Spermatogenesis 2025

Spermatozoon Formation

  • Spermatogenesis: the process of sperm cell formation, occurring in the seminiferous tubules of the testes

  • Developmental Biology: field concerning the processes of growth and differentiation in organisms

Learning Outcomes

  • Discuss the stages of sperm cell formation

  • Describe sperm cell anatomy and physiology

  • Explain the role of spermatogenesis in inheritance

The Sperm & The Egg

Testes Anatomy

  • Testes: paired oval glands in the scrotum, ~5 cm long and 2.5 cm in diameter

  • Mass: each testis weighs ~10–15 grams

Development and Descent

  • Testes develop near kidneys in the abdomen

  • Descent into the scrotum typically begins around the 7th month of fetal development through inguinal canals

Testes Structure

Tunica Vaginalis

  • Serous membrane, derived from peritoneum, covering the testes

Tunica Albuginea

  • A white fibrous capsule of dense irregular connective tissue surrounding the testis

  • Extends inward to form septa, dividing the testis into lobules

Seminiferous Tubules

  • Each lobule contains 1–3 tightly coiled seminiferous tubules, the site of spermatogenesis

  • Cells in Tubules:

    • Spermatogenic Cells: actively forming sperm

    • Sustentacular Cells/Sertoli Cells: support and nourish spermatogenic cells

Cell Development

  • Spermatogonia: stem cells that originate from primordial germ cells, enter testes at 5 weeks of development

  • Dormant in childhood; active at puberty

  • Form primary spermatocytes toward the lumen, then released into seminiferous tubule lumen after maturation

Blood–Testis Barrier

  • Formed by tight junctions between sustentacular cells

  • Protects sperm from immune response by isolating developing gametes

Functions of Sustentacular Cells

  • Support spermatogenic cell nourishment; phagocytize excess cytoplasm

  • Produce fluid for sperm transport; secrete hormone inhibin; regulate testosterone and FSH effects

Leydig Cells

  • Located between seminiferous tubules; they secrete testosterone, promoting masculine traits and libido

Spermatogenesis Overview

  • Duration: 65–75 days in humans; starts with spermatogonia (diploid, 2n)

Spermatogonia Functions

  • Some remain undifferentiated as reserve cells; others develop into primary spermatocytes by passing through the blood–testis barrier

Primary & Secondary Spermatocytes

  • Primary Spermatocytes: diploid, undergo meiosis with DNA replication before division

  • In Meiosis I, homologous chromosomes separate into two secondary spermatocytes (haploid)

    • Secondary Spermatocytes: undergo meiosis II without DNA replication, produce spermatids

Cytoplasmic Separation

  • Cytoplasmic bridges form during spermatogenesis, allowing synchronized sperm production

  • Important for genetic diversity between resulting sperm with X and Y chromosomes

Spermiogenesis

  • Process of developing spermatids into sperm

  • No further cell division occurs; each spermatid becomes a single sperm cell

Sperm Cell Characteristics

  • Transforms into elongated structure with an acrosome

  • Spermiation: release of sperm from connections to sustentacular cells

Structure of Sperm

  1. Head

    • Approximately 4–5 μm; contains the nucleus with 23 chromosomes

    • Acrosome: enzyme-filled vesicle aiding in fertilization

  2. Tail

    • Divided into four regions: neck, middle piece, principal piece, and end piece

    • Mitochondria in middle piece provide energy for motility

    • Principal piece is the longest part; end piece tapers off

Semen Composition

  • Mixture of sperm and seminal fluid

  • Typical ejaculation volume: 2.5–5 mL containing 50–150 million sperm/mL

Sperm Viability

  • Below 20 million sperm/mL indicates potential infertility

  • High sperm numbers needed for fertilization due to low successful reach

Semen Functionality

  • Semen is alkaline (pH 7.2–7.7) to protect sperm in acidic environments

  • Liquefaction occurs post-ejaculation for sperm mobility; abnormal liquefaction can affect sperm movement

Summary

  • Spermatogenesis involves the creation of male gametes in seminiferous tubules of the testes

  • Starts from spermatogonia leading to formation of four haploid spermatids from one diploid primary spermatocyte

  • Components of Sperm: Head (nucleus and acrosome), Tail (locomotion and energy)

References

  • Hurley LL, McDiarmid CS, Friesen CR, Griffith SC, Rowe M. (2018) Experimental heatwaves negatively impact sperm quality in the zebra finch. Proc R Soc B, 285:20172547.

  • Tortora GJ, Derrickson B. (2017). Principles of Anatomy and Physiology (15th Ed). John Wiley and Sons.