Wednesday, February 19, 2014

Facial Development in Utero

Week 10- Continued development of eyelids and ears, facial features are distinct, fingers and toes continue to grow and soft nails form.

This seems like a good time to slightly switch things up a bit and talk about facial features development. Initially I was thinking this would be an easy section to review and cover, well I was wrong. I completely take for granted all that my eyes and ears do on a daily basis, despite their small size they are jam packed with anatomy and physiology. In reviewing the ears I realized it’s a pretty good idea for new parents or parents-to-be to be familiar with the basics of your little ones ears, because you play a large part in the proper development and health of them.

Source: Lennart Nilsson

Let’s start with how the eyes are formed which begin at 9 weeks. Now at this point your baby still looks like an alien, let’s just call it like it is. A really adorable alien, but an alien nonetheless. The eyes start out as little dark spots on the head and have four major players from the embryo contributing to them- the neural tube ectoderm cleverly called neuroectoderm, the neural crest cells, the surface ectoderm, and the mesoderm. It makes sense all of these different layers contribute to the eye, as it is a highly specialized organ. Eye development all begins with outgrowths of the neuroectoderm, a pair of optic vesicles which will eventually become the optic nerves. As these optic vesicles grow they develop a connection to the developing central nervous system. Surface ectoderm thickens to form the lens placode when induced by the optic vesicles. Eventually as both these things grow they will neighbor each other, when this happens the optic vesicle will invaginate and form a cup-like shape, taking angiogenic mesenchyme into its middle. This specialized mesenchyme can make blood vessels and that is just what it does here- converting to the hyaloid artery, which will supply the developing lens. When it is no longer needed it will mostly disintegrate leaving behind a canal in the vitreous humor.  A portion of this embryonic artery will ultimately become part of the adult ophthalmic artery of the retina. The optic cup formed by the optic vesicle develops two distinct layers- the inner layer will become the neural component of the retina this has the light-sensitive elements, photoreceptors, as well as neurons and the outer layer which contains the pigmented epithelial layer. The mesoderm surrounding this cup will develop into the uvea, cornea, and sclera. The lens placode will invaginate forming the lens vesicle. Eventually it will detach from the surface ectoderm and drop into its final position. Ultimately the outermost clear layer, the cornea, will focus light coming into the eye while the iris, the pigmented part, will regulates how much light is let in. The pupil is a hole in the center of the iris that depending on its size lets in varying amounts of light. The sclera is the white part of the eye (I honestly don’t think I ever knew its name). The lens is located just beyond the iris and pupil, it functions to focus light onto the back of the eye, where the retina is located. The retina has neural capabilities and it converts light into electric impulse which are carried by the optic nerve to the brain. 

Your baby’s eyelids are made from the surface ectoderm around the time the eyes start to develop. Shortly after forming the eyelids fuse shut and will remain shut until about 26 to 27th weeks. At this time the baby can open and close them at will. While they are fused shut the baby is able to detect light. By the 28th week eyesight improves.  

To move onto the ears, I mentioned above parents contribute to the development of the baby’s hearing, this starts when the baby is still in the womb. Also most people associate infancy and childhood with ear infections, so knowing the basics of ear anatomy and physiology should help you in case your baby gets ear infections (I will probably spend some time in another post talking about ear infections). The ear is segmented into three section- the outer ear (auricle and the external auditory meatus), the middle ear (tympanic membrane, more commonly known as the eardrum, and the ossicles) and the inner ear (also known as the labyrinthine, contains the semicircular canals, vestibule, and the cochlea). The external auditory meatus is the outside portion of the auditory canal, the opening really. The auditory canal leads to tympanic membrane which connects to the inner ear. The inner ear is a maze of bone covered membranes filled with endolymph. The middle ear conducts and amplifies sound via bone through transference of sound waves to the fluid in the inner ear and from there the inner ear converts the acoustic energy to electrochemical energy for the CNS. In addition to playing a huge role in hearing, the inner ear deals with equilibrium, that is to say balance. 


I got sucked into this great research article by Steven Graven and Joy Browne ‘Auditory Development in the Fetus & Infant’ the articles focuses on how a baby in utero at 25 weeks (or in the NICU) requires auditory stimulation in order to properly develop the key components of the auditory system. The authors explain that while the anatomy of the ear develops in the first 20 weeks, the physiology of it- the neurosensory component of the auditory system develops mostly after 20 weeks. This component is critically developing from about 25 weeks to 5 to 6 months after birth. 28 to 30 weeks is a critical time for the developing system, as the hair cells of the cochlea, the axons of the auditory nerve, and the neurons of the temporal lobe are fine tuning based on what the baby hears going on outside of the womb. This tuning requires voice, language, music, and meaningful background noise to be able to discriminate between varying levels of sound. The hair cells of the cochlea contain stereocilia which as their name suggests function to amplify and convert sound into electric signals to be passed onto the auditory nerve, and so on to the auditory cortex (located next to the temporal lobe).  The fetus can learn the mother’s voice, as well as basic music and common background noises. The authors make it clear this auditory stimulation is required for the little one to distinguish between pitch, intensity, pattern and rhythm. Towards the end of its time in the womb the baby can even distinguish between different emotions and mood based on the sounds it hears. I guess to some extent I knew about this idea before becoming pregnant, since I have heard of parents reading to the baby while the baby was still incubating in there. I just didn’t realize that hearing sounds in its outside environment is critical to the developing of the auditory components. Really fascinating stuff and relevant to know for the greater good of your little one.

Moving on to the overall development-the outer ear, the auricle, is derived from six surface hillocks, which early on look like lumps of clay around an opening (the external meatus) and as time passes they define into the various structures of the outer ear. These surface hillocks come from the first and second pharyngeal arches, shown below as '1' and '2' in the first image. The six hillocks surrounding the primitive ear opening can be seen to the far right, upper corner, as well as what they become in the adult ear below it.

Ok let’s recap the above images- first off, see I told you, your baby looks like an alien. Actually if I overthink about it maybe our preconceived idea of what an alien looks like comes from in utero imaging of baby humans. (Did I blow your mind there or what?) Second these images are really cool because they point out something else I read and was skeptic of until seeing it- the ears actually start down near the ‘neck’ or where the neck will be and then move up. Lastly these pictures show the early development of the eye as well.
The external auditory meatus comes from the first pharyngeal cleft or arch. The pharyngeal arches come from mesoderm, there are six in total and they come about around the fourth week on either side of the developing pharynx (which we haven’t gotten to yet). Each of the six will become a distinct group of muscles, arteries, nerves, and skeletal tissue in the face and neck region. The first arch, among non-ear stuff, gives rise to the a tympanic muscle called the tensor tymphani muscle as well as the two of the three ossicles of the middle ear- the malleus and incus (the second arch gives rise to the third middle ear bone- the stapes). The tympanic membrane comes from ectoderm, endoderm, and mesenchyme.
When talking about the development of the face in general, including the nose and mouth, the overall pattern is similar to the way the outer ear developed. There are lumps of cells, in this case all called ‘prominences’ and they arrange themselves around the embryonic mouth, the stomodeum. Included in these prominences are ones that will give rise to the nose- the lateral and medial nasal prominences as they are called. The lateral gives rise to the sides of the nose, go figure, while the medial will develop into the nasal septum. These groups of cells come about from the various sections of the developing brain. The pharyngeal arches we mentioned above in discussing ear development are relevant to the face in general as they give rise to various bones, muscles, arteries, and nerves of the face including the jaw bone and the muscles responsible for chewing. Week 4 up through week 7 mark big chances for the face, these prominences organize themselves into the major parts of the face. Nasal pits are developed from the nasal cell groupings, and change name again to nasal placode as they continue to take shape. These will ultimately become the olfactory epithelium of the nose. The developing nose shifts to a more middle location of the face and the tip of the nose is visible in profile by week seven. After week seven things start to slow down a bit for the face- there is modest progress up until birth, involving the proportional changes and positioning of the facial features. An interesting point which makes sense upon hearing it, is the brain dictates much of the face’s outward appearance. As the brain grows it forces the forehead outward, moves the eyes to either side, and causes the ears rise up.
Pretty much every image depicting nasal development in utero is creepy and makes your baby-to-be look like her or she is auditioning to be one of the three little pigs. So I went with this illustration because beggars can’t be choosers.


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