Female Reproductive System

The female reproductive system consists of the following organs:

  • ovaries – produce secondary oocytes and hormones (progesterone, oestrogens, relaxin, and inhibin)
  • fallopian tubes – the sites where fertilisation normally occurs; additionally assist in transporting a secondary oocyte to the uterus
  • uterus – cavity in which a fertilised ovum implants, and develops into a fetus for pregnancy and labour
  • vagina – the site which receives the penis during sexual intercourse, and which acts as a passageway during childbirth
  • external organs / vulva / pudendum
female reproductive system
Retrieved from https://www.slideserve.com/csilla/anatomy-and-physiology-the-female-reproductive-system on 9th April 2022

The Ovaries

The ovaries are a pair of irregularly scarred surfaced pale white glands measuring between 2.5 to 3.5cm long, 2cm wide, and 1.25cm thick. They produce gametes, which are secondary oocytes that develop into mature ova following fertilisation, and the hormones oestrogen, progesterone, inhibin, and relaxin.

The ovaries are held in place by 3 types of ligaments, namely the broad ligament of the uterus, which is part of the parietal peritoneum that attaches to the ovaries by the mesovarium; the ovarian ligament, which anchors the ovaries to the uterus; and the suspensory ligament, which attaches the ovaries to the pelvic walls.

female reproductive system
Retrieved from https://quizlet.com/180196971/female-reproductive-system-flash-cards/ on 15th April 2022

Each ovary consists of:

GERMINAL EPITHELIUM – a layer of simple epithelium which covers the ovary’s surface and is continuous with the mesothelium that covers the mesovarium. The germinal epithelium does not give rise to ova.

TUNICA ALBUGINEA – a whitish capsule of dense irregular connective tissue found right beneath the germinal epithelium.

OVARIAN CORTEX – consists of ovarian follicles which are surrounded by dense irregular connective tissue containing scattered smooth muscle cells. The ovarian cortex can be found right beneath the tunica albuginea.

OVARIAN MEDULLA – consists of loose connective tissue which contains blood vessels, lymphatic vessels, and nerves.

OVARIAN FOLLICLES – consist of oocytes in various developing phases and surrounding cells which provide nourishment to the oocytes and secrete oestrogens throughout the follicle’s growing phase; single-layered surrounding cells are referred to as follicular cells, while multiple-layered surrounding cells are referred to as granulosa cells.

GRAAFIAN FOLLICLE – large follicle full of fluid in a close-to-being-ruptured state.

CORPUS LUTEUM – contains the remnants of the mature follicle, following ovulation; the corpus luteum produces progesterone, oestrogens, relaxin, and inhibin, before turning into fibrous scar tissue referred to as the corpus albicans.

Retrieved from https://slideplayer.com/slide/8162919/ on 15th April 2022

The Fallopian Tubes

The fallopian tubes are two hollow fibromuscular cylinders lined by epithelium which extend outwards and backwards around 10cm from the uterine cornu to the ovaries. Each fallopian tube is divided into the following parts:

  1. interstitial (approx. 0.7mm x 2.5cm)
  2. isthmus (approx. 1mm x 2.5cm)
  3. ampulla (approx. 6mm x 5cm)
  4. infundibulum (approx. 10mm wide)
female reproductive system
Retrieved from https://www.britannica.com/science/uterus on 15th April 2022

The fallopian tubes are made up of 3 layers:

INTERNAL MUCOSA – contains ciliated columnar epithelial cells that help move the fertilised ovum along the tube.

MUSCULARIS – is the middle layer. It is composed of an inner circular ring of smooth muscle and an outer thin region of longitudinal smooth muscle; peristaltic contractions of the muscularis along with the ciliary action of the mucosal cells assist the oocyte or fertilised ovum to move towards the uterus.

SEROSA – the outer layer of the fallopian tubes.

The Uterus

The uterus is a pear-shaped hollow organ that projects anteriorly and superiorly over the urinary bladder, measuring about 7.5cm x 5cm at the fundus, and 2.5cm from front to back. The uterine wall thickness measures around 1 to 2cm.

The uterus features the following anatomical subdivisions:

  • fundus – dome-shaped portion superior to the fallopian tubes
  • body – tapering entral portion
  • cervix – inferior narrow portion which opens into the vagina; projects inferiorly and posteriorly, and enters the vaginal wall at almost a right angle
  • isthmus – constricted region measuring around 1cm long found between the body and the cervix

The uterine cavity is the uterine body’s inferior. The cervical canal is the interior of the narrow cervix. The cervical canal opens into the uterine cavity at the internal os into the vagina at the external os.

female reproductive system
Retrieved from https://courses.lumenlearning.com/boundless-ap/chapter/the-female-reproductive-system/ on 15th April 2022

The uterus is made up of 3 layers:

PERIMETRIUM – the outer layer of the uterus becomes the broad ligament laterally, covers the urinary bladder and forms the vesicouterine pouch anteriorly, and covers the rectum and forms the rectouterine pouch a.k.a. pouch of Douglas posteriorly.

MYOMETRIUM – the middle layer of the uterus is thickest and circular in the fundus area and the thinnest and longitudinal in the cervix area; the myometrium responds to oxytocin released by the pituitary during labour and childbirth through contraction coordination which help in expelling the fetus from the uterus.

ENDOMETRIUM – the inner layer of the uterus is highly vascular. It is divided into two layers: the stratum functionalis lines the uterine cavity and sloughs off during menstruation, and the stratum basalis, which is a permanent layer, gives rise to a new stratum functionalis following each menstruation.

Retrieved from https://socratic.org/questions/what-are-the-two-layers-of-the-endometrium on 15th April 2022

Cervical Mucus

Cervical mucus is a secretion produced by the secretory cells of the cervix’s mucosa. It consists of water, glycoprotein, lipids, enzymes, and inorganic salts. Females in their fertile years secrete between 20-60ml of cervical mucus daily, which is more hospitable to sperm at or close to ovulation, when it is less viscous and increasingly alkaline with a pH of 8.5. Viscous mucus forms the cervical plug which stops sperm penetration.

The Vagina

The vagina is a long tubular fibromuscular canal measuring approximately 10cm long which extends from the exterior of the body to the cervix. It acts as a receptacle for the penis during sexual intercourse, an outlet for menstruation, and a passageway during childbirth.

The vaginal mucosa, which is continuous with the uterine mucosa, contains large glycogen stores that, upon decomposing, produce organic acids, which lead to a resulting acidic environment which retards microbial growth.

The Vulva

The vulva a.k.a. pudendum, is the female’s external genitalia. It consists of the following:

  • mons pubis
  • labia majora
  • labia minora
  • clitoris
Retrieved from https://www.cancer.org/cancer/vulvar-cancer/about/what-is-vulvar-cancer.html on 15th April 2022

Mammary Glands

The mammary glands, which lie over the pectoralis major and the serratus anterior, are modified sweat glands which produce milk.

Functions of the breast in relation to lactation include:

  • milk synthesis
  • milk secretion
  • milk ejection

NIPPLE – pigmented projection.

LACTIFEROUS DUCTS – closely spaced duct openings which allow milk ejection.

AREOLA – circular rough-looking pigmented area surrounding the nipple which contains modified sebaceous glands.

SUSPENSORY LIGAMENTS OF THE BREAST A.K.A. COOPER’S LIGAMENTS – strands of connective tissue found between the skin and the deep fascia that provides support for the breast.

MILK – following production, milk passes from the milk-secreting alveoli into secondary tubules, and then into mammary ducts. Mammary ducts close to the nipple expand and form lactiferous sinuses, where milk is stored, and is eventually drained into the lactiferous ducts, which drain into the exterior.

Retrieved from https://www.brainkart.com/article/Mammary-Glands—Anatomy-and-Physiology_18849/ on 16th April 2022

Hormones Related to the Female Reproductive System

female reproductive system
Retrieved from http://www.pharmacy180.com/article/pituitary-gland-3595/ on 5th March 2022

Follicle-Stimulating Hormone (FSH)

  • in females initiates the development of an ova every month, and stimulates cells within the ovaries to secrete oestrogens
  • in males stimulates the testes to produce sperm
  • secretion depends on the hypothalamic regulating factor gonadotropin releasing factor (GnRF), which is released in response to oestrogens in females, and to testosterone in males through a negative feedback system

Luteinizing Hormone (LH)

  • along with oestrogens, in females it stimulates the release of an ovum within the ovary, prepares the uterus for the implantation of the fertilised ovum, stimulates the formation of the corpus luteum in the ovary to secrete progesterone, and prepares the mammary glands for milk secretion
  • in males it stimulates the interstitial endocrinocytes in the testes to develop and secrete testosterone
  • secretion is controlled by GnRF, which works through a negative feedback system

Prolactin (PRL)

  • requires priming of the mammary glands through oestrogens, progesterone, corticosteroids, growth hormone, thyroxine, and insulin
  • initiates and maintains milk secretion by the mammary glands (amount of milk is determined by oxytocin)
  • has an inhibitory and an excitatory negative control system
  • level rises during pregnancy, falls right after delivery, and rises again during breastfeeding, which is why in the 1st two days following birth, mothers do not produce milk but colostrum

NOTE: women on oral contraceptives may experience lack of milk production due to their hormonal effect.

Pituitary Gland Posterior Lobe

The posterior lobe of the pituitary gland a.k.a. neurohypophysis, does not synthesise hormones. It releases hormones to the circulation via the posterior hypophyseal veins to be distributed to target cells in other tissues. The cell bodies of the neurosecretory cells produce Oxytocin (OT) and Antidiuretic Hormone (ADH) / Vasopressin.

Oxytocin (OT)

  • is released in high amounts just before birth
  • stimulates contraction of smooth muscle cells in the pregnant uterus
  • stimulates the contractile cells around the mammary gland ducts
  • affects milk ejection
  • works through a positive feedback cycle which is broken following birthing
  • is inhibited by progesterone, but can work in conjunction to oestrogens
female reproductive system
Retrieved from https://basicmedicalkey.com/uterine-drugs/ on 16th April 2022

The Female Reproductive Cycle

The female reproductive cycle a.k.a. menstrual cycle demonstrates regular cyclic changes seeming as periodic preparations for fertilisation and pregnancy, which, if unsuccessful, results in menstruation where the uterine mucosa (stratum functionalis portion of the endometrium) is shed.

The ovarian cycle features a series of events related to the maturation of an ovum which usually occurs on a monthly basis.

Oestrogen

  • Oestrogen assists in the development and maintenance of the endometrial lining of the uterus, secondary sex characteristics, and breasts
  • Oestrogen helps keep fluid and electrolyte balance
  • Oestrogen increases protein anabolism (process in which amino acids are transformed into proteins) and is synergistic with the Growth Hormone a.k.a. Somatotropin
  • Oestrogen helps in keeping a low blood cholesterol level

NOTE: Moderate levels of oestrogens in the blood inhibit GnRF (Gonadotropin-Releasing Hormone) release by the hypothalamus. This causes the inhibition of FSH (Follicle Stimulating Hormone) secretion by the anterior pituitary gland.

pROGESTERONE

  • Progesterone works in conjunction with Oestrogen in preparing the endometrium for implantation of a fertilised ovum, and in preparation of the mammary glands for milk secretion

NOTE: High levels of progesterone inhibit GnRF (Gonadotropin-Releasing Hormone) and LH (Luteinizing Hormone).

Inhibin

  • Inhibin inhibits the secretion of FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone). This happens so as to inhibit multiple ovum maturation following the release of a mature ovum following ovulation.

Relaxin

  • Relaxin helps relax the uterus through the inhibition of myometrium contractions
  • Relaxin is produced by the placenta during pregnancy to help increase relaxation of the uterine smooth muscle
  • Relaxin increases the flexibility of the pubic symphysis near the end of pregnancy, and may also help in the cervix dilation process in preparation for childbirth
female reproductive system
Retrieved from https://www.easynotecards.com/notecard_set/85750 on 16th April 2022

The Menstrual Cycle

Retrieved from https://sofreshnsogreen.com/wellness/cycle-syncing-guide/ on 16th April 2022

The menstrual cycle can be divided into 3 phases:

THE MENSTRUAL PHASE – the periodic discharge of 25ml to 65ml of blood, tissue fluid, mucus and epithelial cells, caused by a sudden reduction in oestrogens and progesterone. This phase usually lasts for around 5 days.

  1. During this phase, 20 to 25 primary follicles start to produce small amounts of oestrogens.
  2. By the end of menstruation, around 20 of these primary follicles develop into secondary follicles, while surrounding cells increase in number, differentiate, and secrete follicular fluid.
  3. The follicular fluid forces an immature ovum to the edge of the secondary follicle and fills the follicular cavity, whilst secondary follicles produce oestrogens, leading to an elevation of oestrogen levels in the blood.
  4. Ovarian follicle development results from GnRF secretion by the hypothalamus, which then stimulates high FSH production by the anterior pituitary.

THE PREOVULATORY PHASE – the second phase of the menstrual cycle which covers the phase between menstruation and ovulation.

  1. FSH and LH stimulate ovarian follicles to increase oestrogen production, which stimulates the rebuilding of the endometrium, which by the end of this phase doubles to up to 6mm.
  2. With the thickening of the endometrium, short straight endometrial glands develop, and arterioles coil and lengthen whilst penetrating the functionalis.
  3. LH is secreted in increasing quantities as this phase starts to near its end.
  4. A secondary follicle matures into a vesicular ovarian a.k.a. graafian follicle, ready for ovulation. At this time, just before ovulation occurs, the vesicular ovarian starts producing small amounts of progesterone.

OVULATION – the immature ovum in the vesicular ovarian follicle is released into the pelvic cavity around the middle of the menstrual cycle.

  1. Immediately prior to ovulation, high levels of oestrogen inhibit GnRF production by the hypothalamus, which in turn inhibits FSH secretion by the anterior pituitary via a negative feedback effect.
  2. At the same time, high levels of oestrogen work through a positive feedback effect, causing the anterior pituitary to release a high amount of LH which triggers ovulation.
  3. Following ovulation, the vesicular ovarian follicle collapses, the follicular cells enlarge, change, and form the corpus luteum.

THE POSTOVULATORY PHASE – represents the time between ovulation and onset of upcoming menses. This phase is consistent in duration.

  1. Following ovulation, LH secretion stimulates the development of the corpus luteum.
  2. The corpus luteum secretes increasing quantities of oestrogens and progesterone.
  3. FSH secretion increases gradually whilst LH secretion decreases.
  4. During this phase, progesterone becomes the most dominant ovarian hormone.

SEQUELAE

  1. If fertilisation and implantation do not occur, the increasing progesterone and oestrogen levels secreted by the corpus luteum inhibit GnRF and LH secretion.
  2. The corpus luteum degenerates, which causes decreased secretion of progesterone and oestrogens.
  3. The corpus luteum becomes the corpus albicans, whilst the decrease in progesterone and oestrogens trigger another menstrual cycle to begin, along with increased output of GnRF by the hypothalamus and a new output of FSH.
Retrieved from https://quizlet.com/au/232342424/hormonal-control-of-menstrual-cycle-diagram/ on 16th April 2022

Conditions & Operations Related to the Female Reproductive System

Hysterectomy

Hysterectomy is the most common gynaecological operation, commonly indicated in endometriosis, pelvic inflammatory disease, recurrent ovarian cysts, excessive uterine bleeding, and cancer of the cervix, uterus, or ovaries.

There are 3 types of hysterectomies:

  1. Total Hysterectomy – removal of the uterine body and cervix
  2. Partial Hysterectomy – removal of uterine body only (cervix is left in situ)
  3. Radical Hysterectomy – removal of uterine body, cervix, fallopian tubes (and possibly the ovaries), the vagina’s superior portion, the pelvic lymph nodes, and supporting structures
Retrieved from https://www.gleneagles.com.sg/specialties/medical-specialties/women-gynaecology/hysterectomy on 15th April 2022

Cystocoele

Cystocoele is a herniation of the bladder wall into the vaginal cavity.

Retrieved from https://www.chicagourogynecologist.com/center-services/pelvic-organ-prolapse/cystocoele/ on 14th April 2022

Rectocoele

Rectocoele is a herniation of the rectum into the vaginal wall.

Retrieved from https://fascrs.org/patients/diseases-and-conditions/a-z/rectocele-expanded-information on 14th April 2022

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Male Reproductive System

The male reproductive system consists of the following organs:

  • testes
  • ducts system (ductus deferens, ejaculatory ducts, urethra)
  • accessory sex glands (seminal vesicles, prostate gland, bulbourethral gland)
  • supporting structures (scrotum, penis)

The function of the testes is to produce sperm and secrete hormones, while the ducts system is responsible for the transportation and storage of sperm, assistance in sperm maturation, as well as conveyance of the sperm to the exterior.

male reproductive system
Retrieved from https://www.slideserve.com/coy/chapter-28-the-reproductive-systems on 5th April 2022

The Scrotum

The scrotum acts as a supporting structure for the testes, consisting of loose skin and superficial fascia that hangs from the root of the penis. The left testis is suspended lower than the right testis. The spermatic cord passes up the scrotum’s back, through the inguinal ring, and into the pelvic cavity.

The external structure of the scrotum consists of rich sensory innervation, sebaceous glands, darker pigmentation, and sparse hair.

Within the internal structure of the scrotum is the scrotal septum (made up of the superficial fascia and the dartos muscle), which divides the scrotum into 2 sacs.

The testes are kept cooler by the following 3 structures:

  • cremaster muscle of the spermatic cord – relaxes when warm, contracts when cool, thus raising or lowering the scrotum and testes
  • dartos muscle in the scrotal wall – contracts and tautens the scrotum when cool
  • pampiniform plexus of blood vessels in the spermatic cord – acts as countercurrent heat exchanger, cooling blood on its way to the testis
male reproductive system
Retrieved from https://en.wikipedia.org/wiki/Scrotal_septum on 5th April 2022
male reproductive system
Retrieved from https://www.rrnursingschool.biz/unity-companies/scrotum.html on 5th April 2022

The Testes

The testes are paired oval glands measuring around 5cm long with a diameter of 2.5cm. Each testis weighs between 10-15g. During foetal development, the testes originate near the kidneys, from where they start descending through the inguinal canals towards and into the scrotum by the end of the 7th month of pregnancy.

The testis has a fibrous capsule known as Tunica Albuginea. Within the fibrous septa are up to 300 compartments known as lobules, each of which contains up to 3 sperm-producing seminiferous tubules. Between these tubules are clusters of interstitial cells which secrete testosterone.

Retrieved from https://radiopaedia.org/cases/testis-cross-section?lang=us on 5th April 2022

The Seminiferous Tubules

The seminiferous tubules contain the following two types of cells:

  1. spermatogenic cells – produce sperm
  2. sertoli cells – support spermatogenesis
male reproductive system
Retrieved from https://microbenotes.com/spermatogenesis/ on 5th April 2022

Sertoli Cells

Sertoli cells, a.k.a. sustentacular cells, support and protect spermatogenic cells through their development.

Retrieved from https://www.nagwa.com/en/videos/104129276305/ on 5th April 2022

Leydig Cells

Leydig cells a.k.a. interstitial endocrinocytes, are found in clusters within the spaces between adjacent seminiferous tubules. They secrete testosterone.

Retrieved from https://histology.siu.edu/erg/RE028b.htm on 7th April 2022

The Ducts

Sperm cells pass through a number of ducts to exit the body. Once they leave the testes, sperm cells pass through the epididymis, ductus deferens, ejaculatory duct, and urethra.

EFFERENT DUCTULES – carry sperm from the posterior side of the testis to the epididymis thanks to ciliated cell clusters which assist the sperm through.

EPIDIDYMIS – whilst travelling through the epididymis, sperm cells mature, and are then stored in the epididymis’s tail, where they remain fertile for 40 to 60 days.

DUCTUS DEFERENS – sperm cells travel from the epididymis’s tail before uniting with the seminal vesicle duct.

EJACULATORY DUCT – allows the sperm cells through the prostate gland before emptying into the urethra.

SEMINIFEROUS TUBULES – open up into a collection of very short ducts known as straight tubules, which lead into the rete testis.

EFFERENT DUCTS – sperm moves into a series of coiled ducts within the epididymis.

DUCTUS EPIDIDYMIS – efferent ducts empty into a single tube a.k.a. ductus epididymis.

Retrieved from https://quizlet.com/213934544/chapter-27-male-reproductive-system-flash-cards/ on 7th April 2022

Accessory Sex Glands within the Male Reproductive System

male reproductive system
Retrieved from https://www.flexiprep.com/NIOS-Notes/Secondary/Science/NIOS-Class-10-Science-Chapter-24-Reproduction-Part-2.html on 7th April 2022

Seminal Vesicles

Seminal Vesicles are a pair of glands associated with the ductus deferens, posterior to the urinary bladder base and anterior to the rectum. Seminal vesicles secrete an alkaline yellowish secretion, which helps in neutralising the acidic environment of the male urethra and the female reproductive tract. It totals about 60% of semen, containing Fructose (used for sperm ATP production), Prostaglandins (provide sperm motility and viability), and Clotting Proteins (promote semen coagulation following ejaculation).

Prostate Gland

The prostate gland is a single gland situated immediately inferior to the bladder, surrounding the urethra and the ejaculatory duct. It secretes a thin, milky, slightly acidic secretion totaling around 30% of semen, containing Citric Acid (used for sperm ATP production via Krebs’ cycle), Proteolytic Enzymes (promote breakdown of clotting proteins from the seminal vesicles), and Acid Phosphatase.

Bulbourethral a.k.a. cowper glands

Bulbourethral Glands a.k.a. Cowper’s Glands, are pea-sized glands located posterior to the prostate. They produce a clear, slippery fluid during sexual arousal, which helps lubricate the penis’s head in preparation for intercourse. It also neutralises the acidity of residual urine found in the urethra, since this acidity would be harmful to the sperm.

The Penis

The penis provides a passageway for sperm ejaculation and urine excretion through the urethra. It consists of the Root (attached portion), the Body (2 corpora cavernosa, and 1 corpus spongiosum), and the Glans penis.

male reproductive system
Retrieved from https://www.uptodate.com/contents/image?imageKey=EM%2F76391&topicKey=PEDS%2F6587&source=see_link on 7th April 2022

Hormones related to the Male Reproductive System

ANDROGENS – masculinising steroid sex hormones eg. testosterone (normally secreted in both sexes)

OESTROGENS – feminising steroid sex hormones (normally secreted in both sexes)

PITUITARY GONADOTROPHINS – FSH (follicle-stimulating hormone) helps in maintaining spermatogenic epithelium and sertoli cells within the male, and LH (luteinizing hormone) stimulates testosterone production from the Leydig Cells within the testes.

male reproductive system
Retrieved from https://www.austincc.edu/apreview/PhysText/Reproductive.html on 7th April 2022

Testosterone

Testosterone, which is the primary hormone in the testes:

  • promotes male development
  • is responsible for an inhibitory feedback response on the pituitary’s secretion of LH
  • develops and maintains male secondary sex characteristics eg. body hair growth, larynx enlargement and voice deepening, increased stature, etc.
  • exerts a protein anabolic, growth-promoting effect
  • maintains gametogenesis, along with FSH

Testicular Function Control

FSH:

  • maintains gametogenic function, along with androgens
  • is tropic to the Sertoli cells
  • stimulates secretion of androgen-binding protein and inhibin (inhibin feeds back to inhibit FSH secretion)

LH:

  • stimulates testosterone secretion (testosterone feeds back to inhibit LH secretion)
  • is tropic to the Leydig cells

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The Endocrine System – Hypothalamus & Pituitary Gland

The endocrine system is made up of hormone-producing glands within the body which facilitate communication between cells. Glands that make up the endocrine system include the hypothalamus, pituitary gland, and pineal gland, all of which can be found within the brain; the thyroid and parathyroid glands which can be found in the neck; the thymus which is situated between the lungs; the adrenals, which sit on the kidneys; the pancreas, which is found behind the stomach; and the ovaries (women) or testes (men) which are in the pelvic region.

endocrine system hypothalamus and pituitary gland
Retrieved from https://www.blendspace.com/lessons/tv-3ufAxEQI3pQ/group-3-307-313-331-endocrine-system on 3rd March 2022

Within the endocrine system, an endocrine gland or tissue releases an amount of hormone, which amount is determined by the body’s need for that hormone. Through sensing and signalling systems, hormone-producing cells receive information and regulate hormone release amount and duration. Released hormones are carried by the blood to target cells, which contain receptors that bind the hormone, leading to the desired effect. This effect is then recognised by secretory cells through a feedback signal. Once the required hormonal effect is fully accomplished, the hormones are either removed by the liver or the kidneys, or else degraded by the target cells.

Hormonal secretion is regulated by negative feedback control so homeostasis within the body is maintained.

The Hypothalamus

The hypothalamus, which is located below the thalamus, acts as a link between the nervous system and the endocrine system. It receives inputs from various parts of the brain, and sensory signals from internal organs and the retina. Changes are triggered in the hypothalamic activity due to pain, stress, and other emotional factors. The hypothalamus controls the autonomic nervous system and regulates various bodily factors such as temperature, hunger and thirst, sexual behaviour, and defensive reactions.

The Endocrine System - Hypothalamus & Pituitary Gland
Retrieved from https://kids.frontiersin.org/articles/10.3389/frym.2021.534184 on 3rd March 2022

Within the hypothalamus are clusters of specialised neurons – neurosecretory cells, which synthesise the hypothalamic hormones in their cell body. The hormones are transported inside vesicles by axonal transport.

Hypothalamus-Released Hormones

The hypothalamus is an important endocrine gland that produces hormones which, after being released into the blood, travel in the portal veins to a secondary capillary bed found in the anterior lobe of the pituitary, where their effects are produced. Hormones released in this way include:

  • Thyrotropin-releasing hormone (TRH) – related to thyroid gland growth and function
  • Gonadotropin-releasing hormone (GnRH) – related to the reproductive system
  • Growth hormone-releasing hormone (GHRH) – related to growth
  • Corticotropin-releasing hormone (CRH) – related to hormone secretion
  • Somatostatin – related to the growth hormone
  • Dopamine – acts as a neurotransmitter

Hormones which travel in the neurons to the posterior lobe of the pituitary before being released into circulation include:

  • Antidiuretic Hormone (ADH) / Vasopressin – promotes regulation of the amount of water within the body
  • Oxytocin – involved in childbirth and breastfeeding

The Pituitary Gland

The pituitary gland, which measures just about 1.3cm in diameter, is located in the cella turcica of the sphenoid bone. It is attached to the hypothalamus via the infundibulum – a stalklike structure. Pituitary gland hormones regulate body activities. The pituitary gland is divided into two lobes: the anterior lobe and the posterior lobe.

The pituitary gland anterior lobe accounts to around 80% of the pituitary gland. It is involved in growth regulation, metabolism, and reproduction, through its produced hormones. Hormone production happens through stimulation or inhibition by chemical messages originating from the hypothalamus. Thus, hypothalamic hormones act as a link between the nervous system and the endocrine system. They reach the anterior pituitary through the Hypophyseal Portal System.

The pituitary gland posterior lobe is involved in hormone transmission. Hormones originating from neurons within the region of the hypothalamus are secreted directly into peripheral circulation.

The lobes are divided by the pars intermedia – a relatively avascular zone.

The Endocrine System - Hypothalamus & Pituitary Gland
Retrieved from https://www.nature.com/articles/nrdp201692 on 5th March 2022

The 5 Types of Glandular Cells

  1. Somatotroph Cells – produce GH (growth hormone) which is responsible for general body growth
  2. Lactotroph Cells – synthesise PRL (prolactin) which promotes milk production by the mammary glands
  3. Corticolipothroph Cells – synthesise ACTH (adrenocorticotropic hormone) which stimulates hormone secretion, and MSH (melanocyte-stimulating hormone) which is responsible for skin pigmentation
  4. Thyrothroph Cells – produce TSH (thyroid-stimulating hormone), which controls the thyroid gland
  5. Gonadotroph Cells – produce FSH (follicle-stimulating hormone), which stimulates egg and sperm production in the ovaries and testes, and LH (luteinizing hormone), which stimulates other sexual and reproductive activities.
The Endocrine System - Hypothalamus & Pituitary Gland
Retrieved from http://www.pharmacy180.com/article/pituitary-gland-3595/ on 5th March 2022

Growth Hormone (GH)

  • is released through two regulating factors from the hypothalamus, namely GHRF (growth hormone releasing factor) and GHIF (growth hormone inhibiting factor) or Somatostatin
  • causes cells to grow and multiply by increasing the rate at which amino acids enter the cells to be built up into proteins
  • acts on the skeleton and the skeletal muscles firstly by increasing their growth rate, and then maintaining their size when growth is attained
  • increases the rate of protein synthesis a.k.a. protein anabolism
  • promotes fat catabolism by causing cells to change from burning carbohydrates to burning fats to produce energy
  • accelerates rate at which glycogen stored within the liver converts into glucose and releases itself into the blood
  • converts other factors into growth-promoting substances – somatomedins and insulin-like growth factors (IGF), both of which are similar to insulin yet more potent than insulin

Growth Hormone Secretion Stimuli and Inhibition

Retrieved from https://basicmedicalkey.com/normal-endocrine-function/ on 5th March 2022

Prolactin (PRL)

  • requires priming of the mammary glands through oestrogens, progesterone, corticosteroids, growth hormone, thyroxine, and insulin
  • initiates and maintains milk secretion by the mammary glands (amount of milk is determined by oxytocin)
  • has an inhibitory and an excitatory negative control system
  • level rises during pregnancy, falls right after delivery, and rises again during breastfeeding, which is why in the 1st two days following birth, mothers do not produce milk but colostrum

NOTE: women on oral contraceptives may experience lack of milk production due to their hormonal effect.

Melanocyte-Stimulating Hormone (MSH)

  • increases skin pigmentation through the stimulation of melanin granules dispersion in melanocytes
  • secretion is stimulated by the melanocyte-stimulating hormone releasing factor (MRF), or inhibited by the melanocyte-stimulating hormone inhibiting factor (MIF)
  • lack causes the skin to look pallid
  • excess causes the skin to look dark

Thyroid-stimulating factor (TSH)

  • stimulates the synthesis and secretion of hormonal production within the thyroid gland
  • secretion is controlled by the thyrotropin releasing factor (TRF), which is released based on thyroxine blood level, metabolic rate of the body, and other factors through a negative feedback system

Adrenocorticotropic Hormone (ACTH)

  • controls the production and secretion of some adrenal cortex hormones
  • is secreted by the hypothalamic regulating factor called corticotropin releasing factor (CRF), which is released depending on stimuli and hormones through a negative feedback system

Follicle-Stimulating Hormone (FSH)

  • in females initiates the development of an ova every month, and stimulates cells within the ovaries to secrete oestrogens
  • in males stimulates the testes to produce sperm
  • secretion depends on the hypothalamic regulating factor gonadotropin releasing factor (GnRF), which is released in response to oestrogens in females, and to testosterone in males through a negative feedback system

Luteinizing Hormone (LH)

  • along with oestrogens, in females it stimulates the release of an ovum within the ovary, prepares the uterus for the implantation of the fertilised ovum, stimulates the formation of the corpus luteum in the ovary to secrete progesterone, and prepares the mammary glands for milk secretion
  • in males it stimulates the interstitial endocrinocytes in the testes to develop and secrete testosterone
  • secretion is controlled by GnRF, which works through a negative feedback system

Pituitary Gland Posterior Lobe

The posterior lobe of the pituitary gland a.k.a. neurohypophysis, does not synthesise hormones. It releases hormones to the circulation via the posterior hypophyseal veins to be distributed to target cells in other tissues. The cell bodies of the neurosecretory cells produce Oxytocin (OT) and Antidiuretic Hormone (ADH) / Vasopressin.

Oxytocin (OT)

  • is released in high amounts just before birth
  • stimulates contraction of smooth muscle cells in the pregnant uterus
  • stimulates the contractile cells around the mammary gland ducts
  • affects milk ejection
  • works through a positive feedback cycle which is broken following birthing
  • is inhibited by progesterone, but can work in conjunction to oestrogens

Antidiuretic hormone (ADH)

  • affects urine volume; it causes the kidneys to excrete water from fresh urine and return it to the bloodstream, reducing urine volume
  • absence causes an increase in urine output
  • raises blood pressure by constricting arterioles
  • secretion varies based on the body’s needs
  • causes a decrease in sweat
Retrieved from https://slideplayer.com/slide/10623655/ on 6th March 2022

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