Introduction to Embryology

Introduction to Embryology

• Embryology is the study of embryos and their development.
• Embryo = "the unborn, embryo"
• -logia = study
• Embryology is a branch of animal biology.
• Developmental biology studies organisms from embryo development to adulthood
• Embryology focuses on what happens to a fertilized egg during embryo development and the embryo itself (the unborn).

History of Embryology

• Hippocrates, considered the Father of Medicine, was the first to document embryological studies (of birds). He noted the bird embryo was similar to a human embryo.
• Aristotle (384 -322 BC) studied chicken and other embryos, and proposed that they formed from the union of semen and menstrual blood.
• In the Renaissance, many theories were proposed, but they failed to explain embryonic development.
• Microscopes were invented in 1677.
• In 1839, the first microscopic detection of sperm occurred.
• The Noble Qur'an and the Sunnah of the Prophet Muhammed 1426 years ago listed clear evidence about embryology and genetics.
• Dr. Keith Moore, Professor of Embryology at the University of Toronto, Canada, authored 16 Anatomy and Embryology books.

Prenatal Development

• The total life span of an animal can be divided into phases;
  • Prenatal developmental phase(changes occurring before birth).
• In most mammals, this phase is divided into three periods:
  • 1. Pre-embryonic (germinal) period: begins after fertilization until blastula stage and continues until implantation into uterine tissue.
  • 2. Embryonic period: begins after implantation and continues with the division and growth of the blastula resulting in gastrulation to form three layers (ectoderm, mesoderm and endoderm).
  • 3. Fetal period: begins once cells differentiate, includes early body systems and structures like brain, sex organs, and lungs.

Postnatal Development

• Postnatal developmental phase; changes that occur after birth until adulthood.

Importance of Embryology

• Helps to understand normal body structure and pathological conditions (malformations).
• Necessary for drug manufacturers (fertility and infertility drugs).
• Useful for pesticides manufacturers (stopping the reproductive cycle).
• Important to increase endangered species breeding

Branches of Embryology

• Descriptive embryology: describes all embryonic stages
• Comparative embryology: comparing embryonic stages between different organisms.
• Experimental embryology: studying embryos to understand developmental mechanisms.
• Chemical embryology: studying biological, biophysical, biochemical, molecular and physiological changes in different embryo stages.

Reproduction

• Asexual reproduction involves one parent and no gametes. Offspring are genetically the same as the parent.
• Sexual reproduction usually involves two parents, gametes, and fertilization. Offspring are genetically different from each parent.

Basic Concepts of Embryonic Development

• Gametogenesis: The formation of gametes (sex cells; sperms and ova). Involves Meiotic division for reduction of chromosome number
• Fertilization: The fusion of male and female gametes.
• Cleavage: rapid cell division following fertilization, leading to increase in cell number but no increase in the size of the embryo.
• Blastulation: Cells arranged to form a blastula structure
• Gastrulation: The formation of three germ layers from the blastula.
• Organization: The formation of different structures or organs
• Neurulation: The formation of the nervous system

Spermatogenesis

• The process of sperm formation.
• Stages include:
  • Spermatocytogenesis(formation of spermatids)
  • Spermiogenesis(development of spermatids into spermatozoa)
• The process occurs in seminiferous tubules of the testes.
• Spermatogenesis begins after puberty and continues throughout life.
• Not all areas of the testes form sperm at the same time.
• Spermatocytes produce spermatids and then spermatozoa through mitosis and meiosis stages.

Oogenesis

• The process of ovum formation.
• Occurs in the ovaries (female gonads).
• Begins in the fetus before birth and continues until menopause.
• Produces primary oocytes that don't divide further, they either become secondary oocytes or degenerate.

Stages of Ovarian Development

• Primordial follicles
• Growing follicles
• Mature follicles(Graafian) follicle
• Ruptured follicle
• Corpus luteum

General Structure of an Ovum

• Ovum structures vary across animals.
• Includes a nucleus surrounded by cytoplasm, which is enveloped by cell and vitelline membranes.

Secondary egg membranes

• Corona radiata: follicle cells surrounding the secondary oocyte after ovulation (disperse after fertilization).
• Zona pellucida: clear, thick membrane secreted by follicle cells; acts as jelly coat and eventually turns into fertilization membrane after sperm penetration.

Tertiary egg membranes

• Secreted by oviduct glands; absent in mammals.
  • Jelly coat (around amphibian eggs).
  • Albumin and leathery or hard shells (found in reptiles and birds).

Types of Ova

• Based on the amount of yolk: Alecithal (no yolk), Oligolecithal (small amount of yolk), Mesolecithal (moderate amount of yolk), and Polylecithal (large amount of yolk) and based on distribution of yolk: Isolecithal (uniform distribution)/Homolecithal, and Telolecithal (uneven distribution with yolk at one pole/vegetal pole).
• Centrolecithal: yolk is found centrally with the cytoplasm surrounding it, common in insects.

Functions of Yolk

• Supply energy for embryo development.
• Affect embryo size.
• Influence differentiation of ooplasm.
• Determine cleavage patterns.
• Influence gastrulation movements.
• Determine whether development is direct or indirect.

Structure of Spermatozoa (sperm)

• Head
  • Acrosome.
  • Nucleus.
• Neck
  • Proximal Centriole.
  • Distal centriole.
• Middle piece
  • Mitochondria (mitochondrial spiral)
• Tail (flagellum)
  • Axoneme; microtubules.
  • Plasma membrane.

Explore the fascinating field of embryology, the study of embryos and their development. From historical figures like Hippocrates and Aristotle to modern advancements in microscopy, this quiz covers key concepts in the development of embryos across various species. Test your knowledge about this vital branch of animal biology and its implications in developmental biology.