The cervix – a developmental biology primer.
The Fallopian tubes, uterus and cervix develop from Mullerian ducts. These are two tubes that, before sex differentiation, run vertically outside of each gonad before redirecting to the middle of the pelvis.
In males, Mullerian ducts are targeted for degeneration by anti-Mullerian hormone (AMH) released by the testes. In females, AMH is absent; thus, these ducts persist and differentiate.
The upper part of each Mullerian duct remains on either side of the developing ovaries, and each becomes a Fallopian tube.
The lower parts of each Mullerian duct merge to become a common channel that will become the uterus and cervix. At the extreme lower end, this merged channel meets the developing urogenital sinus (UGS).
The UGS is an externally-opening cavity in the developing pelvis, into which urinary and genital tracts open. In both females and males, the upper part of the urogenital sinus becomes the bladder.
In males, the lower part of the UGS becomes the urethra. In females, the lower part of the UGS splits to become the urethra and the vagina.
The merged Mullerian channel fuses with the UGS in the region that will become the upper vagina. Thus, female reproductive anatomy is now a connected passage from paired ovaries>Fallopian tubes, into uterus>vagina (via cervix).
The cervix is the region of female anatomy where the fusion of Mullerian channel (uterus) and UGS (vagina) occurs. It creates a “neck” between these two developing organs. It can be felt as a bulge in the upper vagina.
The cervix has a different morphology (cellular composition) to either uterus or the vagina. It is considered a distinct anatomical structure.
It has a unique muscular and fibrous composition, a dedicated branch of the uterine artery and its own nerve supply (albeit limited, and mostly triggering pressure/cramp sensations).
The cervix acts a gateway between the non-sterile vagina and the sterile uterus, and it sheds cells and mucus as part of this "barrier" function.
It facilitates fertilisation by altering mucus consistency during ovulation and contracting during orgasm. When fertilisation is not achieved, it can open slightly to permit menstrual flow.
In pregnancy, it functions to hold baby safely in the uterus. Towards birth, it softens and, ultimately, dilates hugely during delivery.
The cervix is a very important part of female reproductive anatomy. It is not simply the “top of a vagina”.
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A visual aid. 
