Gametogenesis
Steps:
1. Formation of germ plasm and determination of primordial germ cells
- all gametes arise from PGCs
- sometimes PGCs are specified autonomously by cytoplasmic determinants in the egg that are parceled out to specif cells during cleavage (ie. P-granules)
- cytoplasmic components (ie. proteins, mRNAs) are collectively called the germ plasm
- other times, germ cells are specified by interactions among neighbouring cells (conditional)
- in the nematode Parascaris, cells in the animal pole undergo chromosome diminution (only a portion of the chromosome survives after it fragments just before cell division)
- happens in some of the vegetal cells too, so at the 4-cell stage, only one cell contains a full set of genes
- only cells that have the full chromosome set give rise to germ cells; the rest undergo chromosome diminution to form more somatic cells
- functions of many germ plasm molecules remain unknown, but they may be involved in transcription and translation inhibition
- therefore cells become germ cells because they are forbidden from being anything else
Drosophila
- PGCs form as a group (pole cells) at the posterior pole of blastoderm, surrounded by pole plasm (collection of mitochondria, fibrils, and polar granules, and the mRNA of germ cell-less gene
- knockout of this gene prevents placement of pole plasm in eggs, and subsequent generation is sterile (there are no "grandoffspring")
- other critical determinants:
o oskar - germ cell formation
o nanos - germ cell migration
o vasa - RNA-binding protein critial for initiating differegerntiation and meiosis
o mitochondrial ribosomal RNA (mtrrNA) - directs formation of pole cells
o polar granule component (PGC) - probably involved in pole cell migration to gonads
- similar cytoplasmic localizations are seen in amphibian embryos
- germ plasm of amphibians contains germinal granules and a surrounding matrix
- aggregation requires microtubules, movement to vegetal pole requires kinesin-like protein
** In mammals, there is no obvious germ plasm; germ cell formation is induced at the posterior proximal epiblast by BMP4 and BMP8b from extraembryonic ectoderm
- three genes are expressed in this area:
- fragilis: encodes a transmembrane protein; but can be in PGCs or somatic cells;
- Blimp1: transcriptional inhibitor that specifies PGCs; and
- stella
2. Migration of PGCs into the developing gonads
Drosophila
- pole cells are passively displaced to midgut by gastrulation
- endoderm triggers diapedesis (squeeze between endothelial cells of blood vessels) of PGCs, entering mesoderm from endoderm
o (wunen gene is reponsible)
- PGCs split into two groups and associate with each of developing gonad primordial
- PGCs migrate into gonads (similar mannar to mammals), attracted by Columbus and Hedgehog
Amphibians
- germ plasm becomes associated with endoderm lining the blastocoel floor and becomes concentrated in posterior region of larval gut
- it migrates along dorsal side of gut, along mesentery, then along the abdominal wall and into the genital ridge
- fibronectin and guidance is important
- PGCs divided 3x while migrating; 30 cells colonize the gonads
Mammals
- cells migrate over substrates by extension of filopodia over underlying cell structures
- they can even penetrate monolayers and migrate through cell sheets
- fibronectin is likely an important substrate
- mechanism of pathfinding is unknown; maybe gradient of Sdf1 from genital ridges (from day 10.5) cues PGCs
- Sdf1 binds to receptor CXCR4; this combination is used in lymphocyte/hematopoietic progenitor lines
- cells proliferate even as they migrate, so there are 2500-5000 PGCs in the gonads at day 12, up from 10-100 at the start
Birds and Reptiles
- germ cells derived from epiblast
- they migrate from area pellucida to germinal crescent (zone in the hypoblast in anterior zona pellucida), where they multiply
- they migrate via the bloodstream to region where hindgut is forming, and then into mesentery and genital ridges
3. Meiosis and modifications of meiosis result in sperm/ova
- PGCs divide mitotically before being reduced to haploid
- following the last mitotic division, DNA synthesis doubles DNA to form two sister chromatids joined at a common kinetochore (centromere)
- key aspects of meiosis (that are different from mitosis):
(1) there are two cell divisions, without DNA replication between them
- first division separates homologous chromosomes
- second separates sister chromatids
(2) homologous chromosomes pair together and recombine
- the main difference between oogenesis and spermatogenesis (in many species) is that in oogenesis meiosis is arrested at some point and reinitiated in a smaller population of cells, whereas in spermatogenesis, meiosis and differentiation occur continuously
4. Differentiation of sperm/eggs
Spermatogenesis
- sperm differentiate from PGCs after incorporation into the sex cords
- spermatogenic germ cells are bound to Sertoli cells by N-Cadherin on both cell surfaces; and galactosyltransferase (glycoprotein) molecules on spermatogenic cells that bind to a carbohydrate on the Sertoli cells
o Sertoli cells nourish and protect developing sperm
o level of GDNF secreted by Sertoli cells determines whenever spermatogonia remain stem cells (high), or differentiate into spermatocytes (low)
o GDNF is upregulated by FSH, and therefore may be the connection between endocrine system and Sertoli cells
- spermatogonia secrete BMP8b, which regulates spermatogenesis during puberty
- in spermatogonial division, cytokinesis is incomplete; a spermatogonia will form many syncytial clones
- spermiogenesis/spermateliosis prepare sperm for motility and interaction with egg
- first step: acrosomal vesicle is formed from Golgi apparatus
- while this is going on, nucleus rotates so the cap faces the basement memebrane, so that the flagellum that's forming from the centriole on the other side of nucleus will extend into the lumen
- last stage: nucleus flattens and condenses; histones swapped for protamines
- mitochondria wrap around the flagellum; remaining cytoplasm is lost as a residual body
- development from stem cell to spermatozoon takes 35 days in mice; 65 days in humans
o spermatogonial stage; meiosis stage; and spermiogenesis (maturation/differentiation)
Oogenesis
- stores cytoplasm, enzymes, RNAs, organelles, etc. to initiate and maintain metabolism/development
- the process varies between organisms; some produce thousands of eggs (in which case the oogonia must be self-renewing), others produce only several (oogonia are limited)
- (oogonia are the stem cells that give rise to ova)
- in the human embryo, ~1000 oogonia divide rapidly from 2nd - 7th month of gestation to form ~7 million
- after 7th month, most die, while the rest enter 1st meiotic division and arrest in first prophase (these are called primary oocytes)
- upon puberty, groups of primary oocytes periodically resume meiosis
- only ~400 eggs released during female human's life
5. Hormonal control of gamete maturation and ovulation
- ovulation in mammals occurs:
(1) in response to copulation (minks, rabbits)
(2) periodically (most mammals); ovulation period is called estrus
- periodicity seen in primates is referred to as the menstrual cycle, which integrates:
(1) the ovarian cycles - which matures the egg
(2) the uterine cycle - which makes the implantation environment, and
(3) the cervical cycle - which allows the sperm to come in and fertilize
- these three functions are integrated through the pituitary, hypothalamus, and ovarian hormones
- the first part of menstrual cycle is called proliferative/follicular phase
- hypothalamus releases gonadotropin-releasing hormone
- pituitary releases gonadotropins, follicle-stimulating hormone, and luteinizing hormone
- these hormones cause ovarian follicles to proliferate and secrete estrogen, which enters neurons and turns on mating behaviour
- amount and type of light can stimulate the hypothalamus and pituitary
- with increased estrogen, follicular production of FSH declines, LH contines to increase
- each oocyte is enveloped by a primoridal follicle (single layer of epithelial granulosa cells and a layer of mesenchymal thecal cells)
o granulosa cells continue to grow
o there is also formation of a cavity (antrum)
- following day 10, estrogen rises sharply, accompanied by midcycle burst of LH and FSH, triggering the physical expulsion of the mature oocyte (because LH induced increase in collagenase, plasminogen activator, and prostaglandins within the follicle)
- following ovulation, the luteal phase begins
- the remaining cells of the ruptured follicle become the corpus luteum (under influence of LH)
- it secretes progesterone primarily, but also some estrogen
- progesterone completes the job of preparing the uterine tissue for implantation, and inhibits FSH production to prevent further ova from being released
- menses is the phase where the endometrium degenerates if there is no implantation
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