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Developmental Biology (Meiosis
Cell division that halves chromosome…
Developmental Biology
Meiosis
- Cell division that halves chromosome number and generates gametes (haploid (n))
Meiosis I
- Duplication and crossing over
- Interphase I: The DNA replicates and the cell prepares for division
- Prophase I: Synapsis (pairing up of chromosomes) and crossing over occurs - genetic variation
- Metaphase I: Tetrads/homologs line up on the spindle equator
- Anaphase I: The sister chromatids remain attached, homologous (same alleles) chromosomes separate
- Telophase I: Homologous chromosomes separate into the daughter cells
Meiosis II
- Sister chromatids undergo mitosis-like division
- Prophase II: Chromosomes do not duplicate again
- Metaphase II: sister chromatids line up
- Anaphase II: Sister chromatids separate
- Telophase II & Cytokinesis occurs
- Results in 4 haploid daughter cells
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Male VS Female Meiosis
- Similar:
- Meiosis
- Extensive morphological differences
- Do not survive long if fertilisation doesn't occur
- Different:
- Spermatogenesis: equivalent meiotic divisions resulting in four equivalent gametes
- Oogenesis: Asymmetrical, only one egg is formed and two or three polar bodies.
- Different timing of maturation: oogenic meiosis is arrested at one or more stages (prenatal and puberty) whereas spermatogenic meiosis is rapid and uninterrupted from puberty.
Embryonic Development
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Implantation
- Attaches to the uterine wall - Day 6 after ovulation
- The epiblast and hypoblast form a structure classed the Bi-laminar disk
- At day 8: Syncytiotrophoblast grows into the endometrium
- Secretes human chorionic gonadotropin (HCG)
- HCG can be measured in the uterine to detect pregnancy
- At day 10:
- Endometrium encloses the embryo
- Extra embryonic endoderm will form the yolk sac
- Amniotic cavity forms.
- Gastrulation: Rapid cell movement and proliferation whereby the single-layered epiblast gives rise to the three germ layers
- Endoderm: epithelium lining the gut, respiratory tract, bladder, pancreas, liver, thymus and blood vessels
- Mesoderm: Muscle, skeleton, kidneys, gonads, heart, blood, connective tissue
-Ectoderm: Skin, neural tissue, some connective tissue
- During gastrulation, epiblast cells migrate through the primitive streak
- Cells migrate and displace the hypoblast to form definitive endoderm
- Cells also migrate through the primitive streak to form a middle layer, the mesoderm.
- The ectoderm comes from the embryonic epiblast
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Stem Cells
- Undifferentiated cells that can self renew via mitosis and differentiate into one or more specialised cell types
- Types of stem cells:
- Embryonic
- Adult (bone marrow)
- IPS (induced pluripotent stem cells)
- Defined by their potency (ability to differentiate into many or all other cell types
IPS
- Transforming adult differentiated cells back to stem cells by reprogramming them
- a long trial and error effort to discover four key factors (genes)
- Producing patient specific cell populations
- Avoid ethical concerns
Potency
- Totipotent: Can produce an entire organism (zygote)
- PLuripotent: gives rise to cell derivatives of all three germ layers (embryonic stem cell)
- Embryonic stem cells are derived from the inner cell mass
- Multipotent: gives rise to multiple cell types but does not normally generate all three germ layers (haemotopoietic)
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Invitro Fertilisation
- Mature egg through hormone stimulation
- Remove egg by laprascopy
- Collect sperm
- Invitro fertilisation
- Reimplantation
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Fertilisation
- Binding and fusion of sperm and egg
- Sperm penetrates follicle cells and zona pellucida ( an extracellular matrix)
- Binding of perm to egg surface is an acrosomal reaction (enzyme digestion)
- Cortical reaction (granules inactivate other sperm receptors on egg surface)
- Formation of a diploid zygote form union of haploid sperm and egg
Gametogenesis
- Production of gametes, generated by meiosis
Oogenesis
- Long process - eggs remain within female
- Eggs form in embryo but complete development years later
- Oogonia (1 million in each ovary) divide by mitosis in the embryo
- 1st arrest at prophase
- At puberty FHS causes follicles to mature
- Arrest again at Metaphase II
- When sperm penetrates, meiosis II resumes
- Corpus Luteum secretes oestradiol and progesterone
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Spermatogenesis
- Continuous production
- Millions of sperm produced each day
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Organisation of the Testes
- Sertoli cell: A somatic cell that supplies key signals to support and nurse the germ cells
Structure of a Spermatozoan
Body Axes
- The vertebrate has 3 principle axes:
- Anteroposterior axis: head to tail
- Dorsoventral axis: back to belly
- Left-right axis: certain internal organs are not symmetrically distributed
Stages of Development
- Cleavage divisions: Egg gets divided into a number of small cells
- Gastrulation: generation of the three primary germ layers (endoderm, mesoderm, ectoderm)
- Pattern formation: establishment of body plan
- Morphogenesis: change in form
- Cell differentiation: cells become structurally and functionally different from each other
- Growth: increase in body size
Pattern Formation
- The process by which spatial and temporal patterns of cell activities is organised within the embryo so that a well ordered structure develops
Segmentation
- Involves the axis being divided into a repetitive series of similar but independent developmental units
- Somitogenesis in humans:
- Differential expressions of genes along the long axis apparent in the 3rd week of development
- becomes less obvious with development
- segmentation persists in the adult as vertebrae, ribs, spinal nerves, etc
- Can remember positional values of their location in the body; if you put mesoderm on a wing bud then a leg may grow
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