High school biology
Fertilization and development review
|Gamete||A reproductive (sex) cell. In females, egg; in males, sperm|
|Fertilization||The process in sexual reproduction in which a female gamete and male gamete fuse to form a new cell|
|Zygote||Cell resulting from fertilization|
|Diploid (2n)||Cell that contains two sets of homologous chromosomes|
|Haploid (n)||Cell that contains only a single set of genes|
|Apoptosis||The process of programmed cell death|
|Differentiation||The process by which cells become specialized in structure and function|
Human fertilization and development
Fertilization is the process in which haploid gametes fuse to form a diploid cell called a zygote. To ensure that each zygote has the correct number of chromosomes, only one sperm can fuse with one egg.
Stages of human development
- Zygotic stage: The zygote is formed when the male gamete (sperm) and female gamete (egg) fuse.
- Blastocyst stage: The single-celled zygote begins to divide into a solid ball of cells. Then, it becomes a hollow ball of cells called a blastocyst, attaching to the lining of the mother's uterus.
- Embryonic stage: The major internal organs and external features begin to emerge, forming an embryo. In this stage, the heart, brain, and spinal cord become visible. Arms and legs start to develop.
- Fetal stage: Once the formed features of the embryo begin to grow and develop, the organism is considered a fetus. Differentiation and specialization of structures happens during this time.
Development overview showing the progression from zygote to blastocyst to embryo to fetus.
Differentiation and apoptosis
During development, the number of cells must increase through division so that body axes, tissues, organs, and structures must form. Individual cells become specialized in their structure and function through the process of cell differentiation.
Unnecessary cells also must be removed in order to help form important structures. This occurs is through the process of apoptosis. For example, human hands start out as a paddle-like block of tissue. Eventually, the block was “carved” into fingers by apoptosis of the cells in between the developing fingers.
Microscope images from a scientific paper, showing a developing mouse paw. The cells between the developing digits are stained by a marker that indicates apoptotic cells.
Common mistakes and misconceptions
- Human fertilization occurs in the fallopian tube. Many people believe that human fertilization occurs in the vagina, but this is not the case. Once sperm enter the vagina, they can move through the cervix, into the uterus, and to the end of a fallopian tube. If a sperm is able to fuse with an egg, fertilization occurs.
- Development is sort of pre-determined. While it IS true that all humans in early development look the same, many of the features that later develop in a fetus are already pre-determined by its genes. For example, the biological sex of the fetus is already decided based on whether it received two X chromosomes (one from each parent) or an X from its mother and a Y from its father, despite the fact that sex-specific characteristics do not appear until later in development.
Want to join the conversation?
- Human life begins with a single cell. Over the course of 9 months, this cell becomes trillions of cells. What process is responsible for this?(7 votes)
- Mitosis, mainly. There's an article on Khan Academy showing this process:
- So what happends differently in the fertilization cycle when a woman has twins?(2 votes)
- Two situations can happen :
- in the case of identical twins, the ovaries of the woman release one egg during ovulation, and once this egg gets fertilized it divides once too much, forms twice the amount of cells it was supposed to, and boom you've got two identical bundles of cells that will be the two children. Identical twins share the exact same genes as they come from the exact same egg fertilized by the exact same sperm cell.
- in the case of non-identical twins : sometimes a woman's ovaries can release not one but two eggs during the same cycle. If those two eggs get fertilized by two different sperm cells at around the same time, then you've got two embryos that will be non-identical twins (not sharing the same genome because they come from two different eggs and two different sperm cells).
Hope this helped !(7 votes)
- when does the heart start beating?(3 votes)
- It depends on the animal. For a human the hearts beating at 21-23 days in the fetus.(4 votes)
- How does sexual reproduction in plants differ from asexual reproduction in plants?(2 votes)
- It differs just like it would with animals or other organisms. In sexual reproduction, a male gamete fuses with a female gamete to form a zygote (requiring two parents, leading to more genetic variation, etc). In asexual reproduction, the organism essentially copies itself (one parent, very limited genetic variation). In plants, sexual reproduction looks like pollen, seeds, and plant ovules. An example of asexual reproduction would be where cuttings of plants form roots after being placed in water.(2 votes)
- Is apoptosis a mechanism for cell differentiation?(2 votes)
- Is Human Development and Vertebrate Development the same?(2 votes)
- Can necrosis be a programmed cell death in certain cases?(2 votes)
- Yes. There's been a recent discovery of a process called "Necroptosis", which, as you guessed, is regulated or programmed necrosis.
Because it's a recent discovery and most research has focused on "how necroptosis happens", there's still a lot of research pending on "why necroptosis happens."
A hypothesis (i.e. a plausible idea still not validated by research) is that necroptosis may be used as a mechanism to induce or amplify an inflamatory response.
Source: https://www.nature.com/articles/nature14191(1 vote)
- What does the processes of fertilization not use?(1 vote)
- it uses everything, can you be more specific of exactly what you mean?(2 votes)
- does a female not undergo menstruation throughout the whole pregnancy(1 vote)
- Yes, because the part of menstruation that is "bleeding" is when a female sheds the lining of their uterus. So when the zygote, blastocyst, embryo, or fetus is inside the uterus. It relies on the uterus to develop a placenta and provide food, so it can not shed if that makes sense.(2 votes)
- In people with syndactly, does this apoptosis just not happen properly? What about polydactyly?(1 vote)
- It may have something to do with specific homeobox genes like hox genes and the order in which an organism develops within the uterus, egg, etc.(1 vote)