5.1.2 - Reproduction in Mammals Flashcards
Sexual reproduction in mammals - mechanisms to maximise success
- Internal fertilisation → increase chance that gametes will meet
- Implantation of the embryo into the uterine wall + internal development of embryo → higher chance of survival
- Pregnancy → protects the developing young from the environment + nourishes embryo through placental connection
- Birthed only once it is matured enough to be able to survive (viable)
Hormones
Def: Chemical substances that coordinate physiological functions
Coordinates the reproductive cycle to ensure greater reproductive success:
- Pituitary gland - the master gland. It releases hormones to stimulate/inhibit other organs to release/stop hormones.
- Sex hormoes - specifially affect the growth/functioning of reproductive organs and secondary sex characteristics
Sex hormones
- Androgen
- Testosterone
- Oestrogen
- Progestogen
- Progesterone
Hormones in the female reproductive cycle
Gonadotropic hormones released by pituitatry glands:
- Follicle stimulating hormones: stimulate maturation of follicles in the ovaries of females
- Luteinising hormones - stimulates the final maturation of one ovarian follicle → stimulates ovulation + stimulates the onset of the luteinising phase
- In males, it stimulates secretion of testosterone.
Ovary secretes oestrogen and progesterone
Follicle
A small, fluid-filled sac in the ovary that contains one immature egg
- Follicle cells secrete fluid to push the egg inside it to one side of the follicle
- Graafian follicle: dominant follicle that moves to the surface of the ovary and creates a bulge (this is the one that will cause ovulation)
What is regulated by hormones in the female reproductive cycle
- Ovarian cycle: development of the female gametes/ova in the ovaries.
- Menstrual cycle: the uterus is prepared for implantation of a fertilised egg each cycle. If fertilisation does not take place, the levels of oestrogen and progesteron decrease, resulting in the lining of the uterus tearing away accompanied by bleeding. AKA menstruation
- Maintenance of pregnancy
- Preparation for and maintenance of lactation
Ovarian cycle
- Growth and maturation of an oocyte in preparation for fertilization and reproduction
- Consists of follicular phase, ovulation, and luteinising phase
Follicular phase
- Follicle-stimulating hormone is secreted by the pituitary gland, promoting growth of follicles
- Follicle cells secrete fluid, pushing the egg to one side of the follicle.
- The enlarged dominant follicle moves to the surface of the ovary and creates a bulge. The follicle that does this is called the Graafian follicle
- The Graafian follicle bursts, releasing the egg/ova - ovulation
- The female ovum is now viable for 12-24 hours
- After ovulation, the egg has been released from the follicle, and is known as the corpus luteum
Graafian follicle
Mature follicle that moves to the surface of the ovary and forms a bulge to release the egg
Luteal Phase
- Starts after ovulation, lasts 14 days
- Luteinising hormone production surges due to secretion of oestrogen by follicular cells → ovulation
- Corpus luteum secretes progesterone which builds up uterine wall lining → body prepared for pregnancy
- Main secretor of progesterone until placenta has formed
Menstrual cycle
- Begins with menses (uterine lining sheds, causing bleeding)
- New endometrial lining forms over 5-12 days after menses → preovulation phase
- Ovulation occurs in the ovaries 13-15 days after the start of menstruation
- After ovulation, the enlarged corpus luteum releases progesterone and oestrogen
- Progesterone acts on target cells in the uterus, preparing the endometrium for implantation of the fertilised ovum and pregnancy.
Cells start forming in the endometrium lining. Oestrogen starts the development of the endotrium wall (cell division). The developing and thickness (blood vessels, mucus ) is by progesterone.
Placenta in humans
Direct access to the bloodstream - no maternal cells. Unrestricted access can cause intense fatigue, high blood pressure, plecampsia. Severing the fetus can cause a haemmorage.
Oocyte
Unfertilised egg cell. Contained within a follicle until ovulation
Male reproductive cycle
- Follicle stimulating hormone stimulates the process of spermatogensis (sperm production)
- Luteinising hormone triggers the production of testosterone - development of secondary male characteristics.
As spermatogensis and testosterone production increases - Sertoli cells produce inhibin to stop FSH and LH production in the pituitary gland
Internal environmental conditions that help fertilisation
- Oviducts release a fluid that helps sperm swim up the reproductive tract to the uterus (rheotaxis)
- The presence of progesterone and an akaline pH helps the sperm matures so that it can penetrate the egg. These sperm becomes hypermobile.
Pituitary gland
- Master gland
- Secretes hormones that stimulate or inhibit other endocrine glands
- Secretes 2 Gonadotropic hormones: follicle-stimulating hormone (FSH) and luteinising hormone (LH)
Androgens
- Group of male sex hormones
- Controls development and functioning of male sex organs + secondary sex characteristics
- Present in males and females but higher in males
Testosterone
- An androgen
- Facilitates sperm production (spermatogenesis)
- Secreted by testes
Oestrogens
- Group of female hormones
- Controls development and functioning of female reproductive system + secondary sex traits
- Higher in female but present in male also
- Secreted by ovary
Functions
In the follicular phase of the ovarian cycle, oestrogen is secreted by cells in the lining of the follicle → production of luteinising hormones surge, causing ovulation
In the menstrual cycle, oestrogen and progesteron maintain the uterine wall. A drop in oestrogen levels thus leads to the shedding of the uterine wall
During pregnancy, oestrogen promotes growth of endometrium
Progestogens
Second group of female hormones
Progesterone
- An oestrogen hormone
- Stimulates lactation in mammary glands
- Produced by adrenal cortex and gonads normally, but during pregnancy it is synthesised by the corpus luteum
- Maintains the endometrium and builds uterine lining alongside oestrogen, preparing the uterus for implantation and pregnancy
- Development of mucus and growth of blood vessels
- High levels of progesteron prevent uterine contractions
3 layers sperm has to cross to fertilise an egg
Corona radiata:
- 1st membrane, sperm has to physically push through
- Corona radiata can release enzymes to help sperm penetrate
Zona pellucida:
- 2nd layer with glycoproteins
- When sperm comes into contact with glycoproteins, acrosome sperm head fuses with cell membrane , helping the sperm release enzymes that assist its penetration
Plasma membrane:
- When one sperm penetrates the plasma membrane, surface proteins trigger the release of enzymes by the egg that destroy glycoproteins in the zona pellucida and cause electrical charges, preventing other sperm from entering
HCG
- Human chorionic gonadotropic hormone
- Starts being secreted by the trophoblast, the outer cells of the blastocyst when implantation occurs (trophoblast will later form the placenta, so it can be said that HCG is produced by the placenta)
- Sustains the corpus luteum until the placenta is fully developed → progesterone and oestrogen levels are maintained → endometrium thickness grows, menstruation does not occur
- Suppressing maternal immunal response to protect the embryo.
- Levels rise after conception and continue to rise until 10 weeks in pregnancy.
- Promotes relaxin production, allowing the uterus to relax and accomodate for the growing embryo.
How to EXPEL the baby
- Uterine contraction
- Cervix dilates (pelvis and cervic muscles must soften)
Alcohol intake impact
Can interfere with fetal development as the baby gets nutrients from the mother’s bloodstream
Why does a foetus kick?
It is essential for muscular and skeletal growth/development.
LH
Lutenising hormone
- Secreted by anterior pituitary gland
- Targeted at the ovaries, to trigger ovulation
GnRH
Gonadotropic releasing hormone
- Produced by hypothalamus in the brain
- Stimulates production of FSH and LH by the pituitary gland
- If oestrogen and progesterone levels high in the blood, GnRH will stop stimulating FSH and LH production because it will take this to mean that ovulation has occurred and there is no longer need to produce FSH and LH
Implantation
- Fertilised egg that has begun to undergo cell division as it travells down the fallopian tube implants itself into the endometrium. This is called the blastocyst
- The inner cell mass of the zygote will form the embryo
- Outer cells (trophoblast) will form the placenta
- Once blastocyst is implanted, embryo starts releasing HCG to sustain the corpus luteum’s production of oestrogen and progesterone
Hormones involved in birth
Prostaglandins: Starts the process of labour (contraction of uterine walls). Makes uterus more sensitive to oxytocin (main hormone in contractions)
Oxytocins: coordination of the contraction of the smooth muscles of the uterus + softens the cervix so that baby can be born
Relaxin: relaxes the cervic too
What happens to progesterone and oestrogen levels during birth: these hormone levels decrease and cause stronger constractions.
Beta-endorphins: a form of natural pain relief
Adrenaline: end of labour to cause strong contractions that lead to the birth
Prolactin: sitmulates lactation
Oxytocin
- Secreted by the pituitary gland
- Stimulates uterine contractions and softening of cervix
- Positive feedback loop: stimuli of pressure on the cervix by the baby’s head signal the pituitary gland to release more oxytocin, amplifying contractions
Hormones in uterine contractions
Progesterone + oestrogen
- Drop in progesterone and oestrogen levels cause stronger contractions.
7 month~ Progesterone levels decrease (high levels prevent contractions) while oestrogen continues to increase. This makes the uterus more sensitive to oxcytocin.
Oxytocin:
- Involved in a positive feedback loop where the pushing of the baby’s head against the cervix causes oxytocin the be produced, intensifying uterine contractions, and uterine contractions signal the pituitary gland to release more oxytocin
Adrenaline
- Released at the end of labour to cause strong contractions that lead to birth
Beta-endorphins
Act as natural pain relief
Hormones involved in cervix softening
Prostaglandins, oxytocin, and relaxin
Hormones involved in lactation in mammary glands
Prolactin and oxytocin
