• The sperm acrosome stores large amounts of:
  • Hyaluronidase
  • Proteolytic enzymes
• Hyaluronidase breaks down hyaluronic acid in the material holding granulosa cells together.
• Proteolytic enzymes digest proteins in tissue structures still attached to the ovum.
• When the ovum enters the fallopian tube, it is still surrounded by:
  • Several layers of granulosa cells
  • A thick zona pellucida
• Before fertilization, sperm must:
  Pass through granulosa cells → Penetrate zona pellucida → Reach the oocyte
• Acrosomal enzymes are released to help this penetration.
• Hyaluronidase especially opens pathways between granulosa cells, allowing sperm to move toward the ovum.
• When sperm reaches the zona pellucida, its anterior membrane binds to specific receptor proteins there.
• This triggers rapid breakdown of the acrosome and release of its enzymes—the acrosome reaction.
• Within minutes, these enzymes create a pathway through the zona pellucida, allowing the sperm head to enter.
• Within about 30 minutes, the membranes of the sperm head and oocyte fuse, forming a single cell.
• Genetic material from sperm and oocyte then combines:
  • Half from father
  • Half from mother
• This process is fertilization, after which embryonic development begins.

Why Does Only One Sperm Normally Enter the Oocyte?

• Usually, only one sperm enters the oocyte.
• Within minutes after the first sperm penetrates the zona pellucida:
  • Ca²⁺ enters the oocyte.
  • This causes cortical granules to be released by exocytosis into the perivitelline space.
• Substances from these granules spread through the zona pellucida and:
  • Prevent additional sperm from binding.
  • Cause already-binding sperm to detach.
• Therefore, entry of more than one sperm is normally prevented.
• Entry of multiple sperm is called polyspermia.
• In humans, polyspermia usually causes abnormal development and death of the zygote.

KEY CONCEPT

Sperm reaches ovum → Acrosome reaction → Hyaluronidase + proteolytic enzymes released → Granulosa cells and zona pellucida penetrated → Sperm and oocyte membranes fuse → Fertilization

First sperm enters → ↑ Oocyte Ca²⁺ → Cortical granule release → Zona pellucida blocks other sperm → Prevents polyspermia

Conceptual Examples

• Hyaluronidase → opens spaces between granulosa cells.
• Proteolytic enzymes → digest protein barriers around the ovum.
• Acrosome reaction → releases enzymes needed for sperm penetration.
• Cortical reaction → acts like a block against additional sperm after the first sperm enters.

Abnormal Spermatogenesis and Male Fertility

• The seminiferous tubular epithelium can be damaged or destroyed by several conditions, leading to reduced sperm production or sterility.
• Bilateral orchitis from mumps can damage both testes and may cause sterility in some males.
• Some male infants are born with degeneration of the seminiferous tubular epithelium due to:
  • Strictures of the genital ducts
  • Other reproductive abnormalities
• Excessive testicular temperature can also cause sterility, but this is usually temporary.

Effect of Temperature on Spermatogenesis

• Increased testicular temperature can prevent spermatogenesis.
• High temperature causes degeneration of most cells in the seminiferous tubules, while spermatogonia are relatively preserved.
• The testes are located in the scrotum to keep them cooler than the internal body temperature.
• Normal testicular temperature is about 2°C below core body temperature.
• In cold conditions, scrotal muscles contract and pull the testes closer to the body, helping maintain this temperature difference.
• Therefore, the scrotum provides controlled cooling of the testes, which is important for normal spermatogenesis.
• Without adequate cooling, sperm production may become deficient, especially in hot conditions.

KEY CONCEPT

↑ Testicular temperature → Seminiferous tubular cell degeneration → ↓ Spermatogenesis → Temporary infertility

Scrotum → Keeps testes ≈ 2°C below body temperature → Supports normal sperm production

Conceptual Examples

• Mumps affecting both testes → may damage seminiferous tubules → sterility.
• Excessive heat around testes → ↓ sperm formation.
• Cold environment → scrotum contracts → testes move closer to body → prevents excessive cooling.
• Main purpose of scrotal temperature control → maintain the proper temperature for spermatogenesis.

Cryptorchidism—Failure of the Testes to Descend Into the Scrotum

• Cryptorchidism means failure of one or both testes to descend from the abdomen into the scrotum around the time of birth.
• During fetal development, the testes develop from the genital ridges in the abdomen.
• Normally, about 3 weeks to 1 month before birth, the testes descend:
  Abdomen → Inguinal canals → Scrotum
• If descent is incomplete, a testis may remain in:
  • The abdomen
  • The inguinal canal
  • Another point along its normal pathway
• A testis that remains permanently in the abdomen usually cannot produce sperm.
• Its seminiferous tubular epithelium degenerates, while mainly the interstitial structures remain.
• The higher temperature of the abdomen compared with the scrotum may contribute to this tubular degeneration and sterility.
• Therefore, undescended testes may be surgically moved into the scrotum before adult sexual life.
• Testosterone from the fetal testes normally stimulates testicular descent.
• Many cases of cryptorchidism may occur because abnormal testes produce insufficient testosterone.
• If the testes themselves are poorly developed and cannot produce enough testosterone, surgery may not restore fertility.

Effect of Sperm Count on Fertility

• Average semen volume per ejaculation is about 3.5 mL.
• Each milliliter normally contains about 120 million sperm, although normal values may range from 35–200 million/mL.
• Therefore, an average ejaculate contains about 400 million sperm.
• If sperm concentration falls below about 20 million/mL, infertility becomes more likely.
• Although only one sperm fertilizes the ovum, a very large number of sperm are normally required for successful fertilization.

Effect of Sperm Morphology and Motility on Fertility

• A man may have a normal sperm count but still be infertile.
• In some cases, many sperm have abnormal structure, such as:
  • Two heads
  • Abnormally shaped heads
  • Abnormal tails (Fig. 81.5)
• In other cases, sperm may look normal but are:
  • Nonmotile
  • Poorly motile
• Therefore, fertility depends not only on sperm number but also on normal sperm shape and good motility.

KEY CONCEPT

Cryptorchidism → Testis remains outside scrotum → Higher temperature/tubular degeneration → ↓ Spermatogenesis → Infertility

Normal fertility depends on 3 major sperm factors:

Adequate sperm count + Normal morphology + Good motility

Conceptual Examples

• Undescended abdominal testis → seminiferous tubules degenerate → poor sperm production.
• Sperm count <20 million/mL → infertility becomes more likely.
• Normal count but abnormal heads/tails → fertility may still be reduced.
• Normal-looking but nonmotile sperm → cannot move effectively toward the ovum → infertility.

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