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Most Cells Reproduce by MitosisAnd Cytokinesis
Role of Gametes in SexualReproduction
- • Each parent produces cells called gametes froma pool of precursor cells called germ cells in aprocess called gametogenesis
- – Ova = Eggs in females
- – Spermatozoa = Sperm in males
- • In gametogenesis, diploid germ cells (2n) divideto form haploid gametes (n)
- • Fusion of two gametes (fertilization) produces azygote (2n).
General Principles ofGametogenesis:
Primary Spermatocytes in males
and Primary Oocytes in females
undergo meiosis to produce spermatids and ova
- • First stage is proliferation of primordial germcells by mitosis:
- replication to produce two newcells (daughter cells) with 46
- chromosomesidentical to the original cell
- • Second stage is meiosis: each gamete receives23 chromosomes – one from each homologouspair
- • Union of male gamete (sperm) with femalegamete (egg) results in a cell (zygote) with 46chromosomes
• Starts with diploid germ cell
••Duplication of original DNA yields exact copies ofall 46 chromosomes. The two copies of each(sister chromatids) remain joined together at thecentromere.
• Homologous chromosomes group together inpairs with genes on paternal chromosomesopposite corresponding genes of maternalchromosomes.
2n= 4 chromosomes
• In the first meiotic division(meiosis I) each daughter cellreceives one chromosome from each homologous pair
Among Mammals, Females are the “Default” Sex
- -Female organization progresses normallyin the absence of hormones in mammals.
- • More clinical issues with sexualdifferentiation of males than females.
• Primordial germ cells, called spermatogonia,begin to divide mitotically at puberty,producing primary spermatocytes
• Each primary spermatocyte undergoesdivision I of meisosis to become twosecondary spermatocytes, each with 23chromosomes
• Each secondary spermatocyte undergoes2nd meiotic division into 2 spermatids
• Spermatids differentiate into sperm
MITOSIS DIFERENTIATION BEGINS AT PUBERTY
• Spermatogenesis takes about 64days
• About 200 million sperm areproduced per day
- • They are small (about 20 x 100 μm)
- • Streamlined, built for speed– no ribosomes, ER, Golgi apparatus …
- – head with nucleus and acrosomal vesicle
- – midpiece with mitochondrion
- – long flagellar tail
OOGENESIS MITOSIS I and II
• Primordial germ cells are called oogonia
• Mitosis of oogonia occurs until about 3mo. after conception
• In fetus, all oogonia develop intoprimary oocytes
• Primary oocytes (2-4 million of them)replicate DNA, then meiotic arrest untilovulation
- • Just before ovulation, meiosis I iscompleted.
- • One daughter cell, first polar body, issmall and non-functional
- • Other daughter cell, secondary oocyte,retains almost all the cytoplasm
- • Meiosis II occurs after ovulation, andonly if secondary oocyte is penetratedby a sperm
- • One daughter cell, the ovum, retainsalmost all the cytoplasm
- • The other, the second polar body, isnonfunctional:
- • Net result: each primary oocyteproduces one ovum
- • zona pellucida: extracellular envelopemade of glycoproteins that polymerize to forma gel like layer outside of oocyte
- • Corona radiata: supporting cellsoutside zona pellucida
FERTILIZATION (3 key steps that insure species specificity):
2.Sperm activation/acrosomal reaction
- • Head of a sperm binds to sperm-bindingproteins in zona pellucida.
- • Binding triggers acrosome reaction anddigestion of zona pellucida.
- • Binding of first sperm to receptor onplasma membrane and transport in.
- • Completion of meiosis II
- • Depolarization, inactivation of spermbindingproteins, hardening of zonapellucida – block to polyspermy.
- • Formation of diploid nucleus: Zygotedevelopment begins
Sertoli cells support spermatogenesis in the seminiferoustubules in response to testosterone and FSH.
Development of Oovarian follicle and oocyte
- The fully mature follicle is 1.5 cm in diameter.
- One dominant follicle develops per month. (if more - can resultin fraternal twins)Over 99% of the fetal follicles undergo programmed cell death
- Following the release of the egg from the follicle, the follicledevelops into corpus luteum, and secretes estrogen, progesteroneand inhibin.The corpeus luteum reaches maximal development in 10 days, andthen rapidly degenerates.
- The ovarian cycle of changes in steroid production drivesthe rest of the changes that characterize the menstrual cycleof adult females. The follicular phase is marked by increasinglevels of estrogens whereas the luteal phase is one of increasedprogesterone levels; the transition between the two is ovulation.
REGULATION OF HORMONE SECRETION EARLY TO MID FOLLLICULAR PHASE
- Small increases in thesecretion of LH & FSH leadto follicular maturation,including an increase in thesynthesis and secretion ofovarian steroid hormones(1-7).FSH and LH increase because[estrogen] is low (1), so folliclesgrow.A dominant follicle secretes largeamounts of estrogen (4,5).FSH decreases (6) due toincreased estrogen, causingatresia of non-dominant follicles.Small increases in thesecretion of gonadotropins(LH & FSH) lead to follicularmaturation, including anincrease in the synthesisand secretion of ovariansteroid hormones.As [estrogen] due to thedominant follicle rises, itdecreases FSH and LH. Localpositive feedbackby estrogen causes granulosacells to proliferate.
LATE FOLLICULAR PHASE
- The feedbackrelationship betweenthe ovarian steroidhormones andsecretion from thehypothalamus/anteriorpituitary gland reversesin mid-cycle, elicitingthe large, ovulatorysurge in LH.Ovulation is provoked by a surgein LH and marks the transition tothe luteal phase of the cycle,characterized by high levels ofprogesterone. (8-14).Eventually, (15) a decrease in LHleads to luteolysis, and thewithdrawal of steroid supportfor a thick, active uterus.
- • Sperm must be present between 4 daysbefore ovulation and 2 day after– After ejaculation, sperm remain capable offertilizing an egg for 4-6 days.– Ovulated egg remains viable for 24-48 hours.
- • ~ 1 in 1,000,000 sperm reach the fallopiantube
- • Capacitation: Sperm must reside for hours insecretions of female tract to become capableof fertilizing an egg.
- • If fertilization is successful, the zygoteundergoes mitosis to 16 to 32 cells onthe way to the uterus.
- • About a week after ovulation,implantation begins (imbedding ofblastocyst into endometrium).
- • After a few weeks, the placenta, acombination of interlocking maternaland fetal tissues, forms.
An ectopic pregnancy is one in which
- the fertilized egg implantsin tissue outside of the uterus and the
- placenta and fetus begin todevelop there. The most common site is
- within a Fallopian tube,however, ectopic pregnancies can occur in the
- ovary, the abdomen,and in the lower portion of the uterus (the cervix).
During the first trimester, the chorion is the source of agonadotropin hormone that maintains steroid production by thecorpus luteum in the ovary.
Placental steroidogenesisprovides direct steroidalsupport of the uterus duringthe second and thirdtrimesters: the shift betweenthese steroid sources is acritical transition insuccessful pregnancy.
- The onset of laborand delivery istypically heralded bythe “breaking of thewaters” in referenceto the loss of amnioticfluid (c) through theruptured amniotic sac.Uterine contractions,coupled with themother’s abdominalcontractions, movethe newborn into itsown world.