Glands Flashcards
What are endocrine glands?
secrete hormones directly into the bloodstream (e.g., thyroid gland).
What are exocrine glands?
release their substances either directly onto an epithelial surface (e.g., goblet cell in the gut), or via a system of ducts (e.g., major salivary glands). Can be classified into unicellular (e.g., goblet cell) or multicellular glands.
Multicellular glands are classified based on presence of tubule and acini, presence and configuration of duct (simple or compound).
What are glands formed from?
Epithelial tissue
Thyroid gland location, epithelium and (para)follicular cells secretion
- Location: Anterior neck with right and left lobes, connected to an isthmus.
- Cells are arranged into spherical structures, called follicles, in which the thyroid hormones are stored.
- Each follicle is lined with a single layer of follicular cells (cuboidal epithelium) and surrounded by reticular fibers. Each follicle is filled with the colloid (eosinophilic as lots of protein) which is composed of thyroglobulin. Thyroglobulin contains modified tyrosine amino acids that constitute the thyroid hormones, thyroxine (tetraiodothyronine, T4) and triiodothyronine (T3).
- Parafollicular cells secrete calcitonin (makes bone more dense by taking calcium to bone). Stain lighter than follicular cells with basophilic nucleus and found outside follicles.
- A vascular capillary network surrounds the follicles for entrance of thyroid hormones into the bloodstream.
- Connective tissue septa from the thyroid gland capsule extend into the gland’s interior and divide the gland into lobules.
Posterior pituitary gland subdivisions and hormone secretion
- controlled by the hypothalamus (continuation of it)
- posterior pituitary (neurohypophysis or pars nervosa) is derived from a downgrowth of nervous tissue from the hypothalamus, to which it remains joined by the pituitary stalk.
- subdivisions are infundibulum (pituitary stalk) and the pars nervosa.
- contains the hypothalamo-hypophyseal tract which consists of axons from neurons whose cell bodies are located in the supraoptic and paraventricular nuclei of the hypothalamus.
- secretes two hormones, antidiuretic hormone ( ADH ), also called vasopressin and oxytocin, both of which act directly on non-endocrine tissues. ADH is synthesised in the neurone cell bodies of the supraoptic nucleus, and oxytocin is synthesised in those of the paraventricular nucleus of the hypothalamus. Bound to glycoproteins, the hormones pass down the axons of the hypothalamopituitary tract through the pituitary stalk to the posterior pituitary where they are stored in the distended terminal parts of the axons. Release of posterior pituitary hormones is controlled directly by nervous impulses passing down the axons from the hypothalamus, a process known as neurosecretion .
Anterior pituitary gland (adenohypophysis)
- controlled by the hypothalamus
- anterior pituitary gland (adenohypophysis) arises as an epithelial (ectodermal) upgrowth from the roof of the primitive oral cavity (mouth) known as Rathke’s pouch.
- subdivisions are pars distalis (anterior), pars tuberalis (around pituitary stalk) and pars intermedia (posterior), described by relation to Rathke’s pouch
Pituitary gland histologically
The two major components of the gland, the anterior pituitary AP and the posterior pituitary PP , are easily seen at this magnification. The posterior pituitary is connected to the hypothalamus H by the pituitary stalk P and, like the hypothalamus, is composed of nervous tissue. Note the close proximity of the third ventricle V above the hypothalamus and the optic chiasma O anteriorly.
Acidophils - Hormone-containing granules in the cytoplasm stain with acidic dyes, e.g., eosin
Basophils - Hormone-containing granules in the cytoplasm of these cells stain with basic dyes, e.g., hematoxylin
Cell types and secretion in anterior pituitary
Hormones in the pituitary gland histology
The hormones accumulate and are stored in the distended terminal ends of unmyelinated axons as Herring bodies (pink).
When needed, hormones (oxytocin and ADH) from the neurohypophysis are released and enter the adjacent fenestrated capillaries in the pars nervosa
Hormones in the pituitary gland
Adrenal gland location and zones
- Location: Superior pole of each kidney, surrounded by a dense irregular connective tissue capsule and embedded in the adipose tissue around the kidneys.
- The secretory portion of each adrenal gland consists of an outer cortex (mesoderm) and an inner medulla (neural crest).
- Cortex composed of steroid secreting cells and medulla composed of chromaffin cells.
Pancreas location and gland types
- Located in the curve of the duodenum and divided into a head, body, and tail.
- Is both an exocrine and an endocrine gland.
- The exocrine portion produces an alkaline secretion containing digestive enzymes that empties into the duodenum.
- The endocrine portion secretes insulin, glucagon, and somatostatin that regulate blood glucose levels.
Endocrine pancreas cell types and secretions
- Islets of Langerhans are clumped masses of pale-staining endocrine cells embedded in the exocrine acinar tissue of the pancreas.
- Predominate cell types and secretions:
1. α cell (alpha cell). Secretes glucagon, which elevates glucose levels in the blood (converts glycogen to glucose). Located peripherally
2. β cell (beta cell). Secretes insulin, which lowers blood glucose levels (converts glucose into glycogen). Located centrally and most abundant
3. δ cell (delta cell). Secretes somatostatin (inhibits HCL secretion from parietal cells). Scattered and much less abundant
islet of Langerhans (I) embedded in the exocrine component (E) of the pancreas. The islets, roughly spherical, are composed of much smaller cells than the exocrine pancreas, from which they are separated by a fine fibrocollagenous capsule (C).
Exocrine pancreas
- Compound acinar gland; the acinar cells secrete numerous digestive enzymes that break down proteins, carbohydrates and lipids.
- Ducts begin as centroacinar cells located within the acini.
- Intercalated ducts are lined with low simple squamous to cuboidal epithelium (secrete HCO3).
- Interlobular ducts lead into one or more excretory ducts that empty into the duodenum.
Endocrine summary of glands
Types of exocrine secretions
Merocrine: Secretory product is released by exocytosis of contents contained within membrane-bound vesicles. E.g., pancreatic and parotid gland secretions.
Apocrine: Secretory material is released in an intact vesicle along with some cytoplasm from the apical region of the cell. E.g., mammary gland secretions.
Holocrine: Entire cell is released during the secretory process. E.g., sebaceous glands.
Salivary glands (3)
Parotid – Purely serous
Submandibular and Sublingual – Mixed glands with serous demilunes
As the processes of myoepithelial cells contract, they compress the acinus, facilitating release of the secretory product into the duct of the gland
Salivary gland duct system
Parotid gland location and constituents
- Located in each cheek near the ear, branched (compound) tubuloacinar glands (exclusively serous)
- Has irregular, collagenous connective tissue capsule providing septa that subdivide the glands into lobes and lobules.
- Serous cells of parotid glands secrete abundant α-amylase that initiates hydrolysis of carbohydrates.
- Serous in nature. Thin, watery, protein secretion. Nucleus is round and basally located.
Submandibular gland location and constituents
- The paired submandibular glands are roughly ovoid in shape and are situated just below the mandible.
- Their ducts open into the floor of the mouth, one on each side of the frenulum of the tongue.
- There is a network of myoepithelial cells between the epithelium and the basement membrane of the acini, contraction of the myoepithelial cells squeezing the secretion toward the major ducts.
- The submandibular salivary gland secretes a mixed serous/mucous product. Mucus. Thick, viscous, glycoprotein secretion. Nucleus is flattened and located in the base of the cell.
- The secretory acini empty into intercalated ducts and merge to form intralobular ducts. Intralobular duct modifies the concentration and content of the fluids produced by the secretory acini.
- The intralobular striated ducts fuse to form larger interlobular ducts.
- Interlobular ducts then join to form the major ducts.
