GROWTH AND DEVELOPMENT
Embryology is the study of
development of an organism from
fertilization of the ovum-the single
cell stage-through the period of
In the human, this time frame
encompasses the first 8 weeks of
Week 1: Fertilization , Morula, Blastocyst, Implantation
Week 2: Bilaminar Embryo, Placenta and Membranes,
Week 3: Gastrulation, Neurulation, Early Circulatory System
Week 4: Protoheart beats, Gut tube, Branchial Arches, Limb
Week 5-8: Organogenesis, Face, Limb differentiation, Ear,
Weeks 9-38: Brain growth, neural connections, lots of growth
The development of an animal embryo can be divided into
five major processes:
the process of gamete production.
the fusion of male and female gametes to form
a single-celled zygote capable of undergoing
the mitotic divisions that divide the
cytoplasm into increasingly smaller
cells, without an increase in the total size
of the cell mass.
a stage of cell movement and rearrangement resulting
in three different germ layers of cells.
The three germ layers have different potentials for
tissue specialization and development. Yolk greatly
influences gastrulation and the development of these
three germ layers: ectoderm, endoderm, and
the process whereby organs develop from the three
Several cellular events are essential to the process:
(1) Cell proliferation increases cell number in
preparation for cell differentiation. Cell division
(cycle) times in the embryo are as little as 4 hours, so
there can be a 32-fold increase in cell number in a
(2) Cell migration occurs as cells move into position
to create differentiated cell types.
(3) Cell differentiation is the completion of cell
development, when cells assume their ultimate
Oogonia are not shown in this
figure because they
differentiate into primary
oocytes before birth.
(1) following the two meiotic
divisions, the diploid number
of chromosomes, 46, is
reduced to the haploid number,
(2) four sperms form from one
primary spermatocyte, whereas
only one mature oocyte results
from maturation of a primary
(3) the cytoplasm is conserved
during oogenesis to form one
large cell, the mature oocyte.
The polar bodies are small
nonfunctional cells that
OVARIAN CYCLE AND OVULATION
The ovarian cycle is regulated by the
gonadotrophin releasing hormone produced
by the hypothalamus.
This acts on the pituitary and stimulate the
production of gonadotrophins, the follicle
stimulating hormone (FSH) and the
leutinizing hormone (LH).
During each cycle, 5-15 primordial follicles begin to grow
by the action of FSH.
Usually only one of these, mature and only one oocyte is
The others degenerate and the oocyte and follicular cells
are replaced by connective tissue, forming corpus
The follicular and thecal cells produce estrogens which
induces follicular or proliferative phase of the
endometrium and stimulates the production of LH
(required for follicular maturation and ovulation).
Ovulation and subsequent changes
The primordial follicle matures into graffian follicle under
the influence of FSH.
The primary oocyte completes the first meiotic division.
The surface of the ovary bulges locally and an avascular sopt
The oocyte together with the cells of the region of cumulus
oopherus is released from the ovary.
Cumulus oophorus cells rearrange around zona pellucida to
form zona radiate.
Ovulation and subsequent changes
The oocyte begins the second meotic
The fimbriae collect the oocyte and
guide into the uterine tube.
The granulos cells remaining in the
wall of the follicle gives rise to corpus
luteum that secretes progesterone.
Progesterone together with other
hormones causes the uterine mucosa to
enter progestational or secretory stage.
If fertilization does not occur, corpus
luteum form a scar tissue called corpus
If fertilization occurs it remains as
corpus luteum of pregnancy
(graviditatis) and continues to produce
Fertilization (fusion of the
sperm and egg) normally
occurs in the ampullary
region of the uterine
(fallopian) tube within 24
hours of ovulation.
Once the sperm enters
the egg, the male and
female pronuclei come
into close contact and
replicate their DNA, and
cell division then occurs,
creating a two-cell
Cell division continues as the embryo proceeds along the
uterine tube toward the uterus.
Three days after fertilization, the embryo consists of a
ball of cells called the morula (mulberry).
The cells of the morula undergo compaction, a
process whereby cell-to-cell contacts are
maximized through tight junctions, and inner cells
are segregated from outer cells.
As subsequent cell divisions occur, a small group of
inner cells (the inner cell mass, or embryoblast)
becomes segregated from the outer cells (the outer
cell mass or trophoblast).
Over the next 2 days, fluid is pumped from the
outside to the inside, and the morula is
transformed into a hollow blastocyst.
Inner Cell Mass
There are four fetal membranes—the
amnion, chorion, yolk sac, and
These are thin layers of
tissue which surround
the embryo or fetus and
provide for its nutrition,
and protection; they are
the yolk sac, allantois,
amnion, and chorion.
In the course of development, the
chorion becomes the outermost, and the
amnion the innermost, membrane
surrounding the developing embryo.
As the allantois increases in size, it
expands and becomes closely
associated, if not fused, with the
chorion. The two membranes together
are known as the chorioallantoic
The yolk sac gradually decreases in size
and is eventually incorporated into the
midgut of the embryo.
Placenta is the structure in most mammals that
develops in the uterus along with a fetus to
mediate metabolic exchange.
Nutrients and oxygen in the mother's blood pass
across the placenta to the fetus, and metabolic
wastes and carbon dioxide from the fetus cross in
the other direction; the two blood supplies do not