Gametogenesis Stages

• Gametogenesis is the process of germ plasm converting into specialized sex cells for fertilization.
• Somatic cells (somatoplasm) do not participate directly in gamete production and nourish the germ plasm.
• Gametes and their precursor cells (germ plasm) carry hereditary information to the next generation.
• Primordial germ cells are the precursors to gametes.
• Gametogenesis consists of four major phases:
• Origin and migration of germ cells to the gonads
• Multiplication of germ cells in the gonads through mitosis
• Reduction of chromosome number by meiosis
• Maturation and differentiation of gametes

Origin of Primordial Germ Cells and Their Migration to the Gonads

• The cells destined to become gametes are recognizable early in development, though their early history is not well-understood.
• In frogs and some invertebrates, germ plasm exists in the vegetal pole cytoplasm of the zygote or as specific cells during cleavage.
• The germ-cell lineage can be traced from a circumscribed area in the unfertilized egg, through cleavage near the vegetal pole, and into the endodermal floor of the primitive gut.
• Primordial germ cells of birds, reptiles, and mammals arise in the epiblast of the early embryo, reside temporarily in extraembryonic tissues, and then return to the embryo.
• In birds, they are recognizable in the germinal crescent beyond the future head region of the embryo
• In mammals they can be found in the posterior wall of the yolk sac near the origin of the allantois
• Urodele amphibians (salamanders) have a unique origin of primordial germ cells from embryonic mesodermal cells due to the inductive influence of the vegetal endodermal yolk mass.
• Primordial germ cells are recognizable by:
• Large size and clear cytoplasm
• Histochemical characteristics such as high alkaline phosphatase activity (mammals) and high glycogen content (birds)
• Monoclonal antibodies specific for primordial germ cells have been developed in birds and mammals.
• Primordial germ cells are not formed in the gonads, but in other parts of the embryo.
• Migration to the gonads is determined by several methods:
• Destroying areas where PGCs are first found
• For example, Willier destroying PGCs in a chick resulted in gonads without gametes
• Locating and tracing PGCs based on their specific characteristics
• Using mutant sterile mice strains
• For example, Mintz and Russell showed that sterile mice have few PGCs using histochemical staining for alkaline phosphatase
• In vertebrates, PGCs migrate to the gonads via two principal mechanisms:
• Birds and reptiles: PGCs pass through the walls of blood vessels, enter circulation, recognize gonadal blood vessels, penetrate the walls, and settle in the gonads
• Mammals: PGCs migrate around the posterior gut wall and through the dorsal mesentery
• The migration of mammalian PGCs through the dorsal mesentery is guided by the orientation of extracellular matrix molecules (laminin and fibronectin) and a chemotactic influence from the gonads.
• PGCs occasionally migrate to extragonadal sites, often dying or developing into teratomas (bizarre tumors with scrambled mixtures of tissues).
• Studies show that the number of PGCs increases during migration to the gonads, but the major increase occurs after settling in the gonads.

Proliferation of Germ Cells by Mitosis

• The embryonic gonads are initially populated by a small number of migrating PGCs.
• Germ cells increase by mitosis after settling in the gonads.
• In females, the mitotically active germ cells are called oogonia; in males, they are called spermatogonia.
• The pattern of mitotic activity in the germ cells differs widely between females and males.
• In human females, intense mitotic activity between the second and fifth months of pregnancy brings the population of oogonia from a few thousand to about seven million, however after the seventh month most of the oogonia enter the prophase.
• Most nonmammalian vertebrate oogonia are capable of dividing throughout the reproductive cycle.

Reduction of the Chromosome Number by Meiosis

• Meiosis maintains the normal chromosome number from one generation to the next and is the same in both males and females
• Each haploid gamete must acquire a complete set of chromosomes and the material in maternal and paternal genes are assembled
• Genetic recombination occurs by:
• Random distribution of maternal or paternal chromosomes to daughter cells
• Exchanging portions of homologous chromosomes via crossing over a specific phase
• Meiosis itself does not involve DNA synthesis as it happens when meiotic prophase begins
• The cell at beginning of meiosis has the normal chromosome number, yet the DNA content is double normal
• The genetic material reduction involves two maturation divisions, in which new DNA synthesis does not occur, and lasts from several days to as long as 45 to 50 years.
• The first meiotic division (reductional division) results in two genetically dissimilar daughter cells
• The second division produces two identical daughter cells that are now called gametes

First Meiotic Division - Prophase I

• Is a complex stage usually divided into 5 substages: leptotene, zygotene, pachytene, diplotene, and diakinesis
• Leptotene: Chromosomes are threadlike and are only starting to coil, and each has two chromatids joined by a centromere and synthesized before meiosis
• Zygotene: Homologous paired chromosomes come together and align on a point-for-point basis which is also known as synapsis. This area is called the synaptonemal complex
• Pachytene: Synapsis is completed as the two aligned chromosomal pairs are called a bivalent. The chromosomes thicken and the sister chromatids appear to be held together by a centromere
• Diplotene: Portions of the paired chromosomes overlap as theses called chiasmata. The chromosomes separate into two paired chromosomes while the splitting occurs.
• Diakinesis: The chromosomes shorten further as the splitting continues and the chiasmata moves to the ends. This process is called terminalization - At this point the nucleolus disappears, the nuclear membrane breaks, and the spindle apparatus becomes apparent

Metaphase I

• Tetrads line up along the metaphase plate; maternal chromosomes are on one side while paternal chromosomes are on the other.
• The random alignment ensures genetic differences among individuals

Anaphase I

• Individual paired chromosomes move toward opposite poles of the spindle.
• Sister chromatids of each chromosome remain held together by the centromere.
• Homologous chromosomes move apart, chiasmata are pulled apart, and crossing-over is complete.
• Migration of the maternal chromosome to one pole and the paternal chromosome to the other creates unequal daughter cells.

Telophase I and Interphase

• Two daughter nuclei separate and nuclear membranes may reform.
• Each nucleus has the haploid number of chromosomes (1n), with each chromosome still containing two sister chromatids (2c) connected by a centromere.
• Because the chromosomes of the daughter cells are replicated, no new DNA replication occurs during interphase (interkinesis) between meiosis I and meiosis II.

Spermatogenesis and Oogenesis Comparisons

• Spermatogenesis functional cells produced - Four
• Oogenesis functional cells produced - One
• Spermatogenesis time of occurrence- puberty but continuous
• Oogenesis time of occurrence - In the embryo til puberty
• Spermatogenesis number of arrest period- No prolonged arrest
• Oogenesis number of arrest period - Two periods of arrest to a single
• Spermatogenesis size of cells produced -Spermatozoon is smaller than spermatogonia
• Oogenesis size of cells produced- Mature ova are larger than oogonia
• Spermatogenesis amount of materials deposited- Developing sperm receives little amount of formed materials
• Oogenesis amount of materials deposited - Eggs take up large quantities of materials
• Spermatogenesis growth and maturation period- Happens before cell differentiation
• Oogenesis growth and maturation period-Occurs after cell differentiation

Spermatogenesis

• Spermatogenesis is transformation from primordial germ cells to mature spermatozoa and is divided into 3 principal phases: Mitotic multiplication Meiosis & Spermiogenesis
• Mitotic Multiplication Phase - mitotic cells known as spermatogonia and subdivided into type A & B
• Type-A spermatogonia represent the stem cell population and some of them give rise to type B spermatogonia
• Type-B spermatogonia have committed to leaving the mitotic cycle and finish spermatogenesis.
• Meiotic Phase - after the final round of DNA replication, the type-B cells are used to create two equal daughter cells, and in the human it takes several weeks
• Spermiogenesis/Spermatid Metamorphosis Phase - After two maturation divisions the nucleus is in interphase condition ending when the compacted chromatin constitutes the bulk of the head of the spermatozoon

How Sperm is Made

• Initially mitochondria begin to form a spiral investment around part of the flagellum
• The mature spermatozoon consists of:
• Head
• Nucleus and the acrosome
• Neck
• Proximal centriole
• Middle Piece Proximal part of the flagellum & centrioles & the mitochondrial helix Middle piece
• Tail
• Highly specialized flagellum
• The daughter cells are connected by intercellular bridges
• Sertoli cells serve a variety of functions:
• Target cells for follicle stimulating hormone
• Synthesizing of an androgen-binding protein maintains testosterone
• Maintaining the blood-testes barrier
• Creating an important environment to differentiate sperm cells
• Facilitating the release of mature spermatozoa, and degrading the residual cytoplasm
• Sperm cells from the testes are not capable of fertilizing eggs, unless they continue through the seminal fluid

Oogenesis

• Production of the Ovum and occur in the ovary
• In early development, oogonia undergo mitosis and enlarge into primary
  • Primary oocyte undergoes the first meiotic division, produces two differing sized cells
  • Forms the first polar body and secondary spermatocyte
  • Second meiotic division produces a second polar body and a large ootid.
  • The polar bodies then disintegrate
• Production of mature released eggs are from hundreds to thousands, or only 1 released at one time. Size mainly depends on whether or not the developing fertilized egg will mature in or by the mother
• the Ova and aquatic animals eggs are smaller in size than reptiles and birds since they come equipped with their own yolks

Oogenesis in Amphibians

• The frog eggs (Rana Pipiens) require 3 years and need a relatively leisurely process.
• During the third summer the eggs mature rapidly around 1500 um in diameter, and is hibernated in the autumn There are three batches of eggs produced by one frog at one time

Development in the Amphibian Egg

• The amphibian egg consist of 3 main phases before the deposition of the yolk: Previtellogenesis Vitellogenesis Final Maturation

Previtellogenic Phase

• During pre vitellogenesis, the yolk synthesis does not take place
• There is a qualitative and quantitative synthesis of a large nucleus and cytoplasm of the primary oocyte
• The mitochondria increase in number, the endoplasmic reticulum and Golgi bodies manufacture cortical granules other the the beginning (5S) rRNA molecules

Vitellogenic Phase

• Major changes starts to take place in the cytoplasm
• Non chemical, yet collective term that gives nutrition to the developing embryo and various substances that store for developing the embryo
• In the amphibian egg, proteinaceous material makes up the membrane-bound yolk platelets
• Yolk platelets are most prominent inclusions in the cytoplasm of the amphibian egg produced by the cells of under the influence of estrogens

Final Results of Egg Development

• The result of Yolk Nucleus or Balbiani produces smaller aggregates & transforms this region of cytoplasm to germ plasm Cortical granules have proteins and sugars, which begin to develop around the cytoplasm and ultimately disperse
• The oocyte transforms from late diplotene to early diakinesis
• Vitellogenesis sequentially involves:
  • Induction of vitellogenin synthesis
  • Releases it to circulation
  • Transports vitellogenin to the target tissue cells, and conversion it to storage form

Maturation of Egg

• Is hormonally induced releases of the egg
• Induced reaction releases of the egg from its fiest meiotic block, causing a breakdown of the germinal vesicle a releasing a polar body
• When progesterone begins and causes an electrical depolarization of the plasma membrane
• Secondary oocyte needs RNA while in the G2 -M phase cycle

Oogenesis in Birds

• Hen eggs are a great food material for the growing embryo thats gradually accumulates
• Other egg components (shell etc) are noncellular secretions
• The yolk material gets transported by the follicular cells surrounding the ovum, the yolk is mostly made of the follow
• 50% electrolytes
• 33% fatty material (neutral fats)
• 16% protiens (lipovitellin)
• 1% of carbohydrates & vitamins
• Finally the cytoplasm and nucleus are forced together on the cell surface and yolk occupies all space
• The zone pellucida produces striations for the closely packed microvilli

Oogenesis in Mammals

• Not like other vertebrates, mammals do no replenish their eggs store so mammals have about a million oocytes at birth (many degenerating) and arrested at the diplotene stage when around a layer of follicular cells
• Follicles make way to 3 major stages before the way to ovulation: the pool of follicles, Folliculogenesis & follicle selection
• Phase one- Approx 500,000 make ready to develop into primary follicles by surrounding themselves a layer of cuboidal follicular cells
• The oocytes the release is only at puberty until menopause, they remain arrested in meiosis until puberty
• The second: Folliculogenesis, consists of influence of gonadotropic hormones which cause the outer cellular layer aka theca folliculi that begins to form when the membrana granulosa from granulosa cells forms the fluid filled space antrum and fluid appears. Testosterone begins to produce when thecal cells develop on their surfaces in the second phase.
• Third phase and growing: Is dramatic especially among some growing follicles responding strongly the the circulating gonadrotropins
• After LH mid cycle surge, multiplication drops ending in a anglogenic factor The Dominant selection comes in, with significant amounts of estradiol Tertiary follicle/ Graafian is follicle mature and almost set to rupture Ovulation occurs when the cumulus opphorus releases while ready to respond to releasing ovum, which has almost reached its second meiotic stage

Ovulation

• Ovulation occurs when the LH peak sets motion and follicle continues to swell as follicle fluid and follicle itself increase in size
• Apex and Stigma is marked within a day after characteristics take place for final maturity, The fluid also is expelled
• Once expelled, what left is the corpus luteum which helps secret estrogen with yellow coloring
• When rupturing the antrum bleeds a little forming an endocrine secreting orgin and gets prepared for pregnancy
• Progesterone main functions: Prepare the implantation of the fertilization of the Ovum
• LH and FSH levels of pitrary drops drastically and however pregnancy may lengthen itself at a diameter of 2 to 3 cm.

Pregnancy

• If pregnant then the chorionic gonadotropin are maintained secreted by itself and the placenta.
• To degenerate or not after the corpus luteum has done what it has done happens and becomes a scare tissue
• Accessory coverings, after ovulation. Get protection, and prevent from dessication or shock
• Fertilization process
• Includes membrane contact between sperm and the egg, sperm entry, preventing polyspermy, metabilic actication and fusion of both parent DNA

Affecting factors for Sperm & fusion:

• *Cycle of sex cells Amount of day light Behaviour Approximation

How sea urchins make chance of these meeting higher:

• Move to an aggregate
• Releases same time coordinated
• Stable conditions

Membrane Meeting

• SPERM COATING LAYERS TO GET AND PASS IN FOR ENZYMATIC ACTIVITIES!
• Jelly; Made of peptides to hydrate itself
• Composed of glycoproteins to expand itself through seawater for enzymes
• Vitelline envelope- composed of gluco protien - enzymes
• The cells start cluster together to increase the number of eggs for the enzymes to start working
• The cells start cluster together to increase the number of eggs for the enzymes to start working When they touch with their jelly coats, their chemical messengers (Speract/resact) attract it to the main source.
• Enzymes are able to optimally work

Sperm -Egg

Sticky: Is causes it to clump and due to interaction fertilization is continuesly released and anti fertilization happens for agglutination to start.

• If the pH levels are not optimal, they will be denatured This penetration to the jelly enzyme, there needs to be enough room so that the protein can form and attach, otherwise will die regardless of how much reach the area!
• As components are broken down, is causes a response of extra amounts as it continues to take place
• RECOGNITION from the Egg through Extracellular & Intercellular As this process happens, what may not occur the "Two blocks of cell Polyspermy" which is an infection can occur. Two blocks: fast, but fast acting

Fertilization

Fertilization happens very quick at 2-3 seconds to help with the membrane potential which will cut access when the eggs are far behind. (Change charge for the one and the other to repel) In conclusion, if the movement depending on what you can provide or the reaction may not work with the membrane. The fusion can also be affected if the membrane approches the same original location or not, resulting in nothing Slow block: Takes time but last for real

• Is wave is going crazy by the inner of the other cell to form in that direction! What then happens is high sensitive reaction and a layer is made

Metabloic activity:

Activation of events with a signal transduction.

1. Starts binding the receptor
2. Start process that stimulates major energy with a response!
3. Releasing the egg
4. All starts with membrane and reaction

Mammals reproduction: Sperm

• It begins in the higher vagina or upper, otherwise rodents.
• Needs to be as strong to reach up the end, it goes through quite the physical and chemical tests to even arrive.
• There needs to be high protection or will cause death!
• Needs to have energy to burn

Mammels reproduction: Sperm continued

1. All begins wit the basics in the seminal basic
2. Fluid substances are made the sticky
3. When the Secreting has bulbourethral, energy is acidic, but provides and makes it up the the females

• In the vagina is upper and released with the uterus moving
• Follow, then comes the contraction

4. Alternating contraction happens with then action of a mucus filtering
5. All happens with movement up to the tube which what we have! Acitidty in the vagina is very hostile for bad reaction