Steroid and Thyroid Hormones Transport Quiz

Embryologic Development of Hypophysis (Pituitary Gland)

• The hypophysis (pituitary gland) has a dual embryologic origin, consisting of both epithelial and neural portions.
• The epithelial portion develops from the pharyngeal roof and Rathke's pouch.
• The pouch detaches and becomes the cellular portion, the adenohypophysis.
• The downgrowth of the brain forms the neural portion, the neurohypophysis.
• The neurohypophysis remains attached to the hypothalamus by a neural stalk, called the infundibulum.
• Neurons in the hypothalamus control the release of hormones from the adenohypophysis.

Subdivision of Hypophysis

Adenohypophysis (Anterior Pituitary)

• Pars distalis is the largest part.
• Pars intermedia is a remnant of the pouch and is rudimentary in humans.
• Pars tuberalis surrounds the neural stalk.

Neurohypophysis (Posterior Pituitary)

• Median eminence is located at the base of the hypothalamus.
• Infundibulum is the neural stalk that connects the neurohypophysis to the hypothalamus.
• Pars nervosa is the largest portion and consists of unmyelinated axons and pituicytes.

Vascular and Neural Connections of Hypophysis

• The connection between the hypothalamus of the brain and the adenohypophysis is vascular, forming the hypothalamohypophyseal portal system.
• This system carries releasing and inhibiting hormones from the ventromedial, arcuate, preoptic, and paraventricular nuclei to bind at specific receptors in cells of the pars distalis.
• Superior hypophyseal arteries form a fenestrated primary capillary plexus in the median eminence.
• Secretory neurons in the hypothalamus terminate on the capillary plexus and release hormones.
• Small venules connect to a secondary capillary plexus in the adenohypophysis, forming a portal system.

Cells and Hormones of Adenohypophysis

• There are three cell types based on stains: acidophils, basophils, and chromophobes.
• Acidophils:
• Somatotrophs secrete somatotropin (growth hormone) for cell metabolism and general body growth.
• Mammotrophs produce prolactin that stimulates mammary gland development during pregnancy and maintains milk production after parturition.
• Basophils:
• Thyrotrophs release thyroid-stimulating hormone (TSH) that stimulates the thyroid gland to produce thyroxin and triiodothyronine.
• Gonadotrophs secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
• Corticotrophs secrete adrenocorticotropic hormone (ACTH) to regulate adrenal cortex functions.

Neurohypophysis

• Does not have any secretory cells; secretory neurons are located in the hypothalamus of the brain.
• Releases two hormones from axon terminals: oxytocin and antidiuretic hormone (ADH).

Test your knowledge on how hydrophobic steroid and thyroid hormones circulate on transport proteins and activate cytoplasmic receptors in target cells.