General Considerations In Endocrinology And The Endocrine System
The Endocrine System
A General Overview
Functions of the endocrine system may be considered under the following heading:
- Maintenance of cellular constancy of the body,
- Maintenance of the constancy of body fluids (microhomeostasis).
- Control of metabolic processes
- Growth and differentiation of tissues
- Reproduction and
- Adaptation of changes in the external environment (fluid, food and electrolyte availability).
The effector arms of the endocrine system are hormones or chemical messengers which are secreted by specialized cells and released into circulation in response to specific signals. There are specific cells in the tissues which can recognize hormonal signals because of the presence of specific receptors. Hormones also serve to communicate information between the cells. Endocrine dysfunction may manifest as overt clinical disorders such as Cushing’s syndrome and acromegaly or silent biochemical abnormalities like hyperglycemia or failure of control mechanisms as in gonadal disturbances.
In addition to organized tissues like thyroid and adrenal which are composed of groups of endocrine cells, many tissues like the gut and brain are provided with single cells which secrete hormones. These hormones diffuse to other cells to produce their effect. These cells are called paracrine cells. Biological responses are also under control of other substances such as the neurotransmitters like catecholamines and acetylcholine, prostaglandins, histamine, kinins and slow reacting substance- A (SRS-A). Intracellular chemicals like cyclic 3,5-adenosine monophosphate (Cyclic AMP), cyclic GMP) and calcium act as second messengers for exerting the biological activity of hormones on cells.
Developmentally, the origin of the endocrine system can be traced back to all the three germ layers. Though the components of the endocrine system can be conventionally classified into separate groups, based on their anatomy and location, the explosion of information in this field has brought to light several other tissues which have endocrine function in addition to the well recognized glands.
The Various Endocrine Organs
The Endocrine Glands
The endocrine glands can be broadly grouped into:
The pituitary-thyroid gonadal-adrenal axis (Classical endocrine system): These glands act in unison through feedback control and modulate biological activity of distant target cells and organs.
The neuroendocrine system derived from neurectoderm: This is composed of neurosecretory cells. The central component of this system is formed by the brain-pituitary system and peripheral component is made up of scattered paracrine cells in many organs. The paracrine system is seen mainly in the gastrointestinal tract, adrenal medulla and the ‘c’ cells of thyroid. These paracrine cells are capable of amine precursor uptake and decarboxylation (APUD), which is necessary for the formation of amine or peptide hormones.
Tissues which produce humoral substances which circulate in association with the known hormones e.g prostaglandins and kinins: These exert strong physiological responses and tend to modulate effects. The hormones belong to three chemical classes- peptides, steroids and amines.
The endocrine system and its glands are of huge significance to Man, as they facilitate basic metabolic processes which sustain Man and keep him alive.
© 2014 Funom Theophilus Makama