Human Fetal Development - Part 1

The most enjoyable blogs for me are the ones with no real theme other than a general one, just whatever comes into the mind of the writer.  If I manage to get a thought in my head, it is like a pinball in a Black Rose arcade game.  This past week, I was thinking about embryo development and how complicated that is.  Bovine embryo development, I am familiar with but human, not so much.  Sure, we had four kids but it was kind of like bowling through a blanket; no idea what was happening after the initial roll, so to speak.

DNA is kind of magical stuff. Data is stored in binary format, surprisingly enough. There is roughly one CD worth of data in the human genome; all crammed into a cell nucleus about 6 one-thousandths of a millimeter in diameter. DNA is organized into genes which makes you what you are.  Not just the physical (some of which is also normally binary such as male or female plumbing) but also the unseen stuff like personality, sexuality, intelligence, humour. . . How you respond to your environment is hard wired into you, as we are all both nature and nurture. The more we learn, though, the more nature seems to have the upper hand in so many things.

DNA contains not only the information for your physical and mental self, it also contains the information that turns the various genes on and off when and where appropriate so you are not just one huge blob of identical cells, like yeast or something (Stifle, Edith). The vast majority of this happens during the first few weeks of embryo development (bovine and human).  There is a great deal that can go wrong during that time and estimates are that only 30% of fertilized human eggs result in a live birth (with cattle, it is 70%). The majority of spontaneous abortions or miscarriages have chromosomal abnormalities. The causes of most congenital abnormalities are unknown.

A quick review of ordinary cell division from highschool biology: (Click on pictures to enlarge)

https://lookfordiagnosis.com/mesh_info.php?term=mitosis&lang=1#

The following videos are simple enough even for me to understand and should be watched in the order listed if you are serious about all this.  Number 5 is optional unless you are really serious.

1. Video explaining Cell Structure (7:21) https://youtu.be/URUJD5NEXC8
2. Video explaining DNA to protein (2:42) https://youtu.be/gG7uCskUOrA
3. Video explaining how DNA is replicated (3:27) https://youtu.be/TNKWgcFPHqw
4. Video explaining Mitosis (6:10) https://youtu.be/C6hn3sA0ip0
5. Video explaining DNA structure (5:57) https://youtu.be/o_-6JXLYS-k

When I count days or weeks in this blog post, it is from the time of fertilization of the egg.  Some of the references I ran into counted from the date of the end of the woman’s last period which is confusing as ovulation/fertilization usually occurs about 10-12 days after that so why not start there?

All the cell contents and half the chromosomes are in the egg.  Sperm is not much more than half the chromosomes with a tail.  Relative size is sort of like a rocket ship circling the moon.  Millions of sperm are released but only one enters the membrane of the egg. The reason it takes millions of sperm is that being male, they do not ask directions and most get lost enroute. Once a sperm cell has entered the egg, the membrane becomes impermeable to all other sperm.

Adapted from www.eevatest.com/sites/eevatest.com/files/embry-dev-stage.png

Adapted from dev.biologists.org/content/139/5/829

On Day 1, the pronuclei of the sperm and egg unite to form a nucleus with a full complement of chromosomes at which point the egg becomes a zygote, about the size of a grain of salt, a stage which lasts about 14 days.  One day 1, the single cell becomes two cells.  On Day 2, the two cells divide again becoming 4 cells and by Day 3 it becomes 8 cells.  

By this time the genetic function that was egg-oriented (egg mRNAs decrease) has pretty much wound down and genetic functions reprogrammed to development mode (development mRNAs increase). The overall size of the zygote does not change as more cells are added.  The membrane of the egg becomes the protective membrane (zona pellucida) of the zygote up to Day 6 when the blastocyst hatches and is ready to implant on the uterine wall on Day 7 to 12*.

* For detailed pre-implantation development, see dev.biologists.org/content/139/5/829