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