Embryonic Development

Human embryology is the branch of biology, which deals with the study of the development of embryo during the first eight weeks after fertilisation. The normal period of gestation (pregnancy) is nine months or 38 weeks.

Hematopoiesis originates in the early yolk sac, but as embryogenesis proceeds, this function is taken over by the fetal liver and spleen, and then later by the bone marrow, where it continues throughout life.

Transdifferentiation is the process where one specialized cell type transforms directly into another specialized cell type, without first reverting to a pluripotent stem cell state. This is distinct from dedifferentiation (reversion to stem-like state) and redifferentiation (specialization after dedifferentiation).

Transdifferentiation is the process where a differentiated cell transforms into another differentiated cell type without passing through a pluripotent state. Dedifferentiation is the reverse process where cells become less specialized. Redifferentiation refers to cells regaining their specialized function after dedifferentiation.

The egg-on-side appearance (also called egg-on-its-side sign) is a classic radiological finding in uncorrected transposition of the great arteries (TGA). It results from the abnormal position of the aorta and pulmonary artery on chest X-ray, creating a silhouette that resembles an egg lying on its side rather than standing upright. This sign is specific to TGA and helps differentiate it from other congenital heart diseases.

The nervous system, including nerve cells (neurons), originates from the ectoderm germ layer during embryonic development. The neural plate forms the neural tube, which gives rise to the central nervous system.

The notochord forms early in embryonic development and defines the chordate body plan. It develops before most major organs and provides structural support and signaling functions for the developing nervous system.

The morula stage of embryonic development is enclosed by the zona pellucida, a glycoprotein layer surrounding the oocyte and early embryo. The corona radiata is an outer layer of follicular cells that disperses during fertilization and is not present around the morula stage.

The trophoblast layer of the blastocyst gives rise to extra-embryonic membranes including the chorion, while the inner cell mass forms the embryo proper. The inner cell mass is restricted to embryonic structures, formative cells is not standard terminology, and follicle cells are ovarian structures unrelated to embryonic membranes.

The pseudocoelom is a fluid-filled body cavity lying inside the external body wall of the nematode that bathes the internal organs, including the alimentary system and the reproductive system. It develops from the blastocoel.

A basal body is a protein structure found at the base of a eukaryotic undulipodium (cilium or flagellum).

An embryo takes 3 weeks to develop into a chick.

The stage of the embryo in which all the body parts are identifiable is called foetus.

Transformation of larva into an adult is known as metamorphosis.

In the developing embryos of mammals, the cardiogenic region lies cranial to the prochordal plate and its descendant, the oropharyngeal membrane. The heart tubes form on the endoderm side of intraembryonic coelom in the cardiogenic region. Following the head and lateral body folding, heart and its portion of intraembryonic coelom called pericardial cavity lie in the chest.