Chapter 2: Embryology

INTRODUCTION

A thorough understanding of the underlying anatomy is fundamental to a logical approach to the techniques used in regional anesthesia. An appreciation for the embryologic development of tissues and structures can significantly add to the understanding of functional anatomy as it relates to regional anesthesia. In this chapter, I emphasize the embryologic development of the brain, spinal cord, peripheral and autonomic nervous systems, as well as the musculoskeletal system as it pertains to regional anesthesia. Many excellent comprehensive texts on embryology are available. For this chapter, I have relied heavily on information from primary texts and refer the reader to them for a complete discussion of all embryologic development. The first two, Langman’s Medical Embryology¹ and Basic Concepts in Embryology,² are valuable for their ease of readability, clarity of figures, and clinical correlations. Human Embryology and Developmental Biology³ is a good contemporary, comprehensive explanation of the molecular genetics of embryologic development. The Developing Human: Clinically Oriented Embryology⁴ is the time-tested standard text of embryology.

GENERAL EMBRYOLOGY

The prenatal period is divided into two major periods: the embryonic period, from fertilization through 2 months, and the fetal period, from the third month through birth.

Embryonic Period

The embryonic period is the time when all tissues are formed and, particularly during the second month, all organs are formed. The fetal period is a time of organ growth.² Embryologic development begins with fertilization, the process by which the male and female gametes unite to give rise to a zygote. Approximately 3 days after fertilization, cells of the compacted embryo divide to form a morula, which is composed of an inner and outer cell mass. The inner cell mass gives rise to the tissues of the embryoblast, and the outer cell mass forms the trophoblast, which later contributes to the placenta. After a period of cell division, during which time the morula enters the uterine cavity, the blastocoele forms, and the embryo is known as a blastocyst (Figure 2–1).

By the eighth day of development, the blastocyst is partially embedded in the endometrium. At this time, the trophoblast differentiates into an inner and an outer layer. Lacunae develop in the outer layer, maternal sinusoids are eroded, and by the end of the second week a primitive uteroplacental circulation begins to develop.

The inner cell mass, or embryoblast, differentiates into two layers, the epiblast and the hypoblast, which together form the bilaminar germ disk.

The most characteristic event occurring during the third week of gestation is gastrulation. This is the process that establishes all three germ layers: endoderm, ectoderm, and mesoderm in the embryo (Figure 2–2). Gastrulation begins with the ...