Spermatogenesis
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Production of Fertile Sperm
- hormonal regulation of the testis
- mitotic division of spermatogonia
- meiotic divisions of spermatids
- morphologic transformation of spermatids into spermatozoa
Hormonal Regulation of the Testis
- Pulsatile release of GnRH, LH, FSH and Testosterone
- LH on leydig cells
- production of testosterone
- Testosterone
- negative feedback on hypothalamus and anterior pituitary
- promotes sertoli cell function
- potential direct effects on germ cells
- binds to androgen binding protein (ABP) and is transported down seminiferous
tubule to be eventually used in the epididymis
- FSH on sertoli cells
- stimulates the conversion of testosterone into estradiol
- negative feedback effects of estradiol on hypothalamus and anterior
pituitary
- stimulates the production of inhibin
- negative feedback effects of inhibin on FSH production by anterior
pituitary
- stimulates production of ABP
- stimulates the formation and maitainence of tight junctions between
adjacent sertoli cells
Spermatogenesis
- occurs within the seminiferous tubule
- componets of spermatogenesis
- spermatocytogenesis - mitotic divisions of spermatogonia
- several types of spermatogonia exist
- the names differ among species and classification methods used by different
investigators
- typical sequence of divisions are
- A spermatogonia (several divisions)
- Intermediate spermatogonia
- B spermatogonia
- Primary spermatocyte (formed by a mitotic division)
- meiosis - the sequence is
- primary spermatocyte
- secondary spermatocyte
- round spermatid
- spermiogenesis
- morphological conversion of round spermatid into spermatozoa without
a division
- spermiation
- release of spermatozoa from sertoli cell into lumen of seminiferous
tubule
- many sperm are produced
- as an A type spermatogonia enters proliferating pool, the series of
mitotic and meiotic divisions result in a large number of spermatozoa potentially
produced
- the number of mitotic divisions are species specific
- cytoplasmic bridges exist among a cohort of developing germ cells
- germ cells can degenerate at any stage of development
- degeneration of germ cells effected by
- season
- disease
- trauma or heat
- hormonal
- importance of stem cell renewal
- to prepetuate spermatogenesis
- in case of testicular injury or high heat
- bull
- periodic entry of stem cell type A0 spermatogonia into proliferating
pool as A1
- the periodic entry of the next type A1 spermatogonia into the proliferating
pool occurs at a specific point during spermatogenesis.
- likely the sertoli cell surounding the spermatogonia triggers this
event
- spermiogenesis - round spermatid to elongated spermatid
- golgi phase
- golgi apparatus produces the acrosome which migrates to one end of
nucleus
- centrioles migrate to other end of nucleus and form implantation apparatus
and axoneme
- cap phase
- acrosome forms a distinct cap over nucleus
- golgi moves away from nucleus
- primitive flagellum forms
- acrosomal phase
- acrosome continues to spread around nucleus
- Manchette forms
- maturation phase
- Manchette migrates towards tail and most disappears
- mitochondria migrate toward and cluster on the flagellum
- dense outer fibers and fiborous sheath form
- spermatid is now called a spermatozoa
- spermiation occurs at the end of spermiogenesis
- spermiation
- release of sperm from sertoli cells
- sertoli cell phagocytizes the remaining cytoplasm of the spermatozoa
- only a small amount of cytoplasm is left on the neck of the sperm and
it is called the cytoplasmic droplet
- stages
- specific cellular associations found in the seminiferous tubule
- stages are not of equal length in time
- cycles
- progression through sequence of all stages
- takes 4.5 - 4.7 cycles to produce a spermatozoa
- duration of spermatogenesis
- cycle length
- species dependent
- boar fastest, human the slowest
- waves of the seminiferous epithelium
- important to make sure that some portion of the seminferous tubule
is always releasing sperm and don't clog the pathway
Blood Testis Barrier
- Sertoli cells and associated tight junctions
- induced by FSH
- essential for meiosis
- allows sertoli cells to manipulate environment around primary spermatocyte
so that it is triggered to enter and complete meiosis
- anything that destroys tight junctions will lead to an impairment of
meiosis and production of spermatozoa
- heat, heavy metals, pesticides, lack of FSH and/or testosterone
- first appears at puberty
- mitotic division occur before puberty but not meiotic divisions
- after puberty can be maintained by FSH and/or testosterone
Capacity for Sperm Production
- species dependent
- size of testis
- boar largest of farm animals
- sperm produced/gm of tissue
- involves length and efficiency of spermatogenesis
- boar best among farm animals
- daily sperm production
- involves both size of testis and sperm produced/gm of tissue
- boar best followed by ram, human is poor
Apoptosis important in regulation of Spermatogenesis
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