Topological atlas of the hypothalamus in adult rhesus monkey

Abstract

The prosomeric model explains the embryological development of the central nervous system (CNS) shared by all vertebrates as a Bauplan. As a primary event, the early neural plate is patterned by intersecting longitudinal plates and transverse segments, forming a mosaic of progenitor units. The hypothalamus is specified by three prosomeres (hp1, hp2, and the acroterminal domain) of the secondary prosencephalon with corresponding alar and basal plate parts, which develop apart from the diencephalon. Mounting evidence suggests that progenitor units within alar and basal plate parts of hp1 and hp2 give rise to distinct hypothalamic nuclei, which preserve their relative invariant positioning (topology) in the adult brain. Nonetheless, the principles of the prosomeric model have not been applied so far to the hypothalamus of adult primates. We parcellated hypothalamic nuclei in adult rhesus monkeys (Macaca mulatta) using various stains to view architectonic boundaries. We then analyzed the topological relations of hypothalamic nuclei and adjacent hypothalamic landmarks with homology across rodent and primate species to trace the origin of adult hypothalamic nuclei to the alar or basal plate components of hp1 and hp2. We generated a novel atlas of the hypothalamus of the adult rhesus monkey with developmental ontologies for each hypothalamic nucleus. The result is a systematic reinterpretation of the adult hypothalamus whose prosomeric ontology can be used to study relationships between the hypothalamus and other regions of the CNS. Further, our atlas may serve as a tool to predict causal patterns in physiological and pathological pathways involving the hypothalamus.

Keywords: Alar plate; Basal plate; Holoprosencephaly; Prethalamus; Prosomere; Prosomeric model; Secondary prosencephalon.

Fig. 1

Schematic of progressive regionalization of the hypothalamus according to the prosomeric model. a Dorsal view of the neural plate with intersecting longitudinal (rostro-caudal) zones, named plates, and transverse (medio-lateral) segments named protosegments and neuromeres, forming a mosaic of units with unique molecular profiles. The floor plate and the roof plate are organizers whose signals pattern and specify the alar and basal plates. The roof plate encircles the entire neural plate, and the floor plate terminates rostrally just caudal to the prospective mamillary bodies (Mb). Other organizers, like the acroterminal domain (At), pattern and specify transverse segments. The alar-basal boundary (ABb) divides each neuromere into alar plate and basal plate parts. The diencephalic protosegment (Dien; p3, p2, p1), from which the thalamus arises, lies topologically caudal to the neuromeres of the secondary prosencephalon (SPro: hp1, hp2, At). b Lateral view from the left of the neural tube shows progressive morphogenesis following neural plate specification into a mosaic of units. Hp1 gives rise to the telencephalon (Tel), including the pallium (Pal) and subpallium (SPal), while hp2 contributes the preoptic area (POA) to the SPal. c Lateral view from the left of the neural tube shows progressive morphogenesis and tertiary folding events pushing the hypothalamus to a topographical position ventral to the thalamus, as its name indicates. d Lateral view from the left of the neural tube shows progressive morphological expansion of brain regions by neuromere and plate of origin. Several architectonic landmarks, like the fornix, f, the optic chiasm (oc), and the anterior commissure (ac), in the adult brain are visualized as outgrowths along protosegmental and neuromeric boundaries. The sketches of this figure were adapted from Puelles et al. (2008, 2012) and Puelles and Rubenstein (2015). Color code applies to all subsequent figures. See list of abbreviations for complete terms

Fig. 2

Architectonic boundaries of the caudal hypothalamus in the adult rhesus monkey show developmental origin of anatomical landmarks and hypothalamic nuclei. a, c, e, g Photomicrographs of coronal sections through the hypothalamus of an adult rhesus monkey stained for AChE; b, d, f, h maps of traced architectonic boundaries. Sections are ordered sequentially from caudal a, b to rostral g, h. All nuclei of the caudal hypothalamus are likely of basal plate origin. The SM, the PM, the STN, and the PeM are likely derived from hp1 while the MM and the LM likely arise from hp2. Calibration bar (2 mm) in g applies to a, c, e, and g. For color code see Fig. 1. See list of abbreviations for complete terms

Fig. 3

Architectonic boundaries of the tuberal and medial hypothalamus in the adult rhesus monkey show developmental origin of anatomical landmarks and hypothalamic nuclei. a, c, e Photomicrographs of coronal sections through the hypothalamus of an adult rhesus monkey stained for AChE; b, d, f maps of traced architectonic boundaries. Sections are ordered sequentially from caudal a, b to rostral (e, f). All nuclei of the tuberal and medial hypothalamus are of basal plate origin. The SM and the Pef are likely derived from hp1 while the PreM, the ARC, the TCA, and the VMc likely arise from hp2. The DM and the TM are thought to be derived from the basal plate parts of both hp1 and hp2 (maroon). Proposed architectonic boundaries of the VMs line the VMc medially to meet the ventral boundary of the DM. Proposed boundaries dividing the DM and the TM into likely hp1 or hp2 parts are also shown. For color code see Fig. 1. See list of abbreviations for complete terms

Fig. 4

Continuation of the architectonic boundaries of the tuberal and medial hypothalamus in the adult rhesus monkey show developmental origin of anatomical landmarks and hypothalamic nuclei. a, c, e Photomicrographs of coronal sections through the hypothalamus of an adult rhesus monkey stained for AChE; b, d, f maps of traced architectonic boundaries. Sections are ordered sequentially from caudal a, b to rostral (e, f). All nuclei of the tuberal and medial hypothalamus are of basal plate origin. All nuclei of the rostral hypothalamus are of alar plate origin. The Pef is the rostral-most nucleus of the basal plate part of hp1. The Arc, the TCA, and the VMc arise from the basal plate part of hp2. The DM and the TM are thought to be derived from basal plate parts of both hp1 and hp2 (maroon). Proposed architectonic boundary of the VMs lines the VMc medially and courses dorsally to meet the DM. The LA, the PaM, and the PaP are likely derived from the alar plate part of hp1. Calibration bar (2 mm) in e applies to a, c, e. For color code see Fig. 1. See list of abbreviations for complete terms

Fig. 5

Architectonic boundaries of the rostral hypothalamus in the adult rhesus monkey show developmental origin of anatomical landmarks and hypothalamic nuclei. a, c, e Photomicrographs of coronal sections through the hypothalamus of an adult rhesus monkey stained for AChE; b, d, f maps of traced architectonic boundaries. Sections are ordered sequentially from caudal a, b to rostral (e, f). All nuclei of the rostral hypothalamus are of alar plate origin. The LA, the PaM, and the PaP are derived from hp1 while the RPa, the SO, and the SCH arise from hp2. The POA is excluded from the hypothalamus in the adult rhesus monkey according to Puelles et al. (2012). Calibration bar (2 mm) in e applies to a, c, e. For color code see Fig. 1. See list of abbreviations for complete terms

Fig. 6

Representative sections of each topographic region of the adult hypothalamus visualized with distinct stains. a–c Traced maps of hypothalamic nuclei by prosomere and plate of origin from representative sections from each of the three topographical regions of the hypothalamus of rhesus monkey visualized with acetylcholinesterase (AChE; case AZ). Matched sections were stained for: d–f myelin (case AQ); g–i SMI-32 (case AP); j–l WFA (case AT). WFA was the only stain used that clarified interneuromeric boundaries between p3, hp1 and hp2 (drawn in dashed lines). We visualized medium and large neurons with WFA in the: m magnocellular group of the paraventricular complex (PaM); n lateral hypothalamic area (LA); o arcuate nucleus (ARC); p perimamillary nucleus (PeM); q supramamillary nucleus (SM); r subthalamic nucleus (STN). Calibration bar (2 mm) in sections a, d, g, j apply to all sections within corresponding rows at low magnification. Calibration bar (50 µm) in section m applies to all sections m–r at higher magnification. For color code in traced maps see Fig. 1. See list of abbreviations for complete terms

Fig. 7

Representative sections of each topographic region of the adult hypothalamus visualized with AChE or stains for calcium-binding proteins. a–c Traced maps of hypothalamic nuclei by prosomere and plate of origin from representative coronal sections from each of the three topographical regions of the hypothalamus of rhesus monkey stained for acetylcholinesterase (AChE; case AZ). Representative matched sections from each topographical region of the hypothalamus of adult rhesus monkey stained for the calcium-binding proteins: d–f parvalbumin (PV; case AZ), i–l calbindin (CB; case AZ), o–q calretinin (CR; case AX). t–v Maps of exhaustive plotting of CR neurons (CR map) in representative sections show the distribution and relative density of CR when compared to PV and CB in the hypothalamus. Large PV+ neurons were found in: g STN; and h MM. In stark contrast, large CB+ neurons were only found in: m MM; and n SCH. r, s Small CR+ neurons were only found dispersed between hypothalamic nuclei with a semi-uniform distribution. Calibration bar (2 mm) in sections a, d, i, o, t apply to all sections within corresponding rows at low magnification. Calibration bar (200 µm) in g applies to sections g, h, m, n, r, s at higher magnification. For color code in traced maps see Fig. 1. See list of abbreviations for complete terms

Fig. 8

3D reconstruction of the hypothalamus of the adult rhesus monkey shows developmental origin of anatomical landmarks and nuclei. a Rostral view of the adult hypothalamus bound by the anterior commissure (ac, beige) and preoptic area (POA, grey). b Oblique-dorsal view looking down into the third ventricle with the anterior commissure, the POA, and the relative spatial positions of alar and basal plate derivatives of hp1 and hp2 nuclei. The fornix (f, black) courses exclusively through alar and basal plate parts of hp1 until it diverges caudally into the MM. c Caudal view of the hypothalamus including prethalamic nuclei (PTh) of the diencephalon. The proposed boundaries of the VMs are not shown as it could not be visualized in any stain used in our analysis. For color code see Fig. 1. See list of abbreviations for complete terms

Fig. 9

3D reconstruction of the hypothalamus of the adult rhesus monkey shows developmental origin of nuclei in relation to the fornix. a, b Rostral view of adult hypothalamic nuclei derived from alar a and basal b plate parts of hp1 and hp2 in the rhesus monkey. c, d Caudal view of adult hypothalamic nuclei derived from alar c and basal d plate parts of hp1 and hp2. The proposed boundary of the VMs is not shown as it could not be visualized in any stain used in our analysis. The fornix (f) courses exclusively through alar and basal plate parts of hp1 until it diverges caudally into the MM. For color code see Fig. 1. See list of abbreviations for complete terms

Fig. 10

Schematic summary of architectonic boundaries and hypothalamic nuclei in the adult rhesus monkey by likely prosomere and plate of origin. a Developmental origin of adult hypothalamic nuclei in the rhesus monkey as predicted by the prosomeric model (Puelles et al. ; Puelles and Rubenstein 2015). Architectonic boundaries of the adult hypothalamus in the rhesus monkey are illustrated by relative invariant position (topology) in relation to each hypothalamic prosomere [hp1, hp2, acroterminal domain (At)] and plate of origin (alar and basal plate). Theoretical boundaries of adult hypothalamic nuclei are traced over their likely prosomere(s) and plate of origin. b Sketch of architectonic landmarks and nuclear boundaries of the adult hypothalamus in rhesus monkeys with corresponding developmental origin as predicted by the prosomeric model. c Technical schematic of architectonic landmarks and nuclear parcellation of the hypothalamus in the adult rhesus monkey by developmental origin. Proposed placement of uncertain boundaries or mixed migratory patterns from both hp1 and hp2 are shown in a dashed line. For color code see Fig. 1. See list of abbreviations for complete terms