Growth of the conceptus, development of its shape

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Introduction

The formation of a human embryo is a dynamic process that begins with fertilization and continues through a series of organized and coordinated events that shape the developing organism. This process is often divided into growth of the conceptus and the development of form (morphogenesis).

Growth of the Conceptus

Early Cell Divisions and Formation of the Blastocyst

• Growth begins immediately after fertilization. The zygote undergoes cleavage, producing progressively smaller blastomeres as it travels toward the uterus. Without increasing in overall size, these divisions lead to formation of the morula (12-32 cells) and later the blastocyst, whose two major parts—the trophoblast and embryoblast—set the foundation for extraembryonic tissues and the embryo itself.

Implantation and Early Extraembryonic Development

• Once in the uterus, the blastocyst implants into the endometrium. The trophoblast differentiates into two layers that support implantation and form the early placenta. Meanwhile, the embryoblast gives rise to the bilaminar disc and early extraembryonic membranes such as the amniotic cavity, yolk sac, and chorion.

• These structures grow rapidly and are essential for protection, nutrition, and early blood formation.

Transition to the Embryonic Period

• By the third week, the conceptus begins to grow more complex rather than simply larger. Through gastrulation, the bilaminar disc becomes trilaminar, and three germ layers—ectoderm, mesoderm, and endoderm—are established. This initiates major differentiation events that will build all organs and tissues of the body.

Development of Embryonic Shape (Morphogenesis)

Gastrulation: Establishing the Body Plan

• The formation of the primitive streak marks the onset of gastrulation. Cells migrating through this structure generate the primary germ layers. The resulting trilaminar disc now has a defined cranial–caudal, dorsal–ventral, and left–right axis, forming the framework upon which body form will develop.

Neurulation (weeks 3-4) and Segmentation (weeks 3-5)

• The ectoderm thickens into the neural plate, which folds to form the neural tube, the future central nervous system.
• Adjacent mesoderm segments into somites, establishing the repetitive body pattern from which vertebrae, ribs, and major muscle groups will emerge. This segmentation is a key driver of the embryo’s early structural organization.

Body Folding: Transforming a Flat Disc into a 3D Form

• One of the most crucial events is embryonic folding, occurring around the fourth week.

1. Cephalocaudal folding

• Rapid growth of the neural tube causes the cranial and caudal ends of the embryo to bend ventrally.
• Consequences include:
  • Formation of foregut and hindgut
  • Movement of the developing heart into its ventral position
  • Establishment of the head and tail regions

2. Lateral folding

• As the embryo grows, the sides of the disc curl inward.
• This produces:
  • A cylindrical body shape
  • Formation of the primitive gut tube from the endoderm
  • Closure of the ventral body wall
  • Incorporation of part of the yolk sac into the embryo, with the remaining connection forming the umbilical cord
• These coordinated foldings convert the initially flat conceptus into a fully three-dimensional structure with a recognizable body plan.

The Fetal Period: Growth Over Form

• By the ninth week, the fundamental body shape is established. The fetal period is marked by rapid increases in size, weight, and tissue specialization. Organs grow and mature, limbs elongate, body proportions change, and fat begins to accumulate. At this stage, growth dominates over shaping.

References and Sources

• Langman’s Medical Embryology, 14th Edition — ISBN-13: 978-1-4963-8390-7