Human Embryonic Development: Placental and Somite Formation

Somite Development and Function

Somites are paired blocks of paraxial mesoderm that form sequentially on either side of the neural tube during early embryonic development. They begin forming near Hensen’s node and continue caudally as the embryo elongates.

Somite Differentiation

Each somite differentiates into three main structures:

• Dermatome → forms the dermis of the skin.
• Myotome → forms skeletal muscles of the body and limbs.
• Sclerotome → forms vertebrae and ribs.

Chronology of Development

The number of somite pairs is a reliable morphological indicator of embryonic age. In chick embryos, somite formation begins around 18 hours of incubation. By 24 hours, approximately 4 pairs are visible.

New somite pairs appear at regular intervals (~every 30 minutes), allowing precise staging. Somite count is used in the Hamburger and Hamilton staging system, providing a standardized framework for determining developmental stages.

Axial and Neural Structures

Notochord: A rod-like structure derived from mesoderm beneath the neural tube. It provides axial support, induces neurulation, and contributes to the nucleus pulposus.

Neural tube: Formed by primary neurulation through folding and fusion of neural folds. It develops into the central nervous system, including the brain and spinal cord.

Human Placental Circulation Development

Placental circulation in humans develops through coordinated interactions between the trophoblast, embryoblast, and maternal tissues, establishing the interface for nutrient, gas, and waste exchange.

1. Blastocyst Implantation (Days 6–9)

The trophoblast differentiates into the cytotrophoblast (inner layer) and syncytiotrophoblast (outer layer). The syncytiotrophoblast erodes maternal capillaries, forming lacunae filled with maternal blood, marking the primordial uteroplacental circulation.

2. Chorionic Villi Formation

Primary chorionic villi (Days 11–13): Cytotrophoblast cells penetrate the syncytiotrophoblast, forming finger-like projections.

Secondary and tertiary villi (Weeks 2–3): Secondary villi develop a core of extraembryonic mesoderm. Tertiary villi differentiate into fetal blood vessels, establishing the first fetal capillary network.

3. Establishment of Placental Circulation

Fetal circulation connects via umbilical arteries and veins. Maternal spiral arteries deliver oxygenated blood to the intervillous space. The syncytiotrophoblast acts as the primary barrier for selective exchange.

4. Placental Maturation

From weeks 8–12 onward, the villous tree branches extensively. The placenta matures to meet increasing fetal metabolic demands through a thin trophoblastic barrier and a rich vascular network.

Extraembryonic Membranes

Shelled amniote egg: Includes the shell, chorion, amnion, yolk sac, allantois, and albumen, which collectively protect and nourish the developing embryo.

Trophoblast Differentiation

During the second week, the trophoblast differentiates into:

1. Cytotrophoblast

A single-cell layer of mononucleated cells that retains mitotic activity, providing structural support and supplying cells to the syncytiotrophoblast.

2. Syncytiotrophoblast

An outer multinucleated cytoplasmic mass that invades the endometrium, secretes hCG, and initiates uteroplacental circulation.

Extraembryonic Tissue Development

Extraembryonic cells form structures that support nutrition, gas exchange, and waste removal.

Formation of the Bilaminar Disc

The inner cell mass differentiates into the epiblast (embryo proper) and hypoblast (extraembryonic endoderm). By the third week, the primitive streak forms, initiating gastrulation.

Extraembryonic Membranes

1. Amnion: Forms the amniotic cavity for protection.
2. Yolk Sac: Site for early blood formation.
3. Chorion: Forms the placenta in association with maternal tissues.
4. Allantois: Forms the basis of the umbilical cord.

Sea Urchin Fertilization and Activation

Sperm are attracted to the egg via chemoattractants like resact. Upon fusion, a Ca²⁺ wave triggers egg activation.

Consequences of Egg Activation

• Cortical granule exocytosis: Forms the fertilization envelope to block polyspermy.
• Cytoplasmic alkalinization: Increases pH, stimulating metabolism and protein synthesis.
• Meiosis completion: The egg completes meiosis II and forms the female pronucleus.
• Pronuclear fusion: The male and female pronuclei fuse to form the diploid zygote, initiating the first cleavage.