Endocrine Glands & Hormones

Learning Objectives:

•To understand what the reproductive endocrine glands and hormones are.

•To understand the characteristics and functions of these hormones.

•To understand how the concentrations of these hormones in the blood are

controlled.

BY

ABDUL SAMIK

DEPARTMENT OF VETERINARY REPRODUCTION, FACULTY OF VETERINARY MEDICINE, UNAIR

Endocrine Glands of Reproduction

Body Diagram of Endocrine Glands 

The reproductive process in mammals is governed by the central nervous system.

Information emaniting from a variety of external cues (eg. Visual, auditory, tactile, olfactory) is fed into the centralnervous system and converges on theHypothalamus.

The information is proessed, amplified, transduced to a humoral signal and transmitted to the anterior pituitary gland where it is further amplified and transmitted via the gonadotrophic hormones to gonads.

The latter respond in many ways, one of which is by the secretion of sex hormones.

These, in turn, act on a host of target tissues including the brain and pituitary gland.

This form a vastly complex network of informationtransfer, a network that permits amplification,propagation and integration of signals throughout the body.

Organization of control systemgoverning reproduction

The hypothalamus is located in the brain, at the base of the optic chiasm.

The hypothalamus consists of nervous tissue lying inferior to the two lobes of the thalamus.

The pituitary gland, or hypophysis, is found at the base of the brain below the hypothalamus and the two structures are connected via the infundibulum, or pituitary stalk, which carries both axons and blood vessels.

It secretes hormones that stimulate or suppress the release of hormones in the pituitary gland, in addition to controlling water balance, sleep, temperature, appetite, and blood pressure

Hypothalamus

Brain Diagram of Endocrine Glands 

The hypothalamic boundary is limitted frontally by the optic chiasma, caudally by the mamillary bodies and dorsally by the thalamus.

The hypothalamus surrounds the fluid-filled third ventricle.

It is made up of various types of structuralelements including cell bodies of hypothalamicneurons with their axons and terminals.

Axons and terminals of other neurons with cellbodies lying outside the hypothalamus andaxons passing through from extrahypothalamicneurons.

The cell bodies of the hypothalamic nuclei sendaxonal projections to one of four general regions :1.Other areas of the brain2.Other hypothalamic nuclei3.The median eminence4.The posterior lobe of the pituitary gland

Anatomical organization of theHypothalamo-hypophyseal axis

The median eminence

The median eminence comprises the base of the hypothalamus and is continuous with the pituitary stalk.

It contains few, if any, nerve cell bodies, but consists of axons and terminals of both hypothalamic and extrahypothalamic neurons, glial cells and specialized ependymal cells called tanacytes.

The latter cells line the third ventricle and are of potential importance in the transfer of information from the cerebrospinal fluid to the pituitary gland.

The neural structures of the median eminence contains a capillaryplexus connected with the hypothalamo-pituitary portal system.

The pituitary gland or hypophysis is sometimes called the "master" gland of the

endocrine system, because it controls the functions of the other endocrine glands.

The pituitary gland is located at the base of the brain. The gland is attached to the

hypothalumus (a part of the brain that affects the pituitary gland) by nerve fibers.

The pituitary gland itself consists of three sections:

•the anterior lobe (adenohypophysis)

•the intermediate lobe

•the posterior lobe (neurohypophysis)

The pituitary gland:

Hypothalamus - Pituitary interrelationships

• The anterior pituitary is derived from oral epithelium from the roof of the mouth cavity, which migrates upwards towards the neural tube

• It sits in the sella turcica which is a depression of the sphenoid bone at the base of the skull and lies behind

the sphenoid sinus. • The anterior lobe of the pituitary is

further subdivided into the pars distalis, pars intermedia and pars tuberalis

• The anterior pituitary makes up 75% of the total weight of the pituitary.

• The pars distalis forms the major part of the gland.

• The pars tuberalis surrounds the infundibular stem like a cuff and extends upwards to lie beneath a portion of the median eminence

• The anterior pituitary contains no nerve fibers and terminals and so is not in direct neuronal contact with the hypothalamus.

• The anterior pituitary connected to the brain by a vascular connection, the hypothalamo-hypophyseal portal system.

• The anterior pituitary gland consist of many different cell types classified on the basis of their size, shape and histological staining characteristics.

• Somatotrophs secrete growth hormone (GH), are distributed laterally and make up 50% of hormone secreting cells.

• Thyrotrophs secrete thyrotropin (TSH), are concentrated laterally and make up 10% of cells.

• Gonadotrophs secrete luteinizing hormone (LH) and follicle stimulating hormone (FSH), are randomly distributed

and make up 10% of cells. • Corticotrophs secrete

adrenocorticotrophic hormone (ACTH), b-lipotrophin, a -melanocyte stimulating hormone and b-endorphin, are found in the median portion and make up 15-20%

of cells. • Lactotrophs secrete prolactin, are

randomly distributed and make up 25% of cells. The number of these cells increases in response to increased

oestrogen in pregnancy and lactation.

• The posterior pituitary develops as an extension of the hypothalamus itself.

• The infundibulum is formed from the neuroectoderm of the floor of the third ventricle and develops to form the posterior pituitary.

• The median eminance is also formed from neuroectoderm.

anterior lobe:

• growth hormone

• prolactin - to stimulate milk production

after giving birth

• ACTH (adrenocorticotropic hormone) -

to stimulate the adrenal glands

• TSH (thyroid-stimulating hormone) - to

stimulate the thyroid gland

• FSH (follicle-stimulating hormone) - to

stimulate the ovaries and testes

• LH (luteinizing hormone) - to stimulate

the ovaries or testes

intermediate lobe:

• melanocyte-stimulating hormone - to control skin pigmentation

posterior lobe:

• ADH (antidiuretic hormone) - to

increase absorption of water into the

blood

by the kidneys

• oxytocin - to contract the uterus during

childbirth and stimulate milk production

HYPOTHALAMO-PITUITARY AXIS STRUCTURE AND DEVELOPMENT

Blood Flow • The pituitary receives blood supply

from paired superior and inferior hypophyseal arteries which originate from the internal carotid arteries.

• The superior hypophyseal arteries enter the primary capillary plexus of the median eminance close to the nerve endings of the neuroendocrine

cells of the hypothalamus.

• From here, blood flows down the portal veins to the sinusoidal

vessels of the anterior pituitary

• This provides a direct vascular link between the hypothalamus and anterior pituitary secretory cells.

• There is also thought to be a reverse flow of peptides along the infundibulum back to the brain which may provide a

rapid feedback mechanism.

• Venous drainage is into the dural sinuses surrounding the pituitary

gland.

• Blood supply to the posterior pituitary is from the inferior hypophyseal arteries directly from the systemic blood flow.

Nerve Pathways

• The hypothalamus is innervated from many areas of the brain, including the limbic system, cerebral cortex, thalamus and other areas.

• This is due to its important role in regulating many vital functions such as maintenance of body temperature, appetite, emotions, pain, intellect etc.as well as control of endocrine functions.

• The hypothalamus contains two groups of nuclei with neuroendocrine functions. These are the paired supraoptic and paraventricular nuclei and the hypothalamic-hypophyseotropic nuclei.

• The posterior pituitary contains the termination of axons from the supraoptic and paraventricular nuclei, which abut onto capillaries.

• These cells produce vasopressin and oxytocin, which are stored in and secreted from the posterior pituitary gland.

• The newly synthesised hormones are packaged with the protein neurophysin and move down the nerve fibres by fast axonal transport.

• They then move by exocytosis into the efferent blood vessels of the posterior pituitary. Some of the axons of the supraoptic nuclei terminate in the upper infundibulum so some vasopressin may still be produced following loss of the posterior pituitary.

The anatomy of the Neurohypophysis

• The posterior pituitary is derived from the forebrain during development and is composed predominantly of neural tissue.

• The posterior pituitary lies below the hypothalamus, with which it forms a structural and functional unit: the neurohypophysis.

• The neurohypophysis consists of three parts:

1. the supraoptic and paraventricular nucleii of the hypothalamus (containing the cell bodies of the magnocellular, neurosecretory neurones that synthesize and secrete VP and OT);

2. the supraoptico-hypophyseal tract (which includes the axons of these neurones);

3. and the posterior pituitary (where the axons terminate on capillaries of the inferior hypophyseal artery).

• The supraoptic nucleus (SON) is situated along the proximal part of the optic tract.

• It consists of the cell bodies of discrete vasopressinergic and oxytotic magnocellular neurons projecting to the posterior pituitary along the supraoptico-hypophyseal tract.

• The paraventricular nucleus (PVN) also contains discrete vasopressinergic and oxytotic magnocellular neurons projecting to the posterior pituitary along the supraoptico-hypophyseal tract.

• The PVN contains additional, smaller parvicellular neurons projecting to the median eminence and additional extra-hypothalamic areas including forebrain, brain stem, and spinal cord. Some of these parvicellular neurons are vasopressinergic.

• A group of those projecting via the median eminence co-secrete VP and corticotrophin releasing hormone (CRH), and terminate in the hypophyseal-portal bed of the anterior pituitary.

• These neurons have a role in the regulation of adrenocorticotrophin (ACTH) release.

• The posterior pituitary receives an arterial blood supply from the inferior hypophyseal artery and the artery of the trabecula (a branch of the superior hypohyseal artery), derivatives of the internal carotid artery and its branches.

• The SON and PVN receive an arterial supply from the suprahypophyseal, anterior communicating, anterior cerebral, posterior communicating and posterior cerebral arteries, all derived from the circle of Willis. Venous drainage of the neurohyphysis is via the dural, cavernous and inferior petrosal sinuses.

Schematic representation of the anatomy of the neurohypophysis, and it's major afferent and efferent connections

Anatomy of the Pineal Gland

• The pineal gland is a small organ shaped like a pine cone (hence its name).

• It is located on the midline, attached to the posterior end of the roof of the third ventricle in the brain.

• The pineal varies in size among species; in humans it is roughly 1 cm in length, whereas in dogs it is only about 1 mm long.

• To observe the pineal, reflect the cerebral hemispheres laterally and look for a small grayish bump in front of the cerebellum.

• Histologically, the pineal is composed of "pinealocytes" and glial cells.

How does the retina transmit information about light-dark exposure to the pineal gland?

• Light exposure to the retina is first relayed to the suprachiasmatic

nucleus of the hypothalamus, an area of the brain well known to

coordinate biological clock signals.

• Fibers from the hypothalamus descend to the spinal cord and

ultimately project to the superior cervical ganglia, from which post-

ganglionic neurons ascend back to the pineal gland.

• Thus, the pineal is similar to the adrenal medulla in the sense that it

transduces signals from the sympathetic nervous system into a

hormonal signal.

Structure:

• A flattened ovoid shape, 3 - 5 cm in diameter

• The outer (periphery) ovary is the cortex and is fibrotic. It contains:

– Germ cells, or ova, which are contained in follicles

– Corpora lutea - produce oestrogens and progesterones

– Corpora albicantes - degenerate

• The stroma is the body of the ovary, containing:

– Fine collagen fibres

– Scattered bundles of smooth muscle cells

– Some stromal cells may contain lipid droplets

• The mature ovary will contain many follicle of different sizes and development.

OVARY

Diagramatic Representation of the Ovary:

TESTISMacrostructure: • Paired, ovoid structures • Capsule:

– External: tunica vaginalis – Internal: tunica albuginea

• Gland is divided into intercommunicating, testicular lobules by fibrous septa

– produce androgen-binding globulin:

• this is in response to FSH • similar to plasma sex

hormone-binding globulin • maintains high androgen

levels in the testis and seminal fluid

– produce inhibin • inhibits FSH secretion

– produce seminiferous tubule fluid • Between the seminiferous tubules are

the interstitial LEYDIG CELLS where androgens are produced.

– these cells only have LH receptors, which stimulate steroidogenesis

– Also produce small amounts of oxytocin, endorphins, angiotensins, and prostaglandins

Microstructure:•Each testicular lobule contains seminiferous lobules - site of spermatogenesis •The basement membrane of the seminiferous tubules is lined with SERTOLI CELLS. These:

•produce mullerian-inhibiting substance •convert androgens to oestrogen •act as a supportive cell for the developing germ cells (sperm) •form a blood-testis barrier

A chemical substance secreted by an endocrine gland or group of

endocrine cells that acts to control or regulate specific physiological

processes, including growth, metabolism, and reproduction. Most hormones

are secreted by endocrine cells in one part of the body and then transported

by the blood to their target site of action in another part, though some

hormones act only in the region in which they are secreted.

Chemical types of hormones:

Peptide - Few - Several amino acids

Protein - Long chains of amino acids

Glycoprotein - Protein hormone with carbohydrate molecules

DEFINITION OF HORMONE

HORMONE ACTION

PROTEIN HORMONE STEROID HORMONE

Gland Hormone  Chemical Class Principal Functions 

Ovary   Graafian Follicle

Estrogens (Estradiol)  Steroid Mating behavoir, Secondary sex characteristics, Maintenance of female duct system , Mammary growth 

  Inhibin (Folliculostatin) ProteinRegulates release of FSH from anterior pituitary

Corpus Luteum Progestins (Progesterone) 

Steroid Maintenance of pregnancy, Mammary growth & secretion

  Relaxin  Polypeptide Expansion of pelvis Dilation of cervix 

Testis   Leydig Cells Androgens (Testosterone)  Steroid Male mating behavior, Spermatocytogenesis, Maintenance of male duct system & accessory glands 

  Sertoli Cells Inhibin  Protein Regulates release of FSH 

Adrenal Cortex Glucocorticoids (Cortisol)  Steroid Induction of partruition by fetus Milk synthesis  Stress response

Placenta Human Chorionic Gonadotropin (HCG) 

Glycoprotein LH-like - Involved with establishment of pregnancy in human. Support and maintain CL

 Endometrial Cups  Mare

Equine Chorionic Gonadotropin (eCG) Old name - Pregnant Mare Serum Gonadotropin (PMSG) 

Glycoprotein FSH-like- some LH activity Immunological protection of foal during pregnancy Formation of accessory CL in mare 

  Estrogens/Progestins  Steroids Regulate placental bloodflow Maintenance of pregnancy

  Relaxin  Protein Relaxation/dialation of cervix for parturition 

  Placenal Lactogen  Glycoprotein Stimulates mammary growth & milk secretion

Uterine  Endometrium Graafian Follicles Seminal Vesicles

Prostaglandin F2a (PGF2a) Lipid Regression of CL ,Stimulate myometrial contractions Ovulation Sperm transport

Liver Insulin-like Growth Factors

IGF-I & IGF-II 

Protein

Stimulates steroidogenesis,  Stimulates mammary growth and  fetal development 

Pineal Gland Melatonin  Biogenic amine Control of seasonal reproduction in Mare & Ewe Regulate hair growth

Posterior Pituitary Oxytocin  Octapeptide Stimulate myometrial contractions for sperm transport, parturition & milk ejection

Anterior Pituitary  Follicle Stimulating Hormone (FSH) - Follicotropin

Glycoprotein Stimulate follicle growth Stimulate estrogen production Spermiogenesis in male

  Luteinizing Hormone (LH) -  Luteotropin

Glycoprotein Stimulate ovulation Support CL formation, and progesterone secretion Stimulate testosterone synthesis by leydig cells of testis

  Prolactin  Protein Stimulate milk synthesis  Regulate metabolism for milk synthesis Effects Maternal Behavior

  Adrenalcorticotropic Hormone (ACTH) 

Protein Release of corticosteroids & glucocorticoids from adrenal cortex to initiate parturition

Hypothalamus Gonadotropic Releasing Hormone (GnRH)

Decapeptide Stimulates release of FSH and LH from anterior pituitary

  Dopamine  Biogenic Amine Inhibits release of prolactin 

  Corticotropic Releasing Hormone (CRH) 

Peptide Stimulates ACTH release 

  Growth Hormone-Releasing Hormone (GRH) 

Peptide Stimulates release of growth hormone 

  Oxytocin Octapeptide Produced by the hypothalamus, released at the posterior pituitary.