Endocrine System Flashcards
Description of Hypophysis?
Pituitary (divided into the ‘anterior pituitary’ (mostly pars distalis along with pars tuberalis, and pars intermedia) and ‘posterior pituitary’ (pars nervosa and infundibular stalk)
Location of the Pituitary Gland (Hypophysis)
Sphenoid bone of the skull
Describe the origin of the pituitary gland.
Ectoderm from roof of oral cavity - grows upwards as Rathke’s pouch; the lumen of Rathke’s pouch becomes the residual cleft; seen in the adult pituitary between the pars distalis and the pars nervosa.
Neuroectoderm from diencephalon - grows downwards, becomes the posterior pituitary.
Describe the pars distalis.
Parenchyma of cords or clusters of cells associated with sinusoids; part of adenohypophysis.
What is special about ‘cords or clusters’?
The description of ‘cords or clusters of cells seperated by many capillaries or sinusoids’ is characteristic of many endocrine organs.
What is special about Chromophobes?
DO NOT STAIN
Describe the different Chromophils.
Acidophils: granular, numerous cells, red with H&E
- some produce growth hormone (‘somatotrophs’)
- some produce prolactin (‘lactotrophs’) or (memotrophs)
Basophils: blue with H&E:
- some produce TSH (Thyroid stimulating hormone) (‘thyrotrophs’)
- some produce gonadotropic hormones (LH and FSH) (‘gonadotrophs’)
- some produce ACTH (adrenocorticotropic hormone) (‘corticotrophs’)
Describe Chromophobes.
Chromophobes are usually degranulated chromophils, but can be stem cells or supporting cells.
Describe Pars Tuberalis
Pars tuberalis forms a collar around the neural stalk. Cells are weakly basophilic and are arranged as cords, clusters, or follicles; is part of the adenohpophysis.
Describe Pars intermedia
Pars intermedia is a portion of the adenohypophysis adjacent to neurohypophysis. Looks similar to pars tuberalis. Secretes MSH - melanocyte stimulating hormone and some ACTH
How the hypothalamus “controls” the adenohypophysis.
Releasing and inhibiting factors (hormones) are shynthesized by special secretory neurons in the hypothalamus, transported to and stored in axon terminals ending in the median eminence, which is hypothalamic tissue extending into the pituitary (infundibular) stalk. Upon appropriate stimulus, the releasing and inhibiting factors will be discharged into capillaries at the median eminence and will travel in the hypophyseal portal system to capillaries in the pars distalis. A portal system is made up of veins that are interposed between two sets of capillaries. For example, CHRH (Growth hormone releasing hormone) secreated from axons whose cell bodies are in the hypothalamus would travel in the portal system to the capillaries in the pars distalis where it would cause some acidophils to release growth hormone (somatotropin).
Practice Focus: Adenoma of the Pars Intermedia
This is a slow-growing tumor seen in older horses and dogs. The major clinical signs in the horse relate to the fact that ACTH becomes a significant secretion resulting in hypercortisolism. Typical signs include appearance, weight loss, and poor healing.
What is the Neurohypophysis?
Pars Nervosa plus Pituitary Stalk
Describe the Neurohypophysis.
Contains numerous unmyelinated axons
Cell bodies are in the hypothalamus
Secretions (oxytocin and vasopression) form cell bodies travel down the hypothalamo-hypophyseal tract in unmyelinated axons to pars nervosa.
Swellings called Herring bodies are seen in the pars nervoas; there are acidophilic accumulations of neurosecretory material (oxytocin and vasopressin plus carrier proteins) within the terminal ends of axons that have traveled in the hypothalmic-hypophseal tract from cell bodies in the hypothalamus.
Pituicytes: cells in pars nervosa analogous to neroglia
What is also known as ADH (Anti-diretic hormone)
Vasopressin
Oxytocin and Vasopressin are made and accumulate where?
Oxytocin and Vasopressin are both made in the hypothalamus, but accumulate in the pars nervosa
What is a tract
Accumulation of axons
What is the origin of the Thyroid and where is it located?
The cells that make up most of the thyroid come from an outgrowth from the floor of the buccal cavity. The outgrowth eventually loses its connection to the pharynx (thyroglosal duct)
Two lobes connected by an isthmus (usually) - no isthmus in the dog
What is the structure of the thyroid?
Thyroid follicles which are variable in size are the structural and functional units of the thyroid. They are hollow spheres filled with colloid (a homogeneous, usually acidophilic material made mostly of a glycoprotein called thyroglobulin).
Describe the cell types of the thyroid.
Follicular cells - cuboidal epithelium (individual follicles may have a more columnar or more flattened lining, however). Follicular cells have microvilli on apical edge, and are held together by tight junctions (prevent leakage of thyroglobulin, which is a strong antigen).
Parafollicular cells (“C” cells, “clear cells”) are found as isolated clusters beside the follicle, or (rarely) as single cells within the basal lamina of follicles. They are larger, paler cells than the follicular cells; they produce calcitonin which lowers serum calcium levels).
What is the function of the Thyroid.
Even through the thyroid is classed as an endocrine gland, it has both endocrine and ‘storage’ phases of production (this in unusual among endocrine gl.)
Describe the Follicular cell storage phase.
Synthesize and secrete thyroglobulin, which is stored extracellularly in lumen of follicle, where it is partially iodinated.
Describe the Follicular cell endocrine phase.
Stored thyroglobulin is taken up by follicular cells and broken down (by lysosomal enzymes) to activate thyroid hormones, which are then released into blood. Thyroxine (T4 or tetraiodothyronine) is the prohormone and most abdundant; T3 (triiodothyronine) is less abundant than T4, but it is the active hormone. A deiodinase in liver, kidney, and muscle converts T4 to T3, which increases metabolic rate of target cells, and increases the nmber of mitochondria and hence mitochondrial respiration.
Note in recent years the term ‘thyroid hormone’ is often used to refer to T3.
Describe the colloid
Made up of thyroglobulin, a glycoprotein. Tyrosine amino acyl residues of thyroglobulin are iodinated, and this represents the storage phase of secretion.
