To get started on the developmental angle, I’ve been going over it a bit now trying to get caught up to where I am in my pregnancy. It’s crazy to think how much goes on between the time of conception and when you actually find out you are pregnant. Even as the fertilized egg, known now as the zygote, is moving from the fallopian tubes to the uterus, it is already undergoing rapid division to increase the cell number. But let’s not get too ahead of ourselves. The major early stages of pregnancy are fertilization, implantation, cleavage, and gastrulation. Fertilization is both sex and reproduction, the combining of the female and male genes and the creation of the new organism. This creation begins when the egg’s cytoplasm is activated to start metabolism. The zygote has a lot to accomplish before organ development can occur namely these things are cleavage and gastrulation. Cleavage is the mitotic division to allow the egg’s cytoplasm to be organized into smaller, nucleated cells known as blastomeres. This stage is already underway as the zygote begins to travel to uterus and it marks the beginning of the organization the zygote needs to undergo to set things up for gastrulation. Gastrulation picks up where the earliest organization began and separates the zygote into three distinct germ layers through a series of complex movements of cells. These layers will then give rise to every organ and tissue in the developing fetus. The nomenclature on what to call this baby-in-the-works from the earliest stage starts with zygote, then blastocyst to an embryo to a fetus- depending on the time that has passed and thus the things that have been accomplished in that time. After gastrulation is complete the fetus can move into organogenesis, or the development of organs.
Let's discuss cleavage in a bit more detail. As the blastomere cell number of the zygote increases, in particular the division from 8 to 16 cells is a milestone since this is when the cells cease to be just a ball of cells and separates into two different groups- an outer group of cells that will eventually be known as the trophoblast and an aptly named second group the Inner Cell Mass (ICM). The trophoblast will give rise to extra-embryonic structures, that is things residing outside of the embryo, this includes the embryonic portion of the placenta known as the chorion (more on that later). While the ICM will give rise to the embryo itself and associated structures. This 16 cell structure is known as the morula and after this two layered milestone the structure evolves more so and becomes known as the blastocyst. Now the outer trophoblast layer secretes fluid into the interior of the structure to create a blastocoel, while the Inner Cell Mass (ICM) is balled up on the outer trophoblast layer (similar to how the entire structure will soon nozzle up and embeds itself in the uterine wall). The blastocyst is present around day 5 after fertilization once the zygote’s cells start to organize themselves (and while the organism is still en route in the fallopian tubes).
Implantation occurs about a week after fertilization and marks the end of the journey to the uterus. In the week to week progress this is technically week four of the pregnancy overall. Once the blastocyst arrives safely at the uterus it will shed the outer layer of the egg, the zona pellucida (this layer has previously prevented the blastocyst from improperly implanted in the fallopian tubes as well as had several functions in allowing for fertilization to occur). Now the new, outer layer, the trophoblast will release various factors to both break down some of the endometrium, the innermost epithelial layer of the uterus, and also binding factors to enable latching on. Ultimately the blastocyst burrows into the layer for the long haul. After implantation the organism is then referred to as an embryo up to the 8th week of development. From the 8th week on the term fetus applies.
|Source: Wikimedia Commons, CC BY-SA 3.0|
Fast Facts-Once the ripened egg is in the fallopian tube it only lives for about a day. There are several hundred million sperm that begin the journey to the egg, but only 1 in 10 find its way to the cervix. (Too easy to make a joke about men not asking for directions?)