all joint cartilages including those of the facet joint of the vertebral column, but the accumulation area was reduced in the larger specimens. Glial fibrillary acidic protein (GFAP), one of the intermediate filaments, expressed in a part of the CA9-positive cartilages. Developing elastic cartilages were positive both of CA9 and GFAP. Notably, parts of the tendon or ligament facing to the joint, such as the joint surface of the annular ligament of the radius, were also positive for CA9. A distribution of each matrix components examined was not same as CA9. The bone-tendon and bone-ligament interface expressed CA9, but the duration at a site was limited to 3-4 weeks because the positive site was changed between stages. Thus, in the fetal entheses, CA9 expression displayed highly stage-dependent and site-dependent manners. CA9 in the fetal entheses seemed to play an additional role, but it was most likely to be useful as an excellent marker of mechanical stress at the start of enthesis development.

Expression of carbonic anhydrase in the fetal eye and extra-ocular tissues

Carbonic anhydrases (CAs) plays a critical functional role in the ciliary body and retina for maintenance of microenvironment. With immunohistochemistry using orbital contents from 8 human fetuses (12-16 weeks of gestation), we examined expressions of CAs isozymes-1, 2, 3, 6, 7 9 and 12 and found strong reactivity of CA9 in extra-ocular fibrous tissues in the anterior and posterior eyes. CA9 is known to express in the fetal joint cartilage to maintain pH against hypoxia: actually, in the present specimens, the SO pulley and its tendon was strongly positive for CA9. The CA9-positive anterior fibrous tissues were positive for smooth muscle actin and connected the orbital aspect of the 4 rectus muscle with the palpebral conjunctiva, whereas the posterior tissue was negative for smooth muscle actin and corresponded to the lateral insertion tendon of the orbitalis muscle. The anterior CA9-positve tissues seemed to correspond to the primitive form of the sleeve and pulley system. Any of matrix substances (collagen types I and II, aggrecan, versican, fibronectin, tenascin and hyaluronan) displayed a distribution pattern specific for the CA9-positive fibrous tissues. Therefore, whether or not CA9 was positive in the fibrous tissue seemed not to depend on the tissue components such as the extracellular matrix and intermediate filaments but to suggest a stressful condition such as hypoxia, unsuitable base balance and/or under mechanical stress.