Spermatogenic wave and continuous sperm production
The spermatogenic wave is the staggered arrangement of stages of spermatogenesis (sperm production) along a seminiferous tubule (the tightly coiled tube in the testis where sperm are made).
Remember
Because the stages of spermatogenesis are staggered along each tubule, successive generations of developing sperm cells overlap and several groups are part-way through at once, so sperm are made continuously, not in batches.
Memory aid
A Mexican wave round a stadium
Each section of the crowd stands at a slightly different moment, just as each segment of the tubule is at a slightly different stage.
Making a sperm is slow. Germ cells (the cells that become sperm) take about 64 days in the classic estimate to go from a spermatogonium (the stem cell) to a formed sperm. About every 16 days a new generation of germ cells starts developing, so groups that started at different times are part-way through at once.
Neighbouring segments of the tubule are slightly out of step with one another, so the stages follow each other along its length. That staggered, wave-like arrangement is the spermatogenic wave. Because other segments are at other stages, sperm production is continuous rather than made in batches.
The spermatogenic cycle is a related term. It measures how long a stage takes to come back to the same segment, about 16 days in humans. It is not the time to make a whole sperm, which takes several cycles. Think of the cycle as a clock at one spot on the tubule and the wave as a map of how the stages are laid out along it.
A cross-section (a thin slice) samples one small region of the tubule, so it shows only some of the stages. Within a section, germ cells mature in order from the basement membrane (the thin layer at the outer edge of the tubule) towards the lumen, so spermatogonia lie at the edge and later stages sit closer to the lumen. A mixed picture is normal. It is not evidence of arrest (germ cells stopping at one stage), and it does not mean sperm are made in synchronised batches (germ cells moving through development together).
Key facts
- Wave
- different stages of spermatogenesis appear in sequence along the length of the tubule.
- Cycle
- the same stage reappears in the same segment about every 16 days in humans.
- Whole process
- making a sperm from a spermatogonium takes several cycles, about 64 days in the classic estimate.
- Result
- sperm are produced continuously from puberty, not in batches.
- On a slide
- one cross-section shows only some of the stages.
- Human tubules
- studies describe a helical (spiral-shaped) arrangement, with more than one stage in the same cross-section.
Exam traps
- Sperm development is not completed in about 16 days, because that interval is the spermatogenic cycle and full development takes about 64 days in the classic estimate, so successive generations overlap.
- Germ cells in a tubule do not move through spermatogenesis together in synchronised batches, because a section would then show one uniform stage rather than a mixture.
- A pattern of stages laid out along the tubule describes the spermatogenic wave and not the spermatogenic cycle, because the wave is a pattern in space and the cycle is a length of time.
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