Michael Perry - Diseases and Injuries to the Head, Face and Neck: A Guide to Diagnosis and Management

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The head and neck is without doubt one of the more complex anatomical regions of the body. A number of highly specialised organs and structures are crammed into a relatively small space, each with their own functional, neural, muscular and vascular elements. Many highly specialised and varied tissues exist, notably neural, much of which is contained within a number of complex bony structures (the calvarium and vertebrae). These differ significantly in their structure from conventional long bones. Consequently, the study of head and neck anatomy, not surprisingly can be quite daunting, and trying to understand some of the disorders that affect this region even more so. Yet the anatomy of this region is (in the most part) predictable and reproducible from generation to generation, and this is because of our preprogrammed embryological development. When this process is broken down into bite-size pieces of knowledge, head and neck anatomy and many disorders of the head and neck become much easier to understand and remember. Therefore a brief overview of this fascinating area of development, which the vast majority of us have all gone through, will be presented. This is just an synopsis and will not include all aspects in comprehensive detail. Embryological development of some of the specific organs and structures is also noted in their relevant chapters hereafter.

The term embryogenesis is often used to describe the ongoing process of cellular multiplication and differentiation which occurs in the embryo during the early stages of development. In our species, this all starts from a single cell zygote and ends with a fully developed human being. The normal period of gestation (pregnancy) is around 9 months, or 38 weeks.

This refers to the initial development of the embryo, from its initial stage of fertilisation, until it becomes implanted in the uterus. This takes around 10 days. Approximately 1 day after fertilisation, the fertilised oocyte (egg) or zygote divides into two cells called blastomeres. Subsequent divisions of these structures then occur around every 1224 h. During these early days the zygote does not increase in size. At first, the cells are undifferentiated and remain clumped together as a small sphere. This is enclosed within a zona pellucidaa thick transparent glycoprotein membrane which prevents premature implantation of the cell mass as it passes along the fallopian tubes. This layer thus prevents the development of ectopic pregnancy. After several more divisions, when there are around 16 cells, the cell mass is referred to as a morula (named after its apparent resemblance to the mulberry fruit). At this stage it is still very small and contain very little cytoplasm.