Introduction to Semen, Sperm Morphology and Composition

Introduction to Semen

Semen, also known as seminal fluid, is a biological fluid produced by the male reproductive system. It contains spermatozoa (sperm cells) along with secretions from various male reproductive glands. The release of semen from the body is known as ejaculation.

In forensic science, semen is considered one of the most important biological evidence types, especially in sexual assault investigations, rape cases, and DNA profiling.

Characteristics of Semen

Important characteristics of semen include:

• Normal sperm count ranges from 10⁷ to 10⁸ spermatozoa per milliliter of semen.

• A typical ejaculation releases about 2–5 mL of semen.

• Semen consists of:

  • Seminal fluid (approximately 95%)

  • Sperm cells (approximately 5%)

These characteristics are useful in forensic identification and biological examination.

Morphology of Spermatozoa

A sperm cell is divided into four major parts:

1. Head

• The head is flattened and oval-shaped.

• It contains:

  • Acrosomal cap

  • Acrosome enzymes

  • Nucleus with densely packed chromosomes

The acrosome helps the sperm penetrate the ovum during fertilization.

2. Neck

The neck contains:

• One proximal centriole

• One distal centriole

These structures connect the head to the midpiece.

3. Midpiece

The midpiece contains:

• A filamentous core

• Numerous mitochondria arranged spirally

Mitochondria provide energy for sperm motility.

4. Tail (Flagellum)

The tail is responsible for:

• Movement of spermatozoa

• Motility and propulsion

The flagellum enables sperm cells to travel through the reproductive tract.

Important Constituents of Semen

Semen contains several important proteins and enzymes used in forensic examination.

Semenogelin Proteins

• Semenogelin I and II are secreted from seminal vesicles.

• They help coagulate semen after ejaculation.

Flavin

• Seminal fluid contains flavin.

• Flavin causes semen to fluoresce under ultraviolet (UV) light.

This property helps forensic experts locate seminal stains.

Acid Phosphatase (AP)

Acid phosphatase is an important forensic marker for semen detection.

Features of Acid Phosphatase

• Functions optimally in acidic pH

• High concentration in seminal fluid

• Detectable in dried seminal stains

Stability

• Half-life at 37°C: approximately 6 months

• Detectable up to 1 year at –20°C in dry stains

• Activity decreases in wet conditions

Prostate-Specific Antigen (PSA / P30)

Prostate-specific antigen (PSA), also called P30, is a major protein present in seminal plasma.

Characteristics of PSA

• Concentration: 0.5–2.0 mg/mL

• Molecular weight: 30 kDa

• Hydrolyzes semenogelin proteins

PSA is widely used in forensic laboratories for semen identification.

Stability of PSA

• Detectable in dried semen stains for approximately 3 years at room temperature

• Degrades rapidly in moist conditions

Seminal Vesicle-Specific Antigen (SVSA)

SVSA includes:

• Semenogelin I (SgI)

• Semenogelin II (SgII)

These proteins form the seminal coagulum after ejaculation.

Compared to PSA, semenogelin is present in much higher concentrations and can provide improved sensitivity in semen detection.

Forensic Significance of Semen

1. Semen confirms sexual contact in cases of assault or rape.

2. It provides DNA for identifying or excluding suspects.

3. Seminal stains help link individuals to crime scenes.

4. Semen markers can indicate time since intercourse.

5. Even without sperm, proteins like PSA confirm semen presence.

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