Anatomy
MBBS Anatomy question bank: general anatomy and osteology, regional gross anatomy (upper limb, lower limb, thorax, abdomen, pelvis, head and neck), neuroanatomy, histology and embryology.
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Definition
Gametogenesis is the formation of gametes (sperm & ova) by meiosis; fertilization is their fusion to form a zygote.
Spermatogenesis
- Occurs in seminiferous tubules
- Spermatogonia → spermatozoa (meiosis)
- Continuous from puberty
- Four sperm per spermatogonium
Oogenesis
- Begins in fetal life, then arrested
- One ovum per cycle (+ polar bodies)
- Completed only if fertilised
Fertilization
- In the ampulla of the uterine tube
- Sperm penetrates → cortical reaction
- Restores the diploid number → zygote
Meiosis makes haploid gametes; their fusion restores the diploid zygote. Feature Sperm Ovum Number Millions One per cycle Meiosis Continuous Arrested Product 4 sperm 1 ovum + polar bodies Applied
- In-vitro fertilisation (IVF)
- Non-disjunction → Down syndrome
🔑KEY POINTS TO REMEMBER- Gametogenesis = meiosis → haploid gametes.
- Sperm: 4 per spermatogonium; ovum: 1 + polar bodies.
- Fertilization in the ampulla → diploid zygote.
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).Definition
During the first two weeks, the zygote undergoes cleavage, forms a blastocyst, implants, and becomes a bilaminar germ disc.
First Week
- Zygote → morula → blastocyst
- Blastocyst = inner cell mass + trophoblast
- Zona pellucida shed
- Reaches the uterus by day 4–5
Second Week (‘week of 2s’)
- Bilaminar disc (epiblast + hypoblast)
- Trophoblast → cytotrophoblast + syncytiotrophoblast
- Two cavities: amniotic + yolk sac
- Implantation completed
The first two weeks turn the zygote into an implanted bilaminar disc. Week Key event Week 1 Cleavage, blastocyst Week 2 Bilaminar disc, implantation Applied
- Ectopic pregnancy
- hCG detected (pregnancy test)
🔑KEY POINTS TO REMEMBER- Week 1: zygote → morula → blastocyst.
- Week 2: bilaminar disc, two trophoblast layers, implantation.
- ‘Week of 2s’.
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).Definition
Gastrulation in the third week converts the bilaminar disc into a trilaminar disc with three germ layers — ectoderm, mesoderm and endoderm.
Gastrulation
- Primitive streak appears (in epiblast)
- Cells migrate → form 3 germ layers
- Ectoderm, mesoderm, endoderm
Other Week-3 Events
- Notochord forms
- Neural plate → neurulation begins
- Somites begin to appear
The primitive streak drives cells to form three germ layers. Germ layer Forms Ectoderm Skin, nervous system Mesoderm Muscle, bone, blood Endoderm Gut & gland lining Applied
- Teratomas (contain all 3 layers)
- Sacrococcygeal teratoma (streak remnant)
🔑KEY POINTS TO REMEMBER- Gastrulation: bilaminar → trilaminar via primitive streak.
- Germ layers: ectoderm, mesoderm, endoderm.
- Notochord + neurulation also in week 3.
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).Definition
The fetal membranes (amnion, chorion, yolk sac, allantois) protect and nourish the embryo; the placenta is the fetomaternal exchange organ.
Fetal Membranes
- Amnion — amniotic fluid (cushion)
- Chorion — forms the fetal placenta
- Yolk sac — early blood cells
- Allantois — urinary bladder, umbilical vessels
Placenta
- Fetal (chorion) + maternal (decidua) parts
- Chorionic villi — exchange units
- Functions: nutrition, gas, hormones, barrier
Fetal villi and maternal decidua together build the placenta. Membrane Role Amnion Amniotic fluid Chorion Placenta Yolk sac Early blood cells Allantois Bladder Applied
- Placenta praevia
- Amniocentesis
🔑KEY POINTS TO REMEMBER- Membranes: amnion, chorion, yolk sac, allantois.
- Placenta: fetal chorion + maternal decidua; villi exchange.
- Functions: nutrition, gas, hormones, barrier.
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).Definition
Twinning produces two offspring in one pregnancy; congenital anomalies are structural defects present at birth.
Types of Twins
- Dizygotic (2 eggs) — non-identical
- Monozygotic (1 egg splits) — identical
Congenital Anomalies (causes)
- Genetic (chromosomal, single-gene)
- Environmental (teratogens)
- Multifactorial
Common Anomalies
- Neural tube defects
- Cleft lip / palate
- Congenital heart defects
Twins arise from two eggs or from one that splits. Twin type Origin Dizygotic 2 eggs, 2 sperm Monozygotic 1 zygote splits Applied
- Teratogens (thalidomide, alcohol)
- Folic acid prevents NTDs
🔑KEY POINTS TO REMEMBER- Dizygotic (2 eggs, non-identical) vs monozygotic (1 splits, identical).
- Anomalies: genetic, environmental, multifactorial.
- Folic acid prevents neural tube defects.
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).Definition
Spermatogenesis is the formation of mature spermatozoa from spermatogonia in the seminiferous tubules.
Stages
- Spermatogonia (2n)
- Primary spermatocyte (2n)
- Secondary spermatocyte (n)
- Spermatid → spermatozoa (spermiogenesis)
Features
- Begins at puberty, continuous
- Takes ~64 days
- Needs testosterone, FSH, cool temperature
Meiosis and spermiogenesis turn a spermatogonium into mature sperm. Cell Ploidy Spermatogonium 2n Secondary spermatocyte n Spermatozoon n Applied
- Four sperm per spermatogonium
🔑KEY POINTS TO REMEMBER- Spermatogonium (2n) → spermatozoon (n).
- In seminiferous tubules; ~64 days; needs FSH, testosterone.
- 4 sperm per spermatogonium.
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).Definition
Fertilization is the fusion of sperm and ovum in the ampulla of the uterine tube, forming a diploid zygote.
Steps
- Capacitation of sperm
- Acrosome reaction → penetrate the zona
- Cortical reaction (blocks polyspermy)
- Fusion → restores the diploid number (zygote)
Sperm penetrate the ovum; the cortical reaction then blocks further sperm. Step Effect Capacitation Sperm activated Acrosome reaction Penetration Cortical reaction Blocks polyspermy Applied
- Determines sex (XX / XY)
- In-vitro fertilisation
🔑KEY POINTS TO REMEMBER- Fertilization in the ampulla → diploid zygote.
- Steps: capacitation, acrosome reaction, cortical reaction.
- Sperm determines sex.
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).Definition
The three germ layers (ectoderm, mesoderm, endoderm) give rise to all the tissues of the body.
Ectoderm
- Epidermis, nervous system
- Neural crest derivatives
Mesoderm
- Muscle, bone, connective tissue
- Cardiovascular system, kidney, gonads
Endoderm
- Lining of GI & respiratory tracts
- Liver, pancreas, glands
Each germ layer forms a defined set of tissues. Layer Derivatives Ectoderm Skin, nervous system Mesoderm Muscle, bone, blood, kidney Endoderm Gut lining, liver, lungs Applied
- Teratoma (contains all 3 layers)
🔑KEY POINTS TO REMEMBER- Ectoderm: skin + nervous system.
- Mesoderm: muscle, bone, blood, kidney, gonads.
- Endoderm: gut/respiratory lining, liver, pancreas.
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).Definition
The placenta is the fetomaternal organ for exchange, formed from fetal chorion and maternal decidua.
Structure
- Fetal part — chorionic villi
- Maternal part — decidua basalis
- Intervillous spaces (maternal blood)
Functions
- Respiratory (O₂ / CO₂)
- Nutritive, excretory
- Endocrine (hCG, oestrogen, progesterone)
- Barrier (partial)
Fetal villi bathe in maternal blood, exchanging nutrients and gases. Part Origin Villi Fetal (chorion) Decidua basalis Maternal Applied
- Some drugs/infections cross (TORCH)
- Placenta praevia
🔑KEY POINTS TO REMEMBER- Placenta: fetal chorionic villi + maternal decidua.
- Functions: respiratory, nutritive, endocrine, barrier.
- Partial barrier (TORCH cross).
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).Definition
Implantation is the embedding of the blastocyst into the uterine endometrium, around day 6–7.
Process
- Blastocyst hatches from the zona pellucida
- Trophoblast attaches to the endometrium
- Syncytiotrophoblast invades
- Completed by ~day 10
Normal Site
- Posterior wall of the uterus (fundus / body)
The blastocyst hatches, attaches and burrows into the endometrium. Feature Detail Time Day 6–7 Site Uterine fundus / body Applied
- Ectopic (tubal) pregnancy
- Placenta praevia (low implantation)
🔑KEY POINTS TO REMEMBER- Implantation ~day 6–7, completed by day 10.
- Site: uterine fundus / body.
- Abnormal → ectopic or placenta praevia.
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).Definition
Neural tube defects (NTDs) result from failure of the neural tube to close during the fourth week of development.
Types
- Anencephaly — failure of cranial closure
- Spina bifida — failure of caudal closure
- Meningocele, myelomeningocele
Causes & Prevention
- Folic acid deficiency
- Prevented by periconceptional folic acid
- Detected by ↑ maternal AFP and ultrasound
Failed neural-tube closure causes anencephaly or spina bifida. Defect Site Anencephaly Cranial Spina bifida Caudal (spine) Applied
- Folic acid supplementation
- AFP screening
🔑KEY POINTS TO REMEMBER- NTDs = failed neural-tube closure (week 4).
- Anencephaly (cranial) vs spina bifida (caudal).
- Folic acid prevents; ↑ AFP detects.
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).Definition
Twins are two offspring from one pregnancy, either dizygotic (fraternal) or monozygotic (identical).
Dizygotic (~2/3)
- Two eggs fertilised by two sperm
- Non-identical (like siblings)
- Always 2 amnions, 2 chorions
Monozygotic (~1/3)
- One zygote splits
- Identical (same genes)
- Membranes depend on the timing of the split
Two eggs give fraternal twins; one splitting zygote gives identical twins. Feature Dizygotic Monozygotic Eggs Two One (splits) Genes Different Identical Sex Same or different Same Applied
- Conjoined twins (late split)
- Twin-twin transfusion syndrome
🔑KEY POINTS TO REMEMBER- Dizygotic: 2 eggs, non-identical, always dichorionic.
- Monozygotic: 1 zygote splits, identical.
- Late split → conjoined twins.
📚SOURCES: Langman’s Medical Embryology; Inderbir Singh’s Human Embryology; The Developing Human (Moore).