Human Embryology: Key Concepts

• Human embryology studies the development of a human being before birth.
• Embryology helps understand normal adult body structure relationships and the causes of congenital anomalies.

Key Terms and Concepts

• The developing individual is called an embryo for the first two months, and then a fetus.
• The testis is the male sex organ (gonad), while the ovary is the female sex organ; both produce gametes.
• The male gamete is called spermatozoa, and its production is spermatogenesis.
• The female gamete is called ova, and its production is oogenesis.
• Fertilization is the union of a spermatozoon and an ovum (oocyte) to form a zygote.
• Characteristics of parents are transmitted through codes on strands of DNA.
• Genes are made up of such strands of DNA and are located on chromosomes.
• A typical cell contains 46 chromosomes (diploid number).
• A gamete contains 23 chromosomes (haploid number).
• The diploid number of chromosomes (46) is restored as a result of fertilization.
• Cell multiplication occurs through cell division.
• Ordinary cell division is by mitosis, in which daughter cells are similar to the parent cell.
• A special kind of cell division called meiosis occurs in the testis and ovary for gamete formation.
• Gametes formed from meiosis have a haploid number of chromosomes (23).
• Fetus refers to the developing human after the embryonic period.
• Conceptus includes the embryo and its associated membranes, such as the amnion, chorionic sac, and yolk sac.
• Primordium is the beginning or first discernible indication of an organ or structure.
• Trimester is a period of 3 calendar months.
• Gametes are derived from primordial germ cells (PGCs).
• PGCs form in the epiblast during the 2nd week and move to the yolk sac.
• In the 4th week, PGCs migrate from the yolk sac to the developing gonads, arriving by the 5th week.
• Mitotic divisions increase the number of PGCs.
• In a resting cell, chromosomes (Xmes) are not visible under a light microscope because the chromatin materials are dispersed.
• During cell division, chromosomes condense and become visible.

Mitosis

• Mitosis is a cell division process that results in two daughter cells that are genetically identical to the parent cell.
• Each daughter cell receives a whole complement of 46 chromosomes.
• Before mitosis, each chromosome replicates its DNA. At this point, chromosomes are very long, spread diffusely through the nucleus, and are not visible under a light microscope.
• With the onset of mitosis (prophase), the chromosomes begin to coil, contract, and condense.
• Each chromosome consists of chromatids joined at the centromere.
• During prophase, the chromosomes continue to condense, shorten, and thicken.
• In prometaphase, the chromatids become distinguishable.
• Metaphase involves the alignment of chromosomes in the equatorial plane.
• The doubled structured chromosomes are clearly visible during metaphase.
• Each chromosome is attached by microtubules which extend from centromere to the centriol forming the mitotic spindle
• Anaphase separates the centromere of each chromosome, moving to opposite poles of the dividing cell.
• Telophase involves the uncoiling and lengthening of chromosomes, reformation of the nuclear envelope, and division of the cytoplasm.
• Interphase is the period between two successive cell divisions.

Meiosis

• Meiosis occurs in germ cells to produce gametes.
• It requires two cell divisions (meiosis I and meiosis II) to reduce the number of chromosomes to the haploid number.
• Prophase of the 1st meiotic division has several stages.
• During Leptotene, gametes replicate their DNA, so that each of the 46 chromosomes duplicate into sister chromatids.
• In Zygotene, homologous chromosomes align themselves in pairs (synapsis).
• Pachytene is when the four chromatids are now distinct & form a tetrad.
• Crossover occurs when segments of chromatids break and are exchanged as homologous chromosomes separate.
• As separation occurs, points of interchange are united and form a chiasma.
• During Diplotene, the 2 chromosomes of a bivalent try to move apart & the chromatids involved in crossing over break at the point of crossing & the loose pieces become attached to the opposite chromatid, resulting in the exchange of genetic material.
• Metaphase in Meiosis I – the 46 chromosomes become attached to the spindle at the equator.
• The nuclear membrane disappears during metaphase.
• Anaphase in Meiosis I – no spitting of the centromeres as in mitosis.
• One entire chromosome of each pair moves to each pole of the spindle
• The resulting daughter cells have 23 chromosomes, each with 2 chromatids.
• In Telophase – 2 daughter nuclei are formed & followed by cell division.
• The 2nd meiotic division is similar to mitosis.
• Since crossing over occurred in the first meiotic division, the daughter cells are not identical.

Spermatogenesis

• Spermatogenesis begins at puberty.
• It is the process by which spermatogonia are transformed into spermatozoa.
• At birth, germ cells are found in the sex cords of the testis, with Sertoli cells present as supporting cells.
• Shortly after puberty, the sex cords acquire a lumen and become seminiferous tubules.
• Primordial germ cells give rise to spermatogonial stem cells.
• At regular intervals, cells emerge from these stem cells and form type A spermatogonia.
• Type A spermatogonia undergo a number of mitotic divisions to form type B spermatogonia.
• Type B cells divide to form primary spermatocytes.
• The primary spermatocyte enters a prolonged prophase (22 days), followed by a rapid completion of meiosis I, which forms a secondary spermatocyte.
• Both spermatogonia and spermatids are embedded in Sertoli cells, which provide support, protection, nutrition, and assistance in the release of mature spermatozoa.
• Spermatogenesis is regulated by LH (luteinizing hormone) which binds to Leydig cells and stimulates testosterone production. Testosterone, in turn, binds to Sertoli cells to promote spermatogenesis.
• Spermiogenesis is the transformation of spermatids into spermatozoa.
• The changes in spermiogenesis include the formation of the acrosome, condensation of the nucleus, formation of the neck, middle piece, and tail, and shedding of most of the cytoplasm.
• The spermatozoa then enter the lumen of the seminiferous tubule.

Clinical Correlations

• Primordial Germ Cells are related to Teratomas.
• Abnormal chromosome numbers (Xmal abnormalities) relate to birth defects and spontaneous abortions.
• Euploid: exact multiple of n.
• Aneuploid: an extra chromosome Xme (Trisomy) e.g. trisomy 21 (Down syndrome) or one chromosome Xme missing (monosomy).
• Nondisjunction and translocation cause chromosome abnormalities.
• Abnormal spermatozoa cause clinical conditions.

Chromosomal Abnormalities

• Trisomy 18 (Edwards' Syndrome): Most babies die before or shortly after birth, mental retardation, congenital heart defect, low set ear, flexion of fingers and hands.
• Trisomy 21 (Down Syndrome): Common Xmal anomaly, growth retardation, mental retardation, Craniofacial abnormalities (e.g., upward slanting eye, epicantal fold, flat facies, small ear), and cardiac defect, hypotonia.
• Trisomy 13 (Patau Syndrome): Least common of the trisomies; mental retardation, congenital heart defects, deafness, cleft lip & palate, holoproencephaly, eye defects (microphthalmia, anophthalmia, coloboma)
• Klinefelter syndrome (XXY or XXXY): Taller than average stature, absent, delayed or incomplete puberty, testicular atrophy, small penis, gynecomastia, sterility, weak bones, low energy levels, tendency to be shy and sensitive.
• Turner syndrome (45X): Absence of ovary (gonadal dysgenesis), short stature, wide or weblike neck, low-set ears, broad chest with widely spaced nipples, arms that turn outward at the elbows, lymphoedema, cardiac defects.
• Angelman syndrome: Microdeletion on the long arm of chromosome 15, often smile and laugh frequently, happy, excitable personality, mental retardation, poor motor dev, cannot speak.
• Prader-Willi syndrome (PWS): Microdeletion on the long arm of chromosome 15. If microdeletion on long arm of chromosome 15 affects paternal Chromosome, hypotonia, obesity, mental retardation, hypogonadism, cryptochdism.