Hypothalamus

The pituitary gland, or hypophysis, “hangs” from the base of the brain via a connecting stalk, the infundibulum, which connects it to the hypothalamus. The infundibulum

contains numerous nerve fibers that relay from the hypothalamus to the posterior portion of the pituitary gland. In addition to this nervous pathway between the hypothalamus and the pituitary, numerous small blood vessels pass between the two organs. The pituitary gland has two anatomically and functionally distinct lobes, the neurohypophysis (posterior lobe) and the adenohypophysis (anterior lobe). The posterior lobe arises as an outgrowth of the embryonic brain. It is composed of nervous tissue and forms a neural link with the hypothalamus through the infundibulum. The anterior lobe arises from the epithelial lining of the embryonic pharynx. It consists of glandular epithelial tissue and forms a vascular link with the hypothalamus via the small blood vessels that pass between the two regions.

5Parenchyma consisting of acidophils, basophils, and chromophobes

6 Capillary with red blood cells

7 Parenchyma consisting of 5 6 axons and pituicytes

8 Hypothalamus

9 Cerebrum

10Falx cerebri

11Midbrain

12Pons

16Nasal septum

17Soft palate

19Epiglottis

20Atlas

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Photomicrograph of posterior pituitary

200x

Sagittal section of head and neck with enlarged callout of pituitary gland

Medial view

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The pineal gland, a small reddish-gray body covered with pia mater, is a midline epithelial outgrowth of the embryonic midbrain positioned in a depression between the two superior

colliculi on the midbrain’s dorsal surface. The distal end of this outgrowth becomes a small mass of secretory cells that resemble the shape of a pine cone. It is from this appearance that it derives its name. The pia mater sends septa into the pineal gland that divide it into cords of secretory cells that are intermingled with numerous blood capillaries. The secretory cells of the pineal gland, called pinealocytes, have arm-like processes that contact both neighboring capillaries and the ependymal cells that line the third ventricle. Hormonal secretions produced in the body of the cell are moved through the arm-like processes where they are released by exocytosis into the capillaries and cerebrospinal fluid. Projecting into these cords of tissue are sympathetic postganglionic neurons from the superior cervical sympathetic ganglion. The gland plays a role in integrating photoperiod and affecting circadian rhythms.

12Pons

13Cerebellum

14Sphenoid sinus

19Nasopharynx

20Tongue

21Middle cerebellar peduncle

Sagittal section of brainstem and diencephalon in situ

Medial view

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Dissection of brainstem and diencephalon

Posterior view

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The thyroid gland is a bilobed organ positioned in the anterior neck. This highly vascular organ consists of two lateral lobes of endocrine tissue joined in the middle by a narrow portion of the

gland called the isthmus. It is red-brown in color and is enveloped by a thin layer of connective tissue. This connective tissue capsule sends extensions into the gland that divide the vascular and epithelial core into masses of irregular shape and size. The epithelial cells within the compartments of the thyroid gland form the secretory tissues of the organ. The major thyroid secretory cells are arranged into hollow spheres, each of which forms a functional unit called a follicle. In a microscopic section the follicles appear as rings of follicular cells enclosing an inner lumen fi lled with colloid, a substance that serves as an extracellular storage site for thyroid hormones. Interspersed in the interstitial spaces between the follicles are other secretory cells, the C cells, so called because they secrete the peptide hormone calcitonin.

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Thyroid gland in situ

Anterior view

Photomicrograph of thyroid gland

240x

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The parathyroid glands are small, oval, light brown glands situated on the posterior border of the two lateral lobes of the thyroid gland. The parathyroid glands sit just beneath the connective tissue capsule

of the thyroid gland. There are four parathyroid glands, two superior and two inferior. The endocrine cells of the parathyroid glands are called chief or principal cells. The chief cells form interconnecting columns of cells separated by fenestrated capillaries. The chief cells produce the parathyroid hormone.

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Photomicrograph of parathyroid gland

240x

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Thyroid and parathyroid glands (exposed on left)

Posterior view

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