gastrulation

Recently I read about the biologist Lewis Wolpert. He apparently attained some notoriety years ago when he declared: 'It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life.' Wow, I'd never heard of gastrulation, in spite of being I hope a little more educated on matters biological than most punters, which probably isn't much. So here I am writing and learning about gastrulation.
The first five letters of this word are familiar. Think of gastric ulcers, gastric juices, gastro-enteritis and you think of the stomach, right? Well, maybe. Later we'll have a quick look at etymology.
Gastrulation is a term for a very early phase of embryonic development, in humans as well as in fish, birds, frogs and sea urchins. And a few other creatures. Not surprisingly, gastrulation is complex, and the process varies for different species. I don't feel too confident about being able to explain it, but I'll try.
I'll focus on human gastrulation, because I'm only human. An egg becomes fertilized by a sperm cell in the fallopian tube, and the resultant combo, or diploid, is called a zygote [I'm going back a little way, to help put gastrulation in its context in the development of the embryo]. By the way, the two cells that unite in sexual reproduction to form a zygote are called gametes, and apparently, where the two sexes produce morphologically distinct gametes [as in humans] the female is defined as the one who produces the larger gamete, the egg or ovum. Presumably this is the same for all mammals, and maybe for all sexually reproducing species, and perhaps if we have a species where both sets of gametes are the same size, you wouldn't be able to tell the male from the female of that species? Just speculating.
Anyway, gametes are haploid cells, that is they have half the number of chromosomes as a diploid cell. Once the two gametes have fused to form a diploid zygote [fertilization or conception], the zygote 'cleaves' to form a ball of cells [all diploid cells] called a morula. At this point the cells remain undifferentiated. They're called blastomeres. All of these stages of development, and each of the new words here mentioned - morula, blastomere, cleavage [in the boringly non-Partonesque sense] - could have a separate essay, or a book, dedicated to them, but I'll slide over them for now or I'll never see my way clear to gastrulation. Meanwhile here's a picture to help us with the very early phases of human embryo development.

The morula [about 16 cells] soon transforms into a blastula [day 5 above], consisting of a single layer of parietal [outer] cells and a blastocoelic cavity. Implantation, or attachment to the uterine wall, takes place on the seventh day [probably prophesised in the Bible, given a bit of stretching]. More accurately there's a period known as the human implantation window, which is 6 to 12 days.
The formation of the blastula from the morula, called blastulation, begins the process of cell differentiation. Somehow the undifferentiated cells become two different types of cell, the trophoblasts which form the wall of the blastocyst [as it's called in mammals], and an inner mass of cells called embryoblasts, from which, by the way, embryonic stem cells are taken. Here's an illustration of a blastocyst a little before implantation.

The trophoblasts combine with the maternal endometrium cells to form the placenta in eutherian [placental] mammals. The human blastocyst is made up of 70 to 100 cells, according to Wikipedia, whereas at this site, the pre-implantation human embryo [which is only a blastocyst] contains 200 to 250 cells. What is a dilettante to think? That second site, to be precise, is the 'web-book' Introduction to Developmental Biology, by Frank Lee. 

Anyway, now we come to gastrulation proper. In brief, it's the phase in which the blastocyst generates three germ layers, the ectoderm, mesoderm and endoderm, but this explains nothing. Germ layers are essentially germinal of tissue layers, producing all the tissues and organs of the body. 

What happens first is that the embryoblasts differentiate into hypoblasts and epiblasts. The culumnar epiblast cells give rise, finally, to the three germ layers leading to organs and tissues, while the cuboidal hypoblasts are the originators of  the cells of the yolk sac and the extraembryonic endoderm, that's to say the stuff inside the sac, I think. Actually the shape of the cells varies with species. Within the blastocyst, the epiblasts form adjacent to the trophoblast, while the hypoblasts are closer to the blastocoel. 

The development from blastula to gastrula involves amongt other things the formation of a structure called the primitive streak, which establishes bilateral symmetry, determines the site of gastrulation and initiates the formation of germ layers. What happens is that mesenchymal cells, or embryonic connective tissue cells, line up along the long axis of an elongating blastocyst, creating a prospective midline. This is described as the first symmetry-breaking event in the development of the embryo [I've also been reading about symmetry-breaking in cosmology, so that's interesting]. Cells move along and around the primitive streak in a constant flow. There are two counter-rotating flows that meet at the posterior of the streak. 

My primitive research is still sufficient for me to recognize that the formation of the primitive streak is a complex matter indeed, so I won't get into it here, I'll only get lost in the labyrinth. Interestingly, some bioethicists argue that the formation of the primitive streak [at about the fourteenth day in humans] signifies the creation of a unique, truly human being, presumably through some abstruse argument based on differentiated cell development. On the face of it, this seems another example of the crudity of discontinuous thinking, bemoaned by the likes of Richard Dawkins.

Here's one of the best simple summaries of gastrulation I've found, from UNSW embryology:

Gastrulation means "gut forming" and converts the inner cell mass which then formed the bilaminar embryo (epiblast, hypoblast) into the trilaminar embryo (ectoderm, mesoderm, endoderm).

The process involves the migration of cells from the epiblast layer through the primitive streak to form first the endoderm layer and then a second intermediate layer the mesoderm layer. Once all cells have left the epiblast layer it now becomes the ectoderm layer.

These three germ cell layers (ectoderm, mesoderm, endoderm) will form in a layer specific manner all the future tissues of the developing embryo.

The term itself goes back to the 1870s.  

This is as far as I'm going to go with gastrulation. I've certainly learnt a lot about embrology that I didn't know before, and I'll definitely make a return some time with more embryological terms.