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Print Posted on 11/20/2017 in Pregnancy

Myths and Truth about Embryos: Do They Really Have Tails?

Myths and Truth about Embryos: Do They Really Have Tails?

Perplexity and mysticism about embryos and their development inside uterus existed from the 19th century. In the middle of the 19th century it was postulated that the embryo went through all stages of the evolution of the species, particularly, it was hotly debated that fertilized oocyte grows into the freshwater hydroid, turns into the gilled fish, after that develops into a tailed animal and thereafter embryo has human appearance. And all these irrelevant theories derived from the invalid work of German biologist Ernst Haeckel, who neglected the conscientiousness of scientific investigations and supposed that fetus has gill slits and a tail, therefore, conclusively, being in a womb, fetus goes through all stages of the evolution of the species. This inconclusive hypothesis was the result of profound ignorance, because being based on inconclusive investigations and several ‘valid’ concepts without further verification was declared and documented by Ernst Haeckel and even had been taught at the European medical schools.

Is it truth that embryos have gill slits in the earliest stages of development?

First scandalous postulation, explicated by Ernst Haeckel, was about ‘embryo’s gill slits’ that gradually is absorbed and replaced by the lungs. Biologist thought that embryo develops ‘gills’ and ‘gill slits’ through which it is possible to ‘breathe the oxygen’. The problem with ‘embryo’s gill slits’ was that Ernst Haeckel only described the outward appearance of the embryos and ignorantly left all the details of their internal structure aside.

The scientist mistook the gills for the folding tissue which preceded the head and the neck. The folds became traditionally known as ‘gill arches’, although it would be correct to refer to them as ‘visceral arches’. The embryo has no gill openings, primarily, because there is umbilical cord: a unique connection between the fetus and the placenta with multifunctional peculiarities, and one of its functions is to provide the growing embryo (fetus) with oxygen. The following concept can be made: if the oxygen is delivered through the umbilical cord, how it is possible to develop ‘gill slits’? Furthermore, if the embryo really developed ‘gills’ and ‘gill slits’, then there would be blood vessels all around it, as if it were going to absorb oxygen from water as a gill does.

Myths and truth about embryos: do they really have tails? What is understood under the term ‘embryo tail’? Why ‘embryo tail’ is so essential? What happens to ‘embryo tail’? How does ‘the embryo tail’ disappear?

The ‘embryo tail’ develops during the third gestational week of embryo intrauterine life, acquires its maximal length in the fifth gestational week, and disappears during the seventh gestational week. ‘Embryo tail’ is a temporary structure of vital importance, because it ‘participates’ into neural tube formation, and is, in fact, a part of spinal cord.

There are two types of neurulation mechanisms: primary and secondary. The brain and spinal cord down to the lumbosacral region appear to differentiate from the neuroectoderm of the neural tube during the process of primary neurulation. The caudal (tail) part of the spinal cord derives from mesenchymal cells in the tail bud during secondary neurulation [Sapunar et al., 2001].

As it has been already mentioned, the ‘embryo tail’ develops during the third gestational week, acquires its maximal length in the fifth week, and disappears during the seventh week. Consequently, the existence of embryo tail is not durable: starting from 4th gestational week and up to 6th gestational week, embryo has ‘a tail’ that measures about one–sixth of the size of the embryo itself, with 10–12 vertebrae, particularly, ‘embryo tail’ consists of cranial and caudal (tail) parts of the neural tube. Initially, embryo tail is composed of tail bud mesenchyme, which differentiates into caudal somites, secondary neural tube, notochord and tail gut. Later, these structures gradually regress by cell death thus leading to the disappearance of the ‘tail’ [Sapunar et al., 2001].

In the 4 weeks old embryo, two types of partly overlapping neurulation processes (primary and secondary) neurulation is found in the areas cranial to the closing caudal neuropore and secondary neurulation in the more caudal areas. The region caudal to the still open caudal neuropore forms a temporary embryo tail. In the region close to the caudal neuropore, the cranial neural tube is still in the stage of neural groove. The wall of the neural groove consists of neuroepithelial cells and a thin layer of neuroblasts. The spinal ganglia, ventral and dorsal roots are not yet developed. The notochord is separated from the neural groove by the loose mesenchyme. At the tip of the tail the caudal neural tube is formed of a single lumen surrounded by the neuroepithelium, with the nuclei aligned in several rows. The neural tube is situated in the mesenchyme of the tail bud. The surface of the tail is covered by a thin epithelium. The process of degeneration in the tail region is already present. Numerous dying cells can be observed in the neuroepithelium of the caudal neural tube, as well as in the surrounding mesenchyme [Sapunar et al., 2001].

During the 5th gestational week embryo tail reaches its maximum length. This developmental period is characterized by further differentiation and thickening of the primary neural tube. The lumen of the secondary neural tube is irregular. Sclerotomal cells surround the notochord, thus forming caudal vertebrae [Sapunar et al., 2001].

In the 6–week embryos, signs of the regression of the tail structures are obvious; the secondary (caudal) neural tube consists of an irregular lumen surrounded by a layer of neuroepithelial cells. Connection between the cranial and caudal neural tube forms a transitional zone. The mesenchyme of the tail bud has differentiated into the sclerotomal cells of caudal vertebrae. The tail is short and covered with epithelium. More cranial parts of the neural tube derived during primary neurulation consist of three distinct layers: the neuroepithelial, mantle and marginal layers, respectively. Furthermore, 6th gestational week is distinguished by more dying cells in the tail part. Dying cells characterized by heterochromatic nuclei can be found in the neuroepithelial and mantle layers of this region. In the tail part of the neural tube dying cells are more numerous than in more cranial regions and are distributed throughout the wall of neural tube, as well as in the surrounding mesenchyme [Sapunar et al., 2001].

By 8th gestational week, as the embryo develops into a fetus, the ‘embryo tail’ disappears through absorption by the growing body: the apoptotic type of cell death is observed only in the cranial neural tube that forms during primary neurulation. The other type of cell death characterized by necrotic morphology was observed in the tail mesenchyme and in the caudal neural tube that forms during secondary neurulation. This morphological diversity suggests that besides differences in origin and fate there are different mechanisms of developmental cell death between two parts of the neural tube [Sapunar et al., 2001].

Visible fluffy covering of preemie neonates’ bodies: is it mystics or truth? 

It is well known that being in the womb, starting at the end 12th gestational week, fetus (unborn baby) produces a soft, fluffy ivory–coloured covering of special long fetal hair known as lanugo hair, often still visible in preemies or, in other words, premature born babies. Lanugo hair, which covers whole fetus’s body, including eyelashes, eyebrows, and hair on fetus’s head, except for fetus’s palms, the sole of his/her feet, his/her lips, his/her nails and sides of his/her fingers and toes, — is an essential phenomenon. If your thoughts are concentrated on the question: ‘Why it covers fetus’s body and why it is excessively essential?’, — the inclusive argument would have an emotional simplicity — lanugo hair insulates and regulates fetus’s body temperature.

Preemie neonates may often have lanugo on their bodies, especially if they born too soon the term. By the 22nd gestational week, head and body hair called lanugo thickly covers the fetus. Lanugo hair, appears to be mostly visible starting from the 22nd gestational week, ultimately would have disappeared by the 40th gestational week. If baby is born prematurely, lanugo hair performs a vital protective function: the thermal control system of the preemie may not be ready for the cold air and lanugo hair prevent the neonate from getting cold.

Although lanugo hair is normally shed before birth, — most babies lose their lanugo in the eighth or ninth month of pregnancy, though some can be born with remnants of the fine coating still on their body. Lanugo hair, even in the most extreme cases, will be shed and should not be medically treated, because it will invariably be shed by three to four months after baby’s birth. It is replaced by hair covering the same surfaces called vellus hair, but this hair is finer and more difficult to notice visually.

Despite embryonic development is rather difficult to study, technological advancements and researchers’ accuracy in scientific investigations revealed the essence of ‘what exactly happens inside the uterus: how does embryo grows there’ and all these myths were disproved. The only thing that remains valid –– is fluffiness of the preemie neonates.

REFERENCES:

[1] Sapunar D., Vilović K., England M., Saraga–Babić M. Morphological diversity of dying cells during regression of the human tail. Annals of Anatomy, May 2001; 183(3): 217–222.

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