INTRODUCTION TO EMBRYOLOGY

357GROWTH AND DEVELOPMENT

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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