• Occurs in seminiferous tubules and produces spermatozoa
• Seminiferous tubules in testis Þ epididymus Þ vas deferens
• Cross-section of seminiferous tubules (Fig 1) – leydig cells (steroidogenic cells), Spermatogonium (mitotically proliferating germ cells), Sertoli cells (nursing cells, form tight junctions)

• 6 weeks gestation
• Primordial germ cells arrive at genital ridge and undergo mitotic proliferation (at base of sertoli cells) ~7 million
• 8 weeks gestation
• male germ cells (gonocytes or prespermatogonia) enter mitotic arrest
• female germ cells enter meiosis
• somatic cells of gonad organize the seminiferous tubules
• At puberty
• gonocytes resume mitosis Þ type A spermatogonia (proliferate slowly)
• begins spermatogenesis

Meiosis (Fig 2)

• Type A spermatogonia Þ mitosis Þ primary spermatocytes Þ meiosis
• primary spermatocytes [1] Þ (M1) Þ secondary spermatocytes [2] Þ (M2) Þ spermatids (haploid) [4]
• Incomplete cytokinesis
• Can share products
• No transcription so if mutant, still ok, can get from neighbor

• Spermatid Þ spermatozoan
• Major remodeling (Fig 3)
• DNA becomes heterochromatic (condensed)
• Histones replaced with protamines (smaller and more basic)
• Golgi Þ acrosome (glycoprotein-rich granules)
• Centrioles Þ sperm tail (neck and flagellum)
• Mitochondria Þ midpiece of sperm tail
• Cytoplasm Þ sloughed off Þ residual body

• Meiosis and spermiogenesis = ~60 days
• Pass thru epididymus (~2 weeks) = ~75 days total
• Movement to vas deferens (via fluid, cilia, smooth muscle)
• In vas deferens (lack fluids, more concentrated)
• Spermatozoan is compact and streamlined
• head – condensed nucleus with acrosome (hydrolytic enzymes)
• midpiece – contains basal body emanating the central axoneme (9 + 2)
• tail – flagellum (central axoneme)
• Spermatozoan are not fertile til they reach the epididymus
• Proteins in plasma membrane
• Ability to fertilize and swim
• Ejaculate
• Semen Þ sperm + fluid
• Fluid Þ 60% seminal vessicles; 40% prostate
• 2-3ml Þ 40 – 250 million/ml (10 – 20 million/ml fertile)

• Developing male germ cells undergo incomplete cytokinesis during mitosis and meiosis
• RNA synthesis occurs in diploid spermatogonium but decrease during meiosis
• In mature spermatozoa, the nucleus is inert with respect to gene expression at the time of fertilization