Table 1-3. Effect of Hormones on Breast
| Estrogen | Ducts, nipples, fat |
| Progesterone | Lobules, alveoli |
| Prolactin | Milk production |
| Oxytocin | Milk ejection |
Lactation
The breasts become fully developed under the influence of estrogen, progesterone, and pro-lactin during pregnancy. Prolactin causes the production of milk, and oxytocin release (viathe suckling reflex) causes the contraction of smooth-muscle cells in the ducts to eject the milk from the nipple.
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USMLE Step 2 l Obstetrics
• The first secretion of the mammary gland after delivery is colostrum. It contains more protein and less fat than subsequent milk, and contains IgA antibodies that impart some passive immunity to the infant. Most of the time it takes 1 to 3 days after deliv-ery for milk production to reach appreciable levels.
• The expulsion of the placenta at delivery initiates milk production and causes the drop in circulating estrogens and progesterone. Estrogen antagonizes the positive effect of prolactin on milk production.
• The physical stimulation of suckling causes the release of oxytocin and stimulates pro-lactin secretion, causing more milk production.
OB Triad
Post-Conception Week 1
• Starts at conception
• Ends with implantation
• Yields morula → blastula
OB Triad
Post-Conception Week 2
• Starts with implantation
• Ends with 2-layer embryo
• Yields bi-laminar germ disk
OB Triad
Post-Conception Week 3
• Starts with 2-layer embryo
• Ends with 3-layer embryo
• Yields tri-laminar germ disk
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EMBRYOLOGY AND FETOLOGY
Embryonic and Fetal Development
Postconception Week 1
The most significant event of week 1 is the implantation of the blastocyst on the endometrium. Week 1 begins with fertilization of the egg and ends with implantation of the blastocyst onto the endometrial surface. Fertilization usually occurs in the distal part of the oviduct. The egg is capable of being fertilized for 12–24 hours. The sperm is capable of fertilizing for 24–48 hours.
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Week 1 can be divided into 2 phases:
• The intratubal phase extends through the first half of the first week. It begins at con-ception (day 0) and ends with the entry of the morula into the uterine cavity (day 3). The conceptus is traveling down the oviduct as it passes through the 2-cell, 4-cell, and 8-cell stages.
• The intrauterine phase begins with entry of the morula into the uterus (day 3) and ends with implantation of the blastocyst onto the endometrial surface (day 6). During this time the morula differentiates into a hollow ball of cells. The outer layer will become the trophoblast or placenta, and the inner cell mass will become the embryo.
Postconception Week 2
The most significant event of week 2 is the development of the bilaminar germ disk with epi-blast and hypoblast layers. These layers will eventually give rise to the 3 primordial germ layers.
Another significant event is the invasion of the maternal sinusoids by syncytiotrophoblast. Because b-human chorionic gonadotropin (b-hCG) is produced in the syncytiotrophoblast, this now allows b-hCG to enter the maternal blood stream. b-hCG pregnancy test now can be positive for the first time.
Postconception Week 3
The most significant event of week 3 is the migration of cells through the primitive streak between the epiblast and hypoblast to form the trilaminar germ disk with ectoderm, meso-derm, and endoderm layers. These layers will give rise to the major organs and organ systems.
Postconception Weeks 4–8
During this time the major organs and organ systems are being formed. This is the period of major teratogenic risk.
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• Ectoderm—central and peripheral nervous systems; sensory organs of seeing andhearing; integument layers (skin, hair, and nails).
• Mesoderm—muscles, cartilage, cardiovascular system, urogenital system.
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• Endoderm—lining of the gastrointestinal and respiratory tracts.
Paramesonephric (Müllerian) Duct
This duct is present in all early embryos and is the primordium of the female internal reproduc-tive system. No hormonal stimulation is required.
• In males the Y chromosome induces gonadal secretion of müllerian inhibitory factor (MIF), which causes the müllerian duct to involute.
• In females, without MIF, development continues to form the fallopian tubes, corpus of the uterus, cervix, and proximal vagina.
Female External Genitalia
No hormonal stimulation is needed for differentiation of the external genitalia into labiamajora, labia minora, clitoris, and distal vagina.
Mesonephric (Wolffian) Duct
This duct is also present in all early embryos and is the primordium of the male internal reproduc-tive system. Testosterone stimulation is required for development to continue to form the vas deferens, seminal vesicles, epididymis, and efferent ducts. This is present in males from testicular sources. In females, without androgen stimulation, the Wolffian duct undergoes regression. If a genetic male has an absence of androgen receptors, the Wolffian duct will also undergo regression.
Male External Genitalia
Dihydrotestosterone (DHT) stimulation is needed for differentiation of the external genitaliainto a penis and scrotum. If a genetic male has an absence of androgen receptors, external geni-talia will differentiate in a female direction.
l Reproductive Basics
OB Triad
Post-Conception Week 4-8
• 3 germ layers differentiating
• Greatest risk of malformations
• Folic acid prevents NTD
| Sertoli |
| Leydig | |||||||||||||||||||
| cell |
| cell
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| 5α- | |||||||||||||||||||||
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| Anti-Müllerian | Testosterone | Dihydro- | |||||||||||||||||||
| hormone |
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| testosterone | ||||||||||||||||||
| reductase | |||||||||||||||||||||
| Inhibits Müllerian | Maintains Wolffian | Virilizes urogenital | |||||||||||||||||||
| duct development | duct development | sinus, external gentilia | |||||||||||||||||||
Figure 1-3. Testicular Function
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USMLE Step 2 l Obstetrics
| Hormones Needed for Genital Development | |||||||
| External? | None | ||||||
| Internal? | |||||||
| ì | External? | Androgen | |||||
| Internal? | |||||||
| Table 1-4. Embryology | |||||||
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| Primordia | Female | Male | Major Determinant Factors | ||||
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| Gonadal | |||||||
| Germ cells | Oogonia | Spermatogonia | Sex chromosomes | ||||
| Coelomic epithelium | Granulosa cells | Sertoli cells | |||||
| Mesenchyme | Theca cells | Leydig cells | |||||
| Mesonephros | Rete ovarii | Rete testis | |||||
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| Ductal | |||||||
| Paramesonephric | Fallopian tubes | Testis hydatid | Absence of Y chromosome | ||||
| (Müllerian) | Uterus | Vas deferens | Testosterone | ||||
| Mesonephric | Part of vagina | Seminal vesicles | Müllerian-inhibiting factor | ||||
| (Wolffian) | Gartner’s duct | Epididymis | |||||
| Mesonephric tubules | Epoophoron | Efferent ducts | |||||
| Paraoophron | |||||||
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| External Genitalia | |||||||
| Urogenital sinus | Vaginal contribution | Prostate | Presence or absense of | ||||
| Genital tubercle | Skene’s glands | Bulbourethral glands | testosterone, | ||||
| Urogenital folds | Bartholin’s glands | Prostatic utricle | dihydrotestosterone, and | ||||
| Genital folds | Clitoris | Penis | 5-alpha reductase enzyme | ||||
| Labia minora | Corpora spongiosa | ||||||
| Labia majora | Scrotum | ||||||
Teratology
A 36-year-old woman underwent a barium enema for rectal bleeding on February 1 with estimated radiation dose of 4 rad. Her last menstrual period (LMP) was January 1 and she has 35-day cycles. She was not using any contraception. A urine pregnancy test was positive on March 15. She inquires about the risk to her fetus of teratogenic injury.
Definition. A teratogen is any agent that disturbs normal fetal development and affects subse-quent function. The nature of the agent as well as its timing and duration after conception are critical. There are critical periods of susceptibility with each teratogenic agent and with each organ system.
Stages of Teratogenesis
• From conception to end of second week—The embryo will either survive intact ordie because the 3 germ layers have not yet been formed.
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Chapter 1 l Reproductive Basics
• Postconception weeks 3–8—This is the period of greatest teratogenic risk from for-mation of the 3 germ layers to completion of organogenesis.
• After week 9 of postconception—During this time teratogenicity is low, but adverseeffects may include diminished organ hypertrophy and hyperplasia.
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