What you'll learn
Human reproductive systems covers the structure of the male and female systems and the production of gametes. At CAPE level the detail extends to the histology of the testis and ovary, the stages of spermatogenesis and oogenesis, the structural adaptations of the sperm and egg to their functions, and a systematic comparison of the two processes. By the end of this topic you should be able to describe the structures of both systems and state the function of each part, describe spermatogenesis and oogenesis including where meiosis occurs and when it is completed, relate gamete structure to function, and compare the two processes in terms of timing, number and cytoplasmic division.
Key terms and definitions
Gamete — a haploid sex cell that fuses with another at fertilisation
Gonad — an organ producing gametes: the testis in males, the ovary in females
Spermatogenesis — the production of sperm in the testes
Oogenesis — the production of egg cells in the ovaries
Seminiferous tubule — the coiled tubule within the testis in which sperm are produced
Sertoli cell — a cell in the seminiferous tubule that nourishes and supports developing sperm
Interstitial cell — a cell between the seminiferous tubules that secretes testosterone
Germinal epithelium — the layer of dividing cells from which gametes originate
Acrosome — the vesicle at the head of a sperm containing hydrolytic enzymes
Graafian follicle — the mature follicle containing the secondary oocyte
Corpus luteum — the structure formed from the remains of the follicle after ovulation
Polar body — the small cell produced during oogenesis that receives little cytoplasm and degenerates
Zona pellucida — the glycoprotein layer surrounding the egg cell
Core concepts
The male reproductive system
The testes lie outside the body cavity in the scrotum, which maintains a temperature two to three degrees below body temperature. This is necessary because spermatogenesis is inhibited at core body temperature, and it is a standard examination point.
Each testis contains many coiled seminiferous tubules, lined with germinal epithelium, in which sperm are produced. Between the tubules lie interstitial cells, which secrete testosterone.
Sertoli cells within the tubules extend between the developing sperm, providing nutrients and protection and removing the excess cytoplasm shed during maturation.
Sperm pass into the epididymis, a long coiled tube in which they mature and are stored, acquiring the ability to swim.
The vas deferens carries sperm from the epididymis towards the urethra.
Three glands contribute to semen. The seminal vesicles secrete a fluid rich in fructose, which provides the respiratory substrate for the sperm's mitochondria, along with prostaglandins that stimulate contractions of the female tract. The prostate gland secretes an alkaline fluid that neutralises the acidity of the vagina and of residual urine in the urethra, since sperm are immobilised by acid conditions. The bulbourethral glands secrete mucus for lubrication.
The urethra carries both semen and urine, though not simultaneously, and the penis delivers semen into the vagina.
The female reproductive system
The ovaries produce egg cells and secrete oestrogen and progesterone. Each contains numerous follicles at different stages of development within a connective tissue stroma.
The oviducts, also called fallopian tubes, are lined with ciliated epithelium and smooth muscle. The funnel-shaped opening has finger-like fimbriae that sweep the released egg into the tube. Cilia and peristaltic contractions move the egg towards the uterus, and fertilisation normally occurs in the upper third of the oviduct.
The uterus has a thick muscular wall, the myometrium, which contracts during labour, and an inner lining, the endometrium, which thickens each cycle and into which an embryo implants.
The cervix is the narrow muscular neck of the uterus, producing mucus whose consistency changes through the cycle, becoming thin and watery around ovulation to allow sperm passage and thick at other times.
The vagina receives the penis and forms the birth canal. Its acidic environment inhibits bacterial growth but also immobilises sperm, which is why the alkaline secretions of the prostate matter.
Spermatogenesis
Spermatogenesis occurs in the seminiferous tubules and proceeds from the outer wall inwards, so that the stages can be seen in sequence in a transverse section.
Diploid germinal epithelium cells divide by mitosis to produce spermatogonia, which continue to divide by mitosis throughout adult life, maintaining a continuous supply.
Some spermatogonia enlarge to form primary spermatocytes, which are still diploid.
Each primary spermatocyte undergoes the first meiotic division to produce two haploid secondary spermatocytes.
Each secondary spermatocyte undergoes the second meiotic division to produce two spermatids, so four haploid spermatids result from each primary spermatocyte.
The spermatids then differentiate into spermatozoa, a process involving the loss of most of the cytoplasm, the development of the acrosome from the Golgi apparatus, the arrangement of mitochondria in the midpiece, and the growth of the tail. Sertoli cells assist throughout.
Spermatogenesis begins at puberty and continues throughout life, producing enormous numbers of gametes — of the order of a hundred million per day.
Oogenesis
Oogenesis differs from spermatogenesis in several fundamental ways, and the differences are the most commonly examined part of the topic.
Before birth, germinal epithelium cells in the fetal ovary divide by mitosis to form oogonia, which then enlarge to become primary oocytes. These begin the first meiotic division but arrest partway through, and remain arrested for many years. All the primary oocytes a female will ever have are present at birth.
At puberty, hormonal stimulation causes a few primary oocytes to resume development each cycle. Usually only one completes the first meiotic division, producing two cells of very unequal size: a large secondary oocyte receiving almost all the cytoplasm, and a small first polar body which receives a haploid nucleus but almost no cytoplasm and subsequently degenerates.
The secondary oocyte begins the second meiotic division but arrests again, and it is at this stage that it is released at ovulation.
The second meiotic division is completed only if fertilisation occurs. It produces the mature ovum and a second polar body, which also degenerates.
The unequal cytoplasmic division is functionally essential: it concentrates the cytoplasm, nutrients and organelles into a single large cell able to support the early embryo before implantation, rather than dividing these resources between four cells.
Each primary oocyte therefore yields one functional gamete, whereas each primary spermatocyte yields four.
Follicle development
The developing oocyte is surrounded by follicle cells, and the follicle develops alongside it.
A primary follicle consists of the primary oocyte with a single layer of follicle cells.
As it develops, the follicle cells multiply, a fluid-filled cavity forms, and the structure becomes a secondary and then a mature Graafian follicle, which bulges from the ovary surface.
At ovulation the Graafian follicle ruptures and releases the secondary oocyte, surrounded by the zona pellucida and a layer of follicle cells called the corona radiata.
The remains of the follicle develop into the corpus luteum, which secretes progesterone. If fertilisation does not occur it degenerates after about ten days, and the fall in progesterone triggers menstruation.
Sperm structure and function
The sperm is adapted for motility and for penetrating the egg, and each feature should be paired with its function.
The acrosome at the head is a vesicle derived from the Golgi apparatus containing hydrolytic enzymes, which digest a path through the layers surrounding the egg.
The haploid nucleus occupies most of the head and carries the paternal genetic material.
The midpiece contains numerous mitochondria arranged in a spiral, supplying the ATP required for the movement of the tail. The presence of many mitochondria is a reliable mark.
The tail is a flagellum whose undulating movement propels the sperm.
The cell is streamlined and has very little cytoplasm, which reduces mass and makes movement more efficient.
Egg structure and function
The egg is adapted for nourishing the early embryo and for preventing polyspermy.
It is a very large cell with abundant cytoplasm containing nutrients and organelles to support the zygote and the early cell divisions before implantation, since no external nutrition is available until then.
The haploid nucleus carries the maternal genetic material.
Cortical granules lie beneath the cell surface membrane. On fertilisation they release their contents, which alter the zona pellucida so that no further sperm can penetrate, preventing polyspermy.
The zona pellucida is a glycoprotein layer that contains species-specific receptors to which sperm bind.
The corona radiata is the surrounding layer of follicle cells, which the sperm must penetrate.
The egg is non-motile and is moved passively by cilia and peristalsis in the oviduct.
Comparing the two processes
A systematic comparison is worth holding ready, since it is frequently asked.
Timing differs: spermatogenesis begins at puberty and is continuous throughout life, while oogenesis begins before birth, arrests, and resumes cyclically from puberty until menopause.
Number differs: four functional gametes result from each primary spermatocyte, but only one from each primary oocyte.
Cytoplasmic division differs: equal in spermatogenesis, markedly unequal in oogenesis with polar bodies discarded.
Completion differs: meiosis is completed before release in spermatogenesis, but the second division is completed only on fertilisation in oogenesis.
Size differs: sperm are among the smallest cells in the body, eggs among the largest.
Both, however, involve mitosis to increase cell number, followed by meiosis to halve the chromosome number, and both produce haploid gametes.
Worked examples
Example 1: Explaining the position of the testes (3 marks)
Explain why the testes are located outside the main body cavity.
Spermatogenesis proceeds most effectively at a temperature two to three degrees Celsius below core body temperature.
The scrotum holds the testes outside the body cavity, where heat is lost more readily to the surroundings, so this lower temperature is maintained.
At core body temperature the production of sperm is inhibited and the number and quality of sperm produced falls, so the external position is necessary for fertility. Muscles in the scrotum adjust the distance of the testes from the body in response to temperature, drawing them closer when cold.
Example 2: Explaining unequal cytoplasmic division (4 marks)
Explain the significance of the unequal division of cytoplasm during oogenesis.
Each meiotic division in oogenesis produces one large cell and one very small polar body, so almost all the cytoplasm is retained in a single cell rather than being shared equally among four.
The cytoplasm contains nutrients, mitochondria, ribosomes and other organelles. After fertilisation the zygote divides repeatedly before implantation, and during this period it receives no nutrition from the mother.
Concentrating the cytoplasm into one cell therefore ensures that the resulting egg is large enough to supply the energy and materials needed to sustain the early embryo until implantation is achieved.
The polar bodies receive a haploid nucleus but insufficient cytoplasm to survive, and they degenerate, so each primary oocyte yields a single functional gamete.
Example 3: Relating sperm structure to function (5 marks)
Describe three adaptations of a sperm cell and explain how each contributes to its function.
The acrosome at the head contains hydrolytic enzymes. These are released on contact with the egg and digest a path through the corona radiata and zona pellucida, allowing the sperm to reach and fuse with the egg cell membrane.
The midpiece contains many mitochondria arranged in a spiral. These carry out aerobic respiration to produce the large quantity of ATP required for the continuous movement of the tail during the journey through the female reproductive tract.
The tail is a flagellum that beats in an undulating motion, propelling the sperm forwards so that it can travel from the vagina through the uterus to the oviduct.
The very small quantity of cytoplasm and the streamlined shape also reduce the mass to be moved, making swimming more efficient.
Common mistakes and how to avoid them
The most frequent error is stating that a mature ovum is released at ovulation. A secondary oocyte is released, arrested in the second meiotic division, which is completed only if fertilisation occurs.
Students often say that females produce eggs continuously from puberty. All primary oocytes are present before birth; what happens from puberty is that a few resume development each cycle.
Another common slip is describing polar bodies as failed gametes. They are a necessary consequence of unequal division, which concentrates cytoplasm in one cell.
Many candidates place fertilisation in the uterus. It normally occurs in the upper third of the oviduct.
In sperm structure answers, candidates frequently mention mitochondria without stating why ATP is needed, or the acrosome without naming hydrolytic enzymes.
Finally, candidates often confuse the roles of the seminal vesicles and the prostate. Fructose comes from the seminal vesicles as a respiratory substrate; alkaline secretion comes from the prostate to neutralise acidity.
Exam technique for "Human reproductive systems"
When naming a structure, give its function in the same sentence. Marks are usually awarded for the pairing rather than the label alone.
For spermatogenesis and oogenesis, set the stages out as a sequence with the ploidy stated at each step. Diploid to haploid transitions are frequently credited.
In comparison questions, use the same criteria for both processes — timing, number produced, cytoplasmic division, when meiosis completes — and present them in parallel.
For adaptation questions, always take the explanation through to the functional consequence. Many mitochondria is half an answer; supplying ATP for tail movement completes it.
Be precise about which cell is released at ovulation and which divisions are complete at that point, since this is the single most commonly examined detail in the topic.
Quick revision summary
The testes lie in the scrotum to maintain a temperature two to three degrees below core, and contain seminiferous tubules lined with germinal epithelium, Sertoli cells nourishing developing sperm, and interstitial cells secreting testosterone. Sperm mature in the epididymis, and semen receives fructose and prostaglandins from the seminal vesicles, alkaline fluid from the prostate and mucus from the bulbourethral glands. The ovaries contain follicles, the oviducts are ciliated and are where fertilisation occurs, the uterus has a muscular myometrium and an endometrium for implantation, and the cervix produces mucus that thins at ovulation. Spermatogenesis produces four functional haploid sperm from each primary spermatocyte, continuously from puberty. Oogenesis begins before birth, arrests in the first meiotic division, resumes cyclically from puberty, and produces one functional gamete plus polar bodies through unequal cytoplasmic division, with the second division completed only on fertilisation and a secondary oocyte released at ovulation. Sperm are adapted with an acrosome of hydrolytic enzymes, a midpiece packed with mitochondria and a flagellum; eggs are adapted with abundant cytoplasm, cortical granules preventing polyspermy and a zona pellucida carrying sperm receptors.