Following is an account of the hypothalamic nuclei and areas that can be readily distinguished in the stained sections of the calf brains. By nuclei we mean distinct clumps of cell bodies, distinguished from neighboring structures by cell density, size, and intensity of Nissl staining. By areas we refer to regions without distinct nuclei, which instead contain relatively sparsely distributed cell bodies.
We begin at the rostral edge of the hypothalamus and proceed caudally in the section series in the coronal plane, followed by a medial-to-lateral sequence in sagittal sections. The name of each nucleus or area is followed by its abbreviation, and by the specification of which of the illustrated sections show its location and visible characteristics.
The nucleus of the diagonal band was included in the hypothalamus by Bleier (1983) on the basis of embryological studies of Rose (1942). It forms the anterior pole of the hypothalamus. It is a large and prominent band of cells, remarkably consistent in all mammalian species studied. Its more dorsal vertical (Ndbv) and more ventral horizontal (Ndbh) limbs enclose the anterior portion of the medial preoptic areas. The vertical limb, in the calf as in other species, is continuous with a medial septal nucleus.
Preoptic Area:Poa
coronal, section no.=164
In the calf, as in the cat atlas (Bleier, 1961), there are no prominent nuclear aggregations within or alongside the preoptic area. Rodents and primates, in contrast, exhibit complicated and species-specific collections of prominent individual nuclei. Thus in cattle there are no preoptic nuclei as such. The preoptic area begins rostrally near the anterior end of Ndb, running between Ndb and preventricular portion of the periventricular nucleus Pe medioventrally, and the lateral ventricle, accumbens nucleus, anterior commissure and internal capsule dorsally and laterally. The caudal limit of Poa is arbitrarily set at the level of the anterior edge of the optic chiasm, similar undifferentiated expanses containing scattered cells continue caudally as the anterior hypothalamic area (Aha).
Suprachiasmatic Nucleus: Sch
coronal, section o.= 224; sagittal, section no.=536
The suprachiasmatic nucleus is a group of densely packed small cells dorsal to and adjoining the optic chiasm.
In cattle, Sch is at the rostral edge of the chiasm, at the junction of the chiasm and the supraoptic recess of the third ventricle.
In other mammals, for example, mice (Broadwell and Bleier, 1976) Sch can be located at the caudal edge of the chiasm.
The bovine Sch, in coronal sections, appears to be fusiform and tilted about 45 and infin; with the ventral end more medial and the dorsal end lateral. In most other mammals Sch appears spherical in coronal sections. In cattle, only the ventromedial corner is in contact with the chiasm. A layer of ependymal cells and a cell free zone separate Sch from the third ventricle anteromedially. A particular condensation of cells within Sch, near the center of the nucleus, is evident in coronal sections.
The paraventricular nucleus is a group of mostly large, darkly staining cells. In cattle, unlike some other mammals, it is not readily divisible into subnuclei; instead it is a solid sheet of cells, forming an arc on either side of the periventricular nuclear lining around the upper part of the third ventricle. At its rostrodorsal pole Pa is almost in continuity with the thalamic paraventricular nucleus . The caudal pole of Pa is represented by a few large cells at the base of the thalamus, and the dorsal edge of Pa is never far from the thalamus. In coronal sections there appear to be three caudal lobes extending back from the main bodies of the right and left Pa, one on each side of the ventricle and a third, central one extending across the midline dorsal to the ventricle, which merges the nuclei of the right and left sides.
The supraoptic nucleus is a group of large, darkly staining cells, divided by the optic tracts into anterior and tuberal (or retrochiasmatic) divisions. The anterior division of Soa is larger and extends as a dense column of cells along the antero-dorsal edge of the optic tract, from the level of Sch to the lateral-most reaches of the hypothalamus where it meets the medial nucleus of the amygdala. In the anterolateral edge of Soa the cells become more scattered along the edge of the brain. All cells of Sot are more scattered, and some scattered cells extend from Soa to Sot around the dorsal side of the optic tract as it leaves the chiasm. The bovine Sot does not reach the midline.
Anterior Portion: Pea
coronal, section nos.=164, 188; sagittal, section no.=506.
Pea lies on the midline, astride the rostral face of the anterior commissure, and extends above and below the commissure. It merges with rest of Pe along the walls of the front end of the third ventricle. Pe lies along the walls of the third ventricle, and in the tuberal region it lies between the ventricular wall medially and the ventromedial nucleus laterally. Caudally it merges with the ventral portion of the dorsomedial nucleus and the rostrodorsal edge of the arcuate nucleus.
The ventromedial hypothalamic nucleus in its rostral extent appears, in coronal sections, as a flattened ellipse, with a densely packed outer ring of cells surrounding a more sparsely populated center.
Proceeding caudally, the shape becomes more circular. In the outer ring, cell density is greatest on the dorsomedial portion, corresponding to subnucleus VMHDM of the rat (Paxinos and Watson, 1986).
A region of secondary density lies on the ventrolateral side, corresponding to subnucleus VMHVL of the rat. The cell free ring lying just outside Vm is most pronounced along the dorsal and medial sides of the nucleus, and disappears along the lateral and caudal edges of the nucleus. Thus Vm merges caudally with the arcuate Ar and dorsomedial Dm nuclei.
In sagittal sections, the central sparsity is not evident, and any individual sections shows the nucleus as a mass of cells with no boundaries apparent between the caudal half of Vm, and the neighboring portions of Ar, and Dm.
The arcuate nucleus, also known as the infundibular nucleus, consists of small cells along the border of the tuber cinereum and the infundibular stalk. It merges with Pe rostrally, with Vm rostrodorsally, and with the dorsomedial nucleus Dm dorsally, laterally, and caudally. The remaining boundaries of Ar are formed by the ventricle dorsally and medially, and by the infundibular stalk ventrally.
The dorsomedial hypothalamic nucleus is continuous at its ventrocaudal corner with Pe and Ar, and like them consists of fairly densely packed small cells. Dm extends from this junction dorsally, laterally and somewhat rostrally over the caudal and dorsal aspects of Vm. A boundary between Vm and Dm is difficult to establish in cattle. In general the cells of Dm are smaller and more densely packed than those in Vm.
The premammillary nucleus is a distinct collection of cells lying caudolateral to Dm, lateral to Ar, and rostral to Mm.
The lateral mammillary nucleus is a distinctive collection of large cells, which receives the fibers of the postcommissural fornix.
The medial mammillary nuclei are large round paired collections of small and medial cells. The fibers of the mammillo-thalamic tract (mt) originate in these nuclei and course upward, collecting into the tract at the dorsal surface of the nuclei. The distinction between right and left nuclei is more obscure at the posterior end.
Dorsal to Mm is Pha, a region of scattered cells of various shapes and sizes.
Perimammillary Nucleus: Pem
coronal, section nos.= 434, 456; sagittal, section no.= 506.
A band of cells around, and sharply distinct from, Mm on its caudal, ventral, lateral and dorsal borders is designated Pem. Cells are more sparsely distributed here than in Mm.