Spermatogenesis

Reading

• Chapter 10
• Handout on Spermatogenesis
• Additional diagrams or pictures not found in text

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 a 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 A spermatogonia into proliferating pool
    • FSH regulates the mitotic divisions and efficiency of type A spermatogonia development
    • some type A spermatogonia enter pool that become Intermediate spermatogonia and continue on in development
  • primates
    • FSH controls entry of stem cell type A spermatogonia into proliferating pool
    • fewer division in mitosis than seen in the bull
  • the periodic entry of the next type A 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

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