Hypothalamic-Pituitary Axis 1 Flashcards
What differentiates endocrine glands from other glands within the body?
They are ductless
Describe classic endocrine secretion
glands have high blood flow and are connected to fenestrated capillaries, this allows quick transport of the secretions into the blood to the distal target organs
Describe paracrine secretion
secretions that act locally by being secreted into the extracellular space by a cell where it acts on a neighbouring cell of the same gland (no diffusion into blood)
Describe autocrine secretions
hormones that act directly on the cell that produces it
Describe the major endocrine glands of the upper body
- pineal gland (posterior aspect of the thalamus): secretes melatonin by reacting to light and dark circadian cycles to promote sleep
- hypothalamus (ventral aspect of diencephalon)
- pituitary gland (extends below hypothalamus)
- thyroid gland (bilobed- ventral aspect of trachea)
- parathyroid glands (4 - posterior to thyroid): secretes PTH responsible for regulating calcium levels in the blood
- thymus gland (below thyroid): white cell maturation and training T cells in immune response
Describe the major endocrine glands of the lower body
- adipose tissue (around the hips): produces leptin for regulating satiety levels
- stomach and small intestines: secrete brain-gut peptides important for glucose metabolism
- kidney: EPO hormones to stimulate RBC production and are involved with BP regulation
- adrenal glands: acute and chronic stress response
- pancreas: regulates blood sugar
- gonads: secrete sex hormones and peptide hormones
Describe the 3 mechanisms for controlling secretions from endocrine glands
- humoral release: some factors like metabolites/chemicals induces release of hormones (eg. low calcium in blood is sensed by thyroid gland and parathyroid gland have receptors in their cell membrane to detect calcium, low calcium disrupts signalling inducing release of PTH to stimulate increase in plasma calcium)
- neural: sympathetic neural innervation of adrenal medulla, chromaffin cells innervated by preganglionic sympathetic fibres, neural stimualtions cause release of ACh from nerve endings to stimulate receptors on chromaffin cells which cause release of catecholamines
- hormonal (feed-forward and feedback loops): cascade of hormones. One hormone released by one cell/tissue acts on a downstream group of cells to stimulate the release of other hormones (eg. interaction between hypothalamus and anterior pituitary: hypothalamus releases peptides which are modified nerves that can release hormones in bursts in the anterior pituitary to induce the release of more hormones into circulation)
Describe the anatomical features of the pituitary gland
- small organ found just inferior to optic chiasm
- connected to the hypothalamus via infundibulum/ pituitary stalk arises from medial eminence of hypothalamus
- enclosed in sphenoid bone: sella turcica with diaphragm sella superior and sphenoid sinus inferior
Describe the 2 structures that make up the pituitary gland
Adenohypothysis (anterior pituitary)- mainly neural bodies:
- pars distalis (largest)
- pars tuberalis (upgrowth of tissue from pars distalis during development)
- pars intermedia (line of cells adjacent to posterior lobe)
Neurohypophysis (posterior pituitary)-mainly neural tissue:
- pars nervosa (largely axonal terminals with no nuclei)
- pituitary stalk
Describe the physiological connections between the hypothalamus and pituitary gland
- hypothalamus contains large and small bodied neurons that release of hormones
- neurohypophysis receive neuronal axons directly from hypothalamus nuclei which travel through the pituitary stalk and send out hormones
- adenohypophysis has no neural innervation and so receive hormones from the hypothalamus through portal blood system
Describe the transportation of hormones from large bodied neurons of the hypothalamus
- AVP or oxytocin made in paraventricular or supra-optic nuclei
- neurons extend down and bundle together and then terminate into posterior pituitary and release hormones directly into capillary bed
- hormones flow into systemic circulation
Describe the transportation of hormones from small bodied neurons in the hypothalamus
- found in different nuclei to large bodied neurons and typically send out hormones into portal blood for the anterior pituitary to receive
- released in region of median eminence or further down into larger portal veins
- flow into capillary network around anterior pituitary to enter secondary plexus of hypophyseal system
- travel through fenestrations and released into cells and bind to target cells to cause further release of hormones
What are the 3 stages of the development of the pituitary gland?
Week 4:
- initiation of pituitary organogenesis and formation of Rathke’s Pouch
- evagination of Rathke’s pouch and cell proliferation
- linear determination and cellular proliferation
Describe the process of formation of the 2 lobes of the pituitary gland in development
- begins with thickening of cells in oral ectoderm (oral cavity)
- upgrowth gives rise to diverticulum (Rathke’s pouch) and then anterior pituitary
- extends to neural ectoderm
- downward extension of ventral diencephalon (developing infundibulum and posterior pituitary) forms
- 2 lobes fuses with upward growth from oral cavity
Describe the process of what happens to Rathke’s pouch in development
- Rathke’s pouch eventually constricts at its base and becomes separated from oral epithelium
- cells on anterior border of Rathke’s pouch undergo cell proliferation to form the anterior lobe
- then cells next to the wall undergo slower cell proliferation to form vestigial intermediate lobe
- in adults gap between pars intermedia and intermediate lobe are remnants of lumen of Rathke’s pouch
