Desarrollo aparato genital y Placenta

Desarrollo del Aparato Genital y Placenta

In this class, Professor Stella Maris Goma discusses the development of the genital system and placenta. The lecture is structured into three main parts: phases of intrauterine development, embryology of the genital system, and insights on human placental development.

Phases of Development

• Understanding the changes during intrauterine development is crucial for primary healthcare professionals in health promotion and disease prevention planning.

Recommended Study Materials

• Emphasizes the importance of studying beyond class materials, recommending textbooks like Arteaga Martínez Flores, Moore's Embryology book, and Carson's Embryology book for comprehensive understanding.

Counting Weeks in Pregnancy vs. Development

• Obstetricians count pregnancy weeks from the last menstrual period while embryologists start counting from fertilization day (day 14).

• Clarifies how to convert between pregnancy weeks and developmental weeks by adding or subtracting two weeks accordingly.

Stages of Intrauterine Development

• Pre-embryonic period (weeks 1-3): Formation of embryonic layers - ectoderm, mesoderm, endoderm - which give rise to all body organs.

Embryonic Period

• Embryonic period (weeks 4-8/9): Organ systems begin to develop; vulnerable stage for teratogens that can cause malformations.

Fetal Period

• Fetal period (week 9 until birth): Organs are formed; focus shifts to organ differentiation and maturation for functional readiness.

Embryology of Genital System Introduction

• Human reproduction involves sexual reproduction with distinct gamete production in males (sperm) and females (oocytes) through meiosis.

Chromosomal Determination of Sex

This section discusses how the chromosomal composition determines an individual's sex, starting with the presence of two X chromosomes indicating a female and one X or one Y chromosome determining a male.

Chromosomal Determination Process

• The presence of two X chromosomes results in a female chromosomal sex, while having one X or one Y chromosome leads to a male chromosomal sex.

• Following the chromosomal stage is the gonadal stage where functional gametes are formed exclusively, marking the beginning of phenotypic development.

• Chromosomal determination occurs at fertilization when female and male gametes unite, specifically when pronuclei merge. The SRY gene on the Y chromosome plays a crucial role in testicular development.

• The SRY gene encodes for the protein SRC, acting as a testis-determining factor. Lack of this gene in females leads to default differentiation towards female genital organs.

Gonadal Development Stages

This part delves into gonadal development stages, highlighting the phases of undifferentiated and differentiated gonads during embryonic development.

Gonadal Development Phases

• Gonadal development comprises two sub-stages: undifferentiated gonad and differentiated gonads.

• In the third week of development, mesodermal layers form, including intermediate mesoderm crucial for future gonad and kidney formation.

• By the fourth week, mesodermal lateral division creates parietal and visceral layers around coelomic cavity. Intermediate mesoderm contributes to urinary and reproductive systems' formation.

Differentiation Timeline

This segment outlines the timeline for undifferentiated gonads from fifth to seventh weeks in males and fifth to ninth weeks in females.

Differentiation Timeline Details

• Undifferentiated gonads transition from fifth to seventh weeks in males and fifth to ninth weeks in females.

• Microscopic images depict early genital system structures during this critical developmental period.

Genital Crest Formation

Discusses urogenital crest formation within coelomic cavity during embryonic growth stages.

Urogenital Crest Development

• Examination of urogenital crest reveals internal genital ridge utilized by urinary system. Distinction between genital and urinary crests based on their functions is highlighted.

• Further exploration into internal genital crest emphasizes its epithelial lining derived from lateral mesoderm. Proliferating cells form primary sexual cords aiding germ cell migration.

Germ Cell Migration Process

Explores germ cell migration mechanisms involving primitive germ cells' movement towards developing genital ridges.

Germ Cell Migration Mechanisms

• s Primitive germ cells originate from epiblast during second week before migrating towards developing genital ridges guided by chemotactic factors like fibronectin.

Detailed Overview of Gonadal Development

This section delves into the intricate process of gonadal development, highlighting the key cellular components and their roles in differentiation towards testicular or ovarian pathways.

Cellular Elements in Gonadal Development

• Mesodermal Contributions:

• The intermediate mesoderm of the genital ridge and the mesoderm from the urogenital ridge contribute cells to thicken these cellular clusters.

• Germ Cells Migration:

• Primordial germ cells migrate from the yolk sac to form the gonad, essential for subsequent differentiation.

• Differentiation Factors:

• Presence of Y chromosome leads to testicular differentiation through synthesis of testicular determining factor, influencing cell types like primitive sex cords.

Gonadal Differentiation and Proliferation

• Testicular Differentiation:

• Testosterone synthesis in males inhibits meiosis, promoting proliferation primarily in medullary regions forming testicular cords.

• Ovarian Differentiation:

• In females, absence of testosterone allows meiosis progression leading to oogonia formation without inhibition.

Derivatives and Phenotypic Expression

• Male Derivatives:

• Primordial germ cells give rise to spermatogonia while epithelial-derived cells form testicular cords and Leydig cells.

• Female Derivatives:

• Germ cells differentiate into oogonia initiating meiosis; epithelial cells yield follicular cells and mesodermal cells form theca cells.

Hormonal Influence on Phenotype

• Hormonal Determinants:

Development of Genital System in Embryos

This section discusses the development of the genital system in embryos, focusing on the differentiation and formation of various structures in males and females.

Differentiation of Male and Female Genital Systems

• During the phenotypic stage, female embryos exhibit persistence of Müller ducts, while Wolff ducts regress. In males, due to circulating androgens and anti-Müllerian factor produced by Sertoli cells, Müller ducts regress, allowing Wolff ducts to develop into internal male genital structures.

• In males, Wolff ducts give rise to internal male genital structures such as epididymis, spermatic cord, seminal vesicles, ejaculatory ducts, and prostate gland. The origin of the prostate gland is endodermal.

Formation of Female Reproductive Structures

• In females, without anti-Müllerian factor presence, Müller ducts persist to form fallopian tubes, uterus, and upper two-thirds of the vagina. Wolff duct remnants almost disappear during female development.

• The gonads in both sexes are initially located internally near the urogenital ridge. The cranial part of the urogenital ridge forms ligamentum teres uteri while the caudal part forms gubernaculum. Testes and ovaries descend due to gubernaculum traction influenced by androgens.

Descendence of Gonads in Males

• The descent of testes in males involves ligament transformations: cranial ligament disappears while gubernaculum guides testicular descent towards the scrotum through inguinal canal.

• Testicular descent is more pronounced in males due to androgen influence. After passing through inguinal canal, testes settle in scrotum permanently.

External Genitalia Development

• External genitalia originate from three structures: tubercle gives rise to clitoris or penis; cloacal swellings fuse forming scrotum in males; genital swellings merge into labia majora in females.

Placental Development Before Fourth Month

This segment delves into placental development before the fourth month of pregnancy with a focus on changes induced by progesterone on endometrium leading to residual alterations.

Endometrial Changes Induced by Progesterone

• Progesterone-induced changes lead to residual modifications in endometrium during pregnancy termed decidua. These changes prepare for implantation and early embryonic development.

Syncytiotrophoblast Formation

• Syncytiotrophoblast advances outside embryo releasing lytic enzymes that degrade stroma for nutrient absorption. It secretes human chorionic gonadotropin maintaining progesterone secretion crucial for endometrial changes supporting pregnancy.

Villi Formation for Nutrient Exchange

Detailed Embryonic Development Process

In this section, the speaker delves into the detailed process of embryonic development, focusing on the changes that occur in different weeks of development.

Embryonic Development Stages

• The outer layer of the embryo, when cut transversely in the second week, reveals a soft piece with another population inside known as primary or epithelial villi.

• By the third week, there is uneven growth in the endometrium represented by residual black areas called basal desidia. This phase sees blastocyst trophoblast proliferating and forming cellular columns known as cytotrophoblastic columns.

• These cellular columns send waves of cells that gradually revert to a simple epithelium in incomplete areas, termed cytotrophoblastic shield or plastic armor. Additionally, the intraembryonic mesoderm proliferates and forms a structure within villi resembling an axis.

• Towards the end of the third week, fetal blood vessels start forming within the extraembryonic mesoderm's parietal layer (chorion), leading to tertiary chorionic villi containing fetal capillaries at their center.

Placental Development and Functions

• As development progresses into fourth and fifth weeks, cellular columns from cytotrophoblast undergo significant cell waves resulting in complete formation of plastic shield or armor facilitating placental growth while preventing excessive implantation risks.

• By fifth week, a fully formed shield or armor surrounds the embryo within the amniotic cavity filled with amniotic fluid. The chorionic cavity shows extensive proliferation of villi forming a frond-like appearance known as frondosum chorion while less dense areas are termed smooth chorion.

Understanding Placental Functions

Placenta Structure and Function

In this section, the structure and function of the placenta are discussed, highlighting its role in hematopoiesis and metabolic functions.

Formation of the Placenta

• The placenta serves as a hematopoietic organ, producing blood cells in humans.

• By the fourth month of development, the placenta takes on a discoid shape with two layers: basal plate and chorionic plate.

• The chorionic plate contains chorionic villi responsible for anchoring both placental layers together.

Placental Barrier and Villi Structure

• The placental barrier separates maternal and fetal blood circulation to prevent direct contact.

• Chorionic villi extend from the chorionic plate to anchor into the basal plate, ensuring structural integrity.

Vellosities in Placental Development

• Vellosities play a crucial role in nutrient exchange between maternal and fetal circulations within the placenta.

• Tertiary vellosities are primarily responsible for nutrient exchange due to their intricate structure.

Maturation of Vellosities

• During early pregnancy, immature vellosities facilitate gas exchange through capillary networks within them.

• Oxygen diffuses through various layers including trophoblastic basement membrane before reaching fetal circulation.

Transport Mechanisms in Placental Barrier

Placental Barrier Development in Pregnancy

In this section, the speaker discusses the development of the placental barrier during pregnancy, focusing on changes that occur from the first to the third trimester.

Endothelial Membrane Changes

• The endothelial membrane of the mesenchymal capillary has a basal membrane.

Evolution of Placental Barrier

• The placental barrier is immature but adapts well in the first trimester.

• Challenges arise in the third trimester due to increased metabolic and nutrient demands.

Third Trimester Changes

• Vellosities become more numerous but smaller in size.

• Capillaries become tortuous and prominent, especially at the periphery.

Structural Changes in Placental Barrier

This section delves into structural alterations within the placental barrier as pregnancy progresses, impacting nutrient exchange between maternal and fetal blood.

Capillary Alterations

• Capillaries become dilated and tortuous, mainly located at the periphery for efficient exchange.

Barrier Conformation

• The placental barrier transforms into a single layer with thinning of membranes and fusion of basal membranes.

Membrane Thinning and Nutrient Exchange

Here, details about membrane thinning, nutrient exchange facilitation, and final reflections on placental structure are discussed.

Membrane Fusion

• Membranes basal fuse due to disappearance of mesenchyme at exchange zones.

Layer Reduction

• The six-layered structure reduces to three layers by late pregnancy, enhancing nutrient transfer efficiency.

Concluding Remarks on Placental Structure

Final thoughts on placental structure evolution are shared through an analogy with a tree's branches resembling placentary villi.

Symbolism Analogy

• An analogy is drawn between placentary villi branching and birds symbolizing freedom bringing nutrients to their young.