Hypophysis (Exam II) Flashcards
What is the embryological origin of the adenohypophysis? Of the neurohypophysis?
- The adenohypophysis is derived from the ectoderm of the stomodeum (primitive oral cavity).
- It evaginates from the oral cavity to form Rathke’s pouch, elongates superiorly to meet the neurohypophysis.
- The neurohypophysis is a downgrowth from the neuroectoderm of the diencephalon.
- Note that the adenohypophysis eventually loses its continuity with the oral cavity, but the neurohypophysis retains its connection to the brain throughout life.
What is the definition of a portal system? Does the pituitary have a venous or arterial portal system?
- A portal system is an arrangement of blood vessels in which the blood passes through two separate capillaries beds arranged in series before it returns to the heart. If the vessels that connect the two capillary beds are arteries, it is an arterial portal system. If they are veins, it is a venous portal system.
- The pituitary contains a venous portal system called the hypothalamic-hypophyseal portal system. The first capillary bed is located in the median eminence and infundibulum, and is connected by veins to a second capillary bed located mainly in the pars distalis.
What are the functions of the portal system that is found in the pituitary?
- The first capillary bed (in the median eminence) picks up the releasing and inhibitory factors secreted by the processes of hypothalamic neurons of the parvicellular system. These are carried by veins to the second capillary bed (mainly in pars distalis) where they regulate the secretion of hormones from the acidophils and basophils.
- The second capillary bed then picks up the products of the acidophils and basophils and carries them out of the pituitary toward their target organs.
Trace the life history of a molecule of oxytocin. Where is it synthesized? Where is it secreted? What are its target organs? What effects does it have on these target organs?
Oxytocin is synthesized by the neurons of the magnocellular system in the hypothalamus.
The long cytoplasmic processes of these neurosecretory cells extend into the pars nervosa of the pituitary, forming the hypothalamo-hypophyseal tract.
Oxytocin travels down these long processes and is secreted mainly in the pars nervosa.
There it is picked up by a capillary bed (not officially a part of the portal system of the pituitary but having some cross-connections with it) and carried out of the pituitary.
The main target organs for oxytocin are the mammary glands and the uterus.
In the mammary gland it causes contraction of the myoepithelial cells, which results in secretion of milk.
It is responsible for the efferent limb of the milk ejection (milk let down) reflex.
In the uterus is causes contraction of the uterine smooth muscle in a pregnant woman at the time of birth, thus aiding in the delivery of the infant.
In order for this to occur, increased numbers of oxytocin receptors develop on uterine smooth muscle near the time of delivery.
During parturition, oxytocin secretion is stimulated by pressure exerted by the fetus on the cervix and vagina.
Oxytocin also appears to have effects on the brain, where it enhances maternal behavior patterns.
Do males secrete oxytocin?
Yes, they do. In the male, oxytocin levels increase during ejaculation, and it is possible that it may facilitate sperm transport by stimulating the contraction of smooth muscle in the duct system that leads from the testis to the penis.
Is antidiuretic hormone synthesized in the pituitary? What is its major target organ and what effects does it have there?
No, antidiuretic hormone (ADH), also called vasopressin, is not synthesized in the pituitary. Like oxytocin, it is synthesized in the cell bodies of the hypothalamic neurons of the magnocellular system, carried down their processes into the pars nervosa of the pituitary, and secreted there (i.e., it is secreted, but not synthesized, in the pituitary).
The major effect of ADH is on the kidney, where it stimulates insertion of water channels (aquaporins) into the cell membranes of certain kidney tubules. This allows these tubules to reabsorb water from their lumen, thus concentrating the urine and conserving body water.
What are Herring bodies? Where do you find them?
Herring bodies are dilations of neuronal processes in the hypothalamo-hypophyseal tract caused by the accumulation of electron-dense secretory granules. The granules contain oxytocin, vasopressin, and carrier proteins known as neurophysins.
Herring bodies are found in the pars nervosa. Several such dilations may occur along the length of a single neuronal process, much like a series of beads on a string.
If the pituitary stalk were cut, which type of acidophil or basophil in the adenohypophysis would increase its production of secretory product? Why?
Secretion of prolactin by the mammotrophs (also called lactotrophs) would increase if the pituitary stalk were cut, whereas secretion of all other hormones from the adenohypophysis would decrease dramatically.
This is because the hypothalamus produces an inhibitory factor (dopamine) that tonically suppresses prolactin release, but produces releasing factors that stimulate secretion of all other hormones. Cutting the pituitary stalk would remove all these regulatory inputs, resulting in decreased inhibition of mammotrophs and loss of the required secretory stimuli for all other acidophils and basophils.
Hypophysis consists of:
Adenohypophysis:
- Pars distalis
- Pars tuberalis
- Pars intermedia
Neurohypophysis:
- Infundibulum
- Pars nervosa
Adenohypophysis
- Epithelial in appearance (i.e., has closely packed cells) as opposed to the more fibrillar appearance of the neurohypophysis
- Adenohypophysis is located in the hypophyseal fossa (part of the sella turcica) of the sphenoid bone
- Adenohypophysis is a derivative of an outpocketing of the oropharyngeal ectoderm (Rathke’s pouch)
- Adenohypophysis contains 3 classes of cells by routine LM: Acidophils, basophils and chromophobes. Acidophils & basophils are collectively called chromophils
Pars tuberalis
Surrounds the infundibulum to form the stalk
Pars intermedia
Lies between pars nervosa and pars distalis Often contains cysts (Rathke’s cysts) filled with colloid Contains many basophils
Pars distalis
Forms part of the body of the pituitary (along with pars intermedia and pars nervosa)
Contains more acidophils than pars intermedia
Acidophils
Two types:
- Somatotropes: produce growth hormone (GH, somatotropin)
- Mammotropes: produce prolactin
Basophils
- At least three types:
- Gonadotropes: produce follicle stimulating hormone (FSH) & luteinizing hormone (LH)
- Corticotropes: produce adrenocorticotropin (ACTH) & other derivatives of pro-opiomelanocortin (POMC)
- Thyrotropes: produce thyroid-stimulating hormone (TSH)
- Melanotropes usually considered to be a fourth type of basophil
- Related to corticotropes Produce melanocyte-stimulating hormone (MSH)
- MSH is also a POMC derivative: involved in pigmentation in lower vertebrates. The function and importance in humans is uncertain
Chromophobes
Cytoplasm stains poorly May be undifferentiated cells or degranulated chromophils
What do the 3 arrows indicate?
Pars tuberalis
Pars intermedia
Pars distalis
What are the cell types in the adenohypophysis?
Acidophils, Basophils & Chromophobes
What structure is shown here?
Herring Body.
What is the function of the labeled structure?
Secretory granules cause dilations of axons from hypothalamus to neurohypophysis.
What stain is this? What are the labeled cells?
Trichrome. Basophils, Acidophils & Chromophobes.
Distinguish the two locations of these tissues.
Neurohypophysis
Adenohypophysis
Identify the parts of this gland.
infundibulum
Pars nervosa
What are the 3 layers? What is in the middle?
Pars distalis > pars intermdia > pars nervosa
Colloid in rathke’s cyst (middle)
Where is this tissue? What are the first pair of arrows showing? What about the X?
Neurohypophysis
Pituitcytes
Unmyelinated axons fo hypothalamic neuron
