GENERAL EMBRYOLOGY

IntroductionDevelopmental PeriodsDivided into:A: PrenatalB: Postnatal

A: Prenatal terminology

Oocyte: ovum produced by the ovary. When mature called 2dry oocyte or mature oocyte.Sperm: male germ cell produced by the testisZygote: Result of union between the oocyte and sperm during fertilizationCleavage: Series of divisons taht occur in the zygote resulting in formation of balastomeresMorula: Solid mass of 12-32 blastomeresBlastocyst: appearnce of a fluid filled cavity in the morula changes it into blasytocyst Implantation: the processof entering of the zygote into the endometrium of the utrusGastrula: The appearance of three germinal layers in the blastocystPrimordium: The first trace of an organ or stucture (beginning of first indication).Embryo: The concepetus betwee 3-8 weeks of gestationFetus: Conception between 9 weeks to birth

Abortion (Miscarriage): Premature expulsion of the concepetus from the uterus before it is viable ie before 20 weeks. After that it is called pemature birth. Types of abortion: Threatened, accidental, spontaneous, habitual (expulsion of non viable embryo inin three or more cosecutive pregnancies), complete, criminal (illegal), induced ( either elective or therapeutic).Trimester: One third length of pregnancy

B: Postnatal periodInfancy: first year of age : less than one month is called neonate or newbornChildhood: From 13 months to pubertyPuberty: Occurs between 13-16 years in males and 12-15 years in femalesAdolescence: 11-19 years; from the appearance of pubertal development until attainment of physical, mental and emotional maturity.Adulthood: Attainment of full maturity 18-21 years.

THE CELL CYCLE 

Abbreviation

Description State

A resting phase where the cell

has left the cycle and has stopped dividing.

G0 Gap 0 Quiescent

Cells increase in size in Gap 1. The G1 checkpoint control mechanism ensures that everything is ready for DNA synthesis.

G1 Gap 1 INTER

DNA replication occurs during this phase S Synthesis PH

During the gap between DNA synthesis and mitosis, the cell will continue to grow. The G2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis) phase and divide

G2 Gap2 ASE

Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter cells. A checkpoint in the middle of mitosis (Metaphase Checkpoint) ensures that the cell is ready to complete cell division.

M Mitosis Cell division

Primordial Germ Cells Development begins with fertilization, the processby which the male gamete, the sperm, and thefemale gamete, the oocyte, unite to give rise to a zygote.Gametes are derived from primordial germ cells (PGCs) that are formed in the epiblast during the second week and that move to the wall of the yolk sac. During the fourth week these cells begin to migrate from the yolk sac toward the developing gonads, where they arrive by the end of the fifth week. Mitotic divisions increase their numberduring their migration and also when they arrive in the gonad. In preparation for fertilization, germ cells undergo gametogenesis, which includes meiosis, to reduce the number of chromosomes andcytodifferentiation to complete their maturation

Mitosis

Before mitosis cells replicate their DNA. With onset of mitosis the chromosomes start to coil, condense and contract- Prophase. Chromosomes formed of parallel subunits-chromatids- joined at the centromere –prometaphase. During metaphase chromosomes line up in the equatorial plane, joined by microtubules from the centromere to the centriole, the mitotic spindles.Anaphase: Centomere of each cell divides and chromtids migrate to opposite poles of the spindle.Telophase: Cytoplasm divides and each daughter cell receives half of all double chr. material maintaining the same number of chromosomes as the mother cell.

Meiosis

Meiosis needs two cell divisions (M1 and M2) to reduce the no. of chromosomes to haploid of 23 chromosomes. Cells replicate their DNA so that each of the 46 chromosome is duplicated into sister chromatids. Homologus chromosomes then align in pairs- called synapsis.Interchange of chromatid sgments between homologous pairs take place. Points of interchange are temporarily united and form a X-like structure called chiasma.

In metaphase1 homologous chromatids separate in two daughter cells each having half the No of chromosomes.

In metaphase 11 the centromeres divide and homologous chromatids separate into four daughter cells each having the haploid number of chromosomes.

MEIOSIS

MEIOSIS

Chromosomal Abnormalities

May be numerical, structural or due to gene mutationA: Numerical: AneuploidIt is called Trisomy if there is one extra chromosome or Monosomy if one chr omosome is missing. It might occur in mitosis or meiosis. When occurs in meiosis it is called Nondisjunction. When occurs during mitosis (mitotic disjunction) or mosaism with some cells having abnormal chromosme number and others having normal number.Sometimes chromosomes break and pieces of one chromosme attach to another. Such translocations may be balanced or unbalanced eg Down’s syndrome.Trisomy 21 (Down’s Syndrome):Features: Growth retardation, mental retardation., craniofacial abnormalities, upwrad slanting eyes and low set ears. They are prone to leukemia, infection and thyroid dysfunctions. Incidence: 1: 2000 at 25 years, 1: 300 at 35 and 1: 100above 40 years.

- Trisomy 18: Mental retardation, congenital heart defects, musculoskeletal deformities. Incidence 1: 5000.- Trisomy 13: Mental retardation, holoprosencephaly, congenital heart defects, cleft lip and palate. Incidence: 1; 20000.- Klinefelter syndrome:Occurs only in males. Discovered at puberty. Cells have 47 chr with a sex chr complement of XXY type and a sex chromatin Barr Body. Features are sterility, testicular atrophy, hyalinization of the seminiferous tubules and gynecomastia. Incidence: 1: 500 males.- Turner syndrome: Cells have 45 chromosome (Monosomy 45) and have female appearance. Features: gonadal dysgenesis, short stature, webbed neck, musculoskeletal abnormalities and lymphedema.

B: Structural chromosomal abnormalitiesBreakage of chromosomes result in deletion of parts of a chr.0mosome.- Cri-du-chat syndrome: due to deletion of the short arm of chr 5. Features: Cat-like cry, microcephaly, mental retardation and CHD. Microdeletion of the long arm of chr 15 (on the maternal chr) results in Angelman syndrome. Children are mentally retarded, cannot speak and exibit poor motor development. If the defect is inherited on the paternal chr it results in Prader-Willi syndrome charact by hypotonia, obesity, mental retardation, hypogonadism and cryptorchidism C: Gene mutation:genes usually occur as pairs (allels) except XY chr in males. So that there are two doses for each genetic determinant; one from the father and from the mother. If mutation affect a single dose it is dominant. If it occurs in both doses or on the X chr on the male it is recessive. This type of inheritance is called Mendelian RecessiveType.Gene mutations can cause inborn errors of metabolism eg phenyketonuria, galactosemia.