General Embryology 1 - Lecture 3

General Embryology 1 - Lecture 3

• Developmental anatomy: The science of growth and differentiation of newly formed organisms, from single fertilized egg stage (zygote) to adulthood.
• Embryology: Specific to the prenatal period in mammals (before birth) and pre-hatching period in birds.
• Prenatal period: Stages in mammals
  • Zygote or fertilized ovum period: Begins at fertilization, lasting 2 weeks in humans, all cells are identical.
  • Embryo period: From end of the 2nd week to end of 8th week (or 6th week in some animals like horses and cattle) organs differentiate.
  • Fetus period: Fetus grows in size and weight, continuing until birth.

Stages of Development

• Gametogenesis: Formation of male and female gametes (sperm and ovum).
  • Haploid number of chromosomes (1N).
• Fertilization: Fusion of male and female gametes to form a zygote that has a diploid number of chromosomes (2N).
• Cleavage: Division of zygote into a mass of cells called a blastula.
• Gastrulation: Arrangement of blastula cells into 3 germinal layers (ectoderm, mesoderm, and endoderm) to form a gastrula.
• Organogenesis: Formation of organs. Occurs during the embryonic period.
• Growth and histological differentiation: Increase in length and weight of developing organs, differentiation of tissues occurs during the fetal period.

Spermatogenesis

• Formation of sperms: Taking place within seminiferous tubules of the testes. Process occurs simultaneously from puberty to old age.
• Spermatocytogenesis
  • Multiplication: Spermatogonia divide by mitosis into spermatogonia type A and type B. Spermatogonia type A grows, while type B divides into type A & type B after short rest period.
  • Growth: Spermatogonia type A increases in size to form primary spermatocyte (2n).
  • Maturation: Primary spermatocyte (2n) divides into 2 secondary spermatocytes (1n) through meiosis I. Each secondary spermatocyte divides into 2 spermatids (1n) through meiosis II.
• Spermiogenesis: Non-motile spermatids transform into motile spermatozoa
  • Nucleus shrinks, forms head
  • Golgi forms acrosomal cap at anterior head end
  • Mitochondria form a sheath in the middle piece (energy source)
  • Centriole forms the axial filament (tail)
  • Cytoplasm removed
• Stages: Spermatogonia, primary spermatocyte, secondary spermatocyte, spermatids, spermatozoa. All are haploid after meiosis II.

Oogenesis

• Formation of ova: Occurs in the cortex of the ovary.
• Three stages :
  • Multiplication: Occurs only in prenatal life. Primary germ cells (oogonia) multiply by mitosis and form egg nests.
  • Growth: Oogonia develop into larger primary oocytes which rest until puberty.
  • Maturation: Primary oocyte divides by meiosis I into a secondary oocyte and a first polar body. The secondary oocyte divides by meiosis II to become an ootid (ovum), while the first polar body divides to give 2 polar bodies. Usually completed after fertilization.
  • Stages: Oogonium, primary oocyte, secondary oocyte, ootid/ovum.

Ovarian Follicles

• Developing ovum: Surrounded by a fibrocellular capsule (follicle)
  • Types of follicles: Primordial, primary, secondary, tertiary/Graffian
  • Features:
    • Primordial: Single layer of flattened follicular cells surrounding the germ cell (oocyte)
    • Primary: Oocyte enlarging in size, follicular cells becoming cuboidal
    • Secondary: Follicular cells divide, forming several layers and secreting a homogeneous material (Zona Pellucida).
    • Tertiary/Graffian: A large follicular cavity (antrum) appears, oocyte in eccentric position, surrounded by cumulus oophorus (granulosa cells). Theca interna and externa surrounding the follicle.

Ovulation

• Release of mature ovum from its follicle. Steps:
  • Mature follicle approaches ovary surface, becomes tense/blister-like. Clear avascular spot (stigma) is created.
  • Pressure of follicular fluid ruptures stigma, releases ovum with zona pellucida and corona radiata.
  • Infundibulum of uterine tube catches escaped ovum.
• Types of ovulation: Spontaneous (not linked to coitus), Induced (occurs after coitus).

Corpus Luteum

• Formation: After ovulation, ruptured follicle cavity fills with blood clot (corpus haemorrhagicum). Cells from granulosa and theca interna multiply to create yellowish body (corpus luteum).
• Function: Secretes progesterone (for pregnancy).
  • Degeneration: If no pregnancy, corpus luteum degenerates into corpus albicans. After parturition, corpus luteum degenerates into corpus albicans.

Estrous Cycle

• Definition: Cyclical reproductive process in animals (menstruation in humans).
• Ovarian cycle: Growth, maturation of ovarian follicles, ovulation, corpus luteum formation.
• Uterine cycle: Morphological changes in uterus to prepare for fertilized ovum.

Classification of Animals by Estrous Cycle

• Polyestrous: Repeated estrous cycle throughout the year (sow, cow).
• Seasonal polyestrous: Repeated estrous cycle during specific seasons (cat, sheep, mare).
• Seasonal monoestrous: One estrous cycle per season (bitch).
• Monoestrous: One estrous cycle per year (deer, wild pig).

Phases of Estrous Cycle

• Proestrus: Follicles mature
• Estrus: Ovulation, heat period
• Metestrus: Corpus luteum forms
• Diestrus: Period of rest preceding proestrus