15.2 Gametogenesis in Humans
Learn how diploid precursor cells in the gonads divide and mature to form haploid gametes through spermatogenesis and oogenesis.
Learning Objectives
- State the necessity of gametogenesis in humans.
- Describe the stages of spermatogenesis in the seminiferous tubules, including the multiplication, growth, and maturation phases.
- State the role of hormones (FSH, LH, testosterone) and Sertoli cells in spermatogenesis.
- Describe the stages of oogenesis from pre-birth development, childhood dormancy, to puberty and fertilisation.
- Identify the structures of a sperm and a Graafian follicle.
- Compare and contrast spermatogenesis and oogenesis in terms of location, process, duration, and products.
Comparison of Spermatogenesis and Oogenesis Pathways
Follow the cell division stages side by side, from a primordial germ cell to functional gametes, and see where each pathway pauses.
Spermatogenesis
Continuous process in the seminiferous tubules from puberty onwards, producing four functional sperms per primary spermatocyte.
Oogenesis
Discontinuous process in the ovaries that begins before birth, pauses at prophase I, resumes at puberty, and completes meiosis II only upon sperm entry.
Fertilisation
Fusion of haploid sperm and ovum nuclei restores the diploid chromosome number (2n) in the zygote.
Short Explanation
What is gametogenesis?
Gametogenesis is the biological process by which diploid precursor cells in the gonads undergo division and maturation to form haploid gametes. In males, it is called spermatogenesis and produces mobile sperm cells in the testes. In females, it is called oogenesis and produces a secondary oocyte in the ovaries.
Why is gametogenesis necessary?
Gametogenesis produces haploid (\(n\)) gametes. During fertilisation, the haploid nucleus of the sperm fuses with the haploid nucleus of the ovum in the Fallopian tube to form a diploid zygote (\(2n\)). This restores and maintains the species chromosome number.
Where does it happen?
Spermatogenesis occurs in the seminiferous tubules of the testes. Oogenesis occurs in the ovaries.
Spermatogenesis — three phases
1. Multiplication phase: Primordial germ cells (\(2n\)) divide mitotically repeatedly to form diploid spermatogonia (\(2n\)).
2. Growth phase: Each spermatogonium expands and develops into a diploid primary spermatocyte (\(2n\)).
3. Maturation phase: Each primary spermatocyte undergoes meiosis I to produce two haploid secondary spermatocytes (\(n\)). Each secondary spermatocyte undergoes meiosis II to produce two haploid spermatids (\(n\)). Spermatids then differentiate into mature sperms (\(n\)).
Hormonal regulation of spermatogenesis
FSH and testosterone stimulate spermatogenesis. LH aids the process by stimulating testosterone secretion by interstitial cells in the testes.
Sertoli cells and transport
Sertoli cells provide nutrients to developing sperm cells. The epididymis stores sperm, and the sperm duct (vas deferens) carries sperm out.
Oogenesis — a discontinuous timeline
Before birth: Primordial germ cells divide mitotically to form diploid oogonia (\(2n\)). An oogonium develops into a diploid primary oocyte (\(2n\)), encapsulated by follicular cells to form a primary follicle. The primary oocyte begins meiosis I but is arrested at prophase I.
After birth: A baby girl is born with millions of primary oocytes dormant in prophase I. The number gradually decreases by puberty.
Puberty to menopause: Each month, a primary oocyte resumes and completes meiosis I to form one haploid secondary oocyte (\(n\)) and one haploid first polar body (\(n\)). The secondary oocyte begins meiosis II but halts at metaphase II. The first polar body completes meiosis II to produce two second polar bodies. The secondary oocyte is enveloped by follicular cells to form a secondary follicle, which matures into a Graafian follicle that secretes oestrogen.
Ovulation: The mature Graafian follicle releases the secondary oocyte into the Fallopian tube. The remaining follicle tissues form the corpus luteum.
If fertilisation occurs: A sperm penetrates the secondary oocyte, triggering completion of meiosis II to produce an ovum (\(n\)) and a polar body. The sperm nucleus fuses with the ovum nucleus to form a diploid zygote (\(2n\)). Polar bodies degenerate. The corpus luteum continues to grow and secretes oestrogen and progesterone.
If no fertilisation occurs: The secondary oocyte and corpus luteum degenerate and are removed through menstruation.
Structure of a sperm
Head: contains the nucleus carrying genetic material. Midpiece: packed with mitochondria that generate energy (ATP) for the tail to beat. Tail: flagellum that provides motility for swimming.
Structure of a Graafian follicle
Consists of a large, spherical secondary oocyte surrounded by a gel-like substance and layers of follicle cells.
Comparison of spermatogenesis and oogenesis
Similarities: Both are forms of gametogenesis occurring in the gonads. Both involve mitosis, cell growth, meiosis, and produce haploid (\(n\)) gametes involved in fertilisation.
Differences: Spermatogenesis occurs in the testes, produces four functional sperms per spermatogonium, is continuous from puberty until old age, and produces millions of sperms daily. Oogenesis occurs in the ovaries, produces one functional secondary oocyte and three non-functioning polar bodies per oogonium, is discontinuous, and releases only one secondary oocyte per menstrual cycle.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the biological importance of gametes being haploid (\(n\))?
At which specific stages does meiosis pause in human female oogenesis?
What is the function of the high concentration of mitochondria in the midpiece of a sperm?
Common Mistakes
- Confusing the stage of meiotic arrest: thinking the primary oocyte is arrested at metaphase I instead of prophase I, or that the secondary oocyte is arrested at anaphase II instead of metaphase II.
- Misidentifying the released gamete at ovulation: stating that a mature “ovum” is ovulated. In reality, a secondary oocyte is released at ovulation; the ovum is formed only after sperm entry completes meiosis II.
- Assuming polar bodies can be fertilised. Only the secondary oocyte/ovum is functional; polar bodies contain minimal cytoplasm and degenerate.
- Misattributing Sertoli cell functions: confusing Sertoli cells (which nourish sperm in the seminiferous tubules) with interstitial/endocrine cells that secrete testosterone.
Concept Misunderstandings
Spermatogenesis and oogenesis both start at puberty.
Spermatogenesis starts at puberty and continues throughout life, but oogenesis begins during embryonic/fetal development before the female is born, pausing until puberty.
Meiosis II always completes in every ovulated secondary oocyte.
Meiosis II only resumes and completes if a sperm penetrates the secondary oocyte. Without fertilisation, the secondary oocyte dies and degenerates in metaphase II.
Spermatids are fully functional swimming gametes.
Spermatids are non-motile circular haploid cells that must first undergo a distinct morphological differentiation process to develop a head, midpiece, and flagellar tail.
Summary
- Gametogenesis: Formation of haploid (\(n\)) gametes in the gonads (testes and ovaries) to maintain species chromosome number upon fertilisation (\(2n\)).
- Spermatogenesis: Occurs in the seminiferous tubules, nourished by Sertoli cells, regulated by FSH, testosterone and LH. Progression: primordial germ cell → spermatogonium (\(2n\)) → primary spermatocyte (\(2n\)) → secondary spermatocytes (\(n\)) → spermatids (\(n\)) → sperms (\(n\)). Yield: 4 functional sperms per spermatocyte.
- Oogenesis: Occurs in the ovaries. Begins before birth (halts at prophase I), resumes at puberty, halts at metaphase II, completes only upon sperm entry. Yield: 1 functional ovum and 3 non-functional polar bodies.
- Sperm anatomy: Head (nucleus), midpiece (mitochondria for energy), and tail (locomotion).
- Follicle cycle: Primary follicle → secondary follicle → Graafian follicle → ovulation → corpus luteum.
Short Activity
Objective: test your understanding of gametogenesis, meiotic arrest points, and reproductive structures.
A. Objective Quiz
At which phase is meiotic division arrested in a human secondary oocyte before fertilisation takes place?
Which cell type provides nutrients to developing sperm cells within the seminiferous tubules during spermatogenesis?
B. Fill in the Blanks
The midpiece of a mature sperm is packed with , which generate the energy needed to swim to the Fallopian tube.
After ovulation, the remaining follicular tissues inside the ovary develop into the , which secretes oestrogen and progesterone.
C. Matching / Drag and Drop
Match each reproductive cell or structure to its correct description or characteristic. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Structure / Cell to Its Description
Write the correct structure or cell next to each description.
| Description | Structure / Cell |
|---|---|
| 1. Diploid cell that undergoes meiosis I to form two haploid secondary spermatocytes | |
| 2. Mature ovarian follicle containing a secondary oocyte that secretes oestrogen | |
| 3. Haploid product of meiosis II that undergoes differentiation into a sperm | |
| 4. Small non-functioning cell produced during oogenesis that eventually degenerates |
Answer choices: Primary spermatocyte • Graafian follicle • Spermatid • Polar body
