Introduction to Embryology

Introduction to Embryology

• Embryology is the study of the development of an embryo from fertilization to the fetus stage.
• The embryonic period extends from conception to the third month of development.
• The fetal period lasts from the third month of development until birth.
• A teratogen is a substance that can cause developmental abnormalities.

Terminology

• A zygote is a fertilized egg, marking the earliest stage of embryonic development.
• A morula is a compact ball of 16 cells formed on Day 3.
• The embryoblast, also known as the inner cell mass (ICM), gives rise to the three primary germ layers: endoderm, ectoderm, and mesoderm.
• A blastocyst forms on Day 4 and consists of a trophoblast surrounding the blastocoele and embryoblast.
• The trophoblast forms the placenta.
• The hypoblast forms on Day 8 from the inner cell mass and gives rise to extra-embryonic tissues like the yolk sac, chorion, amnion, and allantois.
• The epiblast, also developing from the ICM, lies above the hypoblast. Epiblast cells are pluripotent, meaning they can differentiate into any cell type.

Gastrulation and Trilaminar Germ Disc

• Gastrulation marks the embryonic phase where the bilaminar germ disc acquires a third germ layer, transforming into the trilaminar germ disc.
• The trilaminar germ disc consists of the ectoderm, endoderm, and mesoderm.
• The ectoderm (outer layer) forms the skin, nervous system, hair, and sensory organs.
• The mesoderm (middle layer) develops into muscles, bones, connective tissue, and the circulatory system.
• The endoderm (inner layer) gives rise to internal organs such as the lungs, liver, and pancreas.

Gametogenesis

• Gametogenesis is the process producing gametes (sperm and egg cells) for sexual reproduction.
• Spermatogenesis involves the production of sperm cells in the testes.
• Oogenesis signifies the production of egg cells in the ovaries.

Oogenesis

• Oogenesis is the process of forming an ovum (egg) in the ovaries.
• It involves a series of stages, including the development of oogonia, primary follicles, growing follicles, primary oocytes, secondary follicles, tertiary follicles, and finally, ovulation.
• At birth, females possess a lifetime supply of developing oocytes, each in Prophase I.
• A secondary oocyte is released each month from puberty until menopause.
• The primary oocyte completes the first meiotic division during puberty.
• The secondary oocyte completes the second meiotic division after fertilization by sperm in the fallopian tube.

Ovulation

• Ovulation is the process of releasing the secondary oocyte from the ovary when the mature Graafian follicle ruptures.
• Usually, only one secondary oocyte is released during an ovarian cycle, which starts at puberty and ends in menopause.
• Ovulation typically occurs around 14 days before the expected menstrual period.

Spermatogenesis

• Spermatogenesis is the process producing sperm cells in the testes.
• The stages involve spermatocytogenesis and spermiogenesis.
• Spermatocytogenesis includes the germ cell division and differentiation into spermatogonia, primary spermatocytes, and secondary spermatocytes.
• Spermiogenesis involves the morphological transformation of spermatids into mature spermatozoa.

Structure of a Mature Spermatozoon

• The sperm consists of the head, neck, middle piece, and tail.
• The head contains the nucleus with haploid chromosomes and the acrosome, which contains digestive enzymes for fertilization.
• The centrioles are found in the middle piece along with mitochondria, which provide energy for locomotion.
• The tail enables the sperm's motility.

This quiz covers the fundamentals of embryology, including the stages of development from fertilization to birth. Learn key terminology such as zygote, morula, and blastocyst, as well as the roles of various structures in embryonic development. Test your understanding of these critical concepts in the study of human development.