Transcript GENERAL EMBRYOLOGY

GENERAL EMBRYOLOGY
Introduction
Developmental Periods
Divided into:
A: Prenatal
B: 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 testis
Zygote: Result of union between the oocyte and sperm during fertilization
Cleavage: Series of divisons taht occur in the zygote resulting in formation of
balastomeres
Morula: Solid mass of 12-32 blastomeres
Blastocyst: 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
utrus
Gastrula: The appearance of three germinal layers in the blastocyst
Primordium: The first trace of an organ or stucture (beginning of first indication).
Embryo: The concepetus betwee 3-8 weeks of gestation
Fetus: 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 in
in three or more cosecutive pregnancies), complete, criminal (illegal), induced (
either elective or therapeutic).
Trimester: One third length of pregnancy
B: Postnatal period
Infancy: first year of age : less than one month is called neonate or newborn
Childhood: From 13 months to puberty
Puberty: Occurs between 13-16 years in males and 12-15 years in females
Adolescence: 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
State
Quiescent
INTER
PH
ASE
Cell division
Description
Gap 0
Gap 1
Synthesis
Gap2
Mitosis
Abbreviation
G0
G1
S
G2
M
A resting phase where the cell has left
.the cycle and has stopped dividing
Cells increase in size in Gap 1. The G1
checkpoint control mechanism ensures
that everything is ready for DNA
.synthesis
DNA replication occurs during this
phase
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
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
Primordial Germ Cells
Development begins with fertilization, the process
by which the male gamete, the sperm, and the
female 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 number
during 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 and
cytodifferentiation 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 subunitschromatids- 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 pairscalled 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 mutation
A: Numerical: Aneuploid
It 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: 100
above 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 abnormalities
Breakage 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 PraderWilli 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 Recessive
Type.
Gene mutations can cause inborn errors of
metabolism eg phenyketonuria, galactosemia.