Glands and endocrine tissue

Question 1

What is a gland

Answer 1

A gland is an epithelial cell or aggregate of cells that are specialised for the secretion of a substance

Question 2

What is an endocrine gland

Answer 2

An endocrine gland is a ductless gland. These glands secrete directly into blood flowing through them, to let the secretion function at distant parts of the body. Their secretions are called hormones. Examples include:
- Pituitary gland
- Thyroid gland
- Parathyroid glnad

Question 3

What is an exocrine gland

Answer 3

An exocrine gland is a ducted gland. These glands secrete into a location or region of the body through a duct, and their secretions are mostly enzymes or lubricants. Examples include:
- Salivary gland
- Pancreas
- Mammary
- Sweat glands
- Sebaceous glands
- Lachymal glands

Question 4

What is Merocrine secretion

Answer 4

Merocrine secretion is through exocytosis where membrane bound compartment approaches the cell surface. It then fuses with the plasma membrane where the contents are in continuity with extracellular space and are dispersed by diffusion. The plasma membrane becomes larger, and the membrane is retrieved, and the cell surface area is stabilised
An example of regulated mercerise secretion is the secretion of insulin.

Question 5

What is Apocrine Secretion

Answer 5

Non membrane bounded structures such as lipids approach the cell surface. It makes contact and pushes up the apical membrane. A thin layer of apical cytoplasm frapes around the droplet. The membrane surrounding the droplet pinches of from the cell and the plasma membrane becomes smaller. The membrane is added to regain the original area.
An example of Apocrine secretion is the breast. In the neonatal period only fats are secreted by apocrine secretion, but during lactation, both fats and proteins are released by apocrine secretion.

Question 6

What is Holocrine secretion

Answer 6

Disintegration of the cell, which releases its contents and discharges the whole cell into the extracellular space.
An example of holocrine secretion is the Sebaceous gland as the secretly cel gradually fills up with secretory granules. The cell organelles degenerate and the cell dies. The plasma membrane breaks and the contents (secretin) empties, dead cells are replaced by mitosis division of the basal cells.

Question 7

How can glands be classified

Answer 7

Glands can either be classified by their mode of secretion (Merocrine, Apocrine or Holocrine), or based on where the end-products are released (Exocrine, Endocrine, Mixed).

Question 8

Describe the methods of endocytosis

Answer 8

Endocytosis is the engulfing of molecules inside the cell via vesicle formation. There are multiple mechanisms such as:
- Phagocytosis where cells (phagocytes), envelops/engulfs other cells/particles
- Pinocytosis where liquid droplets are ingested by cells
- Receptor mediated endocytosis

Question 9

How can exocrine glands be classified and what are each type of salient feature (Simple Structures)

Answer 9

Simple Tubular (large intestine) - Secretory portion of the gland. Is a straight tube formed by secretory goblet cells
Simple Branched Tubular (Stomach) - Mucus secreting glands are in the pylorus
Simple coiled Tubular (Skin) - The eccrine gland in the skin with the secretory portion found deep within the dermis
Simple acinar (Urethra) - The secretory portion is outpouching of the epithelial surface
Simple branched acinar (stomach) - Enlarged secreting mucus cells are in the cardia. The duct is relatively short

Question 10

How can exocrine glands be classified and what are each type of salient feature (Compound features)

Answer 10

Compound tubular (Duodenum) - The submucosal glands of Brunner that also secrete mucus.
Compound acinar (Pancreas) - Alveolar shaped cells are pyramid shaped and serous secreting
Compound tuboacinar (Submandibular salivary glands) - Can be a compound secreting gland producing both serous and mucinous secretions. Have serous endcaps (demilunes)

Question 11

Describe how endocytosis and secretion give transepithelial transport

Answer 11

Molecules may either enter through aqueous channels in the intercellular junction (paracellular transport) or through the lipid cell membranes (transcellular transport). Cells also retrieve material from the plasma membrane through endocytosis. This material can either be recycled to the plasma membrane or degraded in the lysosome. Examples of transcytosis include:
- Amino acids for hormone production
- Steroid hormones
- Thyroxine transport across thyroid follicular cells

Question 12

Explain the mechanism and importance of glycosylation of newly synthesised proteins in the Golgi apparatus

Answer 12

Glycosylation is the covalent attachment of sugars by enzymes to proteins and lipids to form glycoproteins and glycolipids. This:
- Aids protein folding
- Prevents protein digestion by intracellular processes
- Prevents lipid digestion by intracellular lipases
- Cell recognition
- Role on cell to extracellular matrix attachment
This is a critical function of the biosynthetic-secretory pathway of the endoplasmic reticulum and golgiapparatus

Question 13

Describe the role of secretions in cells functions e.g. communication

Answer 13

Secreted substances act as short- or long-distance signals to another cell or tissue type. For examples, neurons secrete neurotransmitters to send a message to neighbouring neurons, whereas the pituitary gland secretes several types of hormones that travel via the bloodstream to act throughout the body.

In other cases, secreted substances play a functional role within an organ or tissue type. In the stomach, for example, gastric glands contain 3 different cell types that secrete components of gastric acid. The mucous cells secrete lubricating mucus, parietal cells secrete hydrochloric acid, and chief cells secrete the precursor to the protein-digesting enzyme, pepsin. These all work in concert to break down food inside the stomach.

Question 14

Describe and discriminate between the simple mechanisms of secretion control (Humoral)

Answer 14

Humoral control - Capillary blood contains low concentrations of calcium which stimulates secretion of parathyroid hormone (PTH) by parathyroid glands. PTH acts to increase blood CA2+

Question 15

Describe and discriminate between the simple mechanisms of secretion control
(Nervous)

Answer 15

Some endocrine secre­tions are solely controlled by nerve im­pulses. Secretion of adrenal medullary hormones secretion of neuro-hypophysial hormones and various releasing hor­mones of hypothalamus are under this category.
Preganglionic sympathetic fibres stimulate adrenal medulla cells to secrete catecholamines

Question 16

Describe and discriminate between the simple mechanisms of secretion control (endocrine)

Answer 16

Some endocrine se­cretions are controlled by other endo­crine glands. For instance, different re­leasing hormones of hypothalamus con­trol the secretion of anterior pituitary hormones. TSH-RH of hypothalamus controls the secretion TSH from anterior pituitary.
The hypothalamus secretes hormones that stimulate other endocrine glands to secrete hormones.

Question 17

Discuss the differences between exocrine and endocrine glands

Answer 17

Exocrine:
- Ducted
- Secretions are mostly enzymes or lubricants
- Secrete in a location of the body through a duct
Endocrine:
- Ductless
- Secretions are hormones
- Secrete directly into blood flowing through them.

Question 18

Describe the molecular and cellular interactions that lead to gland formation

Answer 18

Gland formation (adenogenesis) - In utero development. The growth signal is received (FGF family member). This causes the proliferation of daughter cells, and extracellular protein degradation enzymes produced. The epithelial cells invade space created.
These either form exocrine glands (where central cells die of to produce duct (canaliculisation) and link to mother cells remain causing branching)
They can also form endocrine glands (which produce angiogenic factors to stimulate blood vessel growth in and around epithelial cells. The link to mother cells is broken through apoptosis and there is no branching).

Question 18

Describe the molecular and cellular interactions that lead to branching3

Answer 18

Central cells die of to produce duct (canaliculisation) and link to mother cells remain causing branching.
In the lungs, FGF10 is released by immature fibroblasts (mesenchymal stem cells). Epithelial cells move towards the signal which causes two different fates:
- Tubule elongation where growth factor 1 is active and growth factor 2 is inactive
- Tubule branching where growth factor 1 is active and growth factor 2 is inactive
Elongation and branching are stopped by Sonic the Hedgehog (Shh)

Question 19

Explain how the majority of ductless glands are derivatives of epithelial tissues

Answer 19

Most of the endocrine glands are epithelial derivatives. Endocrine glands are formed by invagination from an epithelial sheet and initially have ducts connecting them to the free surface of the epithelial sheet. During embryonic development, they will lose their ducts and thus are called ductless glands (No ducts).
Angiogenic factors stimulate blood vessel growth in and around epithelial cells. The link to mother cells is broken through apoptosis and there is no branching.

Question 20

Discuss the structure of endocrine glands and their products

Answer 20

Endocrine glands are ductless glands. Some of their products include:
- Glycoprotein and peptide hormones e.g. Insulin, glucagon, prolactin
- Steroid hormones e.g., Oestrogens, androgens, progesterone, cortisol
- Amino acid-derived hormones (Catecholamines) e.g., Adrenaline, noradrenaline, dopamine
- Amino acid-derived hormones (thyroid hormones) e.g., Thyroxine, triiodothyronine
- Lipid hormones e.g., thromboxane, prostaglandin, endocannabinoids

Question 21

Discuss the signalling pathways that regulate endocrine growth and function

Answer 21

All of the pituitary hormones are all subject to constitutive and regulated merocrine secretion. Which is why there is always a small concentration of these hormones in the blood

Question 22

Provide a detailed explanation of a portal circulatory route

Answer 22

Portal circulatory routes differ from the typical circulatory route in the blood passes through two sets of smaller vessels before returning to the heart. Blood from the first set of capillaries collects in portal vessels (sometimes called portal veins) which then begin to branch again to supply a capillary network to a second location before entering a series of veins which will lead to a heart.
One example is the Hypothalamo-hypophseal portal system where blood from the hypothalamus travels through two sets of capillaries before reaching the anterior pituitary.

Question 23

Discuss the approximate anatomical location and salient histological feature of the Thyroid gland

Answer 23

The thyroid gland is found in the front of your neck just below the larynx. It has a right lobe and a left lobe connected by a narrow isthmus.
Parafollicular cells are neuroendocrine cells that migrate into the thyroid during its development in the embryo. They produce the peptide hormone thyrocalcitonin which monitors calcium concentration and decreases CA2+ levels, (it counteracts the function of PTH)
The thyroid gland produces hormones that regulate the body’s metabolic rate controlling heart, muscle and digestive function, brain development and bone maintenance.

Question 24

Discuss the approximate anatomical location and salient histological feature of the parathyroid gland

Answer 24

The parathyroid is a set of four glands located in your neck, just behind the thyroid gland. They constantly monitor plasma calcium concentrations. If plasma calcium is low, then parathyroids make parathyroid hormone (PTH). PTH causes the bones to release calcium in the blood and prevents loss of calcium in the kidneys: absorb more calcium from the GI tract. Classical negative feeback

Question 25

What are the clinical consequences of parathyroid dysfunction

Question 26

Discuss the approximate anatomical location and structure of the Adrenal (suprarenal) glands

Answer 26

The adrenal glands are located at the top of each kidney. It is a mix of endocrine and neuroendocrine tissue. It is made up of the medulla (composed of chromaffin cells, that numerous pre-synaptic sympathetic nerve fibres pass directly through), and the adrenal cortex, which contain:
- Outer layer (Zona glomerulosa)
- Middle layer (Zona fasciculata)
- Inner layer (Zona reticularis)

Question 27

Discuss the salient histological feature in related to the function of the adrenal glands

Answer 27

The adrenal glands are involved in stress response.

Question 28

Describe the structure of the pancreas and how it relates to its function.

Answer 28

the pancreas is made up of Acinar glands, grouped into lobules. It contains numerous zymogen granules. It connects through intercalated ducts to the pancreatic duct which joins with the bile duct to make the common bile duct. It is an intercalated duct lined with cuboidal epithelial cells.

Question 29

Identify the exocrine components of the pancreas

Answer 29

The exocrine functions (acini) produce:
- Trypsinogen (converted to trypsin)
- Chymotrypsinogen (converted to chymotrypsin)
- Lipase
- Amylase
- Ribonuclease
- Deoxyribonuclease
- Gelatinase
- Elastase

Question 30

Identify the endocrine functions of the pancreas

Answer 30

Endocrine function (Isley of Langerhans)
Alpha cell - Glucagon (catabolises glycogen to glucose)
Beta cell - Insulin (Uptake and storage of glucose)
Sigma cell - Somatostatin
PP cell - Pancreatic polypeptide
D-1 cell - Vasoactive intestinal peptide
EC cell - Secretin
E cell - Ghrelin
G cells - Gastrin