Endocrine System

Many animals have a second, slower form of communication - the endocrine system with its chemical messengers.


cell proliferation, growth, differentiation, The integration of these events, as well as the communication and coordination of physiological processes, such as metabolism respiration, excretion, movement, and reproduction, are dependent on chemical messengers - molecules synthesized and secreted by specialized cells.


Chemical messengers can be categorized as follows:


1. Local chemical messengers.

Many cells secrete chemicals that alter physiological conditions in the immediate vicinity.

Most of these chemicals act on adjacent cells and do not accumulate in the blood.

Hormones produced in the gut that help regulate digestion.

In a wound, a substance called histamine is secreted by mast cells and participates in the inflammatory response.


2. Neurotransmitter.

acetylcholine that act on immediately adjacent target cells.

Messengers reach high concentrations in the synaptic cleft, act quickly, and are actively degraded and recycled.


3. Neuropeptides.

Some specialized neurons secrete neuropeptides.

Neuropeptides enter the bloodstream or other body fluids and are transported to nonadjacent target cells where they exert their effects.

In mammals, for example, certain nerve cells in the hypothalamus release a neuropeptide that causes the pituitary gland to release the hormone oxytocin, which induces powerful uterine contractions during the delivery of offspring.


4. Hormones.

Endocrine glands or cells secrete hormones that are transported by the bloodstream to nonadjacent target cells.


5. Pheromones.

Pheromones are chemical messengers released to the exterior of one animal that affect the behavior of another individual of the same species.


Nervous and endocrine systems work together as an all encompassing communicative and integrative network called the neuroendocrine system.


In this system, chemical messengers achieve short and long term regulation of body function in an animal to maintain homeostasis by means of feedback systems.


A hormone is a specialized chemical messenger produced and secreted by an endocrine gland or tissue.


The study of endocrine glands and their hormones is called endocrinology.


By definition, a target cell is one having receptors to which chemical messengers either selectively bind to or on which they have an effect.


Most hormones are proteins, derivatives of amino acids, or steroids: a few are fatty acid derivatives.


Hormones are effective in extremely small amounts.


Only a few molecules of a hormone may be enough to produce a dramatic response in a target cell.


in the target cell, hormones help control biochemical reaction in three different ways:

1. A hormone can increase the rate at which other substances enter or leave the cell

2. It can stimulate a target cell to synthesize enzymes, proteins, or other substances

3. It can prompt a target cell to activate or suppress existing cellular enzymes.


Although hormones are always present in some amount in endocrine cells or glands, they are not secreted continuously.


Instead, the glands tend to secrete the amount of hormone that the animal needs to maintain homeostasis.


Regulation occurs through a feedback control system, where changes in the animal or in the external environment are fed back to a central control unit, where the adjustment is made.


A feedback system that produces a response that counteracts the initiating stimulus is called a negative feedback system.


In contrast, a positive feedback system is one in which the initial stimulus is reinforced.


For example, suppose that the rate of chemical activity in the body cells of a dog slows down.


the hypothalamus responds to this slow rate by releasing more thyrotropin - releasing hormone, which causes the pituitary gland to secrete more thyrotropin, or thyroid - stimulating hormone.


this hormone, in turn, causes the thyroid gland to secrete a hormone called thyroxine.


thyroxine increases the metabolic rate, restoring homeostasis.


The function of a hormone is to modify the biochemical activity of a target cell or tissue.


Two basic mechanisms are involved.


the first, the fixed membrane receptor mechanism, applies to hormones that are proteins or amines.


Because they are water soluble and cannot diffuse across the plasma membrane, these hormones initiate their response by means of specialized receptors on the plasma membrane of the target cell.


The e mobile receptor mechanism applies to steroid hormones.


these hormones are lipid soluble and diffuse easily into the cytoplasm,, where they initiate their response by binding to cytoplasmic receptors.


Binding to a specific receptor site for that hormone on the plasma membrane.


The hormone receptor complex activates the enzyme adenylate cyclase in the membrane.


The activated enzyme converts ATP into a nucleotide called cyclic AMP, which will become the second or intracellular messenger.


Cyclic AMP diffuses throughout the cytoplasm and activates an enzyme called protein kinase, which causes the cell to respond with its distinctive physiological activity.


After inducing the target cell to perform its specific function, cyclic AMP is inactivated by the enzyme phosphodiesterase.


The mobile receptor mechanism involves the stimulation of protein synthesis.


This newly formed hormone receptor complex acquires an affinity for DNA that causes it to enter the nucleus of the cell, where it binds to DNA and regulates the transcription of specific genes to form messenger RNA.


The newly transcribed mRNA leaves the nucleus and moves to the rough endoplasmic reticulum, where it initiates protein synthesis.


Chemicals regulating growth, maturation, and reproduction were among the first hormones to appear during the course of animal evolution.


Porifera - None


Cnidarians - Hydra contain a neuropeptide that stimulates budding, regeneration, and growth.


Also stimulates mitosis in Hydra.


Platyhelminthes - Neurosecretory cells are known.


these cells are located in the cerebral ganglion and along major nerve cords.


The neuropeptides produced by the cells function in regeneration, asexual reproduction,a nd gonad maturation.


Nematodes - Neurosecretory cells associated with the central nervous system.


Controls ecdysis of the old cuticle.

Mollusks - nervous system of mollusks is richly endowed with neurosecretory cells situated in the ring of ganglia that constitutes the central nervous system.


Play a role in the regulation of heart rate, kidney function, and energy metabolism.


Specific hormone stimulates spermatogenesis; another hormone termed egg laying hormone stimulates egg development; and hormones from the ovary and testis stimulate accessory sex organs.


In all snails, the growth of the shell is under neuroendocrine control by way of a growth hormone.


Annelids - well developed endocrine control of physiological function.


generally involved with morphogenesis, development, growth, regeneration, and gonadal maturation.


Arthropods - The endocrine system of a crustacean, controls functions such as ecdysis, sex determination, and color changes.


X-organs are neurosecretory tissues located in the crayfish eye stalks.


Associated with each X-organ is a sinus gland that accumulates and releases the secretions of the X-organ.


Y-organs, are located at the base if the maxillae.

In the absence of an appropriate stimulus, the X-organ produces molt-inhibiting hormone, and the sinus gland releases it.


Another hormone, juvenile hormone, is also involved the morphological differentiation that occurs during the molting of insects.


located just behind the insect brain are the paired corpora allata. These structures produce juvenile hormone.


When the concentration of juvinile hormone in the blood of an insect is high, differentiation is inhibited.


In the absence of an appropriate environmental stimulus, the corpora allata decrease the production of juvenile hormone, which causes the insect larva to differentiate into the pupa.


Echinoderms - neuropeptide called gonad-stimulating substance.


Vertebrate - Vertebrates possess two types of glands.


One type, exocrine glands, secrete chemicals into ducts that, in urn, empty into body cavities or onto body surfaces.


The second type, endocrine glands have no ducts, and instead secrete chemical messengers called hormones directly into the tissue space next to each endocrine cell.


The hormones then diffuse into the bloodstream and are carried throughout the body to their target cells.


Vertebrates other than birds or mammals have somewhat similar endocrine systems, but differences do exist.


1. Hormones with the same function in deferent species may not be chemically identical.


2. Certain hormones are species specific with respect to their function; conversely, some hormones produced in one species may be completely functional in another species.


3. A hormone from one species may elicit a different response in the same target cell or tissue of a different species.


When one compares more ancient groups of vertebrates with more recent ones, one general tendency surfaces - older groups seem to have simpler endocrine systems.


In jawed fishes, three major regions have been identified that secrete neuropeptides.


The two in the brain are the pineal gland of the epithalamus and the preoptic nuclei of the hypothalamus.


The pineal gland produces neuropeptides that affect pigmentation and apparently inhibit reproductive development.



The preoptic nuclei produce various other neuropeptides that control various functions in fishes.


The third major region of fishes that has neuropeptide function is the urophysis.


The urophysis is a discrete structure in the spinal cord of the tail.


The urophysis produces neuropeptides that help control water and ion balance, blood pressure, and smooth muscle contraction.,


In may fishes, amphibians, and reptiles, hormones from the pituitary gland controls variations in skin color.


The major hormones produced by the thyroid gland are thyroxine (T4 and triiodothyronine (T3) which control the rate of metabolism, growth, and tissue differentiation in vertebrates.


In most animals, thyroxine and triiodothyronine regulate overall metabolism.


In amphibians, they play an additional role in metamorphosis.


In jawed fishes and primitive tetrapods, several small glands form ventral to the esophagus and are called ultimobranchial glands. These glands produce the hormone calcitonin that helps regulate the concentration of blood calcium.


Specialized endocrine cells or glands located near the kidneys prepare the animals being discussed for stressful emergency situations.


These tissues and glands produce two hormones (epinephrine or adrenaline, and norepinephrine or noradrenaline) that cause vasoconstriction, increased blood pressure, changes in the heart rate, and increased blood glucose levels.


With some minor exceptions, birds and mammals have a similar complement of endocrine glands.