Ontogenetic Development Auditory system development

Ontogeny: the series of steps that ultimately lead to the mature state. Starts from one cell (a fertilized ovum) that becomes billions of cells organized into a fully functional, complex organism.
Related terms: Ontogeny, embryology, and developmental biology are closely related and sometimes used interchangeably.
Key aspects of ontogeny:
- Morphogenesis: development of form and shape of an organism
- Tissue growth
- Cellular differentiation

Videos linked for optional viewing (soundless):
- https://www.youtube.com/watch?v=1zpV5rzWXMA
Additional videos uploaded to Blackboard:
- Development of inner ear: Embryology of the Ear I (Easy to Understand) – YouTube (≈16 minutes)
- Watch at least the first 6 minutes
- Development of outer and middle ear: Embryology of the Ear II (Easy to Understand) – YouTube (≈14 minutes)
- Start at 1:52
- Development of Pharyngeal Arches (head and neck embryology):
- https://www.youtube.com/watch?v=l5iGrX5hIvM

Early formation (before the neural tube closes):
- By 5\,\text{weeks} (≈35\,\text{days}), two small spots appear on either side of the embryo's head that will become the inner ears.
- These spots fold inward to form sacs beneath the skin; sacs lengthen into a tube; the cochlea (main organ of hearing) takes shape.
Ossicular development (middle ear):
- By around 8\,\text{weeks}, ossicles begin as small clumps of tissue that gradually harden over the following weeks.
- A tube-like cavity forms around these structures to become the middle ear.
Hair cells and neural connection (12 weeks): early cochlear hair cells develop and connect to a nerve that transmits sound impulses to the brain.
By around 22\,\text{weeks} post-conception: fetal hearing of internal sounds (mother’s body) begins, with sounds growing louder as development progresses.

By 22\,\text{weeks} post-conception: fetus can faintly hear sounds inside the mother (breathing, heartbeat, digestion).
By 23\,\text{weeks}: fetus can hear sounds from the outside world, but early sounds are muffled by maternal skin, uterine wall, and amniotic fluid. (only low-pitched sounds at first = male voices are clearer)
By around 26\,\text{weeks}: fetus may begin to respond to sounds with changes in heartbeat, breathing, or movement; loud noises may startle.
By 32-35\,\text{weeks}: middle ear cavity, outer ear canal, and the external part of the ear are fully formed; fetus is ready to listen and respond to sounds at birth.

Newborns can remember some sounds heard in the womb:
- Infants show a clear preference for their mother’s voice over others.
- There is evidence that newborns recognize music heard repeatedly in utero.

20 weeks (anatomical): Nervous system; Cochlea; OHCS develop; energy demands increase.
- Functional: Fetal response (to sounds).
30 weeks (anatomical): Myelination and proliferation of synaptogenesis in the nervous system; Cochlea lateral subsurface cisternae relatively mature.
- Outer/Middle Ear changes: Tympanic membrane (TM) area increases; Ossification of the ossicles; TM structure develops.
- Outer Ear: Size increases.
- Functional: Newborn response; signal detection begins to improve (e.g., responds to specific frequencies).
40 weeks (anatomical): Continued maturation of the auditory structures; tympanic membrane and ossicles more mature; outer ear size larger.
- Functional: Improved frequency responsiveness; thresholds and sensitivity approach later stages of development.

Fetal response to sound becomes more robust as gestation progresses.
At birth, the infant shows adult-like responsiveness trends but with higher thresholds than adults.
Typical newborn auditory thresholds: elevated by ≈ 70\text{ dB}; mature adult-like responsiveness requires a decline in thresholds by roughly 30\text{ dB} relative to neonatal values.
Frequency responsiveness progression:
- Prenatal phase: Responds to low frequencies (e.g., 250\,\text{Hz} and 500\,\text{Hz}) initially.
- Neonatal phase: Responds to higher frequencies (e.g., 1\,\text{kHz} and 3\,\text{kHz}) as maturation proceeds.
By the time of typical birth, the ear and brain have matured enough to begin addressing a wider range of frequencies, with continued development over the first years of life.

After birth, hearing continues to develop across childhood.
The auditory system is not considered fully mature until around 14\text{ to }15\text{ years} of age.
Conceptual takeaway: Prenatal auditory experience lays a foundation for later language and music processing, with early preference for母親の声 and potentially recognizable in-utero music exposure.

Hearing begins in utero; by 22/23\,\text{weeks post conception} a fetus can hear external sounds with muffling from maternal tissues.
By 35\,\text{weeks}, the inner ear and related structures are sufficiently mature to discriminate some sounds, including the mother’s voice.
Postnatally, hearing continues to develop through infancy and childhood, with full maturation around the teenage years (approximately 14\text{ to }15\text{ years}).
The developmental timeline integrates anatomical maturation (nascent cochlea, ossicles, TM, ear canal) with functional milestones (fetal and newborn responses, frequency discrimination, and threshold changes).