BMS2011 Lecture Notes - Lecture 17: Anterior Ramus Of Spinal Nerve, Deep Artery Of Arm, Nerve Plexus
30 May 2018
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Week 9. Gametes, Limb development and
Limb MSK 1
THE GAMETIC ANIMAL
• Not all animals are gametic – instead they undergo asexual reproduction (agamogenesis)
o form of reproduction for multiple species
o short term benefits where population growth is critical
o no genetic diversity
• Types of asexual reproduction:
o Budding eg. Hydras
o Gemmules (internal buds) eg. sponges
o Fragmentation eg. planarians
o Regeneration eg. echinoderms
o Parthenogenesis
• Gamete carries half the genetic information from each gender, when combined forms full
genetic compliment
• Early theories of heredity states that gametes contained a complete preformed individual called
a homunculus
• Oogamy: human oocyte is almost 100,000 times the size of the sperm
• 1 in 6 couples in Australia suffer infertility (most are women)
• Germ cells need to maintain high plural potency
find more resources at oneclass.com
find more resources at oneclass.com
• A great deal of germ cell development takes place in the embryo
• The primordial germ cell journey:
o Specification: around 6 cells in the mouse embryo express the Blimp1 gene which
suppresses expression/transcription of somatic – earliest marker for germ cells
(embryonic stage 5-6.5, after implantation)
-happens very early
o Commitment: after proliferation, a subset of the Blimp1 positive cells express Stella
(second marker) and commit to the germline
o Migration: PGCs undergo both passive and active migration to gonads. Extensive
proliferation occurs during this migration.
-During fourth week of human gestation, PGCs migrate from the yolk sac and into the
developing embryo
o Colonisation: PGCs colonise the gonads -> sex determination
-Oogonia enter meiosis but arrest at prophase 1 of meiosis at week 12 until puberty
-Gonocytes undergo mitosis-arrest-itosis ad do’t eter eiosis util puerty oe
they eter eiosis they do’t stop. At birth, become followed by intermediate
spermatogonia (puberty) and prespermatogonia.
o Very rare to access material to understand – need to be donated after termination
find more resources at oneclass.com
find more resources at oneclass.com
o Tumours of the male germline:
o CIS: carcinoma stays in embryonic stage (inhibited differentiation)
o Seminoma: germ cell tumours of testis
o Treatment: remove testis = high survival rate
o Spermatogenesis and oogenesis:
o PGCs undergo a long journey to the gonads, once in the gonads they are surrounded by:
Support cells (sertoli-testis/granulosa-ovary)
Hormone-producing cells (Leydig-testis/theca-ovary)
o Sperm and eggs are the only meiotic cells (diploid germ cells to haploid gametes)
o Meiosis genetic variation:
Crossing over: recombination during meiosis
Independent assortment: homologous line up and cross randomly on
metaphase plate
Random fertilisation: ovum has 8 million possible chromosome combinations,
so does sperm
o Meiosis nondisjunction events: -> usually cause of infertility
Most cases of aneuploidy results in termination of developing foetus (early in
gestation)
Most common extra chromosomes among live births are 21, 18 and 13
Kleinfelter = two X one Y -> survival is possible
Turner = Only on X -> do survive and can live with
o Oogenesis:
o Female germ cells
5 weeks
Estimated 700-1300 germ cells
16-20 weeks
Estimated 6X106 cells per ovary
Birth
Estimated 0.5-1X106 cells
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Happens very early: commitment: after proliferation, a subset of the blimp1 positive cells express stella (second marker) and commit to the germline, migration: pgcs undergo both passive and active migration to gonads. During fourth week of human gestation, pgcs migrate from the yolk sac and into the developing embryo: colonisation: pgcs colonise the gonads -> sex determination. Oogonia enter meiosis but arrest at prophase 1 of meiosis at week 12 until puberty. Gonocytes undergo mitosis-arrest-(cid:373)itosis a(cid:374)d do(cid:374)"t e(cid:374)ter (cid:373)eiosis u(cid:374)til pu(cid:271)erty (cid:894)o(cid:374)(cid:272)e they e(cid:374)ter (cid:373)eiosis they do(cid:374)"t stop(cid:895). Hormone-producing cells (leydig-testis/theca-ovary: sperm and eggs are the only meiotic cells (diploid germ cells to haploid gametes, meiosis genetic variation: Independent assortment: homologous line up and cross randomly on metaphase plate. Random fertilisation: ovum has 8 million possible chromosome combinations, so does sperm: meiosis nondisjunction events: -> usually cause of infertility. Most cases of aneuploidy results in termination of developing foetus (early in gestation)
