Fertilisation and the first cleavages
Compare sperm-entry barriers in sea urchins and mammals, including exceptions to one-sperm entry. Explain how early cleavage makes more, smaller cells, how early cell cycles vary between animal groups, and how yolk helps shape cleavage.
What a learner can do afterwards
- Describe the electrical fast block and cortical slow block in sea urchins, and distinguish these from mammalian sperm-entry barriers.
- Explain how cleavage increases cell number with little overall growth while retaining DNA replication, and identify limits of the rapid S/M-cycle model.
- Compare whole-egg cleavage in sea urchins with disc cleavage in chickens, relating the difference to yolk amount and distribution.
1 · Read
You have met haploid gametes, each with one chromosome set. In normal human fertilisation, one sperm fuses with one egg, restoring two sets in the zygote. Extra sperm entry is called polyspermy and usually disrupts human development. This is not an animal-wide rule: bird eggs normally admit several sperm, although only one sperm nucleus combines with the egg nucleus.
In sea urchins, sperm-egg fusion rapidly depolarises the egg membrane, changing its voltage. This gives a brief fast block to further sperm fusion. A rise in calcium inside the egg triggers cortical granules to release their contents. These lift and modify the vitelline envelope around the egg, forming a fertilisation envelope. This slow block provides a longer-lasting barrier to extra sperm.
Mammals have no established fast electrical block like the sea urchin's. Cortical-granule release alters the zona pellucida, a coat around the egg. Changes in this coat and at the egg membrane help limit further sperm binding, penetration or fusion. The details differ between species, so a mechanism described in one animal should not be assumed for every animal.
Cleavage consists of mitotic divisions that produce cells called blastomeres. Early cleavage increases cell number with little overall growth, so average cell size falls. In a model where every cell divides once per round, each complete round doubles the cell count. If the divisions are equal and there is no overall growth, divide the starting volume by the final cell count to find each cell's volume.
Little overall growth does not mean no DNA copying. S phase copies DNA before mitosis, allowing daughter cells to retain the chromosome set. In many early non-mammalian embryos, rapid cycles alternate mainly between S phase and M phase, with G1 and G2 absent or short. Mammalian cleavage is generally slower. Early cell-cycle structure varies with species and stage, so the rapid S/M model is not universal.
Yolk amount and distribution help shape cleavage. Holoblastic cleavage divides the whole egg, as in sea urchin eggs with little yolk. Meroblastic cleavage divides only part of the egg. In a chicken's yolk-rich egg, a small disc of cytoplasm above the yolk divides while the large yolk mass remains undivided. Other developmental controls also affect cleavage; yolk is not its only influence.
Sperm-entry barriers differ between animals. Cleavage makes more, smaller cells while retaining DNA copying. Yolk helps shape whether the whole egg or only part divides.
Original lesson, questions and flashcards written for this dataset.
Original reading written for this dataset. OpenStax Biology for AP Courses, section 34.2, was consulted for examples distinguishing fertilisation location from embryo development.
More to read
What an early embryo observation tells you
A rise in cell number does not by itself show that an embryo has grown. During early cleavage, cells divide with little increase in total size. To interpret an observation, compare both cell count and total volume. In an equal-division model with no growth, more cells means a smaller volume per cell. DNA replication still occurs before mitosis; cell size and chromosome copying are different measurements.
The animal's identity also matters. A sea urchin egg briefly changes its membrane voltage after sperm fusion. Its cortical reaction then changes the envelope around the egg, giving a longer-lasting barrier. Mammalian eggs have coat and membrane barriers without an established equivalent electrical fast block. Bird eggs provide another limit to the one-sperm-entry rule: several sperm normally enter, although only one sperm nucleus combines with the egg nucleus.
Look separately at where fertilisation happens and where the embryo develops. Frog gametes meeting in pond water are an example of external fertilisation. Human gametes meet internally. Internal fertilisation does not itself mean live birth: birds also fertilise internally and lay eggs. These categories do not provide a universal ranking of parental effort or offspring survival.
Finally, consider what part of an egg divides. Cleavage passes through a sea urchin egg with little yolk. In a chicken egg, it is confined to a small disc above the large yolk mass. The first pattern is holoblastic and the second is meroblastic. Yolk amount and distribution help explain the contrast, alongside other developmental controls.
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