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Summary Animal Development APES1001 MBBCH I
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Malaaika Siddique Animal Development 2021Animal Development
Overview
- Life begins with zygote (single cell)
- A human embryo at 6 – 8 weeks after conception shows development of distinctive
features
- There are many similar common stages in embryonic development of pigs, frog,
chickens, fish, and nematode
- Fertilisation → zygote → cleavage → formation of blastula → gastrula →
organogenesis
Preformation Hypothesis
- Theory of development from zygote to fully formed human = preformation
- Preformation = egg or sperm contains a miniature infant (homunculus) which becomes
larger during development
- This hypothesis was discredited through studying organisms
• Specific species (model organisms) chosen for the ease with which they can be
studied in the laboratory
• Model organisms → Species that are representative of a larger group and easily
studied → Sea urchins, frogs, chicks, roundworm
• Help understanding of development of embryos because they have developed
along the same evolutionary life and exhibit the same features and development
in the early stages of life
- Humans are NOT model organisms → we are interested in our own development
Development Functions
- Development is determined by:
• Zygotes’ genome
• Cytoplasmic determinants = molecules in the egg
- Development assures 2 main functions: self-organization + self-replication
• Growth = Generation of cell number
• Cell Differentiation = Generation of cell
diversity (specialization of cells in structure
and function)
• Morphogenesis = The organization of
differentiated cells into tissues and organs by
which an animal takes shape
,Malaaika Siddique Animal Development 2021
Developmental Events in Animals
Term Definition
Gametogenesis The formation of gametes
Fertilization Fusion of sperm and egg
Cleavage Cell-divisions that divide the embryo into many cells, resulting in a
hollow ball of cells called the blastula
Gastrulation Blastula folds in on itself, forming multi-layered embryo, the gastrula
Organogenesis Local change in cell shape, and larger changes in cell location results
in rudimentary organs
Morphogenesis Organization of differentiated cells into tissues and organs
, Malaaika Siddique Animal Development 2021
GAMETOGENESIS
- The formation of gametes
- Not considered part of embryonic development
- Gametes = equal haploid no. of chromosomes provided to offspring
- Female gamete cells = egg in ovaries
- Male gamete cells = sperm in testes
Gametogenesis in Humans
Oogenesis
- Oogenesis → Formation of eggs
- Production of primary oocytes is complete before birth
- Only 1 product of meiosis develops into an egg, the rest form polar bodies which
degenerate
- There are long interruptions in oogenesis
- Production of mature gametes ceases at 50
1) Oogonia are produced by primordial germ cells.
2) Oogonia divide via mitosis before birth → forms primary oocyte → these are arrested in
prophase of meiosis I.
Primary oocytes reside in small follicles, cavities lined with protective cells.
1-2 million primary oocytes produced in ovaries
500 fully mature during puberty + menopause
3) During puberty, the primary oocyte divides via meiosis to form secondary oocyte which
get arrested at metaphase II.
4) Beginning of puberty, FSH (follicle-stimulating hormone) stimulates a small no. of
follicles to resume growth and development.
5) Only 1 follicle matures each month, with its primary oocyte completing meiosis I.
6) The second meiotic division occurs, producing the secondary oocyte, but is arrested in
meiosis II. This secondary oocyte is released at ovulation when its follicle breaks open.
7) Only if a sperm penetrates the oocyte, does meiosis II resume.
8) Each of the meiotic divisions undergo unequal cytokinesis, leading to formation of polar
bodies which degenerate.
9) The functional product of complete oogenesis is a single mature egg containing a sperm
head (after fertilisation).
10) The ruptured follicle left behind develops into the corpus luteum. Corpus luteum
secretes progesterone + oestradiol. Helps maintain lining of uterus during pregnancy.
11) If egg is not fertilized, corpus luteum degenerates and a new follicle matures
during the next cycle
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