Development and Inheritance: Human Embryo and Fetal Development

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Gestation and Stages of Human Development

Overview of Gestation

Gestation is the period during which the embryo and fetus develop in the uterus, culminating in birth. It is divided into three main stages:

Pre-embryonic stage: Fertilization to week 2
Embryonic stage: Weeks 3-8
Fetal stage: Week 9 to birth

Human embryo and fetal development stages

Fertilization

Mechanism of Fertilization

Fertilization is the fusion of the sperm and oocyte pronuclei, resulting in a diploid embryo.

Fusion: nsperm + noocyte = 2nembryo
Vaginal environment: The acidic pH (3.8) kills millions of sperm immediately; cervical mucus blocks more.
Survival: Sperm live 3-5 days; oocyte survives ~24 hours.

Oocyte with polar bodies, pronuclei, and zona pellucida

Capacitation of Sperm

Capacitation is the process by which sperm gain the ability to fertilize the oocyte.

Priming: The female reproductive tract modifies sperm, improving motility and thinning the acrosomal membrane.
Acrosomal reaction: Acrosomal enzymes interact with the zona pellucida, allowing sperm penetration.

Stages of sperm capacitation

Oocyte Layers and Sperm Penetration

The oocyte is surrounded by two layers:

Zona pellucida: Inner glycoprotein membrane, reacts with lysosomes.
Corona radiata: Outer layer of granulosa cells, digested by multiple acrosomes.
Chemotaxis: Oocyte releases chemoattractants to guide sperm.

Phases of sperm penetration into oocyte

Prevention of Polyspermy

Fast-Block Mechanism

To prevent polyspermy (fertilization by more than one sperm), the oocyte employs a fast-block mechanism:

Na+ permeability: Depolarizes cell membrane for ~1 minute, blocking additional sperm entry.

Polyspermy block and membrane depolarization Depolarization and fast polyspermy block

Slow-Block Mechanism (Cortical Reaction)

The slow-block involves the cortical reaction:

Zonal inhibiting protein: Causes sperm to release and destroys sperm receptors on oocyte.
Mucopolysaccharides: Join with zona pellucida to form an impenetrable fertilization membrane.

Timeline and mechanisms of polyspermy block

Zygote Formation and Early Cleavage

Zygote Formation

After fertilization:

Meiosis II: Metaphase II begins, polar body ejected, meiosis completed.
Pronuclei: Form and chromosomes intermingle, creating a diploid zygote.
Mitosis: Begins, leading to cellular division.
Twins: Fraternal twins result from fertilization of two separate oocytes.

Stages of zygote formation and pronuclei fusion

Pre-Implantation Development

Cleavage and Blastocyst Formation

Early embryonic development involves several stages:

Blastomere: Initial cellular division.
Morula: 16-cell stage, enters uterus.
Blastocyst: Blastocoel visible, cellular differentiation begins.

Development of the embryo: egg, zygote, morula, blastocyst Egg to blastocyst in female reproductive tract

Blastocyst Structure

Blastocoel: Fluid-filled cavity.
Inner cell mass: Embryo, totipotent cells.
Trophoblasts: Outer layer, nourish egg, form chorionic sac and fetal placenta.
Hatching: Embryo sheds zona pellucida before implantation.

Implantation

Mechanism of Implantation

Trophoblasts: Facilitate implantation; 50-70% fail to implant.
Syncytiotrophoblast: Fused trophoblasts digest endometrial cells and secure blastocyst.

Blastocyst implantation process

Hormonal Changes and Pregnancy Detection

Uterine mucosa: Envelops blastocyst after syncytiotrophoblast fusion.
hCG secretion: Trophoblasts secrete human chorionic gonadotropin (hCG).
Corpus luteum: Produces progesterone and estrogen.
Pregnancy test: Detects hCG a few days after implantation.

Problematic Implantations

Ectopic Pregnancy and Placenta Previa

Ectopic pregnancy: Implantation outside uterus (tubal, ovarian, abdominal).
Placenta previa: Embryo implants in inferior uterus, placenta covers internal os (cervical region).
Symptoms: Profuse bleeding at 24 weeks, antepartum hemorrhage.

Normal placenta vs placenta previa

Embryonic Membranes

Formation and Function of Membranes

Embryonic membranes are essential for protection and nourishment:

Amnion: Develops from trophoblasts, surrounds amniotic fluid and inner cell mass.
Yolk sac: Hypoblast layer, may develop into germ or blood cells.
Allantois: Joins yolk sac, becomes part of urinary bladder and umbilical cord.
Chorion: Fusion of yolk sac and allantois, surrounds all other membranes.

Development of embryonic membranes Embryonic disc and membrane development

Chorion and Placenta Formation

Chorionic villi: Burrow into endometrium, join fetal mesenchymal cells, develop placenta.

Blastocyst cavity and membrane structure Chorionic villi and placenta in uterus

Embryogenesis and Germ Layers

Gastrulation and Germ Layer Formation

Embryogenesis begins at week 3, with the embryonic disc forming three germ layers via gastrulation:

Primitive streak: Emits growth factors, leads to germ layer formation.
Germ layers: Ectoderm (nervous tissue, skin), Mesoderm (muscle, connective tissue), Endoderm (digestive system lining).

Germ layers and tissue development Germ layer differentiation Germ layer derivatives

Development of Placenta

Placental Structure and Function

Chorion: Initiates placental development around 14-16 weeks.
Umbilical arteries: Carry blood away from fetus' heart (low O2).
Umbilical veins: Carry blood toward fetus' heart (high O2).
Blood separation: Prevents maternal cytotoxic T cell attack on fetus.

Placental blood flow and chorionic villi

Embryonic Milestones and Organogenesis

Key Developmental Events

4th week: Heart tubes form.
4-5 weeks: Eye pit, limb buds, rudimentary pulmonary system.
6th week: Uncontrolled limb movements, intestines develop, fingers created by apoptosis.
7th week: Facial features begin.
8th week: Organogenesis, embryo is 3 cm and 8 g.

Embryonic limb development and milestones

Organogenesis and Brain Development

Neurulation and Brain Vesicle Formation

Neurulation: Neural plate forms neural fold, then neural tube, which merges with notochord.
Brain vesicles: Develop over 4-5 weeks; folate is crucial for preventing neural tube defects (e.g., spina bifida, anencephaly).
Three major vesicles: Forebrain (prosencephalon), midbrain (mesencephalon), hindbrain (rhombencephalon).

Brain development from neural tube to vesicles Embryonic folding and primitive gut formation Embryonic folding and gut development

Fetal Development

Fetal Stage and Organ System Maturation

The fetal stage extends from the 9th week until birth, with newborns capable of life outside the womb.

9th-12th week: Sexual differentiation, bipotential gonads influence Müllerian or Wolffian duct disintegration.
Blood shunts: Ductus venosus (bypasses liver), foramen ovale (bypasses lungs), ductus arteriosus (develops pulmonary circuit).
Waste removal: All fetal wastes diffuse through placenta into maternal circulation.

Sexual differentiation and duct development

Fetal Milestones

9-12 weeks: Frequent fetal movement, erythropoiesis by fetal bone marrow, amniotic breathing, urine production, eyes developed but shut.
13-16 weeks: Special sense organs, kidneys well-formed, intestines accumulate meconium (amniotic fluid, cellular debris, mucus, bile).
16-20 weeks: Fetal movements, fetal positioning.
21-30 weeks: Myelination, surfactant production, testes descend.
After 30 weeks: Subcutaneous fat builds up.

Example: Meconium is the first stool of the fetus, composed of ingested amniotic fluid, cellular debris, mucus, and bile. Additional info: Fetal development is marked by progressive organ system maturation and increasing viability outside the womb.