Stages of Embryonic Development: A Comprehensive Overview
Stages of Embryonic Development
Fertilization
Fertilization is the fusion of an egg with a spermatozoon. When the male gamete reaches the female gamete, the latter undergoes a series of electrical and chemical reactions that prevent the entry of more than one male cell.
Segmentation
Once the pronucleus of the egg is functioning with the sperm pronucleus, the egg begins to undergo a series of mitotic cell divisions. Each of these cells is called a blastomere, and the resulting mass of cells is called a morula.
Holoblastic Segmentation
This corresponds to oligolecithal or mesolecithal eggs, where the segmentation is complete. If the yolk is low, the resulting blastomeres will be all the same size, but if there is an appreciable amount of yolk, the cells will be of different sizes: the smallest and closest to the animal pole are called micromeres, and the larger ones closer to the vegetal pole are called macromeres.
Partial Segmentation
This is characteristic of telolecithal eggs. The vegetal pole, where the yolk is concentrated, does not undergo segmentation, but the animal pole or germ does.
Blastulation
Once the morula is formed, the blastomeres begin to migrate from the inside out, leaving a hollow core called the blastocoel. This cavity is delimited by a layer of cells called the blastoderm. In oligolecithal and mesolecithal eggs, this process is easily observable, but in telolecithal zygotes, the migration is minimal, so the blastocoel is very small, and the structure formed by the blastodisc cells is appointed.
Gastrulation
During gastrulation, the origins of what will be the adult organs are established. To this end, the blastula, which has a single layer of cells, undergoes a restructuring that initially leads to two layers: the outer ectoderm and the innermost endoderm. Between these two layers, a new cavity appears, the archenteron or primitive gut, which communicates to the outside by an opening called the blastopore. In some animal cases, a third germinal layer, the mesoderm, also appears.
Gastrulation by Invagination or Emboly
This happens when cells in the blastula’s vegetal pole sink toward the blastocoel, producing a double-layered structure in the shape of a U. The outermost layer is the ectoderm, and the innermost is the endoderm.
Gastrulation by Epibolia
Smaller cells (micromeres) divide more rapidly than macromeres, thus causing a growing layer covering the larger ones.
Gastrulation by Ingression
The cells that migrated earlier to form the blastula migrate back into the interior of the embryo.
Organogenesis
The three germinal tissues differentiate subsequently to give rise to the organs.
- Ectoderm: Originates the neural tube, which differentiates into the brain and nerves. It also originates the sensory components of all sense organs, the crystalline lens of the eye, skin, nails, hair, and the nasal, oral, and anal membranes.
- Mesoderm: Originates the dermis, striated muscle, kidneys, gonads, and the heart.
- Endoderm: Gives rise to the inner lining of the digestive system, lungs, thyroid, liver, pancreas, and bladder.
Experimental Embryology
Foundations
Wilhelm His (1831-1904), a German anatomist, believed that development could be explained in mechanical terms and did not need to recapitulate the entire past of the species, as Haeckel proposed. He argued that the egg is differentiated from its earliest stages, meaning that the initial internal chemical differences become visible in the complex differentiation of the adult.
Eduard Pflüger (1827-1910) initially tried to hybridize different species of frogs, but for some reason, he could not decipher the results of the crossing.
Mosaic Theory
Wilhelm Roux (1850-1924) disagreed with Pflüger about the effect of gravity on the segmentation of the embryo. He thought that during cell division, cells lose a piece of genetic material, which would produce a mosaic of cells with different information, leading to different body parts. In 1888, he used frog eggs that had undergone their first mitotic division and destroyed one of the cells with a hot needle, letting the embryo develop further.
Driesch’s Work
In 1891, Hans Driesch (1867-1942) attempted to repeat Roux’s experiment, but this time using sea urchin eggs and separating the cells mechanically rather than killing them. When he divided the embryo in two, even if it had up to 32 blastomeres, he still obtained two smaller larvae.
Spemann’s Work
Following the results of these two experiments, a discussion arose about which of the proposed theories, preformationism or epigenesis, was more accurate. Hans Spemann (1869-1941) worked with salamander embryos, studying the differentiation of the nervous system, whose origin is ectodermal. In 1901, working with frogs, he cauterized the tissue that would normally form the lens of an embryo in the neurula stage, and as a result, no eye or lens developed. In 1924, working with salamander eggs, he tied them in half with a hair before their first division, ensuring that the nucleus was only in one of the cells. As a result, the side without the nucleus did not develop.