Development of Multicellular Organisms 2023 Final

Faculty of Life Sciences & Medicine 11/2023 Esther Bell [email protected] Department of Anatomy 4BBY1030 Development of Multicellular Organisms Development of the embryo Germline cells are specified and set apart in the early embryo Functional sperm and eggs only produced in the adult Egg and Sp...

Faculty of Life Sciences & Medicine 11/2023 Esther Bell [email protected] Department of Anatomy 4BBY1030 Development of Multicellular Organisms Development of the embryo Germline cells are specified and set apart in the early embryo Functional sperm and eggs only produced in the adult Egg and Sperm derive from Germ Cells (not somatic cells which make up the embryo) Germ-cell formation in the mouse Primordial Germ Cells first seen in proximal epiblast pregastrulation During gastrulation they migrate to the posterior end of the embryo PGCs then migrate to the gonads PGC tend to express a different set of genes to somatic cells and instead express nanog (maintenance of pluripotency) and genes involved in cell adhesion and migration This migration is also thought to occur in humans during the 4th week of development PGCs enter the hindgut endoderm and then migrate via the dorsal mesentery to reach the genital ridges PGC develop at a distance from the gonads (mesoderm lining of abdominal cavity) Migrate to the gonads Differentiate into eggs or sperm Why do PGCs not remain in initial location? To exclude them from the process of laying down the body plan? A way to select healthiest ie those that survive migration Migration path controlled by their environment – chemical signals Germ cells transplanted to new location in blastula can’t get to destination Germ Cells undergo meiosis – gametes with half the number of chromosomes are produced (egg and sperm) therefore the zygote will have Meiosis comprises two cell divisions: Chromosomes are replicated before the first cell division but not before the second so the number is reduced by half Prophase - replicated homologous chromosomes pair up and undergo recombination – DNA sequences are exchanged between homologs; meiotic recombination generates chromosomes with new combinations alleles Oogenesis Germ cells undergo some mitotic division as they migrate to the ovaries and continue to divide in the ovary. Once meiosis starts primary oocytes arrest in prophase The first meiotic division not completed until after ovulation in the adult Second meiotic division after fertilisation Polar Body Small cell which is a product of meiosis during development of the egg. Not involved in embryonic development Spermatogenesis Germ cells enter the embryonic testis where they will become sperm; arrest at G1 stage of the cell cycle After birth they divide by mitosis, forming a population of stem cells (spermatogonia) In sexually mature animal, spermatogonia stem cells give rise to differentiating spermatocytes which undergo meiosis giving rise to spermatids that mature into fully developed sperm Decline of human oocyte numbers with age Around 7 million germ cells in fetal ovaries Born with approximately 1 million oocytes Puberty - 300,000 Only 400 released during a woman’s lifetime Hypotheses to explain mammalian fetal ovarian germline death a) Apoptosis is actively suppressed by growth factors which act through receptors that are expressed on the germ-cell surface b) Failure of meiotic recombination results in germ-cell death by defect c) In germ-cell cysts that form during development, 'nurse' germ cells (yellow) transport macromolecules and organelles (arrows) into one germ cell, which is destined to become the oocyte (green) Tilly, Nature Reviews, 2001 Fertilisation Fusion of the egg and sperm initiates development It is important that only one sperm nucleus enters the egg or there will be too many chromosomes resulting in abnormal development Fertilisation occurs in the fallopian tube Fertilisation Structure of Human Sperm Acrosome: enzymes to digest protective coat around egg Plasma membrane: proteins that bind to egg and facilitate entry Flagellum: movement Mitochondria: energy Once sperm have been deposited in the mammalian female reproductive tract, they undergo a process known as ‘capacitation’ – this facilitates fertilisation Capacitation – membrane remodelling and removal of certain inhibitory factors One or two eggs in humans, less than 100 of the millions of sperm deposited reach the egg(s) Fertilisation of a mammalian egg 1. Sperm penetrates sticky layer of hyaluronic acid and somatic follicle cells – cumulus cells 2. Sperm binds to zona pellucida 3. Penetrates the zona pellucida – layer of fibrous glycoproteins via acrosomal reaction 4. Plasma membrane of sperm fuses with egg plasma membrane 5. Sperm nucleus enter the egg cytoplasm Calcium Wave at Fertilisation The fertilising sperm triggers the calcium wave Calcium needed for the fusion of the cortical granules with the cell membrane Red highest concentration, blue lowest Prevention of Polyspermy – formation of the fertilisation envelope (in sea urchin) A) B) C) D) 10 seconds 25 seconds 35 seconds Fertilisation envelope complete Cortical Granule Exocytosis Due to high concentration of Ca2+ cortical granule membrane fuses with the egg membrane Contents of cortical granules released into space between cell membrane and vitelline envelope Enzymes from cortical granules harden the vitelline envelope and release sperm bound to it Summary: Fertilisation in mammals Other forms of Reproduction Asexual reproduction Budding in Hydra Wnt expression The ‘bud’ appears due to repeated mitotic division of epidermal intestinal cells Grows connected to parent Develops a mouth and tentacles Once fully developed constricts at base and separates Parthenogenesis Development of an embryo from an unfertilized egg cell Development of Female and Male honey bees Worker bee: Worker bees are female. They have a reproductive system that includes a set of ovaries however it is usually non functional so no eggs are produced Drone bee: Drone bees are male Queen bee: Their reproductive system is fully developed with working ovaries Worker vs Queen bee Phenolic acids suppresses ovary growth and is present in the food fed to worker bees Reproduction in Bees Bees have a haploid-diploid sex determination system Females are produced sexually from fertilised diploid eggs Males arise from unfertilised haploid eggs Honey bees Komodo Dragon In parthenogenesis, the mother's half-set of chromosomes doubles up to generate the full complement. Offspring derives all its genes from the mother, but they are not a duplicate of her genome It is thought in these cases the polar body acts as a ”sperm” to turn eggs into embryos Watts et al, Nature 2006 Discovery of facultative parthenogenesis in a new world crocodile Booth et al, Biology Letters, 2023