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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 4th week of embryonic development and subsequently migrate into the future gonad region where they arrive at the end of the 5th week

• 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

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• If the sex pair is XX, the individual is genetically female; if the pair is XY, the individual is genetically male.

• One chromosomes of each pair is derived from the maternal gamete, the oocyte, and one from the paternal gamete, the sperm

• Thus, each gamete contains a haploid number of 23 chromosomes, and the union of the gametes at fertilization restores the diploid number of 46

Meiosis• is the cell division that takes place in the germ cells to generate

male and female gametes, sperm and egg cells, respectively• Meiosis requires two cell divisions: meiosis I and meiosis II, to reduce the number of chromosomes to the

haploid number of 23

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Meiosis 1 Events that occur during meiosis I include the following: synapsis• homologous chromosomes align themselves in pairs, this

process is called synapsis crossingover• Crossovers: this involve the interchange of chromatid

segments between paired homologous chromosomes • Segments of chromatids break and are exchanged as

homologous chromosomes separate.• As separation occurs, points of interchange are temporarily

united and form an X-like structure, a chiasma

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Stages A – D, prophase of 1st meiotic division

E – metaphaseF - AnaphaseG - Telophase(of 1st meiotic division) Cells contain

23double-structuredchromosomes

Cells resulting from 1stmeiotic division

2nd meiotic divisionMetaphase of 2nd meiotic division

DisjunctionCells resulting from 2nd meiotic division

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Summary of Meiosis I Events that occur during meiosis I include the following: Synapsis: pairing of 46 homologous duplicated chromosomes Crossing over: exchange of large segments of DNA Alignment: alignment of 46 homologous duplicated

chromosomes at the metaphase plate Disjunction: separation of 46 homologous duplicated

chromosomes from each other; centromeres do not split. Cell division: formation of two secondary gametocytes (23

duplicated chromosomes, 2N).

Meiosis II• Meiosis II. Events that occur during meiosis II include the

following:– Synapsis: absent.– Crossing over: absent.

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Summary of Meiosis I Events that occur during meiosis I include the following: Synapsis: pairing of 46 homologous duplicated chromosomes Crossing over: exchange of large segments of DNA Alignment: alignment of 46 homologous duplicated

chromosomes at the metaphase plate Disjunction: separation of 46 homologous duplicated

chromosomes from each other; centromeres do not split Cell division: formation of two secondary gametocytes (23

duplicated chromosomes, 2N).

Meiosis II• Meiosis II. Events that occur during meiosis II include the

following:– Synapsis: absent.– Crossing over: absent.

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Alignment: alignment of 23 duplicated chromosomes at the metaphase plate.

Disjunction: separation of 23 duplicated chromosomes to form 23 single chromosomes; centromeres split

Cell division: formation of four gametes (23 single chromosomes, 1N).

Note:

As a result of meiotic divisions,

(a) genetic variability is enhanced through crossover, which redistributes genetic material, and through random distribution of homologous chromosomes to the daughter cells; and

(b) (b) each germ cell contains a haploid number of chromosomes, so that at fertilization the diploid number of 46 is restored

SPERMATOGENESIS• Spermatogenesis is the sequence of events by which

spermatogonia are transformed into mature sperms.• This maturation process begins at puberty• Spermatogonia, which have been dormant in the

seminiferous tubules of the testes since the fetal period, begin to increase in number at puberty

• After several mitotic divisions, the spermatogonia grow and undergo changes

• Spermatogonia are transformed into primary spermatocytes, the largest germ cells in the seminiferous tubules.

• Each primary spermatocyte subsequently undergoes 1st meiotic division (reduction division) to form two haploid secondary spermatocytes, which are approximately half the size of primary spermatocytes

• Secondary spermatocytes then undergo a second meiotic division to form four haploid spermatids

• these haploid spermatids are approximately half the size of secondary spermatocytes.

• The spermatids are gradually transformed into four mature sperm by a process known as spermiogenesis

• The entire process of spermatogenesis, which includes spermiogenesis, takes approximately 2 months

• When spermiogenesis is complete, the sperms enter the lumina of the seminiferous tubules

• Sertoli cells lining the seminiferous tubules support and nurture the germ cells and may be involved in the regulation of spermatogenesis

• Sperms are transported passively from the seminiferous tubules to the epididymis, where they are stored and become functionally mature

• The epididymis is the elongated coiled duct along the posterior border of the testis

• It is continuous with the ductus deferens (vas deferens), which transports the sperms to the urethra

• Mature sperms are free-swimming, actively motile cells consisting of a head and a tail

• The neck of the sperm is the junction between the head and tail• The head of the sperm forms most of the bulk of the sperm and

contains the haploid nucleus. • The anterior two thirds of the nucleus is covered by the acrosome,

a caplike saccular organelle containing several enzymes

• When released, these enzymes facilitate dispersion of the follicular cells of the corona radiata and sperm penetration of the zona pellucida during fertilization

• The tail of the sperm consists of three segments: middle piece principal piece and end piece • The tail provides the motility of the sperm that assists its

transport to the site of fertilization. • The middle piece of the tail contains mitochondria, which

provide the adenosine triphosphate necessary for activity

OOGENESIS• Oogenesis (ovogenesis) is the sequence of events by which oogonia

are transformed into mature oocytes • This maturation process begins before birth and is completed after

puberty • Oogenesis continues to menopause, which is permanent cessation of

the menses (bleeding associated with the menstrual cycles)

Prenatal Maturation of Oocytes • Primordial germ cells (46, 2N) migrate from the wall of the yolk sac

and arrive in the ovary at 5th week and differentiate into oogonia (46, 2N), which populate the ovary through mitotic division.

• A majority of oogonia continue to divide by mitosis but some of them enter meiosis I and undergo DNA replication to form primary oocytes (46, 4N)

• by the 5th month of prenatal development, the total number of germ cells in the ovary reaches its maximum, which is estimated at 7 million

Note: All primary oocytes are formed by the 5th month of fetal life• At this time, cell death begins, and many oogonia as well as

primary oocytes become atretic • By the seventh month, the majority of oogonia have degenerated

except for a few near the surface Note: No oogonia are present at birth• All surviving primary oocytes have entered prophase of meiosis I,

and most of them are individually surrounded by a single layer of flattened, follicular epithelial cells

• The primary oocytes enclosed by this layer of cells constitutes a primordial follicle

• Primary oocytes begin the first meiotic division before birth, but completion of prophase does not occur until puberty

• The follicular cells surrounding the primary oocyte are believed to secrete a substance, oocyte maturation inhibitor (OMI) which keeps the meiotic process of the oocyte arrested

Postnatal Maturation of Oocytes • Beginning during puberty, usually one follicle matures each

month and ovulation occurs, except when oral contraceptives are used

• The total number of primary oocytes at birth is estimated to vary from 600,000 to 800,000

• only approximately 40,000 are present by the beginning of puberty, and fewer than 500 will be ovulated

• As the primary oocyte enlarges during puberty, the follicular epithelial cells become cuboidal in shape and then columnar, forming a primary follicle

• The primary oocyte soon becomes surrounded by a covering of amorphous acellular glycoprotein material, the zona pellucida