Comparative structure of adrenal gland in vertebrates

Adrenal gland comparative structures in vertebrates

1. Adrenal gland
General and comparative study of adrenal cortex and adrenal medulla
Submitted by : Sachin Sharma
Roll No.81221680017
MSc4th semester
Submitted to Dr.AshaGarg Ma’am

2. Adrenal gland Humans
The adrenal glands are small, triangular-shaped
glands located on top of each kidney. That’s why
they are known as supra renal gland
They are essential for producing hormones that
regulate various body functions, including
metabolism, immune response, blood pressure,
and stress responses.
Each adrenal gland consists of two main parts: the
cortex and the medulla. The adrenal cortex
produces steroid hormones such as cortisol and
aldosterone, while the adrenal medulla secretes
catecholamineslike adrenaline and
noradrenaline.
These hormones help the body respond to stress
and maintain homeostasis.

3. Adrenal cortex
The adrenal cortex is the outer region of the adrenal glands, responsible for
producing a variety of steroid hormones critical for homeostasis and the
body’s response to stress.
It is subdivided into three distinct zones, each producing specific hormones:
Zona Glomerulosa:
Cell Type: Cells in this zone are small and arranged in clusters or glomeruli.
Location: The outermost layer of the adrenal cortex.
Primary Hormone: Aldosterone.Function:
Aldosterone is a mineralocorticoid that helps regulate blood pressure and
electrolyte balance by promoting sodium retention and potassium excretion in
the kidneys. This action increases blood volume and, consequently, blood
pressure.

4. Zona Fasciculata
Cell Type: Cells in this zone are larger and
arranged in long, straight cords.
Location: The middle and largest layer of the
adrenal cortex.
Primary Hormone: Cortisol.
Function: Cortisol is a glucocorticoid that plays a
critical role in the body’s response to stress. It
helps regulate metabolism by increasing blood
sugar levels through gluconeogenesis, influencing
fat, protein, and carbohydrate metabolism.
Additionally, cortisol has anti-inflammatory
effects and helps modulate the immune
response.

5. Zona Reticularis:
Cell Type: Cells in this zone are smaller and
arranged in a network or reticular pattern.
Location: The innermost layer of the adrenal
cortex.
Primary Hormones: Androgens (e.g.,
dehydroepiandrosterone, or DHEA).Function:
Androgens are precursor hormones that can be
converted to more potent sex hormones like
testosterone and estrogenin peripheral tissues.
They contribute to the development of secondary
sexual characteristics during puberty and influence
libido and overall energy levels.
The adrenal cortex is under the regulatory control
of the hypothalamic-pituitary-adrenal (HPA) axis.
The hypothalamus secretes corticotropin-releasing
hormone (CRH), which stimulates the anterior
pituitary gland to release adrenocorticotropic
hormone (ACTH). ACTH then stimulates the adrenal
cortex to produce and secrete its hormones. This
regulatory mechanism ensures that hormone levels
are adjusted according to the body’s needs,
particularly in response to stress, circadian
rhythms, and other physiological demands.

6. Adrenal medulla
The adrenal medulla is the inner region of the adrenal
glands, playing a crucial role in the body’s acute stress
response, often termed the “fight-or-flight” response.
It is composed of chromaffincells, which are derived
from the neural crest during embryonic development.
These cells function similarly to neurons and are
responsible for the synthesis and secretion of
catecholamines. The primary hormones produced by
the adrenal medulla include:
Adrenaline (Epinephrine):Function: Adrenaline
prepares the body for rapid action in stressful
situations. It increases heart rate, dilates airways, and
boosts blood flow to muscles while diverting it from
non-essential functions like digestion. This hormone
also promotes the breakdown of glycogen to glucose in
the liver, providing an immediate energy source for the
body.

7. Noradrenaline (Norepinephrine):
Function: Noradrenaline complements the actions of
adrenaline by constricting blood vessels, thereby
increasing blood pressure. It also increases heart
rate and blood flow to muscles. Noradrenaline’s
effects are more localized compared to adrenaline,
which has a more widespread impact on the body.
The release of catecholaminesfrom the adrenal
medulla is regulated by the sympathetic nervous
system. In response to stress signals, the
hypothalamus sends nerve impulses through the
sympathetic nervous system to the adrenal medulla,
prompting the rapid release of adrenaline and
noradrenaline into the bloodstream. This quick
hormonal surge enables the body to respond
immediately to stressful stimuli, enhancing physical
performance and readiness for action.
Together, the adrenal medulla and cortex function
to maintain the body’s balance and respond to both
acute and chronic stressors, ensuring survival and
adaptation in changing environments.

8. Chromaffincells
Chromaffincells are the primary cell type in the
adrenal medulla, playing a crucial role in the body’s
response to stress.
Structure:HistologicalAppearance: Chromaffincells
are large, polygonal cells that appear basophilic
(staining dark with basic dyes) under a microscope.
They are arranged in clusters or cords around blood
vessels.Granules: These cells contain dense-core
granules, which store and secrete catecholamines
(adrenaline and noradrenaline) upon stimulation.

9. Fish
The adrenal glands in fishes, often referred to as the
interrenaland chromaffintissues, differ in structure
and location compared to those in mammals but
serve analogous functions in regulating stress
responses and metabolic processes.
InterrenalTissue:Location: The interrenaltissue in
fishes is analogous to the adrenal cortex in mammals.
It is typically found embedded in the kidney or along
the posterior cardinal vein.Structure: This tissue
consists of clusters of steroidogenic cells that
produce corticosteroids.
ChromaffinTissue:Location: Chromaffincells in
fishes, analogous to the adrenal medulla in
mammals, are often found interspersed with or near
the interrenalcells within the kidney or associated
with the walls of major blood vessels such as the
dorsal aorta.Structure: These cells are similar to
those in the mammalian adrenal medulla and contain
dense-core granules that store catecholamines.

10. Functions
InterrenalTissue:Corticosteroid Production: The interrenalcells produce
corticosteroids, primarily cortisol (in most teleost fishes) or cortisone.
Roles of Corticosteroids: Metabolism: Corticosteroids regulate carbohydrate, protein,
and lipid metabolism, maintaining blood glucose levels and energy balance.
Osmoregulation: They play a crucial role in ion and water balance, especially important
for fishes transitioning between freshwater and seawater environments.
Stress Response: Cortisol is involved in the stress response, helping to mobilize energy
reserves and modulate immune functions during stress
Chromaffincells: CatecholaminesProduction: Chromaffincells synthesize and release
catecholamines, primarily adrenaline (epinephrine) and noradrenaline (norepinephrine).
Roles of Catecholamines
Cardiovascular Regulation: Catecholaminesincrease heart rate, blood pressure, and blood
flow to muscles, aiding in the acute stress response.
Metabolism: They promote the breakdown of glycogen to glucose (glycogenolysis) and the
release of glucose into the bloodstream, providing a quick energy source.Respiratory
Function: Catecholaminesenhance gill perfusion and oxygen uptake, which is vital for
maintaining oxygen supply during periods of increased activity or stress.

11. Amphibians
Adrenal gland of frog contains two main types of cells :
chromaffincells
Corticosteroidogeniccells
Chromaffincells contain a number of regulatory factors :
Catecholamines
Serotonin
Vasotocin
Vasoactive interstitial peptide
Occurance of PNMT( phenyl ethanolamine- N-Methyl
transferase) has been found in sub population of cells which
represent 77% of the chromaffin tissue .

12. Catecholamines All frog chromaffin cells can synthesize dopamine and nor adrenaline . Even they can convert nor
adrenaline into adrenaline. These are significantly stimulated by depolarizing pulse of potassium. Dopamine
induces dose dependent inhibition of Corticosterone and aldosterone release , which is associated with reduced
secretion of prostaglandins .
Serotonin It is present exclusively in PNMT containing chromaffin cells , indicating that serotonin is colocalised
with adrenaline. Serotonin normally remains sequestered in chromaffin vesicles located at the periphery of the
cells. It is rapidly metabolised in tissues . Serotonin is a potent stimulator of corticosteroid secretion in amphibians
. Action of serotonin is mediated through 5-HT4 receptor, same as humans .
Vasotocin The neuropeptide arginin vesotocsin causes stimulation of ACTH release from frog pituitary
corticotrophs. It occurs in chromaffin cells of frog , even all chromaffin cells expresses vasotocin. It stimulate
secretion of both corticosterone and aldosterone from adrenal gland. It is by far the most potent steroidogenic
stimulator. Action of vasotocin is mediated through receptors related to both mammals V2 and oxytocin receptors
positively coupled to PLC.
Vasoactive interstitial peptides It is also sequestered in chromaffin vesicles . It controls the
corticosteroidogenesis .
https://www.researchgate.net/publication/15617943_Neuroendocrine_Communication_in_the_Frog_Adrenal_
Gland

13. Reptiles
Adrenal Gland Structure: The adrenal glands in squamates
consist of two main tissue types: steroidogenic tissue, which
produces steroid hormones, and chromaffin tissue, which
produces catecholamines like norepinephrine and epinephrine.
These tissues are closely associated but vary significantly in
their organization and proportions across different species.
Evolutionary Perspective: The adrenal gland’s structure in
squamates reflects an evolutionary trend towards a more
integrated and compact gland. This evolutionary pathway starts
with the more primitive arrangement seen in fish, where the
tissues are separate, and culminates in the highly organized
structure observed in mammals.

14. Variability in Squamates: In squamates, there is considerable variability in the degree of mixing
between the chromaffin and steroidogenic tissues. This variability is thought to be related to the
phylogenetic history of different species, indicating that evolutionary adaptations may influence
the adrenal gland’s structure and function.
Comparative Anatomy: Unlike amphibians, where adrenal tissues are often dispersed or embedded
in the kidney, squamates typically have adrenal glands that are paired, elongated, and
encapsulated, closely associated with the gonads and main blood vessels.
https://www.researchgate.net/publication/373325251_The_Adrenal_Gland_of_Squamata_Reptilia_
A_Comparative_Overview

15. Birds
Shape and Size: Right and left adrenal glands differ in shape, position,
weight, length, width, and thickness.
 Encapsulation: The glands are encapsulated with thin connective
tissue containing blood vessels.
Tissue Composition: The adrenal parenchyma consists of mixed cortical
and medullary tissues.
 Zones: The gland is divided into subcapsular layer, peripheral zone, and
central zone. The medulla is predominant in the central zone.
Cortico-Medullary Ratio: Approximately 60% cortex and 39% medulla,
with a ratio of 1.6:1.Cell Structure: Cortical cells have small, round to
slightly oval nuclei; medullary cells are polygonal with large, spherical
nuclei.
https://www.jstage.jst.go.jp/article/jpsa/49/1/49_011038/_article

16. Bird Adrenal Organization: Bird adrenal glands are similarly organized to those of
turtles, crocodilians, and most snakes, with variable proportions of chromaffin and
adrenocortical cells.
 Zonation: Some zonation of cell types is observed, resembling the mammalian
pattern but less clearly defined.
 Steroidogenesis: Birds synthesize corticosterone, aldosterone, and 18-
hydroxycorticosterone, with the latter being an aldosterone precursor not found in
plasma.
Hormonal Responses: Bird adrenals respond to ACTH, prolactin, growth hormone,
serotonin, and parathyroid hormone, indicating complex regulation mechanisms.
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/adrenal-gland