endocrine system

local communication methods ( 3 types )

• Proteins called connexions form pore like structures in gap junctions

• Also provide electrical pathway for heart and other things

types of signalling

endocrine versus exocrine glands

• some organs have both functions, like the stomach and pancreas

major structures in the endocrine system

• hypothalamus

• pituatiary gland

• thyroid gland

• parathyroid gland

• thymus

• adrenal glands

• pancreas

• pineal gland

3 main types of hormones

• Steroids formed in smooth ER

• Amines from amino acids

  

peptide and protein hormones

steroid hormones

 All are derived from cholesterol

• Synthesised as needed in the smooth endoplasmic reticulum of gonads, adrenal glands and placenta.

• Largely bound to carrier proteins in the blood  in order to TRAVEL (extends half-life)

• Lipophilic (FAT LOVING) - can cross the plasma membrane - receptors are in the cytoplasm or nucleus (mostly).

• Activate transcription of target genes - alters protein synthesis

amine hormones

• Amino acids with modified groups

  

thyroid hormones properties

absoprtion times of hormones

• Onset can range from seconds to hours depending on pathways activated

• Duration of the response can range from minutes to days.

• Clearance is the removal of hormone from plasma (bulk cleared by liver and kidneys with only a small fraction actually removed by target tissue).

• Half-life is the length of time it takes for the hormone concentration to drop by half. This can range from seconds to days.

receptors and signalling pathways

different for lipophilic and lipophobic

 

types of cell surface receptors

response based on receptor type

Lipophilic hormones (steroid and thyroid hormones) act primarily on intracellular receptors

• 

What determines how big the response elicited by the hormone will be?

what stimulates hormone release

• hormonal

• neural

• humoral

different patters of hormone secretion

example of a neuroendocrine pathway

regulating hormone secretion with feedback

• 

how negative feedback works

factors that influence levels of hormones in plasma

• Rate of secretion (regulated - most important)

• Rate of binding to carrier proteins

• Rate of metabolism (activation/degradation)

Posterior Pituitary (Neurohypophysis)

 

how the posterior pituitary works

• Made up of glandular epithelial cells

• Short axon neurons synthesise hypophysiotropic hormones and release them into capillaries of the hypothalamic-hypophyseal portal system.

 

Hypothalamic – pituitary – adrenal axis (HPA axis)

• Stressors (internal/external) trigger the release of cortisol.

• This involves the release of intermediary hormones from the hypothalamus (CRH:

  • corticotropin releasing hormone) and the

  • anterior pituitary (ACTH: adrenocorticotropic hormone).

Release is regulated via negative feedback.

thyroid gland

• Endocrine gland regulated by the hypothalamus and anterior pituatiary

• Located immediately below the larynx on each side of and anterior to the trachea (BOWTIE)

• The two lobes are joined by the Isthmus

• Normally weighs 15-20g in adults

thyroid gland hormones

 

 

intracellular and whole body affects of thyroid hormones

thyroid follicles

the functional unit of the thyroid

thyroid hormone synthesis = T3 and T4

 Thyroglobulin (large protein) synthesised in follicular cell and exocytosed into the lumen ( as the vesicles in colloid are released)

2. Iodide pump actively pumps iodine into follicular cell

3. Iodine is attached to tyrosine residues on thyroglobulin molecules in follicle lumen

4. Iodinated residues couple to form T3 and T4

5. Thyrogobulin is endocytosed and vesicles fuse with lysosomes

6. Enzymes cleave T3 and T4 which then diffuse into the bloodstream.

7. Iodine (from MIT and DIT) is recycled back into the lumen.

regulation of thyroid hormone secretion

• thyrotropin-releasing hormone (TRH) stimulates release of thyroidstimulating hormone (TSH)

• Negative feedback maintains relatively constant supply of thyroid hormones

• In tissues T4 may be converted to T3 as it is more bio-available

  • T3 and T4 both act on thyroid hormone receptors but T3 has more reactivity

endocrine dysfunction

 

3 main functions of male reproductive system

1. Production of sperm cells in the testes: Sperm cells in males and oocytes (eggs) in females are referred to as gametes.

2. Sustaining and transfer of sperm cells to the female: The ducts and glands in the male reproductive system provide nutrients for the sperm cells produced in the testes, and transport the sperm from the testes through the penis, which is a specialised organ that deposits sperm into the female reproductive system.

3. Production of male sex hormones: Hormones produced by the male reproductive system control the development of the reproductive system itself and of the male body form. These hormones (including testosterone) are essential for the normal function of the reproductive system and reproductive behaviour.

Functional categories of MALE reproductive structures

 

Secondary sexual characteristics are features that are not essential for the reproductive process but are generally considered sexual attractants. In the male, they include body physique, body hair, and voice pitch.

parts of male reproductive system

the scrotum

 

the testes

 

importance of thermoregulation of the testes

how thermoregulation is maintaned

How Testes Temperature is maintained

1. Located outside body within scrotum

Copious temperature receptors and sweat glands in scrotal skin

2.  Cremaster muscle

Skeletal muscle that is under the dermis, Descends from abdomen through spermatid cord

 

Cold: contract to lift testes closer to warm pelvis

Hot: relax to drop testes away from warm pelvis

 3. Dartos muscle

• Smooth muscle

• Within dermis

Resting muscle tone of dartos causes characteristic wrinkling of scrotal surface

 Cold: contracts (increased wrinkling) to reduce surface area for heat loss

Hot: expands (reduced wrinkling) to increase surface area and promote heat loss

 

1. Counter-Current Heat Exchange

• Network of testicular veins (pampiniform plexus) wrap around around testicular artery

• Exchange of heat from artery to vein - material blood cooled; venous blood warmed

 

 

 

spermatogenesis

 process of production of sperm

• Simple, round cell becomes an elaborate, highly specialised cell

• Most immature form of cell found in the basement membrane, most mature towards the lumen

• Begins in outermost layer of seminiferous tubules and proceeds toward the lumen.

• 4 sperm produced for every spermatogonium, begins at puberty

  • Takes approx 65-75 days

 

3 elements:

1. Mitotic proliferation - produces large number of cells

2.  Meiotic division - generates genetic diversity and halves number of chromosomes in daughter cells

3. Spermiogenesis - morphological change in shape of the cell from round to sperm shape

spermatogenesis flow chart

spermiogenesis

• Round spermatid into elongated spermatozoa = 60 µm long

• Adapted for reaching and penetrating an oocyte

• 300 million sperm produced per day

 

4 key events:

1.  Development of acrosome

Over anterior nucleus, vesicle of digestive enzymes

• Help sperm penetrate oocyte by dissolving surrounding structures of oocyte

 2. Development of tail (flagellum)

Sperm motility, as it allows sperm to swim to the oocyte

Help sperm penetrate oocyte through force from the tail

 

3. Condensation of nucleus

DNA undergoes packaging, becoming highly condensed, as the wider the nucleus the more risk of the DNA being damaged

• This Protects DNA

 

4. Shedding of excess cytoplasm

Taken up by Sertoli cells

Streamlined sperm; better motility

male reproductive tract (duct system)

Duct system =

•  Testes produce physically mature sperm that are incapable of fertilising an oocyte

• Tract (ducts) are responsible for functional maturation, nourishment, storage and transport of sperm

epididymis

• Posterior side of testis, can be felt through skin of scrotum

• 7m long tubule

• Coiled and twisted so that it takes up very little space

ductus/ vas deferens

urethra

male accessory glands

 

seminal vesicles

prostate gland

bulbourethral glands

Small (1 cm) paired glands at the Base of penis

• Ducts empty into penile urethra

• Secrete thick alkaline mucus prior to ejaculation

  • Neutralises urethra (acidic from urine) to protect the sperm

  • Lubricates glans penis

semen composition

four main functions of female reproductive system

1. 

catagories of female reproductive system

uterine tubes

uterus

cervix and vagina

Cervix

• Contain mucous glands

 

Water, proteins; lipid, salts etc. Consistency varies during cycle   Near/at ovulation: Less viscous (watery) to allow sperm passage   Other times: More viscous (thicker) to impede sperm penetration   Vagina Passageway for birth and menstrual flow 10cm fibromuscular canal Stratified squamous non-keratinised epithelium Glands producing glycogen   Decomposition produces organic acids Acidic environment retards microbial growth, but … Harmful to sperm

ovaries

folliculogenesis

= development of the follicle

 

• Follicle: oocyte + support (follicle) cells

• Begins before birth

• Maturation of oocyte + increased number of follicle cells

 

Primordial follicle

• Primary oocyte + single layer of flat follicle cells

 

Primary follicle

• Primary oocyte + 1-2 layers of cuboidal follicle cells

• Follicle cells secrete estrogen

 

Secondary follicle

• Enlarged oocyte

• Additional layers of follicle cells

• Multiple, small fluid-filled spaces develop between follicle cells

 

Tertiary follicle

= mature or Graafian) follicle

• Single, large fluid-filled space (antrum)

• Oocyte surrounded by specialised follicular cells called granulosa cells

 

Ovulation = release of oocyte from ovary

• Oocyte and granulosa cells rupture and exit the ovary, most of the follicle and the follicular cells stay behind, it becomes the corpus luteum

 

Corpus Luteum

= Glandular structure left behind in ovary after ovulation

• Empty tertiary follicle collapses

• Remaining follicular cells proliferate to fill space

• Secrete estrogen and progesterone ( follicular cells made estrogen only before ovulation then after they make BOTH)

 

• Lifespan of 10 days, unless pregnancy occurs

• Then after 10 days it becomes the corpus albicans

oogenesis

  

 

ovarian cycle

Primary oocytes are eggs that are suspended in meiosis I just after a female’s birth. The oocytes remain dormant, surrounded by protective follicular cells, until the menstrual cycle begins at puberty. At puberty, the initiation of the ovarian cycle causes the oocyte to mature within a protective structure called a follicle.

Follicular phase (day 0-13)

The ovarian cycle is regulated by hormones produced in the hypothalamus (GnRH) and anterior pituitary gland (FSH and LH). The ovary contains many follicles with oocytes in various stages of development, however typically only one primary oocyte is released during ovulation approximately every 28 days. The ovarian follicle goes through a series of growth phases until it becomes a fluid-filled vesicular (Graffian) follicle.

 

Ovulation (day 14)

 

During ovulation, the vesicular (Graffian) follicle ruptures to release the oocyte into the abdominal cavity, to be picked up by the fimbriae and guided into the uterine tube.

 

Luteal phase (day 15-28)

The ovulated (empty) follicle transforms into the corpus luteum. The corpus luteum acts as a temporary endocrine gland, secreting large amounts of progesterone (and also moderate levels of other hormones such as estradiol and inhibin A). After pregnancy or menstruation, the corpus luteum deteriorates into a scar of dense connective tissue called the corpus albicans.

what is the female reproductive cycle

steps in the female reproductive cycle

Gonadotropin hormone-releasing hormone

phases of female reproductive cycle

Pre-ovulation / Proliferative Phase

• GnRH secreted by the hypothalamus, travels to the anterior pituatiary

• This makes FSH (follicle stimulating hormone) binds to follicular cells casing follicles to grow and develop

• Follicle with most FSH receptors becomes the tertiary follicle

• Estrogen from the follicles then tells stratum functionalis to regrow in uterus

Ovulation

• Then estrogen levels are high there is a positive feedback with more GnRH secreted,  triggering a luteinizing hormone (LH) surge, which triggers the follicule rupturing and so OVULATION, at around day 14

 

Post-ovulatory /Secretory Phase

• Ruptured follicle collapses in on itself and becomes the corpus luteum, LH keeps this structure ALIVE and instructs it to make estrogen and progesterone

• Progesterone switches endometrial glands on and tells them to make mucus

• Negative feedback on GnRH is suppressed for ~10 days whilst oestrogen and progesterone levels are high ( to allow time for egg fertilization)

 

Menstrual phase

• Hypothalamus then picks up on high levels of oestrogen and progesterone in the body

• This means there is no GnRH secreted and so no LH, so corpus luteum becomes the corpus albicans

• Because there is no oestrogen and progesterone being made (with no corpus luteum) the spiral arteries in the stratum basalis layer in the endometrium the blood flow stops in the stratum functionalis layer, KILLING it, the shedding of this layer is menstruation

• This leaves the stratum basalis layer behind and oestrogen and progesterone levels have dropped so negative feedback on GnRH ends and these hormones are made again

how the pill works

• Combination of oestrogen and progesterone (or synthetic versions)

• 21 hormone pills + 7 placebo pills

• Suppresses GnRH and thus FSH and LH (i.e., hold cycle at secretory phase)

• The high levels of oestrogen and progesterone have a negative feedback and supress the GnRH, so no FSH, and no follicle developing and so nothing to ovulat

fertilizations steps

how polyspermy is blocked

Polyspermy blocked

• Depolarisation (Na+ influx) when one sperm has entered the oocyte, changing the electrical charge of it, electrically repelling further sperm

• Cortical granule reaction (Ca2+ influx) hardens zona pellucida = seen below as the green granules

pre- implantation process

= the First week of development

Characterised by:

Fertilisation

Cleavage of zygote

Blastocyst formation

Followed by implantation

 

 

Cleavage of zygote

Once sperm enters oocyte:

Single cell now called a zygote

This is called cleavage

'

Morula

Day 4

Solid ball of cells

Enters uterus day 5

Zona pellucida disintegrates to release…

 

Blastocyst

Cavity appears

Outer cells (trophoblast) become foetal part of the placenta

Inner cell mass (embryoblast) become the embryo

 

• Occurs in the isthmus and the oviduct

• Morula appears at day 4 when the oocyte has entered the uterus

• At day 5 the zona pellucida is dissolved away

• Structure can then implant into the wall, as a blastocyst, centre is fluid surrounded by cells