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The rhombencephalon (plural: rhombencephalons or rhombencephala), or hindbrain is a primary vesicle of the neural tube.
The metencephalon goes on to form the pons, the majority of the cerebellum and the fourth ventricle. The myelencephalon becomes the medulla oblongata, and the central canal 1. The most rostral portion of the cerebellum receives contributions from the mesencephalon, and the floor of the fourth ventricle is derived in part, from the myelencephalon.
At week 22, the typical shape of the brainstem is recognisable, however, the cerebellum does not reach its final configuration until well after birth 2.
The most common structural disorder of hindbrain development is the Chiari malformation.
The embryonic/fetal rhombencephalon is visible with endovaginal ultrasound at ~8-10 weeks as a hypoechoic region in the posterior embryonic/fetal head. Strictly speaking, this is the rhomboid fossa, part of the rhombencephalon complex rather than the rhombencephalon itself 5. The hypoechoic region represents the developing rhombencephalon/hindbrain (medulla, pons, and cerebellum).
This is a normal structure and is reportedly visible in all embryonic/fetal exams at 8-10 weeks 2. It becomes a normal fourth ventricle after the 11th week.
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