INTRODUCTION TO EMBRYOLOGY

357GROWTH AND DEVELOPMENT

Embryology is the study of development of an organism from fertilization of the ovum-the single cell stage-through the period of organogenesis.

In the human, this time frame encompasses the first 8 weeks of pregnancy.

Earliest Development Week 1: Fertilization , Morula, Blastocyst, Implantation Week 2: Bilaminar Embryo, Placenta and Membranes, Gastrulation

Embryonic PeriodWeek 3: Gastrulation, Neurulation, Early Circulatory System Week 4: Protoheart beats, Gut tube, Branchial Arches, Limb BudsWeek 5-8: Organogenesis, Face, Limb differentiation, Ear, External Genitalia

Foetal Period Weeks 9-38: Brain growth, neural connections, lots of growth

The development of an animal embryo can be divided into five major processes:

1) GAMETOGENESIS: the process of gamete production.

2) FERTILIZATION: the fusion of male and female gametes to form a single-celled zygote capable of undergoing development.

3) CLEAVAGE: the mitotic divisions that divide the cytoplasm into increasingly smaller cells, without an increase in the total size of the cell mass.

4) GASTRULATION:

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

mesoderm.

5) ORGANOGENESIS:

the process whereby organs develop from the three germ layers.

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 24-hour period.

(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 phenotype.

Oogonia are not shown in this figure because they differentiate into primary oocytes before birth.

Note that (1) following the two meiotic divisions, the diploid number of chromosomes, 46, is reduced to the haploid number, 23; (2) four sperms form from one primary spermatocyte, whereas only one mature oocyte results from maturation of a primary oocyte; and (3) the cytoplasm is conserved during oogenesis to form one large cell, the mature oocyte.

The polar bodies are small nonfunctional cells that eventually degenerate.

OogenesisOogenesis

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).

Ovulation

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 released.

The others degenerate and the oocyte and follicular cells are replaced by connective tissue, forming corpus atreticum.

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).

The The OvariaOvarian Cyclen Cycle

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 (stigma) appears.

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 division.

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 albicans.

If fertilization occurs it remains as corpus luteum of pregnancy (graviditatis) and continues to produce progesterone.

Fertilization

Fertilization (fusion of the sperm and egg) normally

occurs in the ampullary region of the uterine

(fallopian) tube within 24 hours of ovulation.

Fertilization

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 embryo.

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.

Blastocyst

Trophoblast(outer cells)

Inner Cell Mass(foetal cells)

Chorion(outerembryonicmembrane)

Placenta(embryoniccontributionto nutrientexchange)

Epiblast(Amnionandectoderm)

Hypoblast(Yolk sacandendoderm)

FETAL MEMBRANES

There are four fetal membranes—the amnion, chorion, yolk sac, and allantois.

FETAL MEMBRANES

These are thin layers of tissue which surround the embryo or fetus and provide for its nutrition, respiration, excretion and protection; they are the yolk sac, allantois, amnion, and chorion.

FETAL MEMBRANES

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 membrane.

The yolk sac gradually decreases in size and is eventually incorporated into the midgut of the embryo.

PLACENTA

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 mix.

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