Developmental Anatomy

• Developmental anatomy is the study of the development of an individual from the formation of the zygote until death.
• Embryology is a branch of developmental anatomy that focuses specifically on the embryo.

Embryology

• Embryology is the study of the embryo and deals with the development of the embryo within the womb of the mother.
• It begins with the formation of the zygote and continues until implantation.
• During early development, the embryo exists in a free-floating state in the uterine fluid and is not yet attached to the uterine wall.

Key Terms

• Embryo: The developing organism before implantation.
• Fetus: The developing organism after implantation.

Ontogeny and Phylogeny

• Ontogeny: Development of an individual after birth (throughout life).
• Phylogeny: Evolutionary or racial development of species.
• Ontogeny precedes phylogeny.

Lifespan of an Individual

• Divided into two major periods:
  • Prenatal period: Life inside the uterus.
  • Postnatal period: Life after birth until death.

Prenatal Life

• Spans from zygote formation to parturition (birth).
• Divided into:
  • Embryonic stage: From zygote formation to just before implantation.
  • Fetal stage: Begins after implantation and continues until birth; involves the growth of fetal membranes and organ development.

Postnatal Life

Life after birth, from parturition until death, is divided into several stages:

a. Neonatal Stage

• Continues for 2–3 days after birth.
• The individual undergoes physiological adjustment to the external environment.

b. Prepubertal Stage

• The sex organs develop, but full maturity is not reached.
• The individual is sexually immature and cannot initiate the sex cycle.

c. Pubertal / Puberty Stage

• Sexual maturity is achieved.
• Sex organs are fully developed structurally and functionally.
• The individual begins to show reproductive behavior and starts the sex cycle.

d. Adult Period

• All body systems are fully developed.
• Individual performs optimal physiological functions.
• Considered the most productive and economically significant stage.

e. Senescence / Senile Stage

• Known as old age.
• There is a decline in structure and function of organs, ultimately leading to death.

Embryology as a 4-Dimensional Study

Embryology studies four essential aspects:

1. Source of development – Where the organ originates.
2. Mode of development – How the organ develops.
3. Location – Where the organ is located and its relation with nearby structures.
4. Time – When the organ starts to develop.

Scope of Embryology

Embryology helps to:

1. Explain life processes and aging.
2. Understand evolution.
3. Locate the topographic position of organs.
4. Explore species variation.
5. Study malignancies (e.g., liver tumors).
6. Explain teratology (abnormal growths, e.g., arterial/urogenital anomalies).
7. Support embryo transfer technology.
8. Understand organogenesis (formation of organs in embryos).

Organogenesis

The process by which organs form in the embryo involves:

Step 1: Establishment of Primordium

• A primordium is a cluster of cells that will give rise to an organ.
• It may develop by:
  • Evagination (outward growth)
  • Invagination (inward folding)
    Example: All glands form via invagination.

Differentiation

• Primordia usually develop from one of the three germ layers:
  • Ectoderm
  • Mesoderm
  • Endoderm
• After formation, cells undergo differentiation to develop into specialized structures.
• This is genetically controlled and induced by enzymes and hormones.

Example:

• Mesoderm differentiates into:
  • Connective Tissue (C.T.)
  • Bone
  • Muscle
  • Blood