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ABB Embryology practice questions

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Question 1Gametogenesis and Reproductive Anatomy

Primordial germ cells that will populate the developing gonad initially arise in which location?

  1. The yolk sac wall near the allantois Correct answer.
  2. The gonadal ridge epithelium
  3. The chorionic mesoderm
  4. The amniotic ectoderm

Explanation

Primordial germ cells are specified in the epiblast and migrate through the yolk sac wall near the allantois before traveling to the gonadal ridges. The gonadal ridge contributes somatic support cells, not the germ cells themselves.

Question 2Gametogenesis and Reproductive Anatomy

A primary oocyte arrested in prophase I is surrounded by a single layer of flattened follicular cells. This structure is best termed a:

  1. Secondary follicle
  2. Primary follicle
  3. Primordial follicle Correct answer.
  4. Graafian follicle

Explanation

A primordial follicle consists of a primary oocyte arrested at prophase I surrounded by a single layer of flattened (squamous) follicle cells. Cuboidal follicle cells mark the transition to a primary follicle.

Question 3Gametogenesis and Reproductive Anatomy

The acrosome of a spermatid is derived most directly from which organelle?

  1. The mitochondrion
  2. The centriole
  3. The rough endoplasmic reticulum
  4. The Golgi apparatus Correct answer.

Explanation

Golgi vesicles coalesce over the anterior nucleus of the spermatid to form the acrosomal cap, which contains hydrolytic enzymes. Mitochondria instead reorganize into the midpiece sheath.

Question 4Gametogenesis and Reproductive Anatomy

In the male, the rete testis drains directly into which structure?

  1. The epididymis
  2. The efferent ductules Correct answer.
  3. The ductus deferens
  4. The seminiferous tubules

Explanation

Sperm pass from seminiferous tubules to the rete testis and then through efferent ductules into the epididymis. The efferent ductules are therefore the immediate downstream structure.

Question 5Gametogenesis and Reproductive Anatomy

Which hormone surge is responsible for triggering ovulation?

  1. Progesterone
  2. Luteinizing hormone Correct answer.
  3. Prolactin
  4. Follicle-stimulating hormone

Explanation

A midcycle surge of luteinizing hormone triggers follicular rupture and resumption of meiosis in the oocyte. FSH supports follicular growth but does not itself trigger rupture.

Question 6Gametogenesis and Reproductive Anatomy

The gubernaculum in the male fetus functions chiefly to:

  1. Guide descent of the testis Correct answer.
  2. Secrete testosterone
  3. Close the inguinal canal
  4. Form the tunica vaginalis

Explanation

The gubernaculum is a mesenchymal cord that anchors the testis and guides its descent toward the scrotum. Testosterone and INSL3 drive the process but are not produced by the gubernaculum.

Question 7Gametogenesis and Reproductive Anatomy

Spermatogenesis produces four haploid spermatids from one primary spermatocyte. What is the ploidy and chromatid content of each secondary spermatocyte?

  1. Diploid with paired chromatids
  2. Haploid with paired chromatids Correct answer.
  3. Diploid with single chromatids
  4. Haploid with single chromatids

Explanation

Meiosis I separates homologs, so each secondary spermatocyte is haploid but retains two chromatids per chromosome until meiosis II. Only after meiosis II do single-chromatid spermatids appear.

Question 8Gametogenesis and Reproductive Anatomy

A woman with a 28-day cycle ovulates on day 14. On which day is the oocyte normally completing meiosis II?

  1. Day 10, during follicle selection
  2. Day 21, after fertilization occurs Correct answer.
  3. Day 14, just before ovulation
  4. Day 28, at menstruation

Explanation

The secondary oocyte is ovulated arrested in metaphase II and completes meiosis II only if a sperm penetrates it. Completion therefore occurs after ovulation, around fertilization.

Question 9Gametogenesis and Reproductive Anatomy

Sertoli cells form the blood-testis barrier. Which junction type is primarily responsible for this barrier?

  1. Hemidesmosomes at the basal lamina
  2. Desmosomes between Sertoli and Leydig cells
  3. Gap junctions between Sertoli cells
  4. Tight junctions between adjacent Sertoli cells Correct answer.

Explanation

Occluding (tight) junctions between adjacent Sertoli cells near the basal compartment create the barrier. This isolates adluminal germ cells from immune recognition.

Question 10Gametogenesis and Reproductive Anatomy

Which cell type secretes androgen-binding protein (ABP) under FSH stimulation?

  1. Sertoli cells Correct answer.
  2. Leydig cells
  3. Epididymal principal cells
  4. Peritubular myoid cells

Explanation

FSH stimulates Sertoli cells to produce androgen-binding protein, which concentrates testosterone in the tubule lumen. Leydig cells produce testosterone but not ABP.

Question 11Gametogenesis and Reproductive Anatomy

The corpus luteum secretes progesterone primarily to:

  1. Trigger follicular recruitment
  2. Maintain the secretory endometrium Correct answer.
  3. Initiate menstruation
  4. Induce the LH surge

Explanation

Progesterone from the corpus luteum stabilizes and matures the endometrium to support implantation. Its withdrawal, not its presence, leads to menstruation.

Question 12Gametogenesis and Reproductive Anatomy

In the developing ovary, the absence of SRY permits which structure to persist as the ovary's medullary cords regress?

  1. The paramesonephric (Müllerian) duct
  2. The rete ovarii
  3. The cortical sex cords Correct answer.
  4. The mesonephric (Wolffian) duct

Explanation

Without SRY, the indifferent gonad develops cortical sex cords that give rise to follicles, while medullary cords regress. The paramesonephric ducts form the female tract but are not gonadal cords.

Question 13Gametogenesis and Reproductive Anatomy

Which structure gives rise to the female uterus and upper vagina?

  1. The mesonephric ducts
  2. The urogenital sinus
  3. The genital tubercle
  4. The paramesonephric ducts Correct answer.

Explanation

The fused caudal portions of the paramesonephric (Müllerian) ducts form the uterus and upper vagina. The lower vagina derives from the urogenital sinus.

Question 14Gametogenesis and Reproductive Anatomy

A male infant has a unilateral undescended testis at birth. Which consequence is most directly attributable to the elevated abdominal temperature?

  1. Failure of the epididymis to form
  2. Absence of the cremaster reflex
  3. Loss of Leydig cell testosterone production
  4. Impaired spermatogenesis Correct answer.

Explanation

Spermatogenesis requires the cooler scrotal environment; sustained abdominal temperature impairs germ cell development. Endocrine Leydig function is comparatively preserved.

Question 15Gametogenesis and Reproductive Anatomy

During spermiogenesis, excess cytoplasm is shed as a residual body. This process primarily serves to:

  1. Reduce cell mass for motility Correct answer.
  2. Replicate DNA
  3. Duplicate the acrosome
  4. Increase mitochondrial number

Explanation

Shedding the residual body eliminates excess cytoplasm, producing a streamlined cell suited for flagellar propulsion. DNA replication and acrosome formation occur earlier.

Question 16Gametogenesis and Reproductive Anatomy

Which of the following best describes the location of the ovarian cortex relative to the medulla?

  1. The cortex is the hilum region
  2. The cortex lies within the broad ligament
  3. The cortex is the central vascular zone
  4. The cortex is the outer zone containing follicles Correct answer.

Explanation

The ovarian cortex is the outer region where follicles reside, while the medulla is the central vascular stroma. This arrangement is reversed relative to the testis, where tubules are medullary.

Question 17Gametogenesis and Reproductive Anatomy

A 46,XY fetus lacks functional androgen receptors. Which derivative will fail to develop despite normal testosterone synthesis?

  1. The external genitalia and prostate Correct answer.
  2. The rete testis
  3. The epididymis and ductus deferens
  4. The seminal vesicles

Explanation

Testosterone acts via androgen receptors to drive Wolffian derivatives, but external genitalia and prostate depend on dihydrotestosterone signaling through the same receptor. Loss of receptor function impairs these DHT-dependent structures.

Question 18Gametogenesis and Reproductive Anatomy

A follicle with a well-developed antrum shows a cumulus oophorus. Which meiotic stage is the enclosed oocyte in?

  1. Metaphase II of meiosis II
  2. Telophase II of meiosis II
  3. Prophase I of meiosis I Correct answer.
  4. Metaphase I of meiosis I

Explanation

Antral follicles contain a primary oocyte still arrested in prophase I; meiosis resumes only with the LH surge. Metaphase II arrest applies to the ovulated secondary oocyte.

Question 19Gametogenesis and Reproductive Anatomy

Bilateral absence of Müllerian ducts with normal testes and testosterone production would most likely result from failure of which process?

  1. AMH-mediated paramesonephric duct regression Correct answer.
  2. DHT-driven prostate formation
  3. Testosterone-driven Wolffian duct maintenance
  4. SRY-induced testis determination

Explanation

In a male, anti-Müllerian hormone from Sertoli cells causes regression of the paramesonephric ducts. If AMH signaling fails, Müllerian derivatives persist rather than being absent; conversely, their absence reflects intact AMH action, so the question tests understanding that AMH causes regression.

Question 20Gametogenesis and Reproductive Anatomy

Azoospermia is found in a man with normal Leydig cell function. Which defect best fits an isolated failure of the Sertoli cell barrier and germ cell maturation?

  1. Impaired androgen-binding protein and tight junction formation Correct answer.
  2. Absent LH receptors on Leydig cells
  3. Deficiency of 5-alpha reductase
  4. Failure of gubernaculum descent

Explanation

Sertoli cells form the blood-testis barrier and support germ cell maturation; their dysfunction can cause azoospermia while Leydig steroidogenesis remains intact. LH receptor loss would impair testosterone production instead.

Question 21Fertilization and Early Embryonic Development

Where does fertilization normally occur in the human female reproductive tract?

  1. On the ovarian surface
  2. In the uterine cavity
  3. In the cervix
  4. In the ampulla of the uterine tube Correct answer.

Explanation

Fertilization typically takes place in the ampullary region of the fallopian tube, where sperm meet the oocyte after ovulation. The zygote then travels toward the uterus for implantation.

Question 22Fertilization and Early Embryonic Development

What is the name of the process by which a sperm penetrates the layers surrounding the oocyte to allow fusion of the two gametes?

  1. Acrosome reaction Correct answer.
  2. Capacitation
  3. Cortical reaction
  4. Implantation

Explanation

The acrosome reaction releases enzymes from the sperm's acrosome that digest a path through the zona pellucida. Capacitation is a prerequisite priming step, and the cortical reaction occurs after fusion.

Question 23Fertilization and Early Embryonic Development

Which event prevents polyspermy immediately after the first sperm fuses with the oocyte membrane?

  1. Completion of the second meiotic division
  2. Fusion of the pronuclei
  3. Release of cortical granules and modification of the zona pellucida Correct answer.
  4. Formation of the fertilization cone

Explanation

Cortical granules release their contents into the perivitelline space, altering the zona pellucida so additional sperm cannot bind and fuse. This is the cortical reaction and provides the block to polyspermy.

Question 24Fertilization and Early Embryonic Development

A sperm that has undergone capacitation differs from a freshly ejaculated sperm primarily in that it has:

  1. Fused with the zona pellucida
  2. Completed meiosis
  3. Lost its flagellum
  4. Acquired the ability to undergo the acrosome reaction and fertilize Correct answer.

Explanation

Capacitation involves changes to the sperm membrane, including removal of glycoprotein components, that make the sperm competent to undergo the acrosome reaction. Without capacitation, a sperm cannot fertilize the oocyte.

Question 25Fertilization and Early Embryonic Development

At the time of fertilization, the secondary oocyte is arrested in which stage of meiosis?

  1. Anaphase I
  2. Metaphase II Correct answer.
  3. Prophase I
  4. Telophase II

Explanation

The secondary oocyte is ovulated arrested in metaphase of meiosis II. Sperm entry triggers completion of meiosis II and extrusion of the second polar body.

Question 26Fertilization and Early Embryonic Development

What is the haploid chromosome number restored in the zygote called, and how many pronuclei fuse to form it?

  1. Two pronuclei, one maternal and one paternal Correct answer.
  2. One pronucleus containing all chromosomes
  3. Four pronuclei, two maternal and two paternal
  4. Three pronuclei, one maternal and two paternal

Explanation

The zygote receives one maternal pronucleus from the oocyte and one paternal pronucleus from the sperm, restoring the diploid chromosome number. Their fusion forms the single diploid nucleus of the zygote.

Question 27Fertilization and Early Embryonic Development

Cleavage divisions differ from ordinary mitotic divisions of somatic cells in that during cleavage:

  1. The total volume of the embryo remains essentially unchanged Correct answer.
  2. Chromosomes are not replicated
  3. Cells become haploid
  4. Cell volume increases with each division

Explanation

During cleavage the zygote divides into progressively smaller blastomeres without a net increase in cytoplasmic mass. The overall volume of the embryo stays roughly constant until the blastocyst stage.

Question 28Fertilization and Early Embryonic Development

The morula stage of the human embryo is best described as:

  1. A single-celled zygote
  2. A bilaminar disc
  3. A fluid-filled cavity surrounded by trophoblast
  4. A solid ball of about 16 blastomeres Correct answer.

Explanation

The morula is a compacted solid mass of roughly 16 cells formed after several cleavage divisions. It precedes formation of the blastocyst cavity.

Question 29Fertilization and Early Embryonic Development

In the blastocyst, the inner cell mass gives rise to which structure?

  1. The embryo proper Correct answer.
  2. The trophoblast
  3. The amnion only
  4. The syncytiotrophoblast

Explanation

The inner cell mass (embryoblast) develops into the embryo proper and contributes to some extraembryonic membranes. The outer trophoblast forms the placental tissues.

Question 30Fertilization and Early Embryonic Development

The zona pellucida is lost during which process, allowing the blastocyst to implant?

  1. Gastrulation
  2. Compaction
  3. Neurulation
  4. Hatching Correct answer.

Explanation

Hatching is the escape of the blastocyst from the zona pellucida, which must occur before implantation. Without hatching, the embryo cannot attach to the endometrium.

Question 31Fertilization and Early Embryonic Development

Implantation of the human blastocyst normally begins on approximately which day after fertilization?

  1. Day 14
  2. Day 6 to 7 Correct answer.
  3. Day 1
  4. Day 21

Explanation

The blastocyst reaches the uterus and begins to implant in the endometrium around days 6 to 7 after fertilization. Implantation is completed during the second week.

Question 32Fertilization and Early Embryonic Development

During implantation, the trophoblast differentiates into which two layers?

  1. Ectoderm and endoderm
  2. Epiblast and hypoblast
  3. Cytotrophoblast and syncytiotrophoblast Correct answer.
  4. Splanchnic and somatic mesoderm

Explanation

The trophoblast splits into an inner cytotrophoblast and an outer multinucleated syncytiotrophoblast. The syncytiotrophoblast invades the endometrium to establish the placenta.

Question 33Fertilization and Early Embryonic Development

The bilaminar embryonic disc consists of which two layers?

  1. Yolk sac and chorion
  2. Epiblast and hypoblast Correct answer.
  3. Ectoderm and mesoderm
  4. Trophoblast and amnion

Explanation

In the second week the inner cell mass reorganizes into the epiblast and hypoblast, forming the bilaminar disc. The epiblast later gives rise to all three germ layers.

Question 34Fertilization and Early Embryonic Development

The primitive streak appears during which week of human development?

  1. Fifth week
  2. First week
  3. Third week Correct answer.
  4. Second week

Explanation

The primitive streak forms during the third week and marks the beginning of gastrulation. It defines the body axes and is the site where epiblast cells ingress to form mesoderm and endoderm.

Question 35Fertilization and Early Embryonic Development

Gastrulation in the human embryo produces which three germ layers?

  1. Epiblast, hypoblast, and trophoblast
  2. Mesoderm, hypoblast, and epiblast
  3. Ectoderm, endoderm, and trophoblast
  4. Ectoderm, mesoderm, and endoderm Correct answer.

Explanation

Gastrulation converts the bilaminar disc into a trilaminar disc with ectoderm, mesoderm, and endoderm. These three layers give rise to all body tissues.

Question 36Fertilization and Early Embryonic Development

The notochord is derived from which germ layer?

  1. Mesoderm Correct answer.
  2. Trophoblast
  3. Endoderm
  4. Ectoderm

Explanation

The notochord arises from mesoderm, specifically from the axial mesoderm that migrates through the primitive node. It induces formation of the overlying neural tube.

Question 37Fertilization and Early Embryonic Development

A cell migrating through the primitive streak that ultimately contributes to the lining of the gut most likely became which germ layer?

  1. Neural crest
  2. Definitive endoderm Correct answer.
  3. Paraxial mesoderm
  4. Surface ectoderm

Explanation

Cells that ingress through the primitive streak and replace the hypoblast become definitive endoderm, which forms the epithelial lining of the digestive tract. Ectoderm and mesoderm give rise to other tissues.

Question 38Fertilization and Early Embryonic Development

The neural plate is induced primarily by signals from which structure?

  1. The yolk sac endoderm
  2. The notochord Correct answer.
  3. The amnion
  4. The trophoblast

Explanation

The notochord secretes inductive signals that cause overlying ectoderm to thicken into the neural plate. This begins neurulation in the third week.

Question 39Fertilization and Early Embryonic Development

The neural tube is formed by fusion of which structure?

  1. Neural folds Correct answer.
  2. Somites
  3. Pharyngeal arches
  4. Primitive streak

Explanation

The neural plate folds to form neural folds whose edges elevate and fuse at the midline to create the neural tube. Failure of fusion causes neural tube defects.

Question 40Fertilization and Early Embryonic Development

Failure of the anterior neuropore to close results in which condition?

  1. Cleft palate
  2. Anencephaly Correct answer.
  3. Polydactyly
  4. Spina bifida

Explanation

The anterior neuropore closes around day 25; failure leads to anencephaly because the forebrain does not develop normally. Spina bifida results from failure of posterior neuropore closure.

Question 41Fertilization and Early Embryonic Development

The three primary brain vesicles that form from the cranial end of the neural tube are:

  1. Prosencephalon, mesencephalon, rhombencephalon Correct answer.
  2. Diencephalon, mesencephalon, myelencephalon
  3. Myelencephalon, metencephalon, rhombencephalon
  4. Telencephalon, diencephalon, mesencephalon

Explanation

The cranial neural tube expands into three primary vesicles: prosencephalon, mesencephalon, and rhombencephalon. These later subdivide into the five secondary vesicles.

Question 42Fertilization and Early Embryonic Development

Somites are derived from which subdivision of mesoderm?

  1. Intermediate mesoderm
  2. Visceral mesoderm
  3. Lateral plate mesoderm
  4. Paraxial mesoderm Correct answer.

Explanation

Somites form from paraxial mesoderm flanking the neural tube. They subsequently differentiate into sclerotome, myotome, and dermatome.

Question 43Fertilization and Early Embryonic Development

A researcher observes an embryo with a well-formed primitive streak and a developing notochord but no recognizable somites. Which process is most likely complete?

  1. Organogenesis
  2. Gastrulation Correct answer.
  3. Implantation
  4. Neurulation

Explanation

The presence of a primitive streak and notochord indicates that gastrulation has produced mesoderm and axial structures. Somite segmentation is a later step, so gastrulation is the most advanced completed process.

Question 44Fertilization and Early Embryonic Development

The allantois and the umbilical vessels develop in association with which extraembryonic structure?

  1. The chorion
  2. The yolk sac Correct answer.
  3. The amnion
  4. The decidua capsularis

Explanation

The allantois extends from the yolk sac into the connecting stalk and is associated with umbilical vessel development. The yolk sac also provides early blood formation.

Question 45Fertilization and Early Embryonic Development

Which structure secretes human chorionic gonadotropin (hCG) to maintain the corpus luteum in early pregnancy?

  1. The notochord
  2. The hypoblast
  3. The amnion
  4. The syncytiotrophoblast Correct answer.

Explanation

The syncytiotrophoblast produces hCG, which sustains the corpus luteum so it continues secreting progesterone. This hormonal support is essential until the placenta takes over.

Question 46Fertilization and Early Embryonic Development

Capacitation of a mammalian spermatozoon before fertilization is best described as which of the following?

  1. Completion of the second meiotic division by the oocyte nucleus
  2. Destabilization of the acrosomal membrane so that hydrolytic enzymes can be released
  3. Fusion of the sperm and oocyte membranes into a single continuous bilayer
  4. Removal of glycoprotein coat and seminal plasma proteins from the sperm plasma membrane Correct answer.

Explanation

Capacitation is a conditioning period in the female reproductive tract in which the glycoprotein coat and seminal plasma proteins are stripped from the sperm plasma membrane, making the sperm competent to undergo the acrosome reaction. It is a membrane-surface change, not the acrosome reaction itself.

Question 47Fertilization and Early Embryonic Development

A spermatozoon penetrates the corona radiata and binds to the zona pellucida. Which interaction is principally responsible for this species-specific binding?

  1. Integrin binding to fibronectin exposed on the oocyte surface
  2. Sperm surface proteins binding to ZP3 glycoproteins of the zona pellucida Correct answer.
  3. Fertilin binding to a receptor on the oolemma
  4. Hyaluronidase digesting a channel through the cumulus mass

Explanation

Species-specific sperm binding to the zona pellucida is mediated by sperm surface molecules recognizing ZP3, which then triggers the acrosome reaction. Receptors on the oolemma mediate the later fusion step, after the sperm has traversed the zona.

Question 48Fertilization and Early Embryonic Development

Following fusion of the sperm and oocyte membranes, a wave of intracellular calcium release causes cortical granules to exocytose. What is the immediate consequence of this event?

  1. Resumption of meiosis II by the oocyte
  2. Formation of the male pronucleus
  3. Completion of the acrosome reaction
  4. Hardening of the zona pellucida that blocks polyspermy Correct answer.

Explanation

Cortical granule exocytosis releases enzymes that modify zona pellucida glycoproteins, producing the zona reaction that hardens the zona and prevents additional sperm from binding and fusing. This is the principal block to polyspermy.

Question 49Fertilization and Early Embryonic Development

A zygote carries a normal diploid chromosome complement. Which statement best characterizes the chromosome content of the two pronuclei just before they fuse?

  1. Both pronuclei are haploid, each contributing one chromatid set of chromosomes Correct answer.
  2. Both pronuclei are diploid but the cytoplasm discards half the chromosomes
  3. The male pronucleus is haploid while the female pronucleus is diploid
  4. Each pronucleus is diploid, so fusion would produce a tetraploid cell

Explanation

The male and female pronuclei are each haploid, so their fusion restores the diploid number in the zygote. The oocyte completes meiosis II after sperm entry, leaving a haploid female pronucleus.

Question 50Fertilization and Early Embryonic Development

An embryo is examined in culture and shows a fluid-filled cavity, a distinct outer epithelial layer, and a compact cluster of cells localized to one pole. Which combination of features and fate is most consistent with this stage?

  1. Gastrula stage; the cavity is the yolk sac and the cluster is the primitive streak
  2. Bilaminar disc stage; the cavity is the amniotic cavity and the cluster is the hypoblast
  3. Blastocyst stage; the outer layer forms the trophoblast and the polar cluster forms the inner cell mass Correct answer.
  4. Morula stage; all blastomeres remain totipotent and will form the embryo proper

Explanation

Cavitation with segregation of an outer epithelial trophoblast and an eccentrically placed inner cell mass defines the blastocyst. The trophoblast contributes to placental tissues while the inner cell mass gives rise to the embryo proper and its membranes.