Interesting and a very fundamental question!

Well, viability of younger cells at earlier stages are a great determinant as you have mentioned. But at certain embryonic stages under the two first weeks, neural cells are not differentiated pretty well in function.

I do not have a confirmed answer to why E18 is preferred over earlier (even E16) or later stages (E18-18.5 depending on the line and species of the rodent and its gestation period). But I think that according to many mouse and rodent embryonic databases, at E18 in mice (Theiler Stage 26), nerve fibers anatomically speaking penetrate the epidermis to commence development of tactile sense.

Besides the formation of long whiskers, eyes are also become visible.

Regarding the fact that mouse gestation period is 20 days, most of the functional and anatomical features of nervous system is completed in terms of cellular differentiation and function. Most notably, at and after E17, Spinal trigeminal nucleus interpolaris (SpVi) may receive GABAergic projection fibers from extra-nuclear area before birth, and GABAergic interneurons may form synapses within the SpVi after E17. In addition, GABA action may gradually shift from excitatory to inhibitory between E13 and E17, which is a fundamental and radical change in function.

However, in terms of neuronal culturing, there are no obligatory rules to just use E18 (which is interestingly more of a fetal stage than embryonic) as far as I know. In hippocampal neuron culturing procedure, isolation is mostly done in both E17-18 and/or P1-2 stages.

But for hippocampal formation though CA regions are not evidently separable yet, they can be identified via IHC for the markers Pou3f1. What is a prominent event during development at E18 is the generation and early phase of differentiation of dentate gyrus granular cells that can be visible under microscope and after culturing (being completely identifiable at postnatal stages though). However, the pituitary and pineal glands are well-differentiated.

Spinal cord also is developed similar to adult animals morphologically.

In cortex (both cerebral and cerebellar), cortical lamination and layer differences become distinguishable (just in a gross neuroanatomical aspect. e.g. no barrel field development until P3).

Also, it seems that tissue handling and fixation is rather simpler in, say, E18 compared to E11.5.

The following paper can help you with further details:

Article Histology Atlas of the Developing Prenatal and Postnatal Mou...

Bahman Sadeghi thank you for your excellent and thorough response.

I isolated cells from floxed ~E14 fetuses, transduced them with Cre-containing AAV, and observed gradual knockdown of our protein of interest up until day 12 post-infection. At this time point, there was a rebound effect, and the protein could once again be detected. Assuming no outside contamination, I was considering that this may be due to eventual fate determination of undifferentiated cells in the culture.

Your point about GABAergic action is also relevant to my research, so thank you for including that gem of knowledge in your response, as well as the useful reference link.

Kelly Reihl thanks for your kind comments.

Interesting! It apparently depends on the stage and region that you isolated cells from. Assuming that you have used AAV9 for instance, it appears that the rebound effect which you have observed after 12 days is reliable and cogent in terms of that protein detection.

I wonder why did not you use cells from E18 fetuses in this case? (Though it also depends on the cell type, differentiation and region that you have isolated the cells from.)

Hello

I agree with Bahman Sadeghi; It's a great answer for your question.

Kindly look at link here

https://www.jneurosci.org/content/13/1/285.long

Article Histology Atlas of the Developing Prenatal and Postnatal Mou...

Article Midbrain dopaminergic neuron fate specification: Of mice and...

Good Luck