Differentiation Of Germ Layers And Embryogenesis

After fertilization of the ovum within the uterine tube, cellular mitosis results in formation of a ball of 12 to 16 cells, the morula. A fluid-filled cavity within this embryonic cell mass forms, resulting in a transformation into a blastocyst that begins to penetrate the uterine mucosa on approximately the sixth day postfertilization. The cells of the blastocyst continue to divide with the cells of the future embryo proper (embryoblast) accumulating at one pole. The cells of the primitive embryoblast differentiate into two layers, the epiblast and the hypoblast. These two cellular layers bridge the central cavity of the blastocyst, thus dividing the blastocyst into the amniotic cavity and the yolk sac (Fig. 1-1).

During the third week of gestation, the two-layered embryoblast transforms into a trilaminar embryo as central epiblast cells invaginate between the epiblast and hypoblast layers. Invagination of central epiblast cells creates a longitudinal groove through the midline of the caudal half of the epiblast, the primitive streak. This invagination of epiblast cells is termed gastrulation (Fig. 1-2A,B). Invaginating epiblast cells differentiate to form the mesodermal germ layer, which spreads out to fill the space between the epiblast and hypoblast. Gastrulation proceeds in a cranial to caudal progression and continues through the fourth week of human gestation. These invaginat-ing epiblast cells displace the hypoblast cells to form the endo-derm. The epiblast cells therefore give rise to all three definitive germ layers: ectoderm, mesoderm, and endoderm (Fig. 1-2C).

Maternal sinusoid

Maternal sinusoid

Amniotic cavity

Epiblast [3| Hypoblast I o r

Extraembryonic coelom

Extraembryonic somatopleuric mesoderm

Exocoelomic membrane

FIGURE 1-1. Drawing of a human blastocyst (12 days gestation) that has penetrated the maternal endometrium. An embryoblast has formed that consists of two cell layers: the epiblast above and the hypoblast below.

Amniotic cavity

Epiblast [3| Hypoblast I o r

Extraembryonic coelom

Extraembryonic somatopleuric mesoderm

Exocoelomic membrane

FIGURE 1-1. Drawing of a human blastocyst (12 days gestation) that has penetrated the maternal endometrium. An embryoblast has formed that consists of two cell layers: the epiblast above and the hypoblast below.

Amniotic cavity

Primitive

Amniotic cavity

Primitive

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