| Flexures | |
|---|---|
 Brain of human embryo of four and a half weeks, showing the three flexures | |
| Details | |
| Identifiers | |
| Latin | flexura |
| TE | (embryology)_by_E5.14.3.3.0.0.3 E5.14.3.3.0.0.3 |
| Anatomical terminology | |
Threeflexures form in the part of theembryonicneural tube that develops into thebrain. At four weeksgestational age in thehuman embryo, the neural tube has developed at the cranial end into three swellings – theprimary brain vesicles. The space into which the cranial part of the neural tube is developing is limited. This limitation causes the neural tube to bend, or flex, at two ventral flexures – the rostral cephalic flexure, and the caudal cervical flexure. It also bends dorsally into the pontine flexure. These flexures have formed by the time that the primary brain vesicles have developed into fivesecondary brain vesicles in the fifth week.
The neural tube has a longitudinal axis called theneuraxis, from the future brain area at the cranial end, to theconus medullaris of thespinal cord at the caudal end. By the fourth week in the human embryo, at its cranial end, three swellings have formed as primary brain vesicles.[1] These vesicles form the futureforebrain,midbrain, andhindbrain. The three vesicles need to develop further into five brain vesicles but the space at the cranial end is limited. This causes the neural tube to bend ventrally at two flexures – the first at the cephalic flexure and the second at the cervical flexure. A third flexure is oriented in the opposite dorsal direction as the pontine flexure. By the fifth week further flexion has taken place and the five secondary brain vesicles have formed.[1]
The angle formed by the two ventral flexures, the cephalic flexure and the cervical flexure together, is a right angle in the ventral direction between the axis of the body and the axis of the brain. The pontine flexure is located between these two flexures.
Thecephalic flexure, also known as the mesencephalic flexure, is the first flexure or bend, that forms in the region of midbrain.[2] The caudal part of the midbrain and the rostral part of the hindbrain makes up a mibrain-hindbrain boundary region known as theisthmic organizer.[3] In human embryos, it generally occurs at the end of the 3rd week or the beginning of the 4th.
Thecervical flexure forms between the hindbrain and the spinal cord.[4]
Thepontine flexure, also called therhombic flexure, forms the boundary between themetencephalon and themyelencephalon.[5][6][7] The metencephalon becomes thepons and thecerebellum, and the myelencephalon becomes themedulla oblongata. These two regions develop and fold dorsally at the pontine flexure.[5]