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What is embryology?

The development of a new human being


Why do we study embryology?

1) Foundation of human anatomy - understand anatomy
2) Developmental basis of disease - understand malformations and pathological conditions
3) To understand the mechanism of body formation
4) Development of Reproductive Technologies
5) Many adult illness have developmental roots


What is the percentage of babies born with a significant developmental defect detectable at birth?



What does embryology teach us about anatomy?

1) Developmental biology explains how tissues, organs and body structures formed from simple structures
- e.g. neural tube
2) Formed as a consequence of regulated gene expression


What does embryology teach us about understanding malformations and pathological conditions?

Developmental biology tells us how they happen (allows for risk factors for abnormal develop to be established)
- Congenital anomalies
- Maternal-fetal relationships


What is congenital anomalies due to?

Due to failures in growth, development or differentiation


What is maternal-fetal relationship?

Prenatal diagnosis to determine diseases and mutations in foetus


What is body formation regulated by?

By gene expression, protein expression, etc.
- e.g. Fibroblast Growth Factos (FGF)


What is the neural tube?

- embryo's precursor to the central nervous system, which comprises the brain and spinal cord.
- closes to closed neural tube


If the neural tube doesn't close to form the closed neural tube what happens?

Forms spina bifida


What does FGF do?

Can induce an additional limb


What are examples of reproductive technologies?

In vitro fertilisation and stem cell technologies becoming increasingly important


What is cancer?

- A reversion to embryonic behaviour
- group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body


What is preformationism?

Old belief (1695): a tiny person is inside a sperm


What is the central enigma of development?

1) Cells increase in number
2) Cells increase in type


What happens when cells increase in number?

- One cell turns into billions of cell
- Very important, especially in relation to cancer


What happens when cells increase in type?

Go from one kind of cell (fertilised egg) to hundreds of cell types


What happens to cell once they increase in number and type?

- Cells become arranged in space with respect to each other (your arm and leg are made of the same kinds of cells, deployed differently)


What decides whether the liver cell becomes a liver or whether skin becomes skin? Why?

- DNA is essential to development but the DNA in all cells is identical. Therefore cell differences cannot initially arise from the DNA


What did Dolly the sheep confirm?

Dolly the sheep confirmed that even a differentiated cell still
has all the DNA intact and capable of functioning


What happens in the early stages of development?

1) The fertilised egg divides to form a ball (the morula)
2) This ball becomes hollow (the blastocyst)
3) An Inner Cell Mass forms
4) The outer cells are the trophoblast


Where is DNA found?

DNA found in chromosomes


What is chromosomes?

aggregate of genes that carry heredity info (DNA), formed from condensed chromatin


What is a centromere?

holds 2 chromatids together


What is a gene?

segment of DNA that codes for a trait


What is chromatids?

Identical copies


Usually, humans have how many chromosomes in each cell?

46 chromosomes
- 44 autosomal cells (22 from mom + 22 from dad)
- 2 sex cells (XX or XY)


What is the autosomal arrangement passed down from maternal side?

22 + X


What is the autosomal arrangement passed down from maternal side?

22 + X or 22 + Y


What happens if its Y sperm and X egg?

XY embryo
Usually male


What happens if its X sperm and X egg?

XX embryo
Usually female


What are the possible errors during cell division

- Sex chromosome no. (XXX, XXY)
- Mosaicism
- need a signalling mechanism for it to look like a male/female


What is mosaicism?

not all cells in the embryo have the same chromosome content


How many possibilities are there for siblings to look?

- During spermatogenesis and oogenesis, 46 chromosomes = 22 pairs of autosomes + 1 pair gametes (XX or XY) = 2 to the power of 23 => 8 million possibilities
- Recombination


Why do siblings look different?

1) One pair of homologous chromosomes (Maternal and Paternal - after prophase (4n, when sister chromatids are visible))
2) Crossing over during meiotic prophase 1
3) Meiotic divisions I and II
4) 4 Possible gametes


Review Mitosis and Meiosis



What is mitosis?

mitosis is a part of the cell cycle when replicated chromosomes are separated into two new nuclei.


What is meiosis?

a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell that gave rise to them


What type of cells undergo mitosis?

somatic cells


What type of cells undergo meiosis?

sex cells (gametes)


What are somatic cells?

Diploid (have two copies of chromosomes: matenal and paternal = 46)


What does the resulting cell have after somatic cells undergo mitosis?

Have the same number of chromosomes as the original cell (two daughter cells identical with parent cell)


What are gametes?

Halpid (contain a single set of unpaired chromosomes = 23)


What does the resulting cell have after gametes undergo meiosis?

have half the number of chromosomes as parent cell (brings variety of genetic information) - from 2n to n


Which part of the embryo develops into testis or ovary?

• Each genital ridge can develop into a testis or an ovary – dependant on the genetic signals received.


How long do we not know whether it is a male or female embryo?

• Analogous to 6 week gestation in humans
- Up to 6 weeks, can't tell difference between male and female embryo


What is gestation?

process or period of developing inside the womb between conception and birth


Where does Primordial Germ cells originate in?

In the walls of the yolk sac (4 - 6 weeks)


What happens to the PGC after 6 weeks?

Migrate to genital ridge, invade developing gonad
(6 - 12 weeks)


What does PGC do?

Give rise to the germ line - form sex cells or gametes


What are gamete?

Egg (oocyte) and sperm


What is the yolk sac for?

Before implantation to have a form of nutrition.


Why is the yolk sac small?

Small because eventually it has placenta for source of nutrition, no longer needing the yolk sac


What happens when PGC reaches genital ridge?

they undergo a few more cycles of mitosis (proliferation) & prepare for gametogenesis

– PGCs are now called gonocytes (general term for germ cells responsible for gametogenesis)


What is the difference between male genial ridge and female genital ridge?

Male: gonocytes do not enter meiosis (cell cycle arrest)

Female: gonocytes enter first meiotic prophase (become primary oocytes, 4n) & then the cell cycle arrests


What gene is needed to trigger male sex determination?

SRY gene


What does the SRY gene do?

Activates the SOX9 gene


What does the SOX9 do?

1) Inhibits gene responsible for making ovaries
2) Large set of genes required for making testes
- Instruct cells in genital ridge to become sertoli cells


Why is the SRY gene Y linked?

Because it is found only on the Y Chromosome


What type of cell is the embryo?

Embryo is undifferentiated, move around the body


What can atypical development be caused by?

1) Genetics,
2) Signalling AND
3) Environment
- Additional limb could be to external environment (e.g. Thalidomide)


What contributes to development of both the male and female embryos?

1) Chromsome,
2) Genes AND
3) hormones


What regulate how cells become different from each other during?

1) Positional information AND
2) signalling mechanisms


Where are testes (like ovaries) derived from

1) Intermediate mesoderm that forms the urogenital ridges on the posterior abdominal wall, giving rise to interstitial cells and myoid cells (peritubular contractile cells)
2) Mesodermal epithelium (coelomic mesothelium) that lines the urogenital ridges and gives rise to Sertoli cells
3) Primordial germ cells that migrate from the yolk sac into developing gonads, where they divide and differentiate into spermatogonia