reproduction HL

puberty

• Sequence of developmental changes that form the transition between childhood & sexual maturity

• Onset & progress through puberty is controlled by the brain via the hormone gonadotrophin release hormone (GnRH)

• Sectretion of GnRH starts in a fetus 10 weeks after fertilization - continues until after birth - stops when baby is 4-6 months

• GnRH secretion resumes during teenage years and continues through puberty & adulthood

• Hormone is released from the hypothalamus & acts on the pituitary gland

• GnRH stimulates FSH & LH secretion

puberty in males

• In males, FSH stimulates testis growth and LH stimulates testosterone secretion by Leydig cells 

  • Testosterone causes the development of secondary male characteristics

puberty in females

• In females, FSH stimulates the development of the follicle which secrete estradiol & LH stimulates the development of corpus luteum after ovulation

  • Estradiol causes the development of secondary female characteristics

Gametogenesis

• Process by which diploid precursor cells undergo meiosis to become gametes 

• Occurs in the reproductive organs (gonads) of males and females 

• In males, the gametes are produced in the seminiferous of the testes

• In females, the gametes are produced by the ovaries

spermatogenesis vs oogenisis

	# of gametes produced	Rate of gamete production	Timing of release	Timing of production & release	Amount of cytoplasm
spermatogenesis	4	Millions per day	During ejaculation	Produced from puberty onwards	Little cytoplasm
oogenisis	1	1 per month	At ovulation	Production initiated during foetal development & completed uring menstrual cycle	More cytoplasm than any human cell

oogenisis

• At puberty, some follicles develop each month under the influence of FSH

• Primary oocyte continues to meiosis I, where unequal reduction division takes place

• The primary oocyte develops to sa secondary oocyte & first polar body

• Polary body eventually degenerates 

• Secondary oocute proceeds to meiosis II and stops at prophase II 

polyspermy 

• Fusion of more than one sperm with an egg → non viable zygote

• 2 processes help make it infrequent: acrosome reaction & cortical reaction 

acrosome reaction

• Sperm cells bind to glycoproteins in the zona pellucida → release of the contents of their acrosome → enzymes from acrosome digest the glycoproteins within the zona pellucida → the sperm cell pushes through to reach the plasma membrane of the egg

cortical reaction

• Only the nucleus of the sperm enters the egg causing fertilization → cortical geanules release contents by exocytosis → hardening of the zona pellucida → difficult for other sperms to enter (enzyme also causes changes to the glycoproteisn to which the sperm binds) 

gamete adaptations: egg

• Human egg cell is one of the largest human cells

• Cytoplasm of egg is rich in lipids, proteins, and polysaccharides

• Egg have specialized layers of cells covering the outside which prevent polysperm

gamete adaptations: sperm

• Evolved to be mobile & so contain many mitochondria in the mid piece to provide energy needed to propel the sperm forward (cannot travel backward) 

• Little cytoplasm in sperm head, majority of space taken up by nucleus 

• Arosome cap covers entire head of sperm which contains digestive enzumes that will be used to penetrate the egg cells’ zona pellucida 

• No storage of nutrients for the supply of energy within the sperm, this has to be provided by the mal seminal fluid in which the sperm swims 

blastocyst

• Egg & sperm nuclei combine to form a diploid nucleus & the fertilized cell is now called a zygote 

• Zyote will undergo several mitotic divisions (w/o growth period) to form a solid ball of cells called morula

• After 6-7 days it changes to hollow ball due to unequal cell division & migration - called a blastocyst

• At this point it will migrate from the oviduct to the uterus 

• The toughened zona pellucida breaks down 

• The blastocyst has used up the reserves of the egg cell & needs external supply of food by attaching itself to the endometrium in a process called implantation 

pregnancy

• Human chronic gonadotropin (hCG) is a protein produced by the embryo from the blastocyst stage onwards

• It's production is essential in maintaining pregnancy by preventing the degradation of the corpus luteum 

• During the first 10 weeks, hCG stimulates the corpus luteum to secrete progesterone

• After that, the placenta (trophoblast) starts to secrete progesterone for the remainder of pregnancy 

• The corpus luteum is then degraded

• Pregnancy tests are based on the detection of hCG in urine

placenta

• Made up of fetal tissue in contact with the maternal tissue in the uterus wall

• Fetus also develops membranes that form the amniotic sac, which contains the amniotic fluid used to protect the developing fetus

• Basic functional unit of the placenta is a finger like piece of foetal tissue called chorionic villus

• These villi increase during pregnancy to cope with the increasing femands for the exchange of materials with the mother

• Maternal blood is rich in oxygen & nutrients - they diffuse from maternal blood that collects in the intervillious spaces to the foetal blood vessels in the chronic villi of the placenta 

• CO2 and urea produced by the developing embryo travel from the umbilical blood vessels to the placenta

childbirth

• Childbirth (parturition) is the process by which the fetus is expelled from the body

• At the end of pregnancy, hormones produced by the fetus signal to the placenta to stop secreting progesterone

• Estradiol starts to be secreted, it stimulates contractions in the uterus 

• These contractopns are detected by stretch receptors which signal to posterior pituitary gland to increase oxytocin secretion

• Increased oxytocin secretion makes teh contractions more frequent & more vigorous (positive feedback) 

• Uterine contractions burt the amniotic sac & amniotic fluid leaks out 

hormone therapy

• The onset of menopause is market by a decrease in the levels of estrogene and progesterone

• This results in a range of physical symptoms like hot flashes & vaginal dryness

• Hormone replacement therapy (HRT) is a treatment that relieves the symptoms of menopause

• In HRT, the person undergoing treatment is given medication that contains low doses of estradiol or a combination of estradionl-progestin (synthetic derivative of progesterone)