Embryogenesis

Gametogenesis precedes embryogenesis. Depending whether it is a male or female the formation of the gametes is called spermatogenesis or oogenesis, respectively. In both processes it all starts with the formation of a diploid gamete giving rise through meiosis to a haploid gamete. The formation of spermatocytes starts with an spermatogonia that via duplication, these cells divide via meiosis I and then subsequently divide again in what is called meiosis II, giving rise to four spermatozoa. An analogous process forms the ovum, with duplication and two meiosis, however the byproducts of oogenesis only results in one usable ovum, with the rest being composed of three polar bodies.
Fertilzation happens when the two haploid gametes fuse together, giving rise to a diploid cell that is totipotent, i.e. it is the precursor cell that will give rise to all cells composing an organism. The spermatozoid perforates the corona radiata as well as the primary and secondary egg membrane, which provokes an inflation of the corona radiata that keeps other spermatozoids to get in the ovum. This unicellular body is called zygote and will undergo cleavage. At this point a grey crescent is formed, which defines the animal and vegetal poles of the embryo. The animal pole will become the posterodorsal region of the organism.
As we know, cleavage in deuterostomes (i.e. the most inclusive group containing Echinodermata, Hominidae and their most recent common ancestor) is processed radially. Depending if we have macrolecithal or microlecithal egg (i.e. relative proportion of yolk) cleavage will be meroblastic or holoblastic, respectively. This means that in holoblastic eggs cleavage will occur all the way along the yolked part of the egg; and the opposite situation verified in meroblastic eggs like those of the chick. A special kind of cleavage is seen in placental mammals which is rotational cleavage with a nearly horizontal second cleavage furrow. In the chick the quantity of yolk is so great that the zygote will be pushed to the animal pole forming a cytoplasmic disk that upon continuing division will form the blastodisk (generally called the blastula, blastocyst in placental mammals). The peculiar case of placental mammals reveals an external layer of cells, the trophoblast, that are of maternal origin. This layer prevents the immunological system to recognize the zygote as an foreign cell, since paternal genes also compose it. The trophoblast is an adaptation in Eutheria that allows viviparity. An inner cell mass will keep dividing and it will attach to the trophoblast that will subsequently be incorporated in the walls of the ovary, a process said to be invasive.
The formation of the three main germ layers happens in the course of gastrulation, which is essentially an invagination of the hollow blastula forming a depression called archenteron and the two lips the blastospore, that as we know forms in later development the mouth in deuterosomes. The gastrulation can happen in two main ways by ingression (e.g. in the chick egg) or by involution (e.g. in the amphibian egg. Gastrulation by involution happens follows some specific steps: (i) formation of a cleft that will correspond to the dorsal lip of the gastrula, at this point the outer layer of cells (the ectoderm) and the inner layer of cells (the endoderm) are formed, (ii) further involution (i.e. migration of cells relative to a fixed point) will elongate the archenteron, (iii) chordamesoderm differentiation and epiboly (movement of the ectoderm to envelop the endodermal cells), (iv) as involution goes on the remaining endoderm visibly on the outer surface will become scarcer to the point it forms a yolk plug, (v)a ventral lip is formed and mesoderm starts to differentiate, again as a deuterostome the amphibian egg shows enterocoelic gastulation with outpocketing of the mesoderm, (vi) the next step, neurulation will start to happen.
In chicks due to the macrolecithal condition of the egg, although the process of gastrulation respects the same general principles it is different in some respects. The anatomy of the blastodisk is the following: there is an outer layer called the epiblast, and a lower layer of cells called the hypoblast; the central region is called area pellucida and the outer region area opaca. It all starts with migration of epiblastic cells from the area pellucid outward to lower levels that will form the hypoblast. Then, cells from the area opaca start converging to the upper regions forming a ridge that will give rise to the primitive streak. The primitive streak starts to form when the epiblast cells start to migrate to downwards, the first cells to arrive will form the endoderm, secondly the mesoderm (with the chordamesoderm and paraxial mesoderm) and the superficial cells the ectoderm.
Neurulation is the formation of the neural tube. The neural tube is developmentally derived from the ectoderm and a result of inductive interactions with the chordamesoderm. The chordamesoderm induces the ectoderm which will form the neural plate by means of palisade ectodermic cells, i.e. they undergo a change in volume that result on a rhomboidal to a stretched cell shape. As induction proceeds the neural plate will start to invaginate, which creates the neural crest and the neural groove. The lateral wall of the neural crest will originate the neurogenic placodes and the medial wall the neural crest cells later in development. Invagination of the neural plate will finally reach a point where the two neural crest meet and detach from the remaining ectoderm forming a central tube, later called the neural tube.
At the point that neurulation happens other concomitant processes occur in the embryo, namely: the formation of extraembryonic membranes, and mesoderm differentiation.
Three types of mesoderm are differentiated: the paraxial mesoderm (dermatome, myotome and sclerotome), lateral plate mesoderm (splanchnic and somatic mesoderm) and finally the intermediate mesoderm (nephric ridge and gonadal ridge). Nephric ridge (excretory ducts and kidneys) and gonadal ridge (gonads); lateral plate mesoderm (body cavities).
Especially crucial in amniotic embryos the formation of extraembryonic membranes is a key evolutionary novelty as an adaptation to the terrestrial environment. There are three main membranes that form through different processes: the allantois, the amnion and the extraembryonic coelom. The allantois is formed as an outpocketing of the splanchnic mesoderm and endoderm with a relative motion of the archenteron. The amnion is formed by the somatic mesoderm and ectoderm that fold and then overlap the embryo. The extraembryonic coelom is formed as a progression of the splanchnic mesoderm and ectoderm as well as the somatic mesoderm and endoderm around the yolk. At this point in cleidoid eggs, what happens is that as the embryo goes down the oviduct, albumen (proteins that constitute the egg white are continuously agglomerated around the embryo, further a secondary eggshell membrane is formed and some air is entrapped, and subsequently the shell gland will produce a rigid porous eggshell.
Neural crest cells will migrate to form organ rudiments, by means of fibronectin, which results in the delamination of the epithelial cells (in the form of ectomesenchyme) of the ectoderm near the notochord. Neural crest cells as they reach more ventral parts of the embryo, N-cam starts to be produced and therefore they start to form agglomerate. Neural crest cells form for example the chromatophores in the dermis.
Nevertheless, we have to note that neural crest and neurogenic placodes are very similar embryological compounds although they differentiate in dissimilar products (e.g. neurogenic placodes originate sensory receptors, and neural crest cells originate motor neurons). However they are alike because they migrate, form sensory organs and are ectoderm derivatives.
Organogenesis of the eye is a striking example of a developmental cascade of inductions. What happens is that the neural tube (itself a derivative of the ectoderm by chordamesoderm interaction), is induced by the chordamesoderm and differentiates in the anterior region in the precursors of the tripartite brain. It is the diencephalon by chordamesoderm induction that forms a protrusion called optic vesicle, and nearly reaches the ectoderm. The neural tube optic visicle induces the ectoderm which thickens and forms the lens placode. The lens placode starts to be invaginated to a point that it detaches from the ectoderm and starts differentiating forming the lens visicle, that will further turn into definitive lens . Concomitantly, by chordamesoderm induces the opic vesicle that starts to invaginate and will form the optic cup. The outer layer of the optic cup is ciliated and will become the pigmented retina, the inner layer of the cup will form the neural retina. This way, the ciliated photosensitive layer of ependymal epithelium will be turned to the point where light comes from. The retina is linked to the brain by the optic stalk.
Embryological head organization invertebrates can be summarized by the Goodrich’s diagram that shows a correspondence to the branchiomeres and several structures of vertebrate anatomy. To each branchiomere there is a correspondent somite, nerve and endodermal derivatives. However, the mandibular branchiomere is a special modified anterior one, followed by the hyoidal and glossopharyingeal branchiomeres and a sequence of four vagal branchiomeres. Each branchiomere is bordered by an outpocket: the pharyngeal pouches, of which the spiracle is the most important one.
The role of Hox genes is essential to understand the developmental mechanisms that occur across all Metazoa, because: they are regulatory genes that by means of expression of transcription factors regulate expression of other genes, they are collinear, they play a role in segmentation of the embryo (into different somites, aortic arches, etc.), they are highly conservative genes.