Spermatogenesis
Phase 1: Spermatogonia
- committed spermatogonia are produced by mitosis (stem cell proliferation)
- spermatogonia are located between the basal lamina and the Sertoli membrane (junctional complex)
- Type Ad spermatogonia – self-renewing stem cells; divide to produce either more Ad cells, or differentiated Ap cells
- Type Ap spermatogonia – from Ad cells; committed to differentiation; divide several times, then differentiate into B cells
- all future mitoses involve incomplete cytokinesis – leads to cytoplasmic bridges (Þ synchrony during differentiation)
- Type B spermatogonia – mitosis and differentiation of type B cels produces primary spermatocytes
Phase 2: Spermatocytes
- haploid gametes are produced by meiosis
- Primary spermatocyte: arise from division of type B cells, and rapidly enter meiosis (prophase I, metaphase I, anaphase I)
- Prophase I has six phases:
- (1) preleptotene – cells lose contact with basal lamina, DNA replication is completed Þ 4n
- last time DNA is replicated–allows for ß risk of damage, synchrony of development, and stripping of cell machinery
- (2) leptotene – pairing of sister chromatids; Sertoli processes form tight junctions below the cell (basal side)
- (3) zygotene – luminal Sertoli processes retract (so cell is now past the blood-testis barrier); chromosomes pair up
- (4) pachytene – largest germ cell; crossing-over occurs
- (5) dipoltene – desynapsis; last phase of RNA synthesis Þ allows for synchronization and streamlining of the cells
- (6) diakinesis – preparation for metaphase; sister chromatids are distinctly visible
- Secondary spermatocyte – completes telophase I and meiosis II (no replication of DNA Þ haploid cells)
- Þ theoretical production of 4 spermatids, but about 30% of germ cells don’t make it all the way to mature sperm
- apoptosis: routine; quality control; mediated by Fas/Fas ligand; induced by heat, toxins, nutritional deficiency, etc
Phase 3: Spermatid Phase (spermiogenesis)
- maturation into sperm
- Nucleus – moves to eccentric position; reduction in volume; transcription is arrested
- histones are replaced by protamines (rich in arg and cys) which lay in DNA grooves Þ highly condensed DNA
- microtubules are organized into a cylinder (manchette) around the nucleus; aids in cytoplamic shedding
- Acrosome – Golgi-derived structure; contains lysosomal enzymes, isozymes (many are unique to sperm)
- hyaluronidase (degrades zona pelucida, which is chiefly composed of hyaluronic acid) and acrosin (serine protease)
- protease inhibitors prevent premature activation, and many enzymes are present as zymogens (proacrosin, etc.)
- outer acrosomal membrane lost in activation; inner membrane has egg binding proteins
- Tail – contains axoneme, the classic 9 + 9+ 2 arrangement of microtubules; synthesis initiated by distal centriole
- connecting piece – proximal centriole moves to attach nucleus to the axoneme via 9 coarse microtubular fibers
- middle piece – mitochondria form a helical sheath around the coarse fibers of the connecting piece
- principal piece – longitudinal fibers covered by a fibrous sheath consisting of two longitudinal columns that extends almost to the end of the tail; many connecting ribs cover the middle
- end piece – short segement distal to the fibrous sheath
- Cytoplasm – shed and phagocytized by the Sertoli cells; cytoplasmic droplet may remain at midpiece (lost in epidiymis)
Phase 4: Spermatozoon
- final maturation and storage of sperm: in epididymis (maturation in corpus, storage in cauda)
- transportation takes 6-12 days in humans and occurs mainly through fluid flow; fluid is later reabsorbed to concentrate sperm
- Fertility develops in early corpus and increases as the spermatozoon moves toward the cauda
- Spermatozoa are motile, but motility is inhibited by low pH, low sugar conc., binding proteins and mucous-like substances
- Crosslinking – protamines form cysteine disulfide bonds (greater stability); also altered surface membrane chemistry
- prior to cross-linking, sperm is fertile; afterwards, several steps must occur to regain fertility
- Spermatogenic Cycle – new spermatogonia enter the cycle every 16 days; cycle takes 74 days to complete
- six stages are named based on morphological features; stages are in a helical pattern along the tubule
Sertoli Cells – produce the environment in the tubules (ions, proteins, etc)
- Blood-testis barrier – basal tight junctions of Sertoli cells; allows composition of luminal fluid to be different from blood
- Protection from immune system – blood-testis barrier blocks circulating antibodies; Fas/Fas ligand Þ immunoprivilege
- HLA (class Ia) are not expressed on spermatocytes, spermatids, or spermatozoa
- Androgen binding protein – produced by Sertoli cells; maintains high concentration of testosterone in epididymis
- Sertoli cells maintain connections in the germ cells at all stages of spermatogenesis (aid in synchronization, etc.)
Regulation – FSH and testosterone maintain spermatogenesis
- FSH –Ý mitosis and differentiation of A cells; Ý Sertoli cell protein secretion (FSH receptor density highest at onset of meiosis)
- LH –Ý testosterone production from Leydig cells
- Thyroxine – inhibits Sertoli division, induces differentiation (hypothyroid Þ large testicles)
- Vit A – necessary for spermatogenesis, function is largely unknown
- Testosterone – concentration is 10x higher in testes than in blood – acts on Sertoli and peritubular cells
- some testosterone is converted to dihydroxytestosterone, which has the same action but is 10x more potent
- some processes are testosterone dependent (DNA synth., completion of meiosis, nuclear condensation, elongation, spermiation, elimination of residual cytoplasm) but testosterone alone is not enough to maintain spermatogenesis
- IL-1 – secreted by Sertoli cells at spermiation (release of sperm into duct) Þ induces mitosis of type A cells
- Genetic – X chromosome has androgen receptor with a varied gln repeat in the transactivation domain
- long repeat Þ reduced spermatogenesis; short repeat is associated with Ý risk of prostate cancer
- many microdeletions on the Y chromosome are associated with oligospermy and azoospermy