Embryology Notes

Embryology Overview

  • Embryology is the study of the development of embryos from fertilization to birth.

Objectives of the Section

  • Use of Terminology: Proper vocabulary to describe the embryo and its stages.
  • Dating Pregnancy: Techniques used to date the stages of pregnancy.
  • Congenital Malformations: Description and causes of common congenital malformations.
  • Gamete Structure: Understand the structure of male and female gametes.
  • Hormonal Fluctuations: Overview of how hormone levels change in the menstrual cycle and post-fertilization.
  • Fertilization Process: Details of fertilization from sperm capacitation to the formation of the blastocyst.
  • Zygote Development: Development from the zygote to the pre-implantation blastocyst stage.

Pregnancy Dating Techniques

1. Menstrual Age
  • Defined as the duration from the woman's last menstrual period (LMP).
  • Pregnancy is typically divided into three trimesters.
2. Fertilization Age
  • Beginning from conception, it includes:
    • Pre-embryonic period: Early development and cell division.
    • Embryonic period: Organogenesis phase.
    • Fetal period: Further development leading to birth.

Congenital Malformations

Causes
  • Genetic Causes:
    • Monogenic disorders: caused by single defective genes.
    • Chromosomal abnormalities:
    • Numerical (aneuploidy, e.g., Down syndrome).
    • Structural (e.g., Cri du Chat syndrome).
  • Environmental Causes (Teratogens):
    • Infectious (e.g., TORCH: Toxoplasmosis, Rubella, CMV).
    • Chemical (e.g., Thalidomide, Alcohol).
    • Physical (e.g., radiation).
    • Maternal Diseases (e.g., diabetes).
    • Nutritional (e.g., folic acid deficiency).
Specific Birth Defects
  • Down Syndrome: Growth retardation, cognitive impairment, and heart defects.
  • Toxoplasmosis: Eye inflammation, hearing loss, and microcephaly.
  • Rubella: Can cause heart defects and intellectual disability.
  • Folic Acid Deficiency: Linked to spina bifida and anencephaly.

Gamete Structure

Male Gametes (Sperm)
  • Components: Acrosome, middle piece, nucleus, flagellum (tail).
  • Sperm undergo capacitation in the female reproductive tract before fertilization.
Female Gametes (Oocyte)
  • Encapsulation by zona pellucida and corona radiata.
  • Composed of primary oocyte and cortical granules.

Hormonal Cycle

The hormonal regulation for reproduction follows the H-P-G (Hypothalamus-Pituitary-Gonads) Axis:

  • GnRH (Gonadotropin-releasing hormone): Released by the hypothalamus.
  • FSH (Follicle-stimulating hormone) and LH (Luteinizing hormone): Released by the pituitary gland, influencing the ovaries and causing ovulation.
Follicular Phase
  • Hormone levels fluctuate, with FSH stimulating follicle development.
  • LH plays a crucial role in ovulation, facilitating oocyte release from the Graafian follicle.

Fertilization Process

  1. Capacitation: Sperm undergo capacitation to prepare for fertilization.
  2. Acrosome Reaction: Release of enzymes from the sperm to penetrate the zona pellucida.
  3. Zygote Formation: Fusion of male and female pronuclei to form a zygote.

Early Development

  • Cleavage Stage: Continuous cell division occurs after fertilization (Days 0-4).
  • Morula Formation: 16-32 cells, differentiating into inner cell mass (embryoblasts) and outer cell mass (trophoblasts).
  • Blastocyst Formation: Fluid-filled cavity forms, becoming the blastocyst, which prepares for implantation into the uterine wall.