14.2 Animal Gametes and Fertilization
Key Takeaways
In the seminiferous tubules, one primary spermatocyte produces four sperm, and spermatogenesis continues from puberty.
Sertoli cells support the developing sperm, and Leydig cells secrete testosterone.
Primary oocytes are arrested in prophase I before birth, and the cell that is ovulated is a secondary oocyte arrested in metaphase II.
Meiosis II finishes at fertilization, and unequal cytokinesis means one primary oocyte yields one ovum rather than four.
Human fertilization is usually in the oviduct; the acrosome releases enzymes at the egg coats, and fast and slow blocks prevent polyspermy.
14.2 Animal Gametes and Fertilization
Gamete formation is meiosis plus packaging. A diploid cell copies its DNA, meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids, with no second DNA replication between the divisions. The testis and the ovary differ in timing, cell size, and how many products can fertilize an egg. Counting those products, and knowing which cell is ovulated, is the core of this section.
Spermatogenesis continues from puberty
Spermatogenesis occurs in the seminiferous tubules of the testis. Spermatogonia sit near the outer wall of each tubule and divide by mitosis. Some daughters remain stem cells. Others become primary spermatocytes and enter meiosis. Unlike the ovary, the testis does not finish a lifetime pool of meiotic cells before birth. From puberty onward, new primary spermatocytes keep entering meiosis. The full sequence from spermatogonium to sperm takes roughly two months in humans, and it continues through adult life.
Four sperm from one primary spermatocyte
One primary spermatocyte is diploid, with its DNA already copied. Meiosis I produces two haploid secondary spermatocytes, still with duplicated chromosomes. Meiosis II produces four spermatids, which differentiate into sperm. The nucleus condenses in the head, an acrosome of enzymes caps that nucleus, and mitochondria pack the midpiece to power the flagellum. Cytokinesis is essentially equal, so one primary spermatocyte yields four functional sperm.
Sertoli cells inside the seminiferous tubules support and protect the developing cells and respond to follicle-stimulating hormone. Leydig cells between the tubules secrete testosterone when stimulated by luteinizing hormone. Testosterone supports spermatogenesis and male secondary sex characteristics. Sertoli cells do not supply the testosterone, and Leydig cells do not become sperm.
Oogenesis saves one large cell
Oogenesis in the ovary starts before birth. Oogonia divide by mitosis in the fetus and become primary oocytes, which enter meiosis I and arrest in prophase I. About one to two million are present at birth, and a few hundred thousand remain by puberty. Most never finish meiosis. In a cycle, typically one primary oocyte resumes it.
That primary oocyte completes meiosis I shortly before ovulation. The chromosomes separate, but the cytoplasm does not split equally. The large product is a secondary oocyte. The small product is the first polar body. Both are haploid. The secondary oocyte immediately begins meiosis II and arrests in metaphase II. This secondary oocyte, still paused in metaphase II, is the cell that is ovulated. It is not a finished ovum. Humans do not ovulate an egg that has already completed meiosis II.
Meiosis II waits for the sperm
Fertilization allows the secondary oocyte to finish meiosis II. Sister chromatids separate. The result is the ovum and a second polar body. The first polar body may also divide, but polar bodies stay small and do not become more eggs. Unequal cytokinesis is the reason. Almost all of the cytoplasm, including yolk-like stores, mitochondria, and the machinery the early embryo will use, stays in the one large cell. The meiotic arithmetic still reduces the chromosome number. The packaging does not hand that cytoplasm to four equal daughters.
Count from one starting cell. One primary spermatocyte produces four sperm. One primary oocyte produces one ovum plus polar bodies, not four ova. The ovulated cell is the secondary oocyte in metaphase II, not a primary oocyte and not an ovum that has already finished meiosis II.
| Comparison | Spermatogenesis | Oogenesis |
|---|---|---|
| Where it occurs | Seminiferous tubules of the testis | Ovary |
| When meiosis starts | At puberty, then continues | Primary oocytes enter meiosis before birth |
| Where division pauses | No years-long meiotic arrest | Prophase I until a cycle; metaphase II until fertilization |
| One primary cell yields | Four sperm | One ovum and small polar bodies |
| Cytokinesis | Roughly equal | Unequal, so one cell keeps the cytoplasm |
| Neighboring somatic cells | Sertoli cells support development; Leydig cells make testosterone | Follicle cells surround and support the oocyte |
Fertilization joins one sperm with one egg
In humans, fertilization usually occurs in the oviduct, also called the uterine tube, most often in its ampulla. It does not usually occur in the ovary, and it is not the same event as implantation in the uterus. Cleavage begins as the early embryo travels along the tube. The blastocyst later reaches the uterus.
A sperm has a head, a midpiece full of mitochondria, and a flagellum. In mammals, capacitation in the female tract must occur before fertilization. The acrosome then releases enzymes that help the sperm cross the cumulus cells and the zona pellucida. The plasma membranes fuse, and that fusion is the start of fertilization.
Blocks to polyspermy
Polyspermy, fertilization by more than one sperm, adds extra chromosome sets and disrupts development. A fast block changes the egg membrane within seconds so more sperm cannot fuse. In sea urchins and frogs that block is electrical: the membrane depolarizes. A slow block follows in those animals and in mammals. Cortical granules release enzymes that change the egg coats. Sea urchins lift a fertilization envelope. Mammals alter the zona pellucida so further sperm do not bind. One sperm nucleus remains inside the egg.
After one sperm nucleus enters and meiosis II finishes, egg and sperm chromosomes restore the diploid zygote: one set from each parent. The next stage is cleavage, not another meiosis.
External and internal fertilization
External fertilization is common in many fish and amphibians. Eggs and sperm are shed into water, and the sperm must swim, so drying out prevents fertilization. Internal fertilization is the pattern in reptiles, birds, and mammals: sperm are deposited in the female tract, and the gametes do not need a pond or the sea. Some fish and amphibians also fertilize internally, so the contrast is many, not all, aquatic vertebrates.
Warning
Humans ovulate a secondary oocyte arrested in metaphase II. They do not ovulate an ovum that has already finished meiosis II. One primary oocyte yields one ovum, not four. One primary spermatocyte yields four sperm.
A student counts the functional gametes produced from one primary spermatocyte and one primary oocyte. Which count is right?
One primary spermatocyte yields four sperm, and one primary oocyte yields one ovum.
Both primary cells finish meiosis II before birth and then produce no further gametes.
One primary spermatocyte yields one sperm, and one primary oocyte yields four ova.
Each primary cell produces four equal gametes, so one primary oocyte yields four ova.
Which description of the cell released at human ovulation is accurate?
A secondary oocyte arrested in metaphase II is ovulated, and meiosis II is completed at fertilization.
A mature ovum that has already completed meiosis II is ovulated, and polar bodies form only during spermatogenesis.
Four secondary oocytes of equal size are ovulated together from one primary oocyte.
A primary oocyte that has not yet entered meiosis is ovulated, and meiosis I begins only after implantation.
Which statement about where and how animal fertilization occurs is correct?
The acrosome builds a coat that attracts many sperm, and polyspermy is required to restore the diploid number.
Human fertilization is usually in the oviduct, acrosomal enzymes help the sperm cross the egg coats, and blocks to polyspermy keep additional sperm out.
Reptiles, birds, and mammals usually shed eggs and sperm into open water so fertilization can occur outside the body.
Human fertilization is usually completed in the ovary, and several sperm must enter before cleavage can start.
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