Biology investigatory project on Gametogenesis

8/10/2019 Biology investigatory project on Gametogenesis

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SpermatogenesisSpermatogenesisis the process in which spematozoa are producedfrom male primordial germ cells by way of mitosis and meiosis. Theinitial cells in this pathway are called spematogonia, which yieldprimary spermatocytes by mitosis. The primary spermatocyte dividesmeiotically into two secondary spermatocytes; each secondaryspermatocyte then completes meiosis as it divides into twospermatids. These develop into mature spermatozoa, also knownas sperm cells. Thus, the primary spermatocyte gives rise to two cells,the secondary spermatocytes, and the two secondary spermatocytes

by their subdivision produce four spermatozoa.

Spermatozoa are the mature male gametes in many sexuallyreproducing organisms. Thus, spermatogenesis is the male version ofgametogenesis. In mammals it occurs in themale testes and epididymis in a stepwise fashion. Spermatogenesis ishighly dependent upon optimal conditions for the process to occurcorrectly, and is essential for sexual reproduction.DNAmethylation and histone modification have been implicated in theregulation of this process. It starts at puberty and usually continuesuninterrupted until death, although a slight decrease can bediscerned in the quantity of produced sperm with increase in age(see Male infertility).
http://en.wikipedia.org/wiki/DNA_methylationhttp://en.wikipedia.org/wiki/DNA_methylation


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Spermatogenesis produces mature male gametes, commonlycalled spermbut specifically known as spermatozoa, which are ableto fertilize the counterpart female gamete, the oocyte, duringconception to produce a single-celled individual known as a zygote.This is the cornerstone of sexual reproduction and involves the twogametes both contributing half the normal set of haploidchromosomes to result in a chromosomally normal diploid zygote.

To preserve the number of chromosomes in the offspring whichdiffers between species each gamete must have half the usualnumber of chromosomes present in other body cells. Otherwise, the

offspring will have twice the normal number of chromosomes, andserious abnormalities may result. In humans, chromosomalabnormalities arising from incorrect spermatogenesis can resultin Down syndrome, Klinefelters syndrome, and spontaneous abortion

Location

Spermatogenesis takes place within several structures of the malereproductive system. The initial stages occur within the testes and

progress to the epididymis where the developing gametes mature andare stored until ejaculation. The seminiferous tubules of the testes arethe starting point for the process, where stem cells adjacent to theinner tubule wall divide in a centripetal directionbeginning at the

walls and proceeding into the innermost part, or lumento produceimmature sperm. Maturation occurs in the epididymis.


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Duration

For humans, entire process of spermatogenesis takes 74 days.Including the transport on ductal system, it takes 3 months. Testesproduce 200 to 300 million spermatozoa daily.

Stages

The entire process of spermatogenesis can be broken up into severaldistinct stages, each corresponding to a particular type of cell inhuman. In the following table, ploidy, copy number andchromosome/chromatid counts are for one cell, generally prior toDNA synthesis and division(in G1if applicable). The primaryspermatocyte is arrested after DNA synthesis and prior to division.

Cell typePloidy/Chromosomes in

DNA copy

number/chromatids in

Process entered by cell


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human human

Spermatogonium (types Ad,

Ap and B)Diploid (2N) / 46 2C / 46

Spermatocytogenesis

(Mitosis)

Primary Spermatocyte Diploid (2N) / 46 4C / 2x46 Spermatidogenesis (Meiosis 1)

Two

Secondary SpermatocytesHaploid (N) / 23 2C / 46 Spermatidogenesis (Meiosis 2)

Four Spermatids Haploid (N) / 23 1C / 23 Spermiogenesis

Four functionalSpermatozoids

Haploid (N) / 23 1C / 23 Spermiation

Spermatocytogenesis

The process of spermatogenesis as the cells progress from primaryspermatocytes, to secondary spermatocytes, to spermatids, to sperm.

Spermatocytogenesis is the male form of gametocytogenesis andresults in the formation of spermatocytes possessing half the normalcomplement of genetic material. In spermatocytogenesis, adiploid spermatogonium which resides in the basal compartment ofseminiferous tubules, divides mitotically to produce two diploidintermediate cells called primary spermatocytes. Each primaryspermatocyte then moves into the adluminal compartment of theseminiferous tubules and duplicates its DNA and subsequentlyundergoes meiosis Ito produce two haploid seconadryspermatocytes

, which will later divide once more into haploid spermatids. Thisdivision implicates sources of genetic variation, such as randominclusion of either parental chromosomes, and chromosomalcrossover, to increase the genetic variability of the gamete.
http://en.wikipedia.org/wiki/Spermatocytehttp://en.wikipedia.org/wiki/Spermatocytehttp://en.wikipedia.org/wiki/Spermatocyte


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Each cell division from a spermatogonium to a spermatid isincomplete; the cells remain connected to one another by bridges ofcytoplasm to allow synchronous development. It should also be notedthat not all spermatogonia divide to produce spermatocytes,

otherwise the supply would run out. Instead, certain types ofspermatogonia divide to produce copies of themselves, therebyensuring a constant supply of gametogonia to fuel spermatogenesis.


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Spermatidogenesis

Spermatidogenesis is the creation of spermatids from secondaryspermatocytes. Secondary spermatocytes produced earlier rapidly

enter meiosis II and divide to produce haploid spermatids. Thebrevity of this stage means that secondary spermatocytes are rarelyseen in histological studies.

Spermiogenesis

During spermiogenesis, the spermatids begin to grow a tail, anddevelop a thickened mid-piece, where the microtubules gather and

form an axoneme . Spermatid DNA also undergoes packaging,becoming highly condensed. The DNA is packaged firstly with specificnuclear basic proteins, which are subsequently replaced

with protamines during spermatid elongation. The resultant tightlypacked chromatin is transcriptionally inactive. The golgiapparatus surrounds the now condensed nucleus, becomingthe acrosome. One of the centrioles of the cell elongates to becomethe tail of the sperm.

Maturation then takes place under the influence of testosterone,which removes the remaining unnecessary cytosome and organelles.The excess cytoplasm, known as residual bodies, is phagocytosed bysurrounding Sertoli cells in the testes. The resulting spermatozoa arenow mature but lack motility, rendering them sterile. The maturespermatozoa are released from the protective sertoli cells into thelumen of the seminiferous tubules in a process called spermiation.

The non-motile spermatozoa are transported to

the epididymis in testicular fluidsecreted by the Sertoli cells with theaid of peristaltic contraction. While in the epididymis thespermatozoa gain motility and become capable of fertilization.However, transport of the mature spermatozoa through theremainder of the male reproductive system is achieved via muscle


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contraction rather than the spermatozoon's recently acquiredmotility.

Role of Sertoli cells

At all stages of differentiation, the spermatogenic cells are in closecontact with Sertoli cells which are thought to provide structural andmetabolic support to the developing sperm cells. A single Sertoli cellextends from the basement membrane to the lumen of theseminiferous tubule, although the cytoplasmic processes are difficultto distinguish at the light microscopic level.

Sertoli cells serve a number of functions during spermatogenesis,they support the developing gametes in the following ways:

Maintain the environment necessary for development andmaturation, via the blood testes barrier

Secrete substances initiating meiosis Secrete supporting testicular fluid Secrete androgen binding protien (ABP), which

concentrates testosterone in close proximity to the developinggametes


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Testosterone is needed in very high quantities for maintenanceof the reproductive tract, and ABP allows a much higher levelof fertility

Secrete hormones affecting pituitary gland control of

spermatogenesis, particularly the polypeptide hormone, inhibin Phagocytose residual cytoplasm left over from spermiogenesis They release antimullerian harmone which prevents formation of

the Mllerian Duct / Oviduct. Protect spermatids from the immune system of the male, via the

blood testes barrier

The intercellular adhesion molecules ICAM -1 and soluble ICAM-

1 have antagonistic effects on the tight junctions forming the blood-testis barrier. ICAM -2 molecules regulate spermatid adhesion on theapical side of the barrier (towards the LUMEN) duringspermatogenesis, the cells are closely associated with sertoli cells

which lies at regular interval alone the seminiferous tubules. sertolicells perform the following tasks:

are target cells for follicular stimulating hormones (FSH) synthesize an androgen binding protein that maintain a high levels

of testosterone inside the seminiferous tubules maintain the blood-testes barrier which protect the body's

immune system from destroying the developing sperm cells create an environment that is necessary in the differentiation of

sperm cell degrade the residual cytoplasm that is shed during

spermatogenesis.


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Influencing factors

The process of spermatogenesis is highly sensitive to fluctuations inthe environment, particularly harmones and temperature.Testosterone is required in large local concentrations to maintain theprocess, which is achieved via the binding of testosteroneby androgen binding protein present in the seminiferous tubules.Testosterone is produced by interstitial cells, also known as leydigcells, which reside adjacent to the seminiferous tubules.

Seminiferous epithelium is sensitive to elevated temperature inhumans and some other species, and will be adversely affected bytemperatures as high as normal body temperature. Consequently, thetestes are located outside the body in a sack of skin calledthe scrotum. The optimal temperature is maintained at 2 DegreeCelcius8 C (mouse) below body temperature. This is achieved by


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proapoptotic signals and therefore promote spermatogenic cellsurvival.

The Sertoli cells themselves mediate parts of spermatogenesisthrough hormone production. They are capable of producing thehormones estradiol and inhibin. The Leydig cells are also capable ofproducing estradiol in addition to their main product testosterone.

Oogenesis

ogenesisis the creation of an ovum (egg cell). It is the female form

of gametogenesis; the male equivalent is spermatogenesis. It involves


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the development of the various stages of the immature ovum.

Oogenesis in HumansIn humans, the first part of oogenesis starts in the germinalepethelium, which gives rise to the development of ovarian follicles,the functional unit of the ovary.


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Note that this process, important to all animal life cycles yet unlikeall other instances of cell division, occurs completely without the aidof oo-spindle coordinating centrosomes.

Oogenesis consists of several sub-processes: oocytogenesis,ootidogenesis, and finally maturation to forman ovum (oogenesis proper). Folliculogenesis is a separate sub-process that accompanies and supports all three oogenetic sub-processes.

Cell type Ploidy Process Process completion

Oogonium Diploid Oocytogenesis(Mitosis) Third trimester (forming oocytes)

Primary oocyte DiploidOotidogenesis(Meiosis 1)

(Folliculogenesis)

Dictyate in prophase I for approximately 50

years

Secondary

oocyteHaploid Ootidogenesis(Meiosis 2) Halted in metaphase II until fertilization

Oogonium (Oocytogenesis)> Primary Oocyte (Meiosis I)>First Polar Body (Discarded afterward) + Secondary oocyte (Meiosis II)> Second Polar Body (Discarded afterward) + Ovum

The creation of oogonia

The creation of oognia traditionally doesn't belong to oogenesisproper but, instead, to thecommon process of gametogenesis, whichin the female human begins with the processesof folliculogenesis, oocytogenesis, and ootidogenesis.

Human oogenesis

At the start of the menstrual cycle, some 12-20 primary follicles beginto develop under the influence of elevated FSH to form secondaryfollicles. The primary follicles have formed from primordial follicles,
http://en.wikipedia.org/wiki/Gametogenesis#Common_pathhttp://en.wikipedia.org/wiki/Gametogenesis#Common_path


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which developed in the ovary at around 1030 weeks afterconception. By around day 9 of the cycle, only one healthy secondaryfollicle remains, with the rest having undergone Ovarian follicleatresia. The remaining follicle is called the dominant follicle and is

responsible for producing large amounts of estradiol during the latefollicular phase. Estradiol production depends upon co-operationbetween the theca and granulosa cells. On day 14 of the cycle, an LHsurge occurs, which itself is triggered by the positive feedback ofestradiol. This causes the secondary follicle to develop into a tertiaryfollicle, which then ovulates some 2436 hours later. An importantevent in the development of the tertiary follicle occurs when theprimary oocyte completes the first meiotic division, resulting in the

formation of a polar body and a secondary oocyte. The empty folliclethen forms a corpus luetum which later releases progesteronehormone.

Oocytogenesis

Oogenesis starts with the process of developing oognia, which occursvia the transformation of primordial follicles into primary oocytes, a

process called oocytogenesis. Oocytogenesis is complete either beforeor shortly after birth.

Number of primary oocytes

It is commonly believed that, when oocytogenesis is complete, noadditional primary oocytes are created, in contrast to the maleprocess of spermatogenesis, where gametocytes are continuouslycreated. In other words, primary oocytes reach their maximum

development at ~20 weeks of gestational age, when approximatelyseven million primary oocytes have been created; however, at birth,this number has already been reduced to approximately 1-2 million.

Recently, however, two publications have challenged the belief that afinite number of oocytes are set around the time of birth. Therenewal of ovarian follicles from germline stem cells (originating from


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bone marrow and peripheral blood) has been reported in thepostnatal mouse ovary.

Due to the revolutionary nature of these claims, further experimentsare required to determine the true dynamics of small follicleformation.

Ootidogenesis

The succeeding phase of ootidogenesis occurs when the primaryoocyte develops into an ootid. This is achieved by the processof meiosis. In fact, a primary oocyte is, by its biological definition, acell whose primary function is to divide by the process of meiosis.

However, although this process begins at prenatal age, it stopsat prophase I. In late fetal life, all oocytes, still primary oocytes, havehalted at this stage of development, called the dictyate.After menarche, these cells then continue to develop, although only afew do so every menstrual cycle.


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Meiosis I

Meiosis I of ootidogenesis begins during embryonic development, buthalts in the diplotene stage of prophase I until puberty. The mouseoocyte in the dictyate (prolonged diplotene) stage actively repairsDNA damage, whereas DNA repair is not detectable in the pre-dictyate (leptotene, zygotene and pachytene) stages of meiosis. For


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those primary oocytes that continue to develop in each menstrualcycle, however, synapsis occurs and tetrads form,enabling chromosomal crossover to occur. As a result of Meiosis I,the primary oocyte has now developed into the secondary oocyte and

the first polar body.

Meiosis II

Immediately after meiosis I, the haploid secondary oocyteinitiates meiosis II. However, this process is also halted atthe metaphase II stage until fertilization, if such shouldever occur. When meiosis II has completed, an ootid and

another polar body have now been created.olliculogenesis

Synchronously with ootidogenesis, the ovarian follicle surroundingthe ootid has developed from a primordial follicle to a preovulatoryone.

Maturation into ovum

Both polar bodies disintegrate at the end of Meiosis II, leaving onlythe ootid, which then eventually undergoes maturation into a matureovum.

The function of forming polar bodies is to discard the extra haploidsets of chromosomes that have resulted as a consequence of meiosis.

In vitro maturation

In vitro maturation(IVM) is the technique of letting ovarianfollicles mature in vitro. It can potentially be performed beforean IVF. In such cases, ovarian hyperstimulation isn't essential. Rather,oocytes can mature outside the body prior to IVF. Hence, no (or atleast a lower dose of) gonadotropins have to be injected in the body.However, there still isn't enough evidence to prove the effectiveness

and security of the technique.


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Bibliography

www.merriam-webster.com/dictionary/gametogenesis

https://www.landesbioscience.com/journals/spermatogenesis/

www.sparknotes.com/testprep/books/sat2/.../chapter7section2.rhtm

www.embryology.ch/anglais/cgametogen/oogenese01.html

www.ncbi.nlm.nih.gov NCBI Literature Bookshelf

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Biology investigatory project on Gametogenesis