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GENERAL EMBRYOLOGY

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THE DEVELOPING HUMAN

Introduction• Human development is a continuous process that begins when

an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male

• Cell division, cell migration, programmed cell death, differentiation, growth, and cell rearrangement transform the fertilized oocyte, a highly specialized, totipotent cell, a zygote, into a multicellular human being

• From a single cell to a baby in 9 months, the study of the developmental processes that take place is called Embryology

Note:• Development does not stop at birth,• Although most developmental changes occur during the

embryonic and fetal periods,• important changes occur during later periods of development:

infancy, childhood, adolescence, and early adulthood

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Terminologies• Oocyte ( ovum /egg): refers to the female germ or sex cells

produced in the ovaries• Sperm (spermatozoon): refers to the male germ cell produced in

the testes (testicles) • Zygote: This cell results from the union of an oocyte and a sperm

during fertilization A zygote or embryo is the beginning of a new human being

Developmental Periods: can be divided into prenatal (before birth) postnatal (after birth) period. Stages of prenatal development: • begins at fertilization and embryonic development ends on day 56

(8th week )• The fetal period begins on day 57 and ends when the fetus is

completely outside the mother

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Prefertilization Events Sexual Reproduction• Sexual reproduction occurs when female and male gametes

(oocyte and spermatozoon, respectively) unite at fertilization.• Gametes are direct descendants of primordial germ cells, which

are first observed in the wall of the yolk sac at week 4 of embryonic development and subsequently migrate into the future gonad region.

• Gametes are produced by a process called gametogenesis (formation of gamete)

• In males, this process is called spermatogenesis• In females, it is called oogenesis note :• The sequence of gametogenesis is the same, but the timing of

events during meiosis differs in the two sexes.

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Primodial germ cells in wall of yolk sac

Yolk sac

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• In preparation for fertilization, germ cells undergo gametogenesis. Which include:

meiosis, to reduce the number of chromosomes and cytodifferentiation to complete their maturation

Chromosomes• A single chromosome consists of TWO characteristic regions

called arms. These include: Short arm/ p arm Long arm/ q arm • These two arms are separated by a centromere• During meiosis I, single chromosomes undergo DNA replication,

which essentially duplicates the arms. • This forms duplicated chromosomes, which consist of two sister

chromatids attached at the centromere.

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1) Ploidy and N number• Ploidy refers to the number of chromosomes in a cell • The N number refers to the amount of DNA in a cell In humans, somatic cells (cells of an organism other than the

germ cells) contain 46 single chromosomes The chromosomes occur in 23 homologous pairs, of which

one member (homologue) of each pair is of maternal origin, and the other is of paternal origin to form the diploid number of 46

Note: The term “diploid” is classically used to refer to a cell containing 46 single chromosomes

The 23 homologous pairs of chromosomes is made up of: 22 pairs of matching chromosomes called autosomes and one pair of sex chromosomes