GAMETOGENESIS BY: MARTIN, Ezra D. PLES, Janina Grace C. AAPD2G
Meiosis is the first step in gametogenesis: separation of homologous chromosomes into haploid daughter cellsSpermatogonia and oogonia are the germ cells that will eventually develop into themature sperm or eggPrimary spermatocyte or oocyte: the first step in this development is the duplicationof homologous chromosomes to get ready for meiosisSecondary spermatocyte or oocyte: thefirst meiotic division separates thehomologous chromosomes from eachparentSpermatids or eggs: the second meioticdivision separates the 2 chromatids andcreates 4 haploid cellsIn males, this eventually produces 4sperm cells by the process ofspermiogenesis. In females, it produces1 egg and 3 polar bodies. This allowsthe egg to retain more cytoplasm tosupport early stages of development
Meiosis generates tremendous genetic diversity. How many different types ofgametes can be generated by an individual (male or female) with 23 differentchromosomes?
The timing of meiosis differs in females and malesIn males, the spermatogonia enter meiosis and produce sperm from puberty untildeath.The process of sperm production takes only a few weeks. Each ejaculationhas 100 to 500 million sperm.In females, this process is more complex.The first meiotic division starts beforebirth but fails to proceed. It is eventually completed about one month beforeovulation in humans. In humans, the second meiotic division occurs just before theactual process of fertilization occurs. Thus, in females, the completion of meiosis can be delayed for over 50 years. This is not always good. Only I egg produced In addition, all meiosis is ended in females at menopause.
Homologous chromosomes form the synaptonemal complex which facilitates crossing over and genetic diversity During meiosis, homologous chromosomes join together in pairs to form the synaptonemal complex. Each pair of chromatids is connected by axial proteins. The 2 homologous chromosomes are held together closely by central element proteins. A recombination nodule forms that contains enzymes for cutting and splicing DNA. Chromosomes are cut and joined crosswise at points called chiasmata,seen when they separate. The exchange of genetic material is evident when the chromosomes separate This process is dangerous as it leads to deletions and duplications of genetic material. However, it is also valuable because it increases genetic diversity and facilitates evolution.
In older women, failure of the synaptonemal complex to separate properly can cause genetic disease Down syndrome is trisomy 21. It results in short stature, round face and mild to severe mental retardation. This is the failure of the 2 chromatids to separate during meiosis 2. It results in one oocyte receiving 2 instead of 1 chromatid. In older women, long term association of chromatids (i.e., over 50 years) results in the axial proteins failure to separate. Down syndrome occurs with a frequency of 0.2% in women under 30 but at 3% in those over 45 years of age.
Spermatogenesis occurs in the seminiferous tubulesThe mammalian testes are divided into many lobules, and each lobule contains manytiny seminiferous tubules.Sperm develop in an ordered fashion in these tubules. Cellsstart to mature on the outside and move inward (towards the lumen) as the becomemature sperm.Spermatogonia are the most primative cells. They differentiate as primaryspermatocyteSertoli cells are supporting cells that stretch from the lumen to the edge of thetubule. They nourish the developing sperm. They form a blood-testis barrier tocontrol spermatogenesis (similar to the blood-brain barrier). These cells also inhibitspermatogenesis before puberty and stimulate the process after puberty.
Spermiogenesis is the maturation process into spermThe golgi vesicles combine toform an acrosomal vesicle thatlies over the nucleus. Its full ofenzymesCentosomes start to organizemicrotubules into long flagellaMitochondria start to localize next to theflagella to provide ready energyThe nucleus condenses in size and is stabilized by specialproteins called protaminesThe excess cytoplasm is pinched off as a residual body(no need for organelles and cytoplasmic proteins)Sperm are tiny, but highly specialized missiles for delivering the male genome:Microfilaments shoot the acrosome into the egg to ‘harpoon it’ and pull it in.The acrosome has enzymes for breaking into the egg.The midpiece has large numbers of mitochondria for horsepower.The tail has a powerful flagellum for driving the sperm into the proximity of the egg (inhumans, through the uterus and up into the oviduct.
Spermatogonia and oogonia are stem cellsWhat is a stem cell?Stem cells have 3 properties: 1. They are undifferentiated cells 2. They have potential for self renewal 3. They are able to undergo differentiation to form committedprogenitor cells (a fancy word for all types of differentiatedadult cells such as muscle, bone, skin, etc)
The goal of oogenesis is to produce one egg with massive amounts of cytoplasmIn many organisms, such as frogs and birds, the egg must contain all the nutrientsto support the entire process of embryonic developmentIn humans, the egg does not need to grow so large because the fertilized egg onlyneeds to support growth until it implants in the uterus. The placenta then nourishesdevelopment.In some organisms, such as frogs,oocytes grow to extremely largesize and they have very activechromosomes that synthesize largeamounts of RNA. In contrast tosperm which are tiny cells, oocytesare among the largest cells in thebody.Oocytes contain Lampbrushchromosomes: look like brushesthat were used years ago to cleanlamps. Frog oocytes can contain200,000 times as many ribosomesas a normal cell.
Oocytes have a very small nucleus / cytoplasm ratioMost normal cells have several times as much cytoplasm as nucleus. This allows thenucleus to make enough mRNA and rRNA to keep up with the cytoplasm and cellneeds.In some species, oocytes have a tremendously tiny nucleus to cytoplasm ratio. Theymust have a large amount of cytoplasm and ribosomes to make all of the proteinsneeded for embryonic development.The nucleus is just not large enough to keep up and maintain enough transcription togenerate all of the needed components. However, oocytes have developedspecializations to deal with this problem.1. Ribosomal RNA genes are often amplified in oocytes. This allows more templatesto transcribe more rRNA.
Specializations allow the egg to accumulate cytoplasm: nurse cells allow oocytes of insects to produce massive amounts of RNA In Drosophila melanogaster, the oogonia are called ctyoblasts, and they undergo an unusual specialization They undergo multiple mitotic divisions, but fail to undergo cytokinesis (cell division). Thus, they all remain connected to the original cell as cytocytes One of the lucky cytocysts becomes the oocyte The other 15 become nurse cells. They make large amounts of RNA and nutrients but they send it all to the oocyte. This allows the oocyte to accumulate massive amounts of cytoplasm to support development (15 nuclei instead of 1).
Vitellogenesis is the process of producing the major yolk proteinsYolk: animal eggs contain large amounts of protein, lipid, and glycogen to nourishthe embryo. These materials are collectively called yolk.Yolk is minimal in animal eggs that sustain only the first portion of embryogenesis(humans and many mammals that have a placenta need only support cleavage forseveral days before implantation into the uterus).However, yolk is stored in large amounts in the eggs of birds and reptiles becausetheir eggs have to support the entire process of development.Yolk proteins are synthesized in the liver in vertebrates, or in the fat body of insects(an analogous organ)Animal – vegetal polarity: In eggsthat have a lot of yolk, the yolk isconcentrated in the vegetal pole.The animal pole contains thenucleus and relatively little yolk.The yolk in the vegetal poleinterferes with cytokinesis duringthe process of cleavage leading toincomplete cleavage.
Maturation processes prepare the oocyte for ovulation and fertilizationMost oocytes of different species are arrested in the first meiotic division.Oocyte maturation begins officially when this block is removed and meiosis startsonce again.1. The nuclear membrane breaks down and DNA starts to condense intochromosomes2. The permeability of the oocyte plasma membrane changes so it can functionoutside of the ovary.3. The plasma membrane develops receptors to interact with the spermFertilization occurs at different stages of oocyte maturation: How is oocyte maturation initiated?
Control of oocyte maturation has been studied extensively in frogsOocyte maturation is controlled by hormone interactions between the pituitary andfollicle cells. Pituitary o gonadotropin hormone cprogesterone s triggers oocyte maturation by activating c-mos expressionC-mos activates maturation promoting factor, the same activity as M-phase promotingfactor, that is composed of cyclin B and cyclin dependent kinase 1The exactmechanism isn’t understood. If c-mos is inactivated by antisense oligonucleotides, no oocyte maturation occurs. On the other hand, if extra c-mos is injected it triggers oocyte maturation before it is ready. MPF does many things, although the exact pathways have yet to be found. It causesbreakdown of the nuclear envelopeby phosphorylating nuclear lamins (proteins stabilizing the envelope), it triggerschanges in the oocyte plasma membrane, it stimulatesovulation, and it causescondensation of chromosomes.
Development of mammalian oocytes occurs within the ovaryIn the mammalian ovary, the oocytes are closely associated with somatic cellscalled granulosa cells which aid oocyte maturation and ovulation.The timing of oocyte maturation and ovulation varies in different mammals.Ovulation can be stimulated by seasonal cues, the process of mating, or inprimates, by the monthly cycle regulated by hormones such as estradiol,produced by the granulosa cells.
Eggs are protected by elaborate envelopesVitelline envelope: a glycoprotein layer covers the plasma membrane of all eggs.This acts to protect the egg.Eggs that are deposited in water have a jelly-like coating that surrounds the egg(frogs eggs)Eggs that are deposited on land have particularly elaborate envelopes. The eggs ofbirds have a vitelline envelope, a fibrous layer, an outer layer of albumin (eggwhite), and a shell composed of calcium carbonate. The outer envelopes aresynthesized in the oviduct after the egg has been fertilized.