Introduction to endocrine glands of fishes

The glands that secrete their products into the bloodstream and body tissues along with the central nervous system to control and regulate many kinds of body functions are known as endocrine gland. In fishes various endocrine gland has been found associated with different tasks and functions.

Endocrine glands of fishes: Different types of endocrine glands are found in fishes; such as-

• The pituitary gland or Hypophysis
• Thyroid Gland
• Adrenal gland
• Corpuscles of Stannius
• Ultimobranchial Glands
• Urohypophysis
• Pancreatic islets
• Pineal gland

1. The pituitary gland or Hypophysis:

Location: Ventral surface of brain below optic chiasma.

Origin: Two parts of pituitary gland are derived from two different components. Neurohypophysis develops from the floor of the embryonic diencephalons. Adenohypophysis developes from the dorsal evagination of the ectodermal part of buccal cavity called Rathke’s pouch. This pouch later looses its connection from the buccal cavity and remains permanently connected to neurohypophysis during the rest of the life. The hypophysis in adult fish remains attached with it by a stalk is called infundibular stalk or neurohypophyal stalk and occupies a position on the underside of the brain; in the region of diencephalon.

Part and division of pituitary, their cell types, secretions and action of their hormones: The part of this gland their division, cell types, secretion and functions are given below in a tabular form-

Part of Pituitary	Division	Cell types	Secretions	Functions
Adenohypophysis	Proximal pars distalis	Thyrotrophs	Thyrotropins e.g TSH	Regulates the growth and secretion from thyroid
		Gonadotrophs	Gonadotropin e.g. FSH (follicular stimulating hormone) and LH (leutinizing hormone)	Regulates secretion of gonadal hormone, spermatogenesis and oogenesis
		Somatotrophs	Somatotropins e.g.GH (Growth hormone)	Increase growth and BMR of the fish body
	Rostral pars distalis	Lactotrophs	Prolactin	Regulation of osmoregulation and melanogenesis
		Corticotrophs	Corticotropin viz. ACTH	Regulates secretion of corticotropins from adrenal gland.
	Pars intermedia		MSH and MCH (melanophore dispersing and melanophore contracting hormone)	Regulates the concentration and dispersion of pigments within melanophores.
Neurohypophysis	Pers-nervosa		Vasopression  and oxytocin	Regulates osmoregulation, salt-water balance, mating and egg laying

 

2. Thyroid Gland:

Origin: The thyroid gland in fishes arises from the floor of the pharynx as a median evagination.

Location: The location of thyroid gland varies considerably in different fish species; such as-

• In cyclostomes, follicles of thyroid are dispersed around the ventral aorta and do not form compact capsulated structure.
• In bony fishes, it may lie under the 1^st branchial arch on each side.
• In many teleosts, it is found along the afferent branchial arteries of the gills.
• In other teleosts, the follicles of thyroid migrate to distant unusual localities, such as the liver, kidney, brain, eye, gut, spleen, gonad etc. as in platyfish.

Shape: The shape of the gland is also variable depending on various fish group; such as –

• In cyclostomes, the thyroid is in the form of follicles.
• In many teleosts, thyroid becomes a diffused structure as small masses of follicles.
• In elasmobranches and bony fishes, thyroid is compact structure.
• In dipnoi, thyroid comprises a pair of interconnected lobes.

Histology: The thyroid gland is composed of a large number of follicles forming a shape of a hollow ball and consisting of a single layer of epithelial cells that encloses a fluid filled space. These follicles are bound together by connective tissue. The gland is highly vascular and the epithelium surrounding the follicle may be thick or thin and the height of the cells depends upon its secretory activity. The epithelium mainly composed of two types of cells- the chief cells are cuboidal or columnar in shape with clear cytoplasm and colloid cells, contain droplets of secretory material.

Secretion: Chiefly thyroxine

Function:

• Thyroid hormone’s role is oxygen consumption in fishes have been pointed out though it lacks consistency.
• Thyroid hormone influences osmoregulation in salmon and Gastrosteus.
• Thyroid along with other endocrine glands also influences migration in fishes.
• It is also known to effect growth and nitrogen metabolism in gold fish.
• Thyroxine brings about maturation in fishes.
• Scale and bone formation in fishes is also influenced by thyroxine.

3. Adrenal gland: Adrenal gland in fishes is quite different from that of mammals. The two components of adrenal gland i.e. cortex and medulla are separately found.

Location: One of three layers of cells lying along cardinal veins in the region of the hemapoietic head kidney.

Origin: Mesodermal layer of embryo

Secretion: Adrenaline, Cortisol

Function:

• Promote utilization of steroid fat
• Carbohydrate metabolism
• Water metabolism
• Protein catabolism
• Sodium retention
• Electrolyte metabolism

4. The Corpuscles of Stannius:

Location: Attached to or embedded in kidneys of holosteans and teleosts

Origin: the corpuscles of stannius orginate as outgrowths from the pronephric or the mesonephric duct of the kidney.

Nature: Proteinous

Secretion: Hypocalcin

Colour: Pink or white

Function: Regulates calcium balance

5. The Ultimobranchial Glands: It is also known as post-branchial bodies or suprapericardial bodies or ultimobranchial bodies.
Location: Sac-like structures between ventral wall of esophagus and sinus venosus.
Origin: Ultimobranchial gland develops embryologically from the epithelium of the last or ultimate gill pouch.
Secretion: Calcitonin
Function: Regulates calcium level in blood.

6. Urohypophysis: It is also known as Urophysis or caudal Neusecretory organ.
Location: This gland is in the form of a swelling at the posterior end of the caudal spinal cord i.e. in the tail of the teleosts.
Secretion:Urotensins
Function: Metabolic regulations.

7. Pancreatic islets:
Location: Gut walls in larval Lampreys; hepatopancreas, most bony fishes
Embryonic Origin: Mesoderm
Secretion: Insulin
Function: Carbohydrate metabolism

8. Pineal gland:
Location/Origin:The Pineal organ of the fish arises as a posteromid dorsal evagination of the epithalamus.
Secretion: Melatonin
Function: Photosensory and secretory function

 

REFERENCES:

Kumar S and Tembhre M (1996); Anatomy and Physiology of Fishes, Vikas Publishing House Pvt Ltd, India. pp. 152-156

Lagler KF, Bardach JE, Miller RR, and Passino DR (1977); Ichthyology (2nd ed.), New York: John Wiley & Sons. pp. 336-346.

Pandey KF, Shukla JP (2007); Fish & Fisheries (2^nd ed.), Rakesh kumar Rastogi Publications, Gangotri Shivaji Road, Meerut, India. pp. 208-221.

 

 

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