Embryonic Development

Human development progresses through three phases.⁶ During the first stage of embryonic development, growth occurs through an increase in cell numbers and the elabo­ration of cell products. The second stage of development is one of morphogenesis (development of form), which

PRENATAL DEVELOPMENT

>- The prenatal period, which begins with implantation of the blastocyst, is divided into two periods: the embryonic period and the fetal period.

>- The embryonic period spans the second through the eighth weeks of gestation. This period is marked by the formation of the germ layers, early tissue differentiation, and develop­ment of the major organs and systems of the body.

>- The fetal period extends from the ninth week to birth. Development during the fetal period is largely concerned with rapid growth and differentiation of tissues, organs, and body systems.

includes mass cell movement. During this stage, the move­ment of cells allows them to interact with each other in the formation of tissues and organs. The third stage is the stage of differentiation or maturation of physiologic pro­cesses. Completion of differentiation results in organs that are capable of performing specialized functions.

With the onset of embryonic development, which be­gins during the second week of gestation, the trophoblast continues its rapid proliferation and differentiation, and the embryoblast evolves into a bilaminar embryonic disk. This flattened, circular plate of cells gives rise to all three germ layers of the embryo (i.e., ectoderm, mesoderm, endo-derm). The third week is a period of rapid development, noted for the conversion of the bilaminar embryonic disk into a trilaminar embryonic disk through a process called gastrulation⁶,⁸ (see Fig. 2-3). The ectoderm differentiates into the epidermis and nervous system, and the endoderm gives rise to the epithelial linings of the respiratory pas­sages, digestive tract, and glandular cells of organs such as the liver and pancreas. The mesoderm becomes smooth muscle tissue, connective tissue, blood vessels, blood cells, bone marrow, skeletal tissue, striated muscle tissue, and re­productive and excretory organs.

The notochord, which is the primitive axis about which the axial skeleton forms, is also formed during the third week (see Chapter 49). The neurologic system begins its development during this period. Neurulation, a process that involves formation of the neural plate, neural folds, and their closure, is completed by the fourth week. Dis­turbances during this period can result in brain and spinal defects such as spina bifida. The cardiovascular system is the first functional organ system to develop. The primitive heart, which beats and circulates blood, develops during this period (see Chapter 26).

By the fourth week, the neural tube is formed. The embryo begins to curve and fold into a characteristic C-shaped structure. The limb buds are visible, as are the otic pits (i.e., primordia of the internal ears) and the lens placodes (primordia of the lens of the eyes). The fifth week is notable for the rapid growth of the head secondary to brain growth.

During the sixth week, the upper limbs are formed by fusion of the swellings around the branchial groove. In the seventh week, there is the beginning of the digits, and the intestines enter the umbilical cord (umbilical herniation). By the eighth week, the embryo is human­like in appearance—eyes are open, and eyelids and ear auricles are easily identified.