General embryology
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Developmental anatomy or Embryology (from Greek embryon, "the
unborn, embryo"; and logia mean a branch of learning; a study of a particular subject) is the study of the
formation and development of embryo from conception to birth .
Additionally, embryology encompasses the study
of congenital disorders that occur before birth, known as teratology.
Prenatal /Gestational
period: The prenatal period is defined as the gestational
period, from conception to birth.
Embryologically,
the whole gestation period is subdivided into three period:
a)
Germinal
period/ pre-embryonic period: it extends from 1st to 2ndweek
of development. This period includes fertilization, cleavage division, morula
and blastocyst formation, implantation
and formation of bi-laminar germ disc.
b)
Embryonic
period / period of organogenesis: it extends from the 3rd to 8th
week. During this period , the three germ layers formed from gastrulation (the ectoderm, endoderm,
and mesoderm) form the internal
organs of the organism. Both the germinal
period and embryonic period arethe most
sensitive period to teratogenesis when exposure to a teratogenic agent has the greatest
likelihood of producing a malformation.
c)
Fetal
period: it extends from the 9 week to
birth.
Post - natal
development
1. Infancy (from birth to 1st year)
2. Childhood (from 2nd to 12th year)
3. Puberty (from 13th to 16th year)
4. Adolescence (from 17th to 18th year)
5. Adult (from 19th to 25th year)
Ontogeny , it is the complete life cycle and the process
of development of organism from a
single cell, an egg cell or a zygote, to an adult organism.
It includes prenatal, postnatal
development, maturity, senility and
death of an organism. It is a gradual process of expression of the blue print
of life which store within the genome of zygote.
Phylogeny is the history of the evolution of a species or group.
It is the study of relationships among different groups of
organisms and their evolutionary development.
Ontogeny repeats phylogeny
Ontogeny repeats phylogeny was expressed by
Ernst Haeckel which explains that the development of embryo of an organism from
fertilization to gestation goes through various stages of growth and
development that resembles successive adult stages in the evolution of the
organism's remote ancestors.
The various stages from fertilization of an
organism is known as ontogeny and resemblance with remote ancestors is known as
phylogeny. This theory is also known as theory of recapitulates or biogenetic law.
In the development of human kidney, three stages of
development observed pronephros, mesonephrose and metanaphrose
·
Pronephric kidney appears first in the early
weeks of development in the neck and upper thoracic regions. This kidney
persists in adult forms of some fishes
and cyclostomes but in man it completely disappears.
·
mesonephric kidney appear replacing pronephric kidney, which is appears in the lower thoracic and
lumbar regions and these kidney persists as permanent kidney in most of the fishes and amphibian. In
human mesonephric kidney soon degenerates.
·
metanephric kidney appear replacing mesonephric kidney which appears
first in the lumbo-sacral region and these kidneys persists as permanent kidney
in reptiles, birds and mammals.
So the development of human kidney shows the chronology of
development that man kidney first cross the fish and then replies stage and
ascend and reach the mammalian stage, the final stage of development.
This events explain the famous quote “ontogeny repeats phylogeny” or "Ontogeny recapitulates phylogeny".
Gestational age / Length of the pregnancy: it is calculate
in two ways
1. From 1st day of last menstrual period:
280 days or 40 weeks
2. After fertilization: 266 days or 38 weeks
Common embryological terms:
Conceptus:the embryo and
its adnexa or associated membranes
Embryo:the developing
human during its early stages of development. It extends to the end of the
eighth week
Fetus:after the
embryonic period the developing human is called a fetus.
Abortion:it is defined as the spontaneous or
induced termination of pregnancy before fetal viability.
Menopause: the permanent cessation of the menses
Menarche:establishment
or beginning of the menstrual function.
Embryonic period /period of organogenesis: 3rd
to 8th weeks of intrauterine life is known as
embryonic period, most major organ and organs systems are formed during this
period.
Organizer: a region of the embryo that is capable of
determining the differentiation of other regions.
•
Primary
organizer: primitive streak (it induces the development of notochord)
•
Secondary
organizer: notochord (it induces the development of neural tube)
•
Tertiary
organizer: neural tube (it induces the development of somites of paraxial mesoderm)
Induction: it is the process of influence of an inductor
(chemical substance) on the differentiation of adjacent cells or the
development of an embryonic structure.
Inductor: A tissue elaborating a chemical substance that acts
to determine the growth and differentiation of embryonic parts.
Evocator: a substance or tissue that induces differentiation
of embryonic parts.
Basic process in development
1. Growth
2. Differentiation
3. Induction
4. Migration
5. Apoptosis or program cell death
Growth: means increase in the bulk of tissue which takes
place by
- cells division,
- increasing cell size or
- increased accumulation of intercellular
substance
types of growth or Growth is occurred in three ways
•
Multiplicative
growth: (cell number is increase by cell division) EX: epidermis of skin,
intestinal epi
•
Auxetic
growth: (cell size is increase) EX: oocytes, some neurons
•
Accretionary
growth: (increase accumulation of intercellular substance ) EX: bones,
cartilage
Differentiation: is a complicated process in which groups of cells
assume special characteristics and functions
Totipotent cells: are able to produce a separate embryo under favorable
conditions
Pluripotent cells: are groups of cells which produce specific type of
tissue
Cells of human body
- Somatic
cells
- Germ
cells
Somatic cells of body are three types:
• Permanent
cells: they do not divide in post natal life (after
birth) . Ex: neuron
• Labile cells are cells that multiply constantly
throughout life. ex: epidermis of skin(replace within one month) , intestinal
epithelium(replace daily) ,
• Stable cells are cells that multiply only when needed. Ex : liver, skeletal
muscle
Germ cells:are two types male and
female germ cells
Female germ cell:Oocyte:the female
germ or sex cell produced in the ovaries
• Immature
female germ cells:Oogonia/ Diploid /Chromosome
number: 46 ( 44 autosomes+ XX)/At intrauterine life, they increase their number
by mitosis
•
Mature female germ cells:Ovum ( mature oocyte ) / Haploid/Chromosome number: 23 (22 autosomes +
X) / After puberty uptomenopouse, only one ovum is produced by meiosis in each
month
Male germ
cell: Sperm:the male germ cell produced in the testes.
•
Immature male germ cells :Spermatogonia/Diploid /Chromosome number: 46 ( 44 autosomes+ XY)/ At puberty they start to increase their number
by mitosis and continued until death
•
Mature male germ cells :Spermatozoa / Haploid / Chromosome number: 23 (22 autosomes + X Or 22
autosomes + Y) / At puberty they start
to increase their number by meiosis &
continued until death
Difference
between male & female germ cell
|
•
Female germ cells : |
•
Male germ
cells |
|
•
Only one type: 22X •
22 autosomes •
1 Sex
chromosome X |
•
two types : 22X
& 22Y •
22 autosomes and one sex chromosome either X or Y •
SN: a single male germ cell never contain both X
& Y |
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