Key Points
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This chapter presents the concepts and timing of embryonic development.
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The problems of using staging systems to describe a continuous process are discussed.
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The method behind staging of animal development is presented.
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A revision of the timing of early human development is presented.
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Embryonic and obstetric stages of development are presented.
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The embryonic body plan, main embryological stages and their approximate times are presented.
A variety of staging systems for human embryos were devised in the early years of the past century. To enhance this information, studies on other animals were undertaken and externally similar embryos compared. However, devising a staging system is very different from describing a day-by-day alteration of external characteristics.
Chick and Mouse Embryo Staging Series
In recent years, the external characteristics of laboratory animal species have been available and shown within staging schemes. Computing power now permits the manipulation of external images, sectional information and three-dimensional (3D) representations of internal structures in embryos. Databases of developmental information of laboratory animals have been collated in collaborative projects by those involved in experimental embryology. Chick development was described by Hamburger and Hamilton as a series of 46 stages over the 20-day incubation period. Photographs and movies of all stages of chick development are available on an online database, e-Chick Atlas.
For the mouse similar staging systems have been developed by Theiler based on the Streeter staging of human embryos (see later) and continued by Kaufman. Kaufman noted the care with which specimens needed to be prepared and sectioned and the level of experience necessary for the interpretation of serial sections in order to understand the complexity of spatial and temporal development of body systems and internal organs. The use of magnetic resonance imaging (MRI) to obtain images of both external features and sectional anatomy is available online ( emouseatlas.org ) based on Kaufman’s original images. MRI studies of mouse embryos are also providing a further way of seeing digital images through any plane.
Both chick and mouse databases now display the expression of a range of genes within the developing organs and tissues linked to sections and 3D reconstructions of embryos.
Human Stage Series
Staging of human embryos has always started from a different standpoint to those used in animal series; stages are not a serialisation of external features. Human embryos were first placed in a stage series by Mall, founder of the Department of Embryology of the Carnegie Institution of Washington. His work was continued by George Streeter in the 1940s and O’Rahilly and Müller since that time. The monograph Developmental Stages in Human Embryos has been a mainstay of embryonic developmental stages.
Human embryos are assigned to a stage based on the developmental status of many body systems in concert, not on any one parameter alone. Development from fertilisation averages 266 days, or 9.5 months; the embryonic period, extending from fertilisation to about 58 days, is divided into 23 stages. The embryonic period ends when bone marrow is seen replacing cartilage in the humerus, a time defined by Streeter.
The original studies on human staging were based on 600 embryos within the Carnegie Collection obtained from hysterectomies, and specimens were formalin fixed. It is not clear if all of these embryos were normal. The time at which an embryo enters and leaves a particular stage varies because of a number of factors, including placental health, genetic factors and individual growth rate. In vivo imaging techniques of very early development prompted the revision of some of the ages previously assigned to early embryonic stages. O’Rahilly and Müller note that greatest length, the length of an embryo, exclusive of the lower limbs, is a valuable parameter which can be measured antenatally. By adding the number 29 to the greatest length, within the range 3 to 33 mm, an age in days can be broadly estimated. The revision of stage, the age and the specific measurement of greatest length are taking time to percolate through newer embryological studies. It would be of benefit to all if the references for the staging system used were given and terms used were specified. The main features of the stages of human development are given before a consideration of obstetric staging of the first trimester of pregnancy.
Main Stages in the Embryonic Period
Embryonic development is not apparent before stage 6. Stages 1 to 5 are concerned with setting up the cell populations for implantation; most of the cell lines generated are extraembryonic and involved in establishing the placenta and fetal membranes. In stage 6a, the primordial germ cells are sequestered into the extraembryonic mesoblast, and in stage 6b, the primitive streak appears. From this time, intraembryonic cell populations are generated, and the morphological movements of these populations produce a recognisable embryo. Fig. 3.1 shows the formation of cell populations during stages 1 to 11 matched to estimated age. The proliferation of cells at the primitive streak occurs through stages 6b, 7 and 8, when the notochord is first evident, and provides cell populations which pass within the embryo. By stage 9, the neural populations are becoming defined and result in neurulation and the beginning of somite formation, more clearly seen in stage 10, in which embryos typically have 7 to 12 pairs of somites. The formation of the neural plate and the beginnings of its rostral fusion contribute to the morphological movements of head folding, when the cardiac area, which was rostral to the neural epithelium, becomes ventral and forms a boundary of the cranial intestinal portal. Stages 6b to 10 are concerned with embryogenesis when morphogenetic movements affecting the whole embryo move widely dispersed cell populations closer and into their relevant positions for local interactive processes to commence. A stage 11 embryo is at the gateway of organogenesis, and all body systems can be seen to originate from this point.
The Stage 11 Embryo, Body Plan Stage
The criterion for stage 11 is the presence of 13 to 20 pairs of somites ( Fig. 3.2 ); during this stage, the rostral neuropore closes. Fusion occurs from the rhombencephalon rostrally towards the mesencephalon and from the region of the future optic chiasma towards the mesencephalic roof. The optic primordia have begun evagination towards the surface ectoderm. The otic vesicle which will invaginate and give rise to the inner ear (cochlea and semicircular ducts) has not yet formed but the surface epithelium has thickened and begun to invaginate. Around the developing pharynx, the mandibular processes are present, but the maxillary processes have yet to arise. The second pharyngeal arch, the hyoid, is present but not the third. Ventral to the foregut is the heart, this develops very precociously and is seen in stage 9 embryos as tube-like with a united ventricular portion but still separate atrial components. The specialised cells of the coelom, which will give rise to the myocardium, can be identified as can the matrix produced locally between the endocardium and the myocardium. Cardiac contractions commence at the beginning of stage 10 when the heart has a recognisable ventricle, bulbus cordis and arterial trunk, and a cardiac loop can be distinguished; the organ is already asymmetrical. By stage 11, the sinus venosus, atria, left and right ventricles, truncus arteriosus and substantial dorsal aortae can be identified. The heart is connected to a range of endothelial vessels and plexuses which are most mature cranially and still forming caudally. The fluid in the vascular system contains relatively few cells. It ebbs and flows because of the pulsations of the myocardium, which also cause movement of fluid in the intraembryonic coelom. These combined circulations are sufficient to provide nutrient supply to the embryonic tissues.
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