It is located in the center of each adrenal gland and appears as a thin, gray medulla.
The adrenal medulla is actually a sympathetic ganglion in which postganglionic neurons have lost their axons and have become secretory cells.
The cells secrete when they are stimulated by preganglionic nerve fibers that reach the gland through the splanchnic nerves.
Adrenal medullary hormones work primarily to prepare the body for emergencies.
The adrenal medulla is innervated by sympathetic preganglionic neurons.
Residing here are chromaffin cells, splanchnic nerves, and dilated capillaries.
The cells of the adrenal medulla are arranged in small groups and possess cytoplasmic granules that contain epinephrine .
These cells can be stained using the chromaffin reaction, and appear yellowish-brown in color.
These are called chromaffins and are the equivalent of postganglionic cells.
These chromaffin cells are homologous to sympathetic neurons and, like sympathetic neurons, develop from embryonic neural crest cells.
The adrenal medulla is involved in the release of hormone-like substances into the blood.
Chromaffin cells are responsible for the production of these substances: catecholamines, such as epinephrine (adrenaline), norepinephrine ( norepinephrine ) and dopamine.
Cells release preformed catecholamines from intraneuronal vesicles by exocytosis when activated by the preganglionic sympathetic fiber neurotransmitter.
Epinephrine is released directly into the medullary capillaries and is brought to its site of action through the systemic circulation.
When they enter the bloodstream, they act like hormones rather than neurotransmitters.
The physiological effect it has depends on the neuroreceptors present at the site where the chemical acts.
When these neurons are stimulated, they release chemical messengers, neurotransmitters, into the blood that act like hormones.
Cells that have receptors for epinephrine or norepinephrine respond to these substances and participate in the fight or flight response.
The adrenal medulla has high vascularity, as it has numerous blood vessels and is an important and unique part of the somatic nervous system.
The adrenal glands have two embryonic origins and consequently produce two different types of chemicals.
The outer cortex is of mesodermal origin and releases steroid hormones, while the inner medulla is derived from the ectoderm and secretes adrenergic neurotransmitters (also known as catecholamines.
In the fifth week of development, mesothelial cells enter the mesenchymal layer. Later, the large acidophilic cells differentiate, forming a primitive cortex.
The smaller cells then migrate and engulf the acidophilic cells, this will continue to form the final cortex.
Location of the superrenal glands
The adrenal glands are located in the upper part of the kidneys.
The left and right glands differ slightly in shape and location.
The right gland is more pyramidal and sits on top of the upper pole of the kidney, while the left gland is crescent-shaped and hangs more over the medial side of the left kidney, superior to the hilum.
The right adrenal gland rests on the diaphragm and the anterior medial portion is crossed by the inferior vena cava.
While on the left side, the adrenal gland rests on the left portion of the diaphragm and the upper anterior surface is covered by the peritoneum.
On both sides, the adrenal glands are enclosed in Gerota’s renal fascia, which separates it from the kidney.
Morphology of the adrenal glands
The adrenal glands are essentially made up of two structures: the adrenal medulla and the adrenal cortex.
There is an outer fat capsule just deep in the kidney fascia that functions as an additional protective layer. Deep within the capsule is the adrenal cortex.
These two endocrine organs in the adrenal gland are one surrounding the other.
The outer adrenal cortex secretes steroid hormones.
The cortex can be subdivided into the zona glomerulosa, zona fasciculata, and zona reticularis:
- The zona glomerulosa: it is composed of small rounded cells that are responsible for the secretion of mineralocorticoids such as aldosterone. Aldosterone regulates the absorption of water in the distal convoluted tubules, which consequently alters the blood pressure of the body.
- The zona fasciculata: it is significantly thicker than the other two cortical layers. It is made up of vacuolated cells arranged in parallel rows. This layer is responsible for secreting glucocorticoids to increase the overall blood glucose level in an effort to provide more energy for a system under stress .
- The zona reticularis: consists of smaller cells that stain darker in relation to the cells of the aforementioned layers. Here, adrenal androgens are produced, which serve as precursors to testosterone .