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.

UTERUS, UTERINE TUBES, AND OVARIES

The body of the uterus narrows from the fundus, the rounded, superior part of the body, to the isthmus, constricted region between the body and cervix.

The cervix of the uterus is its tapered vaginal end that is nearly cylindrical in shape.

The lumen of the cervix, the cervical canal, has a constricted opening at each end.

The internal os communicates with the cavity of the uterine body and the external os communicates with the vagina.

Uterus The uterus

a thick-walled, pear-shaped muscular organ

The uterus consists of two major parts:

Body, the expanded superior two thirds Cervix, the cylindrical inferior one third

UTERUS, UTERINE TUBES, AND OVARIESThe walls of the body of the uterus consist of three layers

Perimetrium, the thin external layer Myometrium, the thick smooth muscle layer Endometrium, the thin internal layer

The perimetrium is a peritoneal layer that is firmly attached to the myometrium.

During the luteal (secretory) phase of the menstrual cycle, three layers of the endometrium can be distinguished microscopically (see Fig. C)

A thin, compact layer consisting of densely packed, connective tissue around the necks of the uterine glands A thick, spongy layer composed of edematous connective tissue containing the dilated, tortuous bodies of the uterine glands A thin, basal layer containing the blind ends of the uterine glands

During the luteal (secretory) phase of the menstrual cycle, three layers of the endometrium can be distinguished microscopically (see Fig. C)

A thin, compact layer consisting of densely packed, connective tissue around the necks of the uterine glands A thick, spongy layer composed of edematous connective tissue containing the dilated, tortuous bodies of the uterine glands A thin, basal layer containing the blind ends of the uterine glands

The walls of the body of the uterus consist of three layers

Perimetrium, the thin external layer Myometrium, the thick smooth muscle layer Endometrium, the thin internal layer

The perimetrium is a peritoneal layer that is firmly attached to the myometrium.

About the sixth day, the blastocyst implants by attaching itself to the uterine epithelium

The uterus consists of three layers, endometrium, the mucosal lining, myometrium, thick layer of smooth muscles and perimetrium, the peritoneal covering of the outside wall.

During the ovarian cycle the endometrium passes through the follicular or proliferative phase, secretory or progestational phase and the menstrual phase.

UTERUS, UTERINE TUBES, AND OVARIES

At the peak of its development, the endometrium is 4 to 5 mm thick.

The basal layer of the endometrium has its own blood supply and is not sloughed off during menstruation.

The compact and spongy layers, known collectively as the functional layer, disintegrate and are shed during menstruation and after parturition (delivery of a baby).

Proliferation phase:It begins at the end of the menstrual phase under the

influence of estrogen.

Secretory phase:This phase begins 2-3 days after ovulation in response to

progesterone produced by the corpus luteum. Implantation occurs during this phase. During this period three layers of the uterine mucosa can be distinguished (superficial compact layer, intermediate compact layer and a thin basal layer).

Menstrual phase:If fertilization does not occur the shedding of compact and

spongy layer begins, making the initiation of the menstrual phase.

The Uterine CycleThe Uterine Cycle

2nd week of development

After implantation, over the next several days, the blastocyst invades this tissue.

By this time, the trophoblast has differentiated into two layers: an invasive outer multinucleated cytoplasmic mass called the syncytiotrophoblast, and an inner proliferative, the cytotrophoblast.

SYNCYTIOTROPHOBLASTCYTOTROPHOBLAST

The embryoblast reorganizes into two layers, the epiblast dorsally and the hypoblast ventrally

EPIBLAST DORSALLY HYPOBLAST VENTRALLY

Two cavities are formed, the amniotic cavity dorsal to the epiblast and the yolk sac cavity ventral to the hypoblast

AMNIOTIC CAVITY AND THE YOLK SAC CAVITY

The epiblast and hypoblast appear as a slightly elongated disc (the bilaminar germ disc) and it is the epiblast that will give rise to all of the

tissues of the embryo.

Proliferation of epiblast cells at the margins of

the disc forms amnioblasts that line the amniotic cavity.

In a similar fashion, a primitive yolk sac is

created by proliferation of hypoblast cells at the

disc margins.

Thus, the embryonic disc is suspended between these two cavities.

Two layers of extraembryonic mesoderm are formed between the embryo and its cavities and cytotrophoblast.

Extraembryonic mesoderm is formed by the cells derived from the yolk sac and forms a connective tissue network.

Initially, this tissue forms as a single layer, but it soon separates into two layers: a layer around the yolk sac, which is the extraembryonic splanchnic mesoderm, and a layer over the amnion and on

the inner surface of the cytotrophoblast, which is the extraembryonic somatic mesoderm.

By 12 to 14 days, cells of the syncytiotrophoblast erode uterine blood vessels, and maternal blood fills spaces (lacunae) that form in the syncytium, bringing nutrients closer to the developing embryo.

3rd week of development

Gastrulation which results in the development of three germ layers, ectoderm, endoderm and mesoderm, occurs during this week.

3rd week of development

Gastrulation begins with the formation of a primitive streak, which later appears as a narrow groove, on the surface of the epiblast.

3rd week of development

The cephalic end of the primitive streak is called primitive node.

The primitive node consists of a slightly elevated area surrounding the small primitive pit.

3rd week of development

Cells of the epiblast migrate toward the primitive streak.

Upon arrival at the primitive streak they become flask shaped and detach from the epiblast and lie beneath it.

This type of cell movement is called invagination.

3rd week of development

Some of the invaginating cells displaces the hypoblast, giving rise to endoderm.

Some of the other cells occupy the region between the newly formed endoderm and the epiblast forming the mesoderm.

The cells remaining in the epiblst forms the ectoderm.

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