let-7c expression in the zebrafish brain.
let-7a, let-7b and let-7c are expressed in both proliferating and differentiating cells. let-7b and let-7c differ in their sequence in only one nucleotide located outside the seed region. They share similar regional expression in the larval brain with two differences: let-7b is expressed in the retinal ciliary marginal zone and pineal cells whereas let-7a and let-7c are absent (table A). let-7a, let-7b and let-7c mainly conserve their regional expression between larval and adult brain (tables A, F).
let-7a, let-7b and let-7c are expressed in many proliferating and differentiating cells of the larval fore-, mid- and hindbrain with the exception of some areas such as hypothalamic nuclei (caudal hypothalamus, diffuse nucleus of inferior lobe, lateral torus) interpeduncular nucleus, locus coereleus, raphe and reticular formation. We detected only minor differences at the regional level between larval and adult brain expression. For example, let-7b and let-7c are expressed in some adult but not larval hypothalamic lateral torus and superior raphe cells (tables A, F). A. transverse section through the larval telencephalon showing let-7c expressing cells in the ventral (Sv) and dorsal (Sd) subpallium and pallium (P).
B. transverse section through the larval caudal telencephalon and epithalamus showing let-7c expressing cells in the dorsal subpallium (Sd), pallium (P) and habenula (Ha).
C. transverse section through the larval diencephalon and optic tectum showing let-7c expressing cells in the ventral thalamus (VT), dorsal thalamus (DT), periventricular (Pr) and migrated (M1) pretectum and optic tectum (TeO, proliferative zone-m, periventricular gray zone-pgz, longitudinal torus-TL).
D. transverse section through the larval diencephalon and midbrain showing mainly periventricular (arrowheads) let-7c expressing cells in the periventricular (PT) and lateral migrated (M2) posterior tuberculum, tegmentum (T), semicircular torus (TS) and tectal periventricular gray zone (pgz).
E. oblique transverse section through the larval caudal hypothalamus, midbrain and hindbrain showing let-7c expressing cells in the semicircular torus (TS), optic tectum (TeO), isthmic area (Is), cerebellar plate (CeP), and medulla oblongata (MO).
F. transverse section through the larval retina (dorsal to the right) devoid of let-7c expressing cells. The arrow points at the ciliary marginal zone.
G. oblique transverse section through the larval hindbrain at the level of the otic capsule (caudal to section E) showing let-7c expressing cells in the semicircular torus (TS), optic tectum (TeO), granular cerebellar eminence (EG), cerebellar plate (CeP) and medulla oblongata (MO).
H. oblique transverse section through the larval hindbrain at the level of the octaval ganglion (caudal to section G) showing let-7c expressing cells in the granular cerebellar eminence (EG), medulla oblongata (MO) and rhombic lip (RL).
I. transverse section through the adult rostral telencephalon showing let-7c expressing cells in the dorsal telencephalon/pallium (P) and olfactory bulb (OB).
J. transverse section through the adult caudal hindbrain showing let-7c expressing cells in the inferior reticular formation (IRF), posterior octaval (PON) and caudal octavolateral (CON) nuclei and facial lobe (LVII).
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