Layers imagable with optical coherence tomography
Using optical coherence tomography (OCT) there are 18 layers that can be identified in the retina. The layers and anatomical correlation are as follows:

From innermost to outermost, the layers identifiable by OCT are as follows:

Development
Retinal development begins with the establishment of the eye fields mediated by the SHH and SIX3 proteins, with subsequent development of the optic vesicles regulated by the PAX6 and LHX2 proteins. The role of Pax6 in eye development was elegantly demonstrated by Walter Gehring and colleagues, who showed that ectopic expression of Pax6 can lead to eye formation on Drosophila antennae, wings, and legs. The optic vesicle gives rise to three structures: the neural retina, the retinal pigmented epithelium, and the optic stalk. The neural retina contains the retinal progenitor cells (RPCs) that give rise to the seven cell types of the retina. Differentiation begins with the retinal ganglion cells and concludes with production of the Muller glia. Although each cell type differentiates from the RPCs in a sequential order, there is considerable overlap in the timing of when individual cell types differentiate. The cues that determine a RPC daughter cell fate are coded by multiple transcription factor families including the bHLH and homeodomain factors.In addition to guiding cell fate determination, cues exist in the retina to determine the dorsal-ventral (D-V) and nasal-temporal (N-T) axes. The D-V axis is established by a ventral to dorsal gradient of VAX2, whereas the N-T axis is coordinated by expression of the forkhead transcription factors FOXD1 and FOXG1.
The utterance of "hmm" is key for listeners to understand that the speaker is currently engaged in thought; if the speaker thought silently instead, listeners may be unsure if the speaker had finished their utterance. "Um" and "er" are also used during thoughtful absorption; however, typically the extent of the absorption of thought is more limited since "um" and "er" are usually spoken mid-sentence and for shorter periods of time than "hmm". For this reason, thoughtful absorption is typically associated with the utterance of "hmm". "Huh" – the universal syllable
Research has shown that the word/syllable "huh" is perhaps the most recognized syllable throughout the world. It is an interrogative. This crosses geography, language, cultures and nationalities. See also
References
Further reading
Clark, HH. ; Fox Tree, JE. (May 2002). "Using uh and um in spontaneous speaking" (PDF). Cognition. 84 (1): 73–111. CiteSeerX 10.1.1.5.7958. doi:10.1016/S0010-0277(02)00017-3. PMID 12062148. S2CID 37642332. Archived from the original (PDF) on 2013-11-10. Corley, Martin; Stewart, Oliver W. (2008). "Hesitation Disfluencies in Spontaneous Speech: The Meaning of Um" (PDF). Language and Linguistics Compass. 2 (4): 589–602. doi:10.1111/j.1749-818X.2008.00068.x. hdl:20.500.11820/0e5f2f2f-7383-42c5-a7ba-63f2587ad877. ISSN 1749-818X. Daniel, Ari (5 Feb 2015). "Are we witnessing the death of uh? Um, maybe—and not just in English". Eklund, Robert (2004). Disfluency in Swedish human-human and human-machine travel booking dialogues (PDF) (Thesis). Linköping Studies in Science and Technology Dissertation No. 882 (Corrected ed.). Department of Computer and Information Science, Linköping University. ISBN 91-7373-966-9. ISSN 0345-7524. OCLC 940753621. Retrieved 11 August 2021. Erard, Michael (2008). Um...: Slips, Stumbles, and Verbal Blunders, and What They Mean. Anchor. ISBN 978-1-4000-9543-8.