**2. Definition and localization**

The hypothalamus is a small, central region of the human brain formed by nervous fibers and a conglomerate of nuclear bodies with various functions. The hypothalamus is considered to be a link structure between the nervous and the endocrine system, its main function being to maintain the homeostasis of the body.

The hypothalamus is located under the thalamus from which it is separated by the hypothalamic sulcus of Monro. The sulcus is located at the lateral wall of the third ventricle and extends anteroposteriorly from the interventricular foramen of Monro (that assures the communication between the third, diencephalic ventricle and the frontal horn of each lateral ventricle) up to the level of Sylvius cerebral aqueduct. The hypothalamus is limited anteriorly by the *lamina terminalis*, a gray matter layer of triangular aspect extended above the chiasma optique, in between the two anterior horns of the fornix. *Lamina terminalis* also forms the anterior wall of the third ventricle and contains the organum vasculosum, a circumventricular structure characterized by the absence of blood–brain barrier and thus highly sensitive to osmotic variations of the blood [5]. The superior wall of the hypothalamic region participates in the formation of the inferolateral wall of the third ventricle of the brain and has close relations with the white matter structure that surrounds it, called the fornix. The fornix is a C-shaped white cerebral structure that connects various parts from the brain (hypothalamic nuclei with hippocampal region, thalamic nuclei with hypothalamus's mammillary bodies). Even if its function is not clearly understood, its relation with memory is known, and recent studies are testing its deep brain stimulation as a treatment in advanced Alzheimer's disease [6]. Posteriorly, the hypothalamus extends up to the periaqueductal gray substance and the tegmentum of the superior part of the brainstem.

Only on the inferior surface of the brain, the hypothalamus can be visualized from the optic chiasm and the anterior perforated substance anteriorly to the posterior cerebral peduncles of the midbrain and the mammillary bodies, dorsally (**Figure 1**). The mammillary bodies are small, round white-matter structures that belong to the limbic system. They are involved in memory due to their connections with the hippocampal region and also in maintaining the sense of direction [7]. The hypothalamus is limited laterally by the optic tracts in their direction toward the lateral geniculate bodies, an important relay of the optical pathway. Inside the delimited area on the exterior surface of the brain, a small prominence, called tuber cinereum or infundibulum connects the hypothalamus with the posterior lobe of the underneath pituitary gland. The pituitary or the hypophyseal gland is located at the base of the brain, in a depression of the sphenoid bone called the sella turcica.

#### **2.1. The hypothalamus—hypophyseal complex**

the anatomical description was accepted *per se* and very few research papers have questioned

The columnar morphologic model is based on the division of the forebrain in functional longitudinal units, placing the telencephalon in the most rostral region and the diencephalon caudally, in between the telencephalon and the midbrain, while the hypothalamus if formed

In the last decades, mapping of the genes involved in hypothalamic development allowed the identification of a disparity between the morphological, classic boundaries of this region and the molecular ones. According to Puelles' Prosomeric model [3], the initially proposed longitudinal axis of the brain is bent due to the first mesencephalic flexure of the embryo. This condition puts the diencephalon rostrally between the telencephalon cranially and the midbrain caudally and sets the hypothalamus independent from the diencephalon as a distinct

An important role in hypothalamic development is assigned also to the presence of specific signaling centers (Wingless-Int protein family–Wnt, Hedgehogs family–Hh, and Bone mor-

The hypothalamus is a small, central region of the human brain formed by nervous fibers and a conglomerate of nuclear bodies with various functions. The hypothalamus is considered to be a link structure between the nervous and the endocrine system, its main function being to

The hypothalamus is located under the thalamus from which it is separated by the hypothalamic sulcus of Monro. The sulcus is located at the lateral wall of the third ventricle and extends anteroposteriorly from the interventricular foramen of Monro (that assures the communication between the third, diencephalic ventricle and the frontal horn of each lateral ventricle) up to the level of Sylvius cerebral aqueduct. The hypothalamus is limited anteriorly by the *lamina terminalis*, a gray matter layer of triangular aspect extended above the chiasma optique, in between the two anterior horns of the fornix. *Lamina terminalis* also forms the anterior wall of the third ventricle and contains the organum vasculosum, a circumventricular structure characterized by the absence of blood–brain barrier and thus highly sensitive to osmotic variations of the blood [5]. The superior wall of the hypothalamic region participates in the formation of the inferolateral wall of the third ventricle of the brain and has close relations with the white matter structure that surrounds it, called the fornix. The fornix is a C-shaped white cerebral structure that connects various parts from the brain (hypothalamic nuclei with hippocampal region, thalamic nuclei with hypothalamus's mammillary bodies). Even if its function is not clearly understood, its relation with memory is known, and recent studies are testing its deep brain stimulation as a treatment in advanced Alzheimer's disease [6]. Posteriorly, the hypothalamus extends up to the periaqueductal gray substance and the tegmentum of the superior

phogenetic family–FgF) that modulates cell proliferation and neurulation [4].

from the ventral most part of the diencephalic vesicle [2].

posterior part of the forebrain [2, 3].

**2. Definition and localization**

maintain the homeostasis of the body.

part of the brainstem.

its validity.

4 Hypothalamus in Health and Diseases

The pituitary gland is a three-lobe structure: anterior, posterior and intermediate lobe, with different embryological origin.

The anterior lobe, pars anterior, or adenohypophysis is derived from the anterior wall of Rathke's pouch, an ectodermal structure that also forms the primitive oral cavity and the pharynx [8]. The anterior gland contains a heterogeneous cellularity that synthesized and secreted hormones in the blood stream: the majority of the cells are somatotrope cells that produced the human growth hormone (hGH) or somatotropin hormone (STH), a peptide that promotes growth in childhood. The production of the somatotropic hormone is under the control of the hypothalamic growth-releasing hormone (GRH) produced by the arcuate nucleus. The next hormones produced in high quantity by the anterior gland of the hypophysis are the corticotrope ones (adrenocorticotropic hormone—ACTH, melanocyte-stimulating hormone—MSH, and beta-endorphins). This group of hormones is under the control of the hypothalamic corticotropin-relasing hormones (CRHs) derived from the paraventricular nuclei. In smaller percentages, the adenohypophysis has population of cells that produced

**Figure 1.** Inferior surface of the brain with hypothalamic visualization at this level.

thyrotropes, gonadotropes, and lactotropes. Thyrotropes respond to signals from the hypothalamic thyrotropin-releasing hormone (TRH) produced in the paraventricular nuclei and further synthesize the hormone responsible for thyroid hormones production—thyroid stimulating hormone (TSH). Luteinizing hormones (LHs) and follicle stimulating hormones (FSHs) are secreted by gonadotrope cells of the gland under the influence of pulsatile secretion of gonadotropin-releasing hormone (GRH) produced in hypothalamus preoptic area. The secretion of prolactine (PRL) from the lactotropes is stimulated by hypothalamic thyrotropinreleasing hormone (TRH) and inhibited by the dopamine [9].

an anterior and posterior region. The anterior region is also referred to as the prechiasmatic region, due to its location above the chiasma optic, while the posterior region is called the mammillary region. The infundibular region is situated between the previous two regions.

Anatomy and Function of the Hypothalamus http://dx.doi.org/10.5772/intechopen.80728 7

From a structural point of view, the hypothalamus is formed by gray matter conglomeration of neurons that organize in nuclei and also by white-matter substance formed by myelinated

The anterior region of the hypothalamus is located above the optic chiasm and is referred to as the supraoptic area. It contains the following nucleus: supraoptic, preoptic and medial preoptic, the suprachiasmatic and the anterior hypothalamic nucleus, alongside with the paraventricular one (**Figure 2**). The supraoptic nucleus produces vasopressin or the antidiuretic hormone (ADH) that is stored in the posterior lobe of the pituitary gland and is responsible for blood pressure control and water balance of the organism. The preoptic region alongside with the anterior hypothalamic nucleus is involved in cooling (thermoregulation) of the body through the sweating process. The preoptic nucleus is also involved in the habit of eating and in reproduction while the medial preoptic region is involved in cardiovascular control as a response to stress [10]. The suprachiasmatic nucleus is situated above the optic chiasm and is involved in the circadian rhythm. The paraventricular nucleus (named after its location near the third diencephalic ventricle) represents an important autonomic center of the brain

The central part as the hypothalamus is located above tuber cinereum and is named the tuberal area. It is composed of two parts, anterior and lateral, and contains the following nucleus: dorsomedial, ventromedial, paraventricular, supraoptic, and arcuate (**Figure 2**). The ventromedial area is involved in controlling the habits of eating and the feeling of satiety [12]. The arcuate or infundibular nucleus is responsible for orexigenic peptides secretion: ghrelin,

nervous fibers.

involved in stress and metabolism control [11].

**Figure 2.** Schematic representation of hypothalamic nuclei (sagittal section).

orexin, or neuropeptide Y [11].

Hypothalamic hormones reach the adenohypophysis through a vascular system. Hypothalamus exerts its effects over the anterior part of the gland through the hypothalamo-hypophyseal portal system, a special vascular system formed by fenestrated capillaries. The proximal vascular structure of the portal system is the anterior hypophyseal artery, branch from the ophthalmic segment of the internal carotid artery [9]. Through it, hypothalamic hormones are transported to the primary plexus, located near the infundibulum of the hypothalamus. From this region, hormones are drained into the second vascular venous plexus of the hypothalamo-hypophyseal portal system that surrounds the adenohypophysis [9]. This vascular system allows hormones to diffuse through the wall, inside of the gland. The hypophyseal vein further drains the blood into the venous sinuses of the dura mater and from here in the venous system of the body.

The posterior wall of Rathke's pouch forms the intermediate lobe of the gland [8]. It is absent or of small size in adults. In children, it is the part of the gland responsible for skin pigmentation through the secretion of the melanocyte stimulating hormone (MSH) or "intermedins" [9]. Pars intermedia also produces corticotrophin-like intermediate lobe peptide (CLIP) and adrenocorticotrophic hormone (ACTH) [9].

The posterior lobe of the gland, pars distalis or neurohypophysis derives from the neuroectoderm [9]. It is an inferior extension of the hypothalamus and is mainly from its neural fibers. The connection between the hypothalamus and the posterior lobe of the gland forms the infundibular stalk. Through this complex, hormones synthetized in the hypothalamus nuclei are transported and deposited in the posterior gland where they are stored in presynaptic vesicles and then released into the blood stream. The supraoptic nuclei of the hypothalamus are responsible for the secretion of antiduretic hormone (ADH) or vasopressin, the hormone involved in maintaining the water balance in organism and thus in preventing dehydration. The paraventricular nuclei produce oxytocin, a hormone released during labor, in the presence of uterine contractions.

The hypothalamus intervenes along with the pituitary gland the majority of the endocrine and metabolic functions of the body through a double-sense transport of hormones between the two structures.
